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Continuous stable isotope records from marine sediment cores spanning the Pliocene have been used to assess the oceans' response to major perturbations in the climate system as the oceans play an integral role in regulating the global distribution of heat and gases. The Early to mid-Pliocene has previously been characterized as a time of relative warmth followed by Late Pliocene Southern Hemisphere cooling and bipolar glaciation at ~2.7 Ma. Previous studies have predominantly focused on the Atlantic and Equatorial Pacific Oceans. In this study, we extended the deep water benthic foraminifera stable isotope record from Ocean Drilling Program (ODP) Site 1123 in the southwest Pacific, back to the warm Early Pliocene. This is a high-latitude site at the gateway where the abyssal waters enter the Pacific Ocean and provides information about the connection between the Southern Ocean and the Pacific. We identify a dichotomy between the deep southwest Pacific and South Atlantic δC records spanning the mid-Pliocene and suggest that this is most likely the result of variations in the relative contributions of Northern versus Southern Hemisphere deep waters to the different basins. At 3.6 Ma, δC values start to decrease; this is interpreted to represent alteration in preformed values as a result of increased remineralization of carbon caused by a reduction in deep ocean ventilation in the Southern Ocean. This is likely the consequence of a greater extent and seasonal duration of sea ice in the Southern Ocean from Antarctic Ice Sheet expansion and cooling.

A 24-meter jumbo piston core (NBP0101 JPC41) collected from an inner shelf basin in Iceberg Alley reveals an approximately 2000-year history of unusually high primary productivity. Iceberg Alley, an ~ 85 km long and 10–20 km wide cross-shelf trough on the Mac.Robertson Shelf, East Antarctica, reaches depths of 850 m and is bounded on either side by shallow banks that are lined with grounded icebergs. The sediments are laminated on a mm- to cm-scale throughout and are highly biosiliceous. Microscopic examination of smear slides, quantitative diatom slides, and sediment thin sections reveals that the sediments are visually dominated by the diatom Corethron pennatum, a large and lightly silicified species notable for its long and narrow shape; the valves, girdle bands and spines are all exceptionally well-preserved, suggesting rapid sedimentation. Other common species include sea ice-related Fragilariopsis, such as F. curta and F. cylindrus, with lesser contribution from other large diatoms, including Rhizosolenia spp. and Chaetoceros Ehrenberg subg. Chaetoceros. Chaetoceros Ehrenberg subg. Hyalochaete Gran resting spores, typically associated with large early-season blooms and common in many laminated sedimentary sections around the Antarctic margin, are surprisingly rare. Laminae with any significant terrigenous component are also very rare. Individual laminations appear to represent blooms, and in some cases sub-seasonal events are likely preserved. We suggest that this productive system is associated with the continuous presence of low-salinity meltwater derived from a combination of sea ice melt and grounded icebergs, which may be a source for a steady supply of micronutrients such as iron to the surface mixed layer.

Trace elemental ratios preserved in the calcitic skeleton of bamboo corals have been shown to serve as archives of past ocean conditions. The concentration of dissolved barium (BaSW), a bioactive nutrientlike element, is linked to biogeochemical processes such as the cycling and export of nutrients. Recent work has calibrated bamboo coral Ba/Ca, a new BaSW proxy, using corals spanning the oxygen minimum zone beneath the California Current System. However, it was previously unclear whether Ba/Cacoral records were internally reproducible. Here we investigate the accuracy of using laser ablation inductively coupled plasma mass spectrometry for Ba/Cacoral analyses and test the internal reproducibility of Ba/Ca among replicate radial transects in the calcite of nine bamboo corals collected from the Gulf of Alaska (643–720 m) and the California margin (870–2054 m). Data from replicate Ba/Ca transects were aligned using visible growth bands to account for nonconcentric growth; smoothed data were reproducible within ~4% for eight corals (n = 3 radii/coral). This intracoral reproducibility further validates using bamboo coral Ba/Ca for BaSW reconstructions. Sections of the Ba/Ca records that were potentially influenced by noncarbonate bound Ba phases occurred in regions where elevated Mg/Ca or Pb/Ca and coincided with anomalous regions on photomicrographs. After removing these regions of the records, increased Ba/Cacoral variability was evident in corals between ~800 and 1500 m. These findings support additional proxy validation to understand BaSW variability on interannual timescales, which could lead to new insights into deep sea biogeochemistry over the past several centuries.

In Central America, seasonal and interannual shifts in the position of the Intertropical Convergence Zone (ITCZ) control the hydrologic budget. To better understand long-term changes in regional ITCZ-driven precipitation we re-examined a coral δ 18 O record from a Porites lobata coral head near Secas Island (Core ID: S1) (7°59′ N, 82°3′ W) in the Gulf of Chiriqui on the Pacific side of Panama. Linsley et al., (1994) originally published the 277-year time series and first described the presence of a narrow-band decadal cycle (period near 9–12 years) in δ 18 O. The original study did not present potential drivers for the decadal cycle, although they ruled out the influence of the sun spot cycle. Our re-analysis of this record supports the original interpretation that coral δ 18 O is largely responding to variations in precipitation and associated river discharge, but with a new proposed mechanism to explain the decadal mode. There is no similar decadal cycle in gridded instrumental sea surface temperature from the area, suggesting that the decadal coral δ 18 O signal results from hydrologic changes that influence coastal δ 18 O seawater . The decadal component in S1 δ 18 O is also coherent with a decadal mode embedded in the Pacific Decadal Oscillation (PDO) Index that we suggest has tropical origins. We speculate that the coral's temporary δ 18 O deviation (1900–1930) in the decadal mode from the corresponding bands in rainfall and the PDO can be ascribed to a weak PDO in addition to local Panama gap wind variability and its effect on moisture transport from the Atlantic to the Pacific. Ultimately, the Secas Island coral δ 18 O series records ITCZ-driven precipitation dictated by both the Atlantic and Pacific basins.

Inhabiting areas of the ocean where paleoenvironmental records are sparse, deep-sea corals represent valuable yet largely untapped Holocene records of intermediate and deep ocean variability. δ18O and δ13C were analyzed in nine live-collected deep-sea gorgonian corals (Isididae and Coralliidae) in order to further develop the “lines” paleotemperature method. Least squares linear regression analysis for full lifespan δ18O vs. δ13C (corrected for δ18Owater and δ13CDIC) was utilized to yield equations of the form y = mx + b. δ18O and intercept values were found to be a function of temperature, and to approximate calcite δ18O equilibrium. The corals in this study extend the previously reported calibration ( Hill et al., 2011 ) over a broader range of temperatures from 5 °C to 11.2 °C. When combined with the data from Hill et al. (2011) , a new expression for the relationship between the δ18Ointercept value and temperature is proposed: T ° C = − 4.12 ± 0.38 δ 18 O intercept + 12.33 ± 0.75 R 2 = 0.90 , p value 0.0001 . Error estimates are ± 0.7 °C for corals living at cold temperatures (2 °C), ± 1.4 °C in warmer waters (11 °C), and ± 0.5 °C at the mean water temperature of the data set (4.6 °C). The first multi-specimen verification of the “lines” method was performed on three co-located bamboo (Isididae) corals and found to give nearly coincident δ18O intercepts. Detailed intraspecimen sampling reveals δ18O and δ13C isotopic variability within coeval portions of the skeleton. In one specimen, “lines” method analysis was utilized on multiple samples taken from the same temporal increment of the skeleton, yielding multiple δ18O intercepts. Calculated temperatures using the calibration proposed here describe a temperature range of 7.9 to 10.3 °C, which approaches the temperature range of 11.1 ± 0.7 °C at the coral collection location.

article OCRL), a parameter subject to multiple regional and local environmental influences. Location-specific calibration of δ 18 OCRL is a necessary first step for developing long-term paleoceanographic reconstructions. Here we present four new coral δ 18 O stratigraphies from the Republic of Palau (7.5°N 134.5°E), and compare our records with instrumental measurements for the period 1950-2008. We also compare our results with a previously pub- lished coral record from Palau. We employ a new sea surface salinity (SSS) product and validate its utility for coral-based paleoclimate calibrations. We not only examine differences among the records but also identify strong and regionally coherent environmental signals. We find that SSS variability is the dominant influence on δ 18 OCRL in Palau, while sea surface temperature (SST) is of secondary importance. Our results show that time-averaging multiple δ 18 OCRL records into a single composite series produce greater correlations with instru- mental data and indices than individual stratigraphies alone. Our results are consistent with observations of a strengthening of the hydrological cycle in the WPWP region over the past 50 years, though the magnitudes of long term linear trends differ among the different Palau δ 18 OCRL records. Interannual and interdecadal

Warm intervals within the Pliocene epoch (5.33–2.58 million years ago) were characterized by global temperatures comparable to those predicted for the end of this century1 and atmospheric CO2 concentrations similar to today2, 3, 4. Estimates for global sea level highstands during these times5 imply possible retreat of the East Antarctic ice sheet, but ice-proximal evidence from the Antarctic margin is scarce. Here we present new data from Pliocene marine sediments recovered offshore of Adélie Land, East Antarctica, that reveal dynamic behaviour of the East Antarctic ice sheet in the vicinity of the low-lying Wilkes Subglacial Basin during times of past climatic warmth. Sedimentary sequences deposited between 5.3 and 3.3 million years ago indicate increases in Southern Ocean surface water productivity, associated with elevated circum-Antarctic temperatures. The geochemical provenance of detrital material deposited during these warm intervals suggests active erosion of continental bedrock from within the Wilkes Subglacial Basin, an area today buried beneath the East Antarctic ice sheet. We interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the East Antarctic ice sheet was sensitive to climatic warmth during the Pliocene.

The late Pliocene, ~ 3.3–3.0 Ma, is the most recent interval of sustained global warmth in the geologic past. This window is the focus of climate reconstruction efforts by the U.S. Geological Survey's Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) Data/Model Cooperative, and may provide a useful climate analog for the coming century. Reconstructions of past surface ocean conditions proximal to the Antarctic continent are essential to understanding the sensitivity of the cryosphere to this key interval in Earth's climate evolution. An exceptional marine sediment core collected from the southwestern Ross Sea (78° S), Antarctica, during ANDRILL's McMurdo Ice Shelf Project preserves evidence of dramatic fluctuations between grounded ice and productive, open ocean conditions during the late Pliocene, reflecting orbitally-paced glacial/interglacial cycling. In this near-shore record, diatom-rich sediments are recovered from interglacial intervals; two of these diatomites, from ~ 3.2 Ma and 3.03 Ma, are within the PRISM chronologic window. The diatom assemblages identified in PRISM-age late Pliocene diatom-rich sediments are distinct from those in mid-Pliocene and later Pliocene/Pleistocene intervals recovered from AND-1B, and comprise both extant taxa with well-constrained ecological preferences and a diverse extinct flora, some members of which are previously undescribed from Antarctic sediments. Both units are dominated by Chaetoceros resting spores, an indicator of high productivity and stratification that is present at much lower abundance in materials both older and younger than the PRISM-age sediments. Newly described species of the genus Fragilariopsis, which first appear in the AND-1B record at 3.2 Ma, are the most abundant extinct members of the PRISM-age assemblages. Other extant species with established environmental affinities, such as Fragilariopsis sublinearis, F. curta, Stellarima microtrias, and Thalassiothrix antarctica, are present at lower abundances. Environmental inferences drawn from extant diatom assemblages are in good agreement with those from Chaetoceros resting spores and the Fragilariopsis radiation. All three lines of evidence indicate the onset of late Pliocene cooling in the Ross Sea near-shore environment at 3.2 Ma, with intensification and possible regional persistence of summer sea ice by 3.03 Ma. An important implication of this research is the indication that the Ross Ice Shelf fluctuated dramatically on orbital timescales at a time when nearshore Antarctic conditions were only modestly warmer than present.

et al., Sea-level rise (SLR) is one of the most conspicuous examples of the environmental impact of recent climate change. Since SLR rates are not uniform around the planet, local and regional data are needed for proper adaptation plans. Pb-dated sediment cores were analyzed to determine the trends of sediment accretion rates (SARs) at three tropical saltmarshes in the Estero de Urias lagoon (Gulf of California, Mexico), in order to estimate the SLR trends during the past ~100 years, under the assumption that these ecosystems accrete at a similar rate to SLR. A chemometric approach, including multivariate statistical analysis (factor analysis) of geochemical data (including δC; δN; C/N ratios; and Br, Na, and Cl as proxies for marine transgression) was used to identify the marine transgression in the sediment records. Based on core geochemistry, only one of the three cores provided a long-term record attributable to marine transgression. SLR trends, estimated from SARs, showed increasing values, from a minimum of 0.73 ± 0.03 mm yr at the beginning of the 20th century and up to 3.87 ± 0.12 mm yr during the period 1990–2012. The estimated SLR trend between 1950 and 1970 was comparable to the tide gauge records in Mazatlan City for the same period. Results showed the caveats and strengths of this methodology to reconstruct decadal SLR trends from the sedimentary record, which can be used to estimate long-term SLR trends worldwide in regions where monitoring data are scarce or absent., This work has been supported by the grants CONACyT CB2010-153492, PAPIIT-IN203313, CONACYT PDCPN2013-01/214349, and the CONACYT fellowship to JLSP.

Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87 m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600 m and 40Ar/39Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well‐dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre‐existing strata. The combined age model implies relatively rapid (1 m/2–5 ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin‐wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies: 1. The base of the cored succession (1275–1220 mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5 Ma by several reworked volcanic clasts; 2. A late-Miocene sub-polar orbitally controlled glacial–interglacial succession (1220–760 mbsf) bounded by two unconformities correlated with basin‐wide reflectors associated with early development of the terror rift; 3. A late Miocene volcanigenic succession (760–596 mbsf) terminating with a ~1 my hiatus at 596.35 mbsf which spans the Miocene–Pliocene boundary and is not recognised in regional seismic data; 4. An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (590–440 mbsf), separated from; 5. A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (440–150 mbsf) by a 750 ky unconformity interpreted to represent a major sequence boundary at other locations; 6. An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150–80 mbsf), and; 7. A late Pleistocene glacigene succession (80–0 mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment.

Laminated sediments are unique archives of palaeoenvironmental and palaeoceanographic conditions, recording changes on seasonal and interannual timescales. Diatom-rich laminated marine sediments are examined from Dumont d’Urville Trough, East Antarctic Margin, to determine changes in environmental conditions on the continental shelf from 1136 to 3122 cal. yr BP. Scanning electron microscope backscattered electron imagery (BSEI) and secondary electron imagery are used to analyse diatom assemblages from laminations and to determine interlamina relationships. Diatom observations are quantified with conventional assemblage counts. Laminae are primarily classified according to visually dominant species identified in BSEI and, secondarily, by terrigenous content. Nine lamina types are identified and are characterized by: Hyalochaete Chaetoceros spp. resting spores (CRS); CRS and Fragilariopsis spp.; Fragilariopsis spp.; Corethron pennatum and Rhizosolenia spp.; C. pennatum; Rhizosolenia spp.; mixed diatom assemblage; Stellarima microtrias resting spores (RS), Porosira glacialis RS and Coscinodiscus bouvet; and P. glacialis RS. Formation of each lamina type is controlled by seasonal changes in sea ice cover, nutrient levels and water column stability. Quantitative diatom assemblage analysis revealed that each lamina type is dominated by CRS and Fragilariopsis sea ice taxa, indicating that sea ice cover was extensive and persistent in the late Holocene. However the lamina types indicate that the sea ice regime was not consistent throughout this period, notably that a relatively warmer period, ~3100 to 2500 cal. yr BP, was followed by cooling which resulted in an increase in year round sea ice by ~1100 cal. yr BP.

The deep-sea coral, Enallopsammia rostrata, a member of the Dendrophylliidae family, is a major structure-forming species that creates massive dendroid colonies, up to 1 m wide and 0.5 m tall. Living colonies of E. rostrata have been collected using the PISCES submersibles from three locations from 480 to 788 m water depth in the Line Islands (similar to 160 degrees W) in the Equatorial Pacific. We have applied to these colonies a high sensitivity, low blank technique to determine U-series ages in small quantities (70 +/- 15 mg) of modern and near modern calcareous skeletons using MC-ICP-MS (Multi-collector Inductively Coupled Plasma Mass Spectrometer). The application of this method to living slow-growing colonies from a range of sites as well as the observations of axial growth patterns in thin sections of their skeletons offer the first expanded and well constrained data on longevity, growth pattern and mean growth rates in E. rostrata. Absolute dated specimens indicate life spans of colonies ranging from 209 +/- 8 yrs to 605 +/- 7 yrs with radial growth rates from 0.012 to 0.072 mm yr(-1) and vertical extension rates from 0.6 to 1.9 mm yr(-1). The linear growth rates reported here are lower than those reported for other deep-sea scleractinian corals (Lophelia pertusa and Madrepora oculata). The U-series dating indicates that the growth ring patterns of E. rostrata are not consistent with annual periodicity emphasizing the importance of absolute radiometric dating methods to constrain growth rates. Slow accretion and extreme longevity make this species and its habitat especially vulnerable to disturbances and impacts from human activities. This dating method combined with observation of growth patterns opens up new perspectives in the field of deep-sea corals since it can provide quantitative estimates of growth rates and longevity of deep-sea corals in general.

Recent advances in the chronology and the palaeoclimatic understanding of Antarctic ice core records point towards a larger heterogeneity of latitudinal climate fluctuations than previously thought. Thus, realistic palaeoclimate reconstructions rely in the development of a tight array of well- constrained records with a dense latitudinal coverage. Climatic records from southernmost South America are critical cornerstones to link these Antarctic palaeoclimatic archives with their South American counterparts.At 548S on the Island of Tierra del Fuego,Lago Fagnanois located in one of the most substantially and extensively glaciated regions of southernmost South America during the Late Pleistocene. This elongated lake is the largest (� 110km long) and non-ice covered lake at high southern latitudes. A multi-proxy study of selected cores allows the characterisation of a Holocene sedimentary record. Detailed petrophysical, sedimentological and geochemical studies of a complete lacustrine laminated sequence reveal variations in major and trace elements, as well as organic content, suggesting high variability in environmental conditions. Comparison of these results with other regional records allows the identification of major known late Holocene climatic intervals and the proposal for a time for the onset of the Southern Westerlies in Tierra del Fuego. These results improve our understanding of the forcing mechanisms behind climate change in southernmost Patagonia. Copyright # 2009 John Wiley & Sons, Ltd.

Distributions of Mg and Sr in the skeletons of a deep-sea coral (Caryophyllia ambrosia) and a shallow-water, reef-building coral (Pavona clavus) have been obtained with a spatial resolution of 150 nm, using the NanoSIMS ion microprobe at the Museum National d'Histoire Naturelle in Paris. These trace element analyses focus on the two primary ultra-structural components in the skeleton: centers of calcification (COC) and fibrous aragonite. In fibrous aragonite, the trace element variations are typically on the order of 10% or more, on length scales on the order of 1-10 mu m. Sr/Ca and Mg/Ca variations are not correlated. However, Mg/Ca variations in Pavona are strongly correlated with the layered organization of the skeleton.

We present reconstructions of multidecadal- to centennial-scale shifts in the hydrologic balance of the northern Altiplano during the middle Holocene using stable isotopic, geochemical, and sedimentologic evidence from Lake Titicaca sediments. Large downcore changes in the δ13Corg of bulk organic matter, previously shown to be a site-specific lake-level proxy, indicate that extended periods of lake-level regression occurred in response to sustained reductions in mean annual precipitation and/or increases in air temperature. Supporting evidence from a floating interval of finely-laminated sediments indicates that salinity and stratification were enhanced at Lake Titicaca during the middle Holocene. The age model used in this study is based on 7 AMS 14C age dates estimated from bulk decalcified sediment from a 60 cm sequence spanning 7200–5600 14C cal yr BP. Simulations from a hydrological-energy balance model of the Titicaca Basin indicate that the ∼ 38 m drop in lake level that preceded the maximum lowstand at ∼ 6200 yr BP would require a significant decrease in mean annual precipitation of up to 50% and/or a significant increase in mean annual temperature of up to 8 °C. Today, the Altiplano is often affected by drought during periods of enhanced summertime westerly wind flow, which reduces the delivery of moisture from the east. This investigation supports the idea of sustained periods of enhanced westerly air flow during the mid-Holocene and provides additional submillennial-scale information about the timing and magnitude of regional drought. Results of our frequency-domain analysis of the floating lamination interval indicate significant power at a decadal periodicity (10–12 yr) associated with the Schwabe cycle of solar activity. Frequency domain analysis also reveals power in a 75 year time series of modern lake level. We suspect that solar influence on large-scale atmospheric circulation features results in changes to the moisture balance in the Titicaca basin over decadal and possibly century-scale time periods.

An ultra-high-resolution post-glacial laminated sediment record from Mertz Ninnis Trough, East Antarctic Margin (EAM), has been analysed using SEM backscattered electron imagery, secondary electron imagery and quantitative diatom abundance. Laminations are classified using visually dominant diatom species and terrigenous content. Four biogenic diatom ooze laminae types, one diatom-bearing terrigenous lamina type and one diatom-bearing terrigenous sub-lamina type have been identified. Diatom ooze lamina types comprise near-monogeneric Hyalochaete Chaetoceros spp. resting spore laminae, laminae characterised by Corethron pennatum, laminae characterised by Rhizosolenia spp. and mixed diatom assemblage laminae. Diatom-bearing terrigenous lamina and sub-lamina types comprise mixed diatom assemblage terrigenous laminae and sub-laminae characterised by Porosira glacialis resting spores. Formation of each of these lamina types is controlled by seasonal changes in nutrients, oceanographic regimes and the Mertz Glacier Polynya dynamics.

The Nathaniel B. Palmer 01-01 cruise produced a SeaBeam map showing unprecedented detail of the bathymetry in the Mertz Trough of East Antarctica. In addition, seismic reflection surveys and sediment core collection were completed in the region. The morphology of the Mertz Trough is combined with core data to interpret the sequence of events that occurred in this area since the Last Glacial Maximum. These complementary data indicate that an ice sheet once covered the Mertz Trough, which deposited diamicton and formed mega-scale glacial lineations during glacial maximal conditions and grounding-line wedges during recession. An erosional feature caused by subglacial meltwater breaching at least one of the grounding-line deposits is also recognized, along with a fan of sediment deposited seaward of the breach. Sediment cores from the Mertz Trough consist of two distinct units, the diamicton deposited subglacially and a diatom mud and ooze, deposited after the ice retreated. The latter unit has been preferentially deposited in deeper areas of the trough as a hemipelagic drape and shows that a change in the nature of the diatom unit occurred about 3300 14C yr BP.

The palaeoceanography and climate history of the East Antarctic Margin (EAM) are less well understood than those of West Antarctica. Yet, the EAM plays an important role in deep ocean circulation and the global ocean system and has likely done so in the past. Deglacial-age marine sediments from the EAM provide clues about its past role during this critical period of rapid climate change. Several deep basins across the EAM such as Iceberg Alley (∼67°S, 63°E) on the Mac.Robertson Shelf (MRS) accommodate thick marine sequences that archive the deglaciation in the form of diatom-rich, continuously laminated (varved) sediments. These laminated sediments are pristinely preserved and contain seasonal and long-term information on the cryospheric and palaeoceanographic changes associated with the rapid retreat of the glacial ice sheet across the MRS. We present results of microfabric analysis of the lower ∼2 m of deglacial varves from jumbo piston core JPC43B (Iceberg Alley). Backscattered electron imagery (BSEI) of polished thin sections and scanning electron microscope secondary electron imagery (SEI) of lamina-parallel fracture surfaces are used to analyze the varves. One hundred and ninety-two laminations are investigated and their nature and temporal significance are discussed in terms of seasonal deposition and cyclicity of diatom species. Our high-resolution palaeodata record exceptionally high diatom production and silica flux associated with the retreat of the East Antarctic Ice Sheet, and seasonal sea-ice changes along the EAM. This information is invaluable for assessing cryospheric-oceanographic variation and, therefore, the local and regional response to this period of rapid climate change. Varves are made up of lamina couplets comprising (i) thickly laminated to thinly bedded orange/orange-brown very pure diatom ooze dominated by Hyalochaete Chaetoceros spp. vegetative cells and resting spores, and (ii) brown/blue-grey terrigenous angular quartz sand, silt and clay with an abundant mixed diatom flora. The colour variation between these two types of lamination is striking. Using floristic and textural information we interpret the diatom oozes as spring flux and the terrigenous laminae as summer flux. Each couplet pair represents one annual cycle and reflects seasonal changes in nutrient availability and stratification associated with the cyclical advance and retreat of seasonal sea-ice. The diatom oozes can reach up to ∼7.5 cm in thickness indicating enormous silica flux to the sea floor associated with ice sheet retreat.

We present and compare AMS-14C geochronologies for sediment cores recovered from Lake Titicaca, South America. Radiocarbon dates from three core sites constrain the timing of late Quaternary paleoenvironmental changes in the Central Andes and highlight the site-specific factors that limit the radiocarbon geochronometer. With the exception of mid-Holocene sediments, all cores are generally devoid of macrophyte fragments, thus bulk organic fractions are used to build core chronologies. Comparisons of radiocarbon results for chemically defined fractions (bulk decalcified, humate, humin) suggest that ages derived from all fractions are generally coherent in the post-13,500 yr BP time interval. In the pre-13,500 yr BP time interval, ages derived from humate extracts are significantly younger (300–7000 years) than ages from paired humin residues. Gross age incoherencies between paired humate and humin sub-fractions in pre-13,500 yr BP sediments from all core sites probably reflect the net downward migration of humates. Ages derived from bulk decalcified fractions at our shallow water (90 m) and deep water (230 m) core sites consistently fall between ages derived from humate and humin sub-fractions in the pre-13,500 yr BP interval, reflecting that the bulk decalcified fraction is predominantly a mixture of humate and humin sub-fractions. Bulk decalcified ages from the pre-13,500 yr BP interval at our intermediate depth core site (150 m) are consistently older than humate (youngest) and humin sub-fractions. This uniform, reproducible pattern can be explained by the mobilization of a relatively older organic sub-fraction during and after the re-acidification step following the alkaline treatment of the bulk sediment. The inferred existence of this ‘alkali-mobile, acid-soluble’ sub-fraction implies a different depositional/post-depositional history that is potentially associated with a difference in source material. While internally consistent geochronologies can be developed for the Lake Titicaca sequence using different organic fractions, mobile organic sub-fractions and fractions containing mobile sub-fractions should generally be avoided in geochronology studies. Consequently, we believe humin and/or bulk decalcified ages provide the most consistent chronologies for the post-13,500 yr BP interval, and humin ages provide the most representative ages for sedimentation prior to 13,500 yr BP interval. Using the age model derived from the deep water core site and a previously published isotope-based lake-level reconstruction, we present a qualitative record of lake level in the context of several ice-core records from the western hemisphere. We find the latest Pleistocene lake-level response to changing insolation began during or just prior to the Bolling/Allerod period. Using the isotope-based lake-level reconstruction, we also find the 85-m drop in lake level that occurred during the mid-Holocene was synchronous with an increase in the variability of ice-core δ18O from a nearby icecap, but was not reflected in any of the polar ice-core records recovered from the interior of Antarctica and Greenland.

A composite high-resolution diatom stratigraphy from three piston cores and one box-core in the deep sub-basin of Lake Titicaca reveals large moisture variations during the past 30 kyr in the Altiplano region. Diatom sequences indicate orbital and millennial-scale variability in water level and salinity. The pelagic freshwater diatom species Cyclotella andina and Cyclotella stelligera dominate Glacial-age sediments, suggesting that the lake was above its present outlet. Generally, wet conditions continued until 11 000 cal yr BP, as indicated by high percentages of freshwater planktonic diatoms. Large pulses of benthic diatom species between about 11 000 and 10 000 cal yr BP suggest brief intervals of large-amplitude declines in lake level. During the early Holocene (10 000–8500 cal yr BP), a freshwater diatom assemblage suggests overflowing conditions. Pelagic freshwater diatoms are replaced ca. 8500 cal yr BP by the salinity-indifferent species Cyclotella meneghiniana and by benthic taxa, indicating the beginning of lake regression. During the mid-Holocene (6000–3500 cal yr BP), the abundance of the saline taxon Chaetoceros muelleri , coupled with high abundances of epiphytic and epipelic diatoms, indicates maximum salinity and lowest lake levels in the entire 30 000 year record. Lake transgression began ca. 4000 cal yr BP, and the lake achieved modern levels by about 1500 cal yr BP. These water-level changes imply changes in effective moisture, most likely resulting from large precipitation changes. Precipitation was high throughout the Last Glacial Maximum (21 000–18 000 cal yr BP), likely due to an enhanced South American Summer Monsoon during peak summer insolation in the Southern Hemisphere. In contrast, the mid-Holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon.

Approximately 600 km of high-resolution seismic reflection data were collected to investigate the late-Quaternary stratigraphic development of Lake Titicaca. The focus of this report is on two seismic sequence boundaries, which are interpreted as erosional surfaces formed at times of low lake level. The younger erosional surface occurs as much as 90 m below the present lake level and up to 8 m below the present sediment–water interface. This erosional surface is interpreted to be coeval with a well-documented early- to mid-Holocene lowstand, dated between ∼8000 and 3600 cal yr BP. An earlier and previously unknown erosional surface occurs at a sub-bottom depth of approximately 30 m, and as much as 240 m below the present lake level, which implies a major late-Pleistocene lowstand of Lake Titicaca. By extrapolation of sedimentation rates from the upper ∼14 m of sediment, we estimate the age of this older lowstand at >90 000 cal yr BP. Both lowstands of Lake Titicaca indicated by the seismic data are likely to have been a response to climatic change in the region.

Suspended and sea ice particulate organic carbon (POC) collected from the Ross s 13 In r Sea, Antarctica, shows seasonal enrichments in C unrelated to at osphe ic pCO2 varia- tions. The carbon isotopic composition of phytoplankton bloom-related POC (53Cpoc) col- lected from the southwestern Ross Sea during a 1992 bloom event is strongly correlated with POC concentration. This correlation may be attributed to high growth rates, species-specific fractionation, input from sea ice communities enriched in 3C, increased heterotrophic recy- cling, or high rates of bloom-related CO2(aq) drawdown in the upper water column. While all these factors likely contribute to POC 3C enrichment, we favor the latter hypothesis because of the link betweenC enrichment and partial isolation of the surface water CO2 pool in ar- eas of sea ice melt-induced stratification of the upper water column. The CO2 drawdown hy- pothesis is further supported by dissolved inorganic carbon 53C enrichments of up to 2-3%0 in the vicinity of phytoplankton blooms. Patterns of isotopic composition and organic carbon concentration in Ross Sea surface sediments resemble that of the upper water column, par- ticularly in areas of frequent, annually recurrent phytoplankton blooms. Overall, surface sediment 5C values are 1-3%o enriched relative to surface water POC, with the highest 53C values found in areas that receive large amounts of phytoplankton bloom detritus. Future downcore carbon isotopic analyses may prove useful as tracers of bloom dynamics in melt- water-influenced areas, where large enrichments in 3C coincide with high productivity.

We have developed a 101 year (1893–1994) subseasonal oxygen (δ18O) and carbon (δ13C) isotopic time series from Clipperton Atoll in the eastern Pacific using the coral Porites lobata. In agreement with Linsley et al. [1999] we find that seasonal and interannual coral δ18O variability at Clipperton results from variability in both water temperature and salinity. Three new coral time series demonstrate the reproducibility of a secular coral δ18O trend of −0.35‰ since 1910 but show varying δ13C trends. Strong decadal-scale variability in the δ18O record appears related to the Pacific Decadal Oscillation (PDO) through postulated changes in the strengths of the Equatorial Counter Current and North Equatorial Current. Interannual variability in this coral δ18O record is directly related to the El Nino Southern Oscillation (ENSO) and isolation of this frequency band indicates reduced ENSO variability in the eastern equatorial Pacific in the period 1925 to ∼1940, in agreement with instrumental and other Pacific coral records.

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.

New evidence from piston cores and high-resolution seismic reflection data shows that water levels in Lake Titicaca were as much as 100 m below the present level during the early to mid-Holocene (between .6 and 3.814C kyr BP). Climatological and modelling studies indicate that Lake Titicaca rainfall depends on convective activity in upwind Amazonia; the lake-level data therefore suggest a drier Amazon Basin during this time. This view is bolstered by an excellent match between the Titicaca lake-level curve and decreased methane concentrations in Greenland ice, previously ascribed to drying of low-latitude wetlands (Blunier et al., 1995). The postglacial history of Lake Titicaca fits a global pattern of lake-level change in the tropics, characterized by opposite phasing between the Southern and Northern Hemispheres. This pattern is most likely the result of orbital controls over the intensity of summer insolation.

We measured time series of the vertical particle flux at three locations in the Ross Sea, Antarctica, between January 1990 and February 1992 as part of an interdisciplinary project focusing on the accumulation and recycling of organic C and biogenic Si on a polar shelf. We estimate area-wide annual average fluxes through the deep water column of 5 g organic C m−2 yr−l and 30 g biogenic Si m−2 yr−1, values similar to the highest annual average fluxes to the subsurface reported for other areas of the Antarctic continental shelf. Total particle and biogenic Si fluxes are highest during January and February in the southwestern Ross Sea, beneath a seasonally recurrent bloom of the diatom Fragilariopsis curta. Organic C fluxes are highest in the central Ross Sea, consistent with a surface water algal assemblage dominated by the prymnesiophyte Phaeocystis. While organic C flux decreases with depth at all three sites, the result of remineralization within the water column, biogenic opal fluxes are higher in near-bottom traps than at 230 m at the two western Ross Sea sites. Some biogenic opal must be supplied to these deep traps via horizontal advection and possibly resuspension. Fecal pellets and large aggregates contributed between 4 and 70% of the vertical flux and settled at rates of 60 to >400 m d−1. Maximum particle fluxes occur 2 to 10 weeks after surface waters become ice free. We discuss three hypotheses to explain lags between production and settling: (1) advection from surface waters with different ice cover characteristics, (2) lags in the development of a grazing Zooplankton community, and (3) early season windinduced inhibition of primary production. Interannual variability in surface wind stress is empirically linked to variability in biogenic fluxes. Windiness and relative phasing of the annual cycles of ice cover and air temperature may be responsible for the development of different algal communities in the central versus western Ross Sea.

During the middle and late Eocene (similar to 48-34 Myr ago), the Earth's climate cooled(1,2) and an ice sheet built up on Antarctica. The stepwise expansion of ice on Antarctica(3,4) induced crustal deformation and gravitational perturbations around the continent. Close to the ice sheet, sea level rose(5,6) despite an overall reduction in the mass of the ocean caused by the transfer of water to the ice sheet. Here we identify the crustal response to ice-sheet growth by forcing a glacial-hydro isostatic adjustment model(7) with an Antarctic ice-sheet model. We find that the shelf areas around East Antarctica first shoaled as upper mantle material upwelled and a peripheral forebulge developed. The inner shelf subsequently subsided as lithosphere flexure extended outwards from the ice-sheet margins. Consequently the coasts experienced a progressive relative sea-level rise. Our analysis of sediment cores from the vicinity of the Antarctic ice sheet are in agreement with the spatial patterns of relative sea-level change indicated by our simulations. Our results are consistent with the suggestion(8) that near-field processes such as local sea-level change influence the equilibrium state obtained by an ice-sheet grounding line.

Quantitative microscopic analyses of sediment trap samples collected in the Ross Sea between January 1990 and February 1992 reveal striking temporal and spatial differences in algal bloom composition and size. Trap samples from the southwestern Ross Sea (site A, 76°30′S, 167°30′E) were dominated by the diatom Fragilariopsis curia (maximum of 92%), a species associated with both sea ice and the retreating ice edge in the Ross Sea. This species was probably seeded by melting congelation ice. Highest flux of diatom valves to both upper and lower traps in the 1991–1992 season occurred as a distinct event in mid-February 1991, after which flux decreased by 1 to 3 orders of magnitude. As overall flux decreased, an increase in relative abundance of Chaetoceros resting spores and the open water species Thalassiosira antarctica was observed in the lower trap, suggesting some localized lateral advection. In the south central Ross Sea (site B, 76°30′, S 175°W), algal diversity was much higher, with a greater contribution of nondiatom material. The prymnesiophyte Phaeocystis antarctica, dinoflagellates, and cysts of unknown affinity were much more common than at site A, as were the diatoms Fragilariopsis cylindrus, Fragilariopsis kerguelensis, and Thalassiosira gracilis. Highest diatom flux occurred later (February 20 to March 6,1991) at this site and was an order of magnitude lower as compared to maximum flux at site A. Distinct differences in bloom size and composition between sites A and B may be a function of upper water column structure, differences in the amount of sea ice melting, and the type of sea ice present at the time of ice breakout (congelation versus pack ice). Despite rich surface productivities, lower silica flux to the seafloor at site B results from the higher proportion of nonsiliceous algae. At site C (72°30′S, 172°30′E), in the northwestern Ross Sea, diatoms again dominated algal flux; however, assemblage composition differed from that observed at both sites A and B. Relative abundance of F. curta was lower, averaging 60–70%, while the remainder of the assemblage was made up of F. cylindrus, other more robust species of Fragilariopsis, and Chaetoceros resting spores. Algal flux at site C is intermediate in style between sites A and B. Significantly, spatial differences in diatom assemblages noted above appear to be reflected in seafloor surface sediments, suggesting that downcore diatom data provide an interpretable record of paleoproductivity in the Ross Sea.

We examined the cycling of organic carbon and biogenic silica in the water column and upper sediments of the Ross Sea, seeking to understand the processes leading to the formation of opal-rich, organic-poor sediments over much of the Southern Ocean. Between January, 1990 and December, 1994 we conducted three cruises, performing tracer incubation studies (14C, 15N, 30Si, 32Si) to measure rates of primary production, nitrate-based “new” production, biogenic silica production and biogenic silica dissolution in the upper 50 m over most of the Ross Sea shelf in spring, mid summer and late summer. We deployed sediment traps from January, 1990 to early March, 1992 to measure the mid-water (250 m) and near-bottom gravitational fluxes of particulate organic carbon, nitrogen and biogenic silica year-round at three sites, and obtained sediment cores at 15 sites to assess the accumulation rates of organic carbon and biogenic silica in all known sediment regimes on the shelf. At 9 of those sites we also measured nutrient efflux from the sediments, enabling us to calculate benthic recycling fluxes of organic matter and opal. These data permit estimates of the annual production, near-surface recycling, vertical sinking flux, delivery to the seabed, benthic regeneration and long-term burial of both organic and siliceous material, integrated over a 3.3 × 105 km2 area that covers 75–80% of the Ross Sea shelf. The resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column, as well as in the upper seabed. Selective preservation of silica within the upper 50 m is not indicated, and both organic matter and silica are transported from 250 m to the sea floor with virtually 100% efficiency. The SiO2/C mass ratios for surface-layer production, 250-m sinking flux, delivery to the seabed and long-term burial are approximately 0.85, 6.1, 6.2 and 27, respectively. This progressive enrichment in silica results in long-term burial of 5.8% of the biogenic silica and 0.17% of the organic carbon produced by phytoplankton in the surface layer, a factor of 30 greater preservation efficiency for silica than for carbon. Nevertheless, the ratio of opal burial to opal production in the Ross Sea is only about twice the apparent global average of 3% and

The Ross Sea, Antarctica, with its high rates of primary productivity and biogenic accumulation, provides an important location to test the validity of a one-dimensional particle-settling model. As part of an interdisciplinary field project performed from 1990 to 1992 to examine cycling and accumulation of biogenic matter in the Ross Sea, water-column particulate and current data were collected at three sites. At each of the sites, a current meter and sediment trap were placed 240 m below the water surface, and a similar set of instruments was located 40 m above the seabed. The moorings were deployed for 1- to 2-years duration. The current-meter records showed that the speed of flow in the southwestern Ross Sea is relatively slow ( 50 cm s−1) and least variability in direction. To examine the validity of a one-dimensional approximation for fluxes of biogenic material, two models were developed to determine the net displacement of particles settling through the water column. Current-meter data and particle-settling characteristics were incorporated in both models. One model produced a time-varying, linearly interpolated current field between the moorings in which particle advection was evaluated. The second model used time-averaged progressive-vector plots to estimate lateral particle advection. Results show that particles are displaced the least at the southwestern site ( 50 km). The pattern in displacement trends correlates well with observed sediment types and accumulation rates at each site. A one-dimensional model for the settling of biogenic material is most applicable at the southwestern site and least applicable at the northwestern site.

A 2-year (1993-1994) study was conducted in the Galapagos Islands (Ecuador) to determine the relationship between δ18O in skeletal carbonate and sea surface temperature (SST) in three species of reef-building corals: Pavona clavus, Pavona gigantea, and Porites lobata. Coral samples were grown at 3, 10, and 3 m depth at Bartolome Island, Champion iIsland, and Urvina Bay (Isabela Island), respectively. Hourly measurements of SST and sea surface salinity (SSS) were taken at each site immediately adjacent to colonies which were stained biannually to establish the chronology of growth. In addition, surface waters were sampled periodically (bimonthly to monthly) at each site to determine variation in δ18O seawater. Results indicate the mean annual SSTs were similar between sites, varying from 22.9°C at Champion to 23.8°C at Urvina Bay. Comparisons of monthly SST averages between instrumental and remote sensing (satellite, 1° × 1° grid) data show a high correspondence (r2 ranging from 0.84 to 0.94), indicating that remote sensing data are useful for interpreting the δ18O record in corals when instrumental data are lacking. Here δ18Ocoral analyses of eight specimens show that coralline aragonite is a reliable indicator of SST in Galapagos. In general, higher-resolution coral sampling/year resolved more of the monthly variation in SST, up to 97% at a sampling resolution of 1.4 samples per millimeter of linear skeletal growth. Comparisions of the δ18Ocoral signal among and between species at the same site showed consistent seasonal patterns of variation closely tracking SST. In addition, comparisons between sites were highly concordant, with some differences reflecting local variation in SST. Seasonal patterns, however, were essentially the same over the entire region. Thus we conclude that the δ18Ocoral signal from coral skeletons in Galapagos can be used to interpret regional changes in SST variation.

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well-behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.

A reconstruction of late Pleistocene surface water carbon isotopic (δ13C) variability is presented from Ocean Drilling Program (ODP) site 769 in the Sulu Sea in the western tropical Pacific. The Sulu Sea is a shallowly silled back arc basin with a maximum sill depth of 420 m. Site 769 was drilled on a bathymetric high in 3643 m of water and has average late Pleistocene sedimentation rates of 8.5 cm/kyr. The oxygen isotope record (δ18O) of Globigerinoides ruber at site 769 shows a strong correlation with the SPECMAP stacked δ18O record, attesting to the continuity of sediment archive at the site. Surface δ13C displays consistent glacial-interglacial variability which averages ∼0.9‰ and has varied from 0.75 to 1.1‰ over the last 800 kyr. Comparison to surface water δ13C records in the South China Sea and western tropical Pacific suggests that the glacial-interglacial surface δ13C variability is regional in scale. Planktonic δ13C data from ODP site 677 in the eastern Pacific is also coherent with the site 769. Additionally, we have found that the site 769 surface δ13C record is coherent at periods of 100 and 41 kyr with deepwater δ13C records from the Pacific. The highest correlation occurs with the deep eastern Pacific, where benthic δ13C data from cores RC13-110 and ODP site 677 closely match the Sulu Sea surface water record. We evaluate several possible controls of surface water δ13C in the Sulu Sea that may explain the coherent timing with Pacific deepwater δ13C records. These include variations in terrestrial organic matter flux to the basin, the upwelling of subsurface water and productivity changes, and the influx of western Pacific intermediate water to the Sulu Sea. Our preferred explanation involves a region of upper intermediate water upwelling in the far western Pacific which has been shown to outgas CO2 from subsurface waters into surface waters. Upwelling also occurs in the area of Panama Basin site 677. These equatorial upwelling zones could potentially provide a route by which Pacific intermediate water can modulate the δ13C composition of certain Pacific surface water locations. Future reconstructions of late Pleistocene surface water δ13C variability in the western Pacific and Indonesian seas will be required to further evaluate the source of the glacial-interglacial surface water δ13C change.

We measured stable oxygen isotope ratios and skeletal growth rates in the massive corals Pavona clavus and P. gigantea from the west coast of Isabela Island, Galapagos, to assess interannual to decadal climate variability in the eastern Pacific. Comparisons of instrumental data sets show that sea surface temperatures (SST) in the Galapagos region are representative of a broad portion of the eastern equatorial Pacific. The site is especially well-suited for long-term studies of the El Nino/Southern Oscillation (ENSO) phenomenon, as it lies within the eastern Pacific “center of action” for thermal anomalies associated with ENSO. The P. gigantea isotope record is nearly monthly in resolution, spans the period 1961–1982, and shows strong correlation with a Galapagos instrumental SST record (r = −0.90 for annual averages). Cross-spectral analysis shows that SST can explain greater than 80% of the variance in δ18O at both the annual cycle and within the high-frequency portion of the ENSO band (3-5 years). The P. clavus record is annual in resolution, extends from 1587 to 1953 A.D., and was obtained from a 10-m diameter colony preserved within the Urvina Bay uplift. Because seawater δ18O variations in the region are very small, we interpret the Urvina Bay coral δ18O record in terms of annual average SST. The isotopic record appears to be a very good, but not perfect, indicator of ENSO events and shows good correspondence with the historical ENSO reconstruction of Quinn et al. (1987). A number of low δ18O excursions that we observe during the 17th and 18th centuries very likely represent ENSO events that are missing from the historical tabulations. Most interannual δ18O variations between 1607 and 1953 A.D. represent annual average temperature excursions of 1° to 2.5°C. During the Little Ice Age, the annual δ18O series correlates well with many North American tree ring records and shows low temperatures during the early 1600s and early 1800s, and relatively warmer conditions during the 1700s. Unlike most northern hemisphere tree ring and instrumental records, we see no evidence at this site for warming between 1880 and 1940 but rather observe a slight cooling ( 28% of the total variance. The main ENSO mode is centered at 4.6 years and accounts for 12% of the total variance. Additional significant oscillations occur at periods of 3.3, 6, 8, 11, 17, 22, and 34 years. Both annual growth rate and δ18O show variance at periods equivalent to the solar and solar magnetic periods (e.g., 11 and 22 years, respectively). In addition, the amplitude of the 11-year δ18O cycle generally varies with the amplitude of the solar cycle, supporting previous suggestions that the solar cycle may modulate interannual to decadal climate variability in the tropics. The dominant oscillatory modes, both within the ENSO and interdecadal frequency bands, shift to shorter periods from the early to middle 1700s and again from the middle to late 1800s. This may reflect major reorganizations within the tropical ocean-atmosphere system and suggests that tropical Pacific climate variability is linked across timescales ranging from years to decades.

[1] The Ross Sea polynya is among the most productive regions in the Southern Ocean and may constitute a significant oceanic CO2 sink. Based on results from several field studies, this region has been considered seasonally iron limited, whereby a “winter reserve” of dissolved iron (dFe) is progressively depleted during the growing season to low concentrations (∼0.1 nM) that limit phytoplankton growth in the austral summer (December–February). Here we report new iron data for the Ross Sea polynya during austral summer 2005–2006 (27 December–22 January) and the following austral spring 2006 (16 November–3 December). The summer 2005–2006 data show generally low dFe concentrations in polynya surface waters (0.10 ± 0.05 nM in upper 40 m, n = 175), consistent with previous observations. Surprisingly, our spring 2006 data reveal similar low surface dFe concentrations in the polynya (0.06 ± 0.04 nM in upper 40 m, n = 69), in association with relatively high rates of primary production (∼170–260 mmol C m−2 d−1). These results indicate that the winter reserve dFe may be consumed relatively early in the growing season, such that polynya surface waters can become “iron limited” as early as November; i.e., the seasonal depletion of dFe is not necessarily gradual. Satellite observations reveal significant biomass accumulation in the polynya during summer 2006–2007, implying significant sources of “new” dFe to surface waters during this period. Possible sources of this new dFe include episodic vertical exchange, lateral advection, aerosol input, and reductive dissolution of particulate iron.

[1] We calculate net community production (NCP) during summer 2005–2006 and spring 2006 in the Ross Sea using multiple approaches to determine the magnitude and consistency of rates. Water column carbon and nutrient inventories and surface ocean O2/Ar data are compared to satellite-derived primary productivity (PP) estimates and 14C uptake experiments. In spring, NCP was related to stratification proximal to upper ocean fronts. In summer, the most intense C drawdown was in shallow mixed layers affected by ice melt; depth-integrated C drawdown, however, increased with mixing depth. ΔO2/Ar-based methods, relying on gas exchange reconstructions, underestimate NCP due to seasonal variations in surface ΔO2/Ar and NCP rates. Mixed layer ΔO2/Ar requires approximately 60 days to reach steady state, starting from early spring. Additionally, cold temperatures prolong the sensitivity of gas exchange reconstructions to past NCP variability. Complex vertical structure, in addition to the seasonal cycle, affects interpretations of surface-based observations, including those made from satellites. During both spring and summer, substantial fractions of NCP were below the mixed layer. Satellite-derived estimates tended to overestimate PP relative to 14C-based estimates, most severely in locations of stronger upper water column stratification. Biases notwithstanding, NCP-PP comparisons indicated that community respiration was of similar magnitude to NCP. We observed that a substantial portion of NCP remained as suspended particulate matter in the upper water column, demonstrating a lag between production and export. Resolving the dynamic physical processes that structure variance in NCP and its fate will enhance the understanding of the carbon cycling in highly productive Antarctic environments.

Downcore diatom data from a deep basin in Granite Harbor, Antarctica, provide new, high-resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years. The influence of global-scale climatic change, during the time of the Medieval Warm Period and possibly the Little Ice Age, is revealed by long-term changes in floral assemblages, as recorded by changes in the relative concentration of Nitzschia cylindrus. The diatom data suggest that Granite Harbor experienced less annual ice and pack ice cover during both these periods, possibly because of higher temperatures during the Medieval Warm Period and stronger offshore winds during the Little ice Age. The sedimentary record reveals several distinct, short-lived events, in which nearly monospecific blooms of Corethron criophilum and Chaetoceros spp. occurred, possibly the result of strong stabilization of the upper water column and were most likely a response to regional, rather than global conditions. Although short-lived, these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor.

[1] We examined controls on the carbon isotopic composition of sea ice brines and organic matter during cruises to the Ross Sea, Antarctica in November/December 1998 and November/December 2006. Brine samples were analyzed for salinity, nutrients, total dissolved inorganic carbon (ΣCO2), and the 13C/12C ratio of ΣCO2 Particulate organic matter from sea ice cores was analyzed for percent particulate organic carbon (POC), percent total particulate nitrogen (TPN), and stable carbon isotopic composition (δ13CPOC). ΣCO2 in sea ice brines ranged from 1368 to 7149 μmol kg−1, equivalent to 1483 to 2519 μmol kg−1 when normalized to 34.5 psu salinity (sΣCO2), the average salinity of Ross Sea surface waters. Sea ice primary producers removed up to 34% of the available ΣCO2, an amount much higher than the maximum removal observed in sea ice free water. Carbonate precipitation and CO2 degassing may reduce sΣCO2 by a similar amount (e.g., 30%) in the most hypersaline sea ice environments, although brine volumes are low in very cold ice that supports these brines. Brine ranged from −2.6 to +8.0‰ while δ13CPOC ranged from −30.5 to −9.2‰. Isotopic enrichment of the ΣCO2 pool via net community production accounts for some but not all carbon isotopic enrichment of sea ice POC. Comparisons of sΣCO2, and δ13CPOC within sea ice suggest that ep (the net photosynthetic fractionation factor) for sea ice algae is ∼8‰ smaller than the ep observed for phytoplankton in open water regions of the Ross Sea. These results have implications for modeling of carbon uptake and transformation in the ice-covered ocean and for reconstruction of past sea ice extent based on stable isotopic composition of organic matter in sediment cores.

The Santa Barbara Basin (SBB) record of laminated sediment, well known for its excellent time resolution, shows the presence of an unusual once-in-a-millenium event, in the period 1835–1840, at a depth of 55–57 cm below the surface of the seafloor. This “Macoma Event” is the brief flourishing of a macrofaunal community in the deep center of the SBB, from which large organisms are normally excluded by the lack of oxygen. Shells of the pelecypodMacoma leptonoidea Dall and associated fossils, which congregate in a thin layer dated at 1840, provide evidence for a sudden local extinction of macrobenthos by suffocation. Various stratigraphic evidence suggests thatM. leptonoidea colonized the basin floor for a few years prior to 1840 as a result of temporarily decreased productivity and increased oxygen content in the water spilling over the sill into the basin. Historical eyewitness reports indicate unusually severe winter storms from southeasterly directions during the 1830s in the SBB. We suggest that coastal upwelling was severely reduced during 1835–1839 with the effect of reducing the intensity of the oxygen minimum layer. TheMacoma Event apparently is one manifestation of large-scale climatic anomalies felt throughout North America, and which were possibly linked to the major volcanic eruption of Cosiguina, Nicaragua, in 1835.

Geological evidence shows that the ice sheet and climate in Antarctica has changed considerably since the onset of glaciation around 34 million years ago. By analysing this evidence, important information concerning processes responsible for ice sheet growth and decay can be determined, which is vital for appreciating future changes in Antarctica. Geological records are diverse and their analyses require a variety of techniques. They are, however, essential for the establishment of hypotheses regarding past Antarctic changes. Numerical models of ice and climate are useful for testing such hypotheses, and in recent years there have been several advances in our knowledge relating to ice sheet history gained from these tests. This paper documents five case studies, employing a full range of techniques, to exemplify recent insights into Antarctic climate evolution from modelling ice sheet inception in the earliest Oligocene to quantifying Neogene ice sheet fluctuations and process-led investigations of recent (last glacial) changes.

Precisely dated Antarctic continental margin and Southern Ocean geological records show that the early Pleistocene interglacial Marine Isotope Stage 31 (MIS-31) was characterized by warmer than present surface waters with reduced sea-ice and enhanced high latitude marine carbonate production. Micropaleontologic, isotopic, and paleomagnetic evidence from drill cores at 77°S (Cape Roberts Project-1) and 53°S (ODP Site 1094) indicate circumantarctic changes in sea surface temperature and water mass stratification that are in phase with high southern latitude insolation changes during MIS-31. These changes imply a significant, though as yet unquantifiable reduction in Antarctic ice volume. This study supports the hypothesis that the interhemispheric antiphased relationship of the precession cycle attenuates a potentially significant Antarctic ice volume signal in the deep sea oxygen isotope record. The implications are that Antarctic marine ice sheets may be more susceptible to warming and high insolation driven retreat than has been widely recognized.

The central and northern Peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins. We have examined the Cenozoic stratigraphy of the onshore portions of the Sechura Basin (5–7°S) and Pisco Basin (13–16°S), two shelf basins which have accumulated marine sediment discontinuously since the mid to late Eocene. Cenozoic sediments in the Pisco Basin were deposited during at least three major transgressive cycles. Each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones, and capped by biogenic deposits including diatomites, diatomaceous mudstones, dolomitic horizons, and phosphate deposits. Stratigraphic nomenclature for the Pisco Basin has recently evolved; a stratigraphy presented here includes the Eocene Caballas Fm., upper Eocene Los Choros fm., upper Eocene to lowermost Oligocene Yumaque fm., uppermost Oligocene to middle Miocene Chilcatay fm., and upper Miocene to Pliocene Pisco Fm. Major hiatuses in the Pisco Basin span the Late Cretaceous to middle Eocene, early to late Oligocene, middle Miocene, and late Pliocene/Pleistocene to Recent. Cenozoic sediments of the Sechura Basin were deposited within at least 4 major transgressive cycles with hiatuses during the Paleocene to middle Eocene, Oligocene, early to middle Miocene, and late Miocene. Based on recent biostratigraphic studies, sediments enriched in biogenic components accumulated between about 40-36 Ma, 24-16 Ma, and 11-3 Ma in the Pisco Basin and between 40-37 Ma and 8.5-4.5 Ma in the Sechura Basin. In both basins, the most diatomaceous sediments are restricted to the Late Eocene and Late Miocene through Pliocene. The temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the Late Eocene. The occurrence of diatomites and phosphorites is diachronous between the Pisco and Sechura Basins and between the Peruvian forearc and other circum-Pacific Monterey Formation analogs, a reflection of the strong influence of local tectonism on sedimentation patterns. The volume of Neogene sediments along the Peruvian forearc is nearly twice that of the Monterey Fm.; despite basin-to-basin facies diachroneity, these deposits very likely contributed to fluctuations of the late Miocene carbon/CO 2 system by acting as large carbon sinks.

[1] We investigate the functioning of the ocean’s biological pump by analyzing the vertical transfer efficiency of particulate organic carbon (POC). Data evaluated include globally distributed time series of sediment trap POC flux, and remotely sensed estimates of net primary production (NPP) and sea surface temperature (SST). Mathematical techniques are developed to compare these temporally discordant time series using NPP and POC flux climatologies. The seasonal variation of NPP is mapped and shows regional- and basin-scale biogeographic patterns reflecting solar, climatic, and oceanographic controls. Patterns of flux are similar, with more high-frequency variability and a subtropical-subpolar pattern of maximum flux delayed by about 5 days per degree latitude increase, coherent across multiple sediment trap time series. Seasonal production-to-flux analyses indicate during intervals of bloom production, the sinking fraction of NPP is typically half that of other seasons. This globally synchronous pattern may result from seasonally varying biodegradability or multiseasonal retention of POC. The relationship between NPP variability and flux variability reverses with latitude, and may reflect dominance by the large-amplitude seasonal NPP signal at higher latitudes. We construct algorithms describing labile and refractory flux components as a function of remotely sensed NPP rates, NPP variability, and SST, which predict POC flux with accuracies greater than equations typically employed by global climate models. Globally mapped predictions of POC export, flux to depth, and sedimentation are supplied. Results indicate improved ocean carbon cycle forecasts may be obtained by combining satellite-based observations and more mechanistic representations taking into account factors such as mineral ballasting and ecosystem structure.

10 figures, 3 tables., The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene “climatic optimum” (14–10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14–10 Ma). Sequence WL-S9 forms under a polar regime during the Pliocene–Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope-forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability. Validation of our inferences about the nature and timing of Wilkes Land glacial sequences can be achieved by deep sampling (i.e., using IODP-type techniques). The most complete record of the long-term history of glaciation in this margin can be obtained by sampling both (1) the shelf, which contains the direct (presence or no presence of ice) but low-resolution record of glaciation, and (2) the rise, which contains the distal (cold vs. warm) but more complete record of glaciation. The Wilkes Land margin is the only known Antarctic margin where the presumed “onset” of glaciation unconformity (WL-U3) can be traced from shelf to the abyssal plain, allowing links between the proximal and the distal records of glaciation to be established. Additionally, the eastern segment of the Wilkes Land margin may be more sensitive to climate change because the East Antarctic Ice Sheet (EAIS) is grounded below sea level. Therefore, the Wilkes Land margin is not only an ideal location to obtain the long-term EAIS history but also to obtain the shorter-term record of ice sheet fluctuations at times that the East Antarctic Ice Sheet is thought to have been more stable (after 15 Ma-recent)., This paper has been possible through a coordinated approach to understanding glacial history in the Wilkes Land margin. The approach has depended upon the considerable efforts of coproponents and contributors of site survey data for two IODP drilling proposals, Proposal #482 (Escutia et al.) and an IODP Ancillary Project Letter (Dunbar et al.). Besides the coauthors, coproponents and contributors include, in alphabetical order, Xavier Crosta, Eugene Domack, Takemi Ishihara, Amy Leventer, Rick Murray, Phil O'Brien, and Manabu Tanahashi. This manuscript was greatly improved thanks to the constructive reviews provided by Eugene Domack, German Leitchenkov, and Phil O'Brien.

[1] Radiocarbon (14C) in the skeletal aragonite of annually banded corals track radiocarbon concentrations in dissolved inorganic carbon (DIC) in surface seawater. As a result of nuclear weapons testing in the 1950s, oceanic uptake of excess 14C in the atmosphere has increased the contrast between surface and deep ocean 14C concentrations. We present accelerator mass spectrometric (AMS) measurements of 14C/12C ratios (Δ14C) in Porites corals from the Mentawai Islands, Sumatra (0°S, 98°E) and Watamu, Kenya (3°S, 39°E) to document the temporal and spatial evolution of the 14C zonal gradient in the tropical Indian Ocean. The rise in Δ14C in the Sumatra coral, in response to the maximum in nuclear weapons testing, is delayed by 2–3 years relative to the rise in coral Δ14C from the coast of Kenya. Kenya coral Δ14C values rise quickly because surface waters are in prolonged contact with the atmosphere. In contrast, wind-induced upwelling and rapid mixing along the coast of Sumatra entrains 14C-depleted water from the subsurface, which dilutes the effect of the uptake of bomb-produced 14C by the surface ocean. Bimonthly AMS Δ14C measurements on the Mentawai coral reveal mainly interannual variability with minor seasonal variability. Singular spectrum analysis of the Sumatra coral Δ14C record reveals a significant 3-year periodicity. These results lend support to the concept that interannual variability in Indian Ocean upwelling and sea surface temperatures is related to ENSO-like teleconnections over the Indo-Pacific basin.

[1] The pack ice ecosystem of the Ross Sea was investigated along a 1470-km north-south transect during the spring 1998 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea (ROAVERRS). Snow and sea ice thickness along the transect varied latitudinally, with thinner snow and ice at the northern ice edge and thin new ice in the vicinity of the Ross Sea polynya. Relative to springtime observations in other sea ice regions, algal chlorophyll a (Chl a) concentrations were low. In contrast, particulate organic carbon (POC), total particulate nitrogen (TPN), and POC:Chl a were all high, indicating either that the ice contained substantial amounts of detritus or nonphotosynthetic organisms, or that the algae had a high POC:Chl a ratio. The abundance of Chl a, POC, and TPN in the sea ice was related to ice age and thickness, as well as to snow depth: older ice had thinner snow cover and contained higher algal biomass while new ice in the polynya had lower biomass. Older pack ice was dominated by diatoms (particularly Fragilariopsis cylindrus) and had vertical distributions of Chl a, POC, and TPN that were related to salinity, with higher biomass at the ice-water interface. Fluorescence-based measurements of photosynthetic competence (Fv/Fm) were higher at ice-water interfaces, and photosynthesis-irradiance characteristics measured for bottom ice algae were comparable to those measured in pack ice communities of other regions. Nutrient concentrations in extracted sea ice brines showed depletion of silicate and nitrate, depletion or regeneration of phosphate and nitrite, and production of ammonium when normalized to seawater salinity; however, concentrations of dissolved inorganic nitrogen, phosphorous, and silica were typically above levels likely to limit algal growth. In areas where pack ice and snow cover were thickest, light levels could be limiting to algal photosynthesis. Enrichment of δ13C-POC in the sea ice was correlated with the accumulation of POC, suggesting that carbon sources for photosynthesis might shift in response to decreasing CO2 supply. Comparisons between new ice and underlying waters showed similar algal species dominance (Phaeocystis antarctica) implying incorporation of phytoplankton, with substantially higher POC and TPN concentrations in the ice.

Corals offer a rich archive of past climate variability in tropical ocean regions where instrumental data are limited and where our knowledge of multi-decadal climate sensitivity is incomplete. In the eastern equatorial Pacific, coral isotopic records track variations in ENSO-related changes in sea-surface temperature; further west, corals record variability in sea-surface temperature and rainfall that accompanies zonal displacement of the Indonesian Low during ENSO events. These multi-century records reveal previously unrecognised ENSO variability on time scales of decades to centuries. Outside the ENSO-sensitive equatorial Pacific, long-term trends towards recent warmer/wetter conditions suggest the tropics respond to global forcings. New coral paleothermometers indicate that surface-ocean temperatures in the tropical southwestern Pacific were depressed by 4–6°C during the Younger Dryas climatic event and rose episodically during the next 4000 yr. High temporal-resolution measurements of Sr/Ca and δ 18 O in corals provide information about the surface-ocean hydrologic balance and can resolve the seasonal balance between precipitation and evaporation. Radiocarbon measurements in corals, coupled with ocean circulation models, may be used to reconstruct near-surface ocean circulation, past mixing rates, and the distribution of fossil fuel CO2 in the upper ocean. Most recently, seasonal to interannual variations in the radiocarbon of corals from the equatorial Pacific have been linked to the redistribution of surface waters associated with the ENSO.

High-resolution seismic reflection profiles of the sediments of Lake Titicaca, Peru-Bolivia, suggest that lake levels in the recent past were considerably lower than today. Incised channels on the major deltas extend to depths of 85 m below modern lake level. Erosional truncation of onlapping seismic reflectors is found at similar depths. This interpretation of the seismic data is supported by analyses of sediment cores from the lake, which indicate that there was a significant drop in lake level during the early to mid-Holocene.