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An ocean of climate impacts Large decreases in Atlantic meridional overturning circulation accompanied every one of the cold Northern Hemispheric stadial events that occurred during the heart of the last glacial period. These events, lasting on average around 1000 years each, have long been thought to result from changes in deep ocean circulation. Henry et al. used a suite of geochemical proxies from marine sediments to show that reductions in the export of northern deep waters occurred before and during stadial periods (see the Perspective by Schmittner). This observation firmly establishes the role of ocean circulation as a cause of abrupt glacial climate change during that interval. Science , this issue p. 470 ; see also p. 445

Approximately synchronous with the onset of Heinrich Stadial 1 (HS1), δ13C decreased throughout most of the upper (~1000–2500 m) Atlantic, and at some deeper North Atlantic sites. This early deglacial δ13C decrease has been alternatively attributed to a reduced fraction of high-δ13C North Atlantic Deep Water (NADW) or to a decrease in the NADW δ13C source value. Here we present new benthic δ18O and δ13C records from three relatively shallow (~1450–1650 m) subpolar Northeast Atlantic cores. With published data from other cores, these data form a depth transect (~1200–3900 m) in the subpolar Northeast Atlantic. We compare Last Glacial Maximum (LGM) and HS1 data from this transect with data from a depth transect of cores from the Brazil Margin. The largest LGM-to-HS1 decreases in both benthic δ13C and δ18O occurred in upper waters containing the highest NADW fraction during the LGM. We show that the δ13C decrease can be explained entirely by a lower NADW δ13C source value, entirely by a decrease in the proportion of NADW relative to Southern Ocean Water, or by a combination of these mechanisms. However, building on insights from model simulations, we hypothesize that reduced ventilation due to a weakened but still active Atlantic Meridional Overturning Circulation also contributed to the low δ13C values in the upper North Atlantic. We suggest that the benthic δ18O gradients above ~2300 m at both core transects indicate the depth to which heat and North Atlantic deglacial freshwater had mixed into the subsurface ocean by early HS1.

The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6±0.4)×10−3‰/(μmolkg−1) [CO32−] and pressure (−4.9±1.7)×10−5‰m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat=δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ≅0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ≅0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Despite decades of use as a paleoceanographic proxy, considerable uncertainty still surrounds the temperature dependence of benthic foraminiferal δ18O. Widely applied paleotemperature equations may mix non-equilibrium foraminifera with equilibrium synthetic calcite, resulting in temperature sensitivities that are too large. Warm-water foraminiferal calibrations may give temperature sensitivities that are too small for very cold waters. Here we combine new core top measurements from the Florida Straits and the Arctic Ocean with published data to derive new δ18O:temperature relationships for three groups of benthic foraminifera. We derive a quadratic equation for Cibicidoides and Planulina that agrees well with equilibrium synthetic calcite, and that should be applicable over all oceanographic temperatures. We find that Uvigerina is not at equilibrium and is isotopically heavier than Cibicidoides and Planulina by 0.47‰, in contrast to the historically used 0.64‰. Hoeglundina elegans is further enriched and appears to be slightly heavier than equilibrium aragonite. Finally we discuss the implications of the Florida Straits observations for the hypothesis that benthic foraminifera precipitate their shells from a pH-dependent mixture of bicarbonate and carbonate ions.

Antarctic Intermediate Water (AAIW) is a key player in the global ocean circulation, contributing to the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and influencing interhemispheric heat exchange and the distribution of salinity, nutrients and carbon. However, the deglacial history of AAIW flow into the North Atlantic is controversial. Here we present a multicore-top neodymium isotope calibration, which confirms the ability of unclean foraminifera to faithfully record bottom water neodymium isotopic composition ( e Nd ) values in their authigenic coatings. We then present the first foraminifera-based reconstruction of e Nd from three sediment cores retrieved from within modern AAIW, in the western tropical North Atlantic. Our records reveal similar glacial and interglacial contributions of AAIW, and a pronounced decrease in the AAIW fraction during North Atlantic deglacial cold episodes, Heinrich Stadial 1 (HS1) and Younger Dryas (YD). Our results suggest two separate phases of reduced fraction of AAIW in the tropical Atlantic during HS1, with a greater reduction during early HS1. If a reduction in AAIW fraction also reflects reduced AMOC strength, this finding may explain why, in many regions, there are two phases of hydrologic change within HS1, and why atmospheric CO2 rose more rapidly during early than late HS1. Our result suggesting less flow of AAIW into the Atlantic during North Atlantic cold events contrasts with evidence from the Pacific, where intermediate-depth e Nd records may indicate increased flow of AAIW into the Pacific during the these same events. Antiphased e Nd behavior between intermediate depths of the North Atlantic and Pacific implies that the flow of AAIW into Atlantic and Pacific seesawed during the last deglaciation.

Several periods of massive iceberg discharge into the North Atlantic and widespread cooling marked the last glacial period. Reconstructions of northward flow along the Florida margin suggest that not all cold events were associated with a change in the strength of the Atlantic meridional overturning circulation.

The Arctic Ocean has an important role in Earth's climate, both through surface processes such as sea-ice formation and transport, and through the production and export of waters at depth that contribute to the global thermohaline circulation. Deciphering the deep Arctic Ocean's palaeo-oceanographic history is a crucial part of understanding its role in climatic change. Here we show that sedimentary ratios of the radionuclides thorium-230 ((230)Th) and protactinium-231 ((231)Pa), which are produced in sea water and removed by particle scavenging on timescales of decades to centuries, respectively, record consistent evidence for the export of (231)Pa from the deep Arctic and may indicate continuous deep-water exchange between the Arctic and Atlantic oceans throughout the past 35,000 years. Seven well-dated box-core records provide a comprehensive overview of (231)Pa and (230)Th burial in Arctic sediments during glacial, deglacial and interglacial conditions. Sedimentary (231)Pa/(230)Th ratios decrease nearly linearly with increasing water depth above the core sites, indicating efficient particle scavenging in the upper water column and greater influence of removal by lateral transport at depth. Although the measured (230)Th burial is in balance with its production in Arctic sea water, integrated depth profiles for all time intervals reveal a deficit in (231)Pa burial that can be balanced only by lateral export in the water column. Because no enhanced sink for (231)Pa has yet been found in the Arctic, our records suggest that deep-water exchange through the Fram strait may export (231)Pa. Such export may have continued for the past 35,000 years, suggesting a century-scale replacement time for deep waters in the Arctic Ocean since the most recent glaciation and a persistent contribution of Arctic waters to the global ocean circulation.

Q1 (ENSO)) show abrupt changes around 6000 yrs B.P., but most coastal sedimentary records do not span this time period. Establishing longer records is essential for understanding mid-Holocene patterns of storminess and their climatic drivers, which will lead to better forecasting of how climate change over the next century may affect tropical cyclone frequency and intensity. Storms are thought to be an important mechanism for transporting coarse sediment from shallow carbonate platforms to the deep-sea, and bank-edge sediments may offer an unexplored archive of long-term hurricane activity. Here, we develop this new approach, reconstructing more than 7000 years of North Atlantic hurricane variability using coarse-grained deposits in sediment cores from the leeward margin of the Great Bahama Bank. High energy event layers within the resulting archive are (1) broadly correlated throughout an offbank transect of multi-cores, (2) closely matched with historic hurricane events, and (3) synchronous with previous intervals of heightened North Atlantic hurricane activity in overwash reconstructions from Puerto Rico and elsewhere in the Bahamas. Lower storm frequency prior to 4400 yrs B.P. in our records suggests that precession and increased NH summer insolation may have greatly limited hurricane potential intensity, outweighing weakened ENSO and a stronger West African Monsoon—factors thought to be favorable for hurricane development.

A KNOWLEDGE of the age of marine sediments is necessary to determine the timing of events and rates of processes in the marine realm, and the relationships among marine and other climatically sensitive records. The establishment of an accurate chronology for Pleistocene marine sediments beyond the range of radiocarbon dating (approximately the past 45 kyr) has therefore been a goal of palaeoceanographers for decades. Early attempts1,2 based on measurements of the radionuclides 230Th and 231Pa were beset with problems, and subsequent studies focused on tying fluctuations in marine sediment oxygen-isotope records to events such as the formation of coral reef terraces and changes in the Earth's magnetic polarity3,4, and tuning the resultant chronologies to the Earth's orbitally driven insolation variations5–8. But these chronologies (especially the age and duration of the last interglacial period) have been challenged by several studies9–12, raising questions about the fundamental cause of Pleistocene climate fluctuations. Here we report the direct U–Th dating of aragonite-rich marine sediments from the Bahamas, and present an accurately dated marine oxygen-isotope record for the last two interglacials. We obtain dates of 120–127 kyr BP for the last interglacial and 189–190 kyr BP for the late stage 7 interglacial. These dates are in accord with the general theory of orbitally forced climate fluctuations and demonstrate the potential of our direct-dating approach for developing an absolute chronology for the Pleistocene marine oxygen-isotope record.

Near-surface reaction of CO2-bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO2 storage, and perhaps for direct capture of CO2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes (“at the outcrop”) rather than ex situ processes (“at the smokestack”).

Model Behavior The initial pulse of warming during the last deglaciation, which defined the start of an interval called the Bølling-Allerød, occurred abruptly about 14,500 years ago. To date, the most detailed simulations used models of intermediate complexity, not with more sophisticated Coupled Global Climate Models (CGCMs) that can synchronously couple both oceanic and the atmospheric components. Overcoming practical and technical challenges, Liu et al. (p. 310 ; see the Perspective by Timmermann and Menviel ) performed such a simulation using CCSM3, a state-of-the-art ocean-atmosphere CGCM. In contrast to previous studies, which indicated that the Bølling-Allerød was triggered by a nonlinear bifurcation between modes of deep ocean circulation in the Atlantic, the results suggest that the event was a transient response caused by the cessation of meltwater input into the surface ocean in the North Atlantic region.

The North Atlantic Oscillation controls winter climate variability in eastern North America and Europe. Coral-derived records of sea surface temperature in Bermuda suggest that multidecadal variability of the North Atlantic Oscillation has increased in the past few decades relative to the early nineteenth century.

Although North Atlantic deep-water formation was greatly reduced during the last glacial maximum, bottom-water currents were as vigorous as at present. However, they were weakened during periods of North Atlantic surface freshening. A strong correlation can be seen between bottom-water-current strength and Greenland air-temperature records, thus confirming a close connection between ocean circulation and abrupt climate change. Past changes in the freshwater balance of the surface North Atlantic Ocean are thought to have influenced the rate of deep-water formation, and consequently climate1,2. Although water-mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3, there has been little dynamic evidence to support this linkage. Here we present a 25,000 year record of variations in sediment grain size from south of Iceland, which indicates vigorous bottom-water currents during both the last glacial maximum and the Holocene period. Together with reconstructions of North Atlantic water-mass distribution, vigorous bottom currents suggest a shorter residence time of northern-source waters during the last glacial maximum, relative to the Holocene period. The most significant reductions in flow strength occur during periods that have been associated with freshening of the surface North Atlantic. The short-term deglacial oscillations in bottom current strength are closely coupled to changes in Greenland air temperature, with a minimum during the Younger Dryas cold reversal and a maximum at the time of rapid warming at the onset of the Holocene. Our results support a strong connection between ocean circulation and rapid climate change.

The Gulf Stream transports approximately 31 Sv (1 Sv = 10(6) m(3) s(-1)) of water and 1.3 x 10(15) W of heat into the North Atlantic ocean. The possibility of abrupt changes in Gulf Stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries. Given the limited length of the instrumental record, our knowledge of Gulf Stream behaviour on long timescales must rely heavily on information from geologic archives. Here we use foraminifera from a suite of high-resolution sediment cores in the Florida Straits to show that the cross-current density gradient and vertical current shear of the Gulf Stream were systematically lower during the Little Ice Age (ad approximately 1200 to 1850). We also estimate that Little Ice Age volume transport was ten per cent weaker than today's. The timing of reduced flow is consistent with temperature minima in several palaeoclimate records, implying that diminished oceanic heat transport may have contributed to Little Ice Age cooling in the North Atlantic. The interval of low flow also coincides with anomalously high Gulf Stream surface salinity, suggesting a tight linkage between the Atlantic Ocean circulation and hydrologic cycle during the past millennium.

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 69 (2005): 2547-2558, doi:10.1016/j.gca.2005.01.001.

Distinct cyclicity in lithology and microfaunal distribution in sediment cores from the Mendeleev Ridge in the western Arctic Ocean (water depths ca. 1.5 km) reflects contrasting glacial/interglacial sedimentary patterns. We conclude that during major glaciations extremely thick pack ice or ice shelves covered the western Arctic Ocean and its circulation was restricted in comparison with interglacial,modern-type conditions. Glacier collapse events are marked in sediment cores by increased contents of ice-rafted debris,notably by spikes of detrital carbonates and iron oxide grains from the Canadian Arctic Archipelago. Composition of foraminiferal calcite N 18 O and N 13 C also shows strong cyclicity indicating changes in freshwater balance and/or ventilation rates of the Arctic Ocean. Light stable isotopic spikes characterize deglacial events such as the last deglaciation at ca. 12 14 C kyr BP. The prolonged period with low N 18 O and N 13 C values and elevated contents of iron oxide grains from the Canadian Archipelago in the lower part of the Mendeleev Ridge record is interpreted to signify the pooling of freshwater in the Amerasia Basin,possibly in relation to an extended glaciation in arctic North America. Unique benthic foraminiferal events provide a means for an independent stratigraphic correlation of sedimentary records from the Mendeleev Ridge and other mid-depth locations throughout the Arctic Ocean such as the Northwind and Lomonosov Ridges. This correlation demonstrates the disparity of existing age models and underscores the need to establish a definitive chronostratigraphy for Arctic Ocean sediments. ; 2003 Elsevier B.V. All rights reserved.

We have measured Zn/Ca ratios in two taxa of Holocene-aged benthic foraminifera from throughout the world's oceans. Zn/Ca is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and Ba/Ca, by bottom water saturation state with respect to calcite. Measurements on “live-collected” foraminifera suggest that the saturation effect occurs during growth and is not a postdepositional artifact. Zn/Ca could be a sensitive paleoceanographic tracer because deep water masses have characteristic Zn concentrations that increase about tenfold from the deep North Atlantic to the deep North Pacific. In addition, since Zn/Ca responds to a different range of saturation states than Cd/Ca, the two may be used together to evaluate changes in deep water carbonate ion (CO32−) concentration.

We present empirically based calibrations of our measurements made on a Finnigan MAT252 equipped with a Kiel Device to Vienna Pee Dee belemnite, using an enriched δ18O standard. Calibrations include corrections for biases caused by the differences in isotopic composition of carbonate standards measured on the two parallel extraction lines of the Kiel Device and for decreases in the isotopic difference between the reference and sample gas caused by mixing in the source. After correcting for these biases, the precision of 2200 NBS19 analyses (10–300 µg ) is ±0.07 for δ18O and ±0.03 for δ13C. We have shared our standard enriched in δ18O with 18 laboratories engaged in paleoceanographic research, producing the first large-scale interlaboratory calibrations for this community. Using correction procedures reported here, water mass reconstructions using data produced on multiple mass spectrometers may now be possible with a precision approaching the level necessary to reconstruct temperature-salinity and density variability in the deep ocean.

As it passes through the Florida Straits, the Gulf Stream consists of two main components: the western boundary flow of the wind-driven subtropical gyre and the northward-flowing surface and intermediate waters which are part of the ‘global conveyor belt’, compensating for the deep water that is exported from the North Atlantic Ocean1. The mean flow through the Straits is largely in geostrophic balance and is thus reflected in the contrast in seawater density across the Straits2. Here we use oxygen-isotope ratios of benthic foraminifera which lived along the ocean margins on the boundaries of the Florida Current during the Last Glacial Maximum to determine the density structure in the water and thereby reconstruct transport through the Straits using the geostrophic method—a technique which has been used successfully for estimating present-day flow3. Our data suggest that during the Last Glacial Maximum, the density contrast across the Florida Straits was reduced, with the geostrophic flow, referenced to the bottom of the channel, at only about two-thirds of the modern value. If the wind-driven western boundary flow was not lower during the Last Glacial Maximum than today, these results indicate a significantly weaker conveyor-belt component of the Gulf Stream compared to present-day values. Whereas previous studies based on tracers suggested that deep waters of North Atlantic origin were not widespread during glacial times, indicating either a relatively weak or a shallow overturning cell, our results provide evidence that the overturning cell was indeed weaker during glacial times.

We present a new method for the quantitative reconstruction of upper ocean flows for during times in the past. For the warm (T>5°C) surface ocean, density can be accurately reconstructed from calcite precipitated in equilibrium with seawater, as both of these properties increase with decreasing temperature and increasing salinity. Vertical density profiles can be reconstructed from the oxygen isotopic composition of benthic foraminifera. The net volume transport between two vertical density profiles can be calculated using the geostrophic method. Using benthic foraminifera from surface sediment samples from either side of the Florida Straits (Florida Keys and Little Bahama Bank), we reconstruct two vertical density profiles and calculate a volume transport of 32 Sv using this method. This agrees well with estimates from physical oceanographic methods of 30–32 Sv for the mean annual volume transport. We explore the sensitivity of this technique to various changes in the relationship between temperature and salinity as well as salinity and the oxygen isotopic composition of seawater.

Our understanding of oceanic variability on timescales longer than the time span of direct oceanographic measurements (about a century for most common measurements) relies on our capability to interpret the marine sediment record. Sediment observations have reached the point where hypotheses regarding oceanic conditions during specific time intervals of the geological past can be tested. An interval of preeminent interest is the Last Glacial Maximum (LGM, ca. 20 kyr BP), when large ice sheets occupied North America and northern Europe, and global sea level was reduced by more than 100 m. Much effort has been devoted to estimating oceanic conditions during the LGM, in particular in the Atlantic basin. Hypotheses regarding the ocean circulation during the LGM are particularly relevant, given the postulated role of ocean circulation in climate change. Here we report on a test of the null hypothesis that observations from glacial sediments in the Atlantic basin are consistent with the modern circulation.

Ocean circulation is closely linked to climate change on glacial–interglacial and shorter timescales. Extensive reorganizations in the circulation of deep and intermediate-depth waters in the Atlantic Ocean have been hypothesized for both the last glaciation1,2,3,4,5,6 and the subsequent Younger Dryas cold interval3,6,7,8,9,10, but there has been little palaeoceanographic study of the subtropical gyres11,12,13. These gyres are the dominant oceanic features of wind-driven circulation, and as such they reflect changes in climate and are a significant control on nutrient cycling and, possibly, atmospheric CO2 concentrations. Here we present Cd/Ca ratios in the shells of benthic foraminifera from the Bahama banks that confirm previous suggestions11,12 that nutrient concentrations in the North Atlantic subtropical gyre were much lower during the Last Glacial Maximum than they are today (up to 50% lower according to our data). These contrasting nutrient burdens imply much shorter residence times for waters within the thermocline of the Last Glacial Maximum. Below the glacial thermocline, nutrient concentrations were reduced owing to the presence of Glacial North Atlantic Intermediate Water. A high-resolution Cd/Ca record from an intermediate depth indicates decreased nutrient concentrations during the Younger Dryas interval as well, mirroring opposite changes at a nearby deep site3,9. Together, these observations suggest that the formation of deep and intermediate waters — North Atlantic Deep Water and Glacial North Atlantic Intermediate Water, respectively — wax and wane alternately on both orbital and millennial timescales.

Five δ13C records from the deep ocean, extending back to 1.3 Ma, were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41- to 100-ka climate transition. These data show that significant perturbations in mean ocean carbon chemistry were associated with the mid-Pleistocene climate transition. Notable features of the last 1.3 Myr are (1) a pronounced ∼0.3‰ decrease in mean ocean δ13C between 0.9 and 1.0 Myr, followed by a return to pre-1.0 Ma values by 400 ka B.P., which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean, possibly associated with an increase in global aridity (and decrease in the size of the biosphere) across the 41- to 100-ka transition; (2) no change in the Atlantic-Pacific (A-P) δ13C gradient over the last 1.3 Myr, suggesting no change in mean ocean nutrient content accompanied the addition of light carbon; and (3) stronger vertical nutrient fractionation in the North Atlantic in the middle Pleistocene between sites 607 and 552, suggesting weaker North Atlantic Deep Water formation at this time relative to the early and late Pleistocene. We also find evidence for a more pronounced deep recirculation gyre in the western North Atlantic basin in the early Brunhes, as evidenced by “aging” of deep northern basin water (site 607) relative to deep water in the equatorial Atlantic (site 664).

Stable isotopic measurements of G. sacculifer and C. wuellerstorfi in a core from the western equatorial Atlantic imply that there are parallel, suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3. Low values of G. sacculifer δ18O accompany high values of C. wuellerstorfi δ13C, linking warmer sea surface temperatures (SSTs) in the tropics with increased production of lower North Atlantic Deep Water (NADW). The amplitude of the δ18O oscillations is 0.6‰ (or 2°–3°C), which is superimposed on a glacial/interglacial amplitude of about 2.1‰. Using the G. sacculifer δ18O data, we calculate that surface waters were colder during stage 2 than calculated by CLIMAP [1976, 1981]. The longer-period (>2 kyr) oscillations in air temperature recorded in the Greenland and Antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial Atlantic. The magnitude of these oscillations in tropical SST is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone. The apparent synchroneity of equatorial SST and polar air temperature changes, as well as the amplitude of the SST changes at the equator, are consistent with the climate effects expected from changes in the atmosphere's greenhouse gas content (H2Ovapor, CO2, and CH4).

We investigated glacial-interglacial changes in the circulation and carbon cycling in the western North Atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of Bahamian benthic foraminifera. Potential temperature-salinity-depth relations show that modern thermocline (∼200–1000 m) and deep (∼1000–2000 m) waters in the Providence Channels, Bahamas, originate in the Sargasso Sea and are typical of the subtropical gyre. Gradients in the stable isotopic compositions of late Holocene Planulina and Cibicidoides species from the bank margins (∼400 to 1500 m depth) reflect temperature, nutrient, and isotopic gradients of modern subtropical gyre waters. The difference between the δ18O of glacial maximum and late Holocene foraminifera is ∼2.1‰ for the upper 900 m of the water column and ∼1.6‰ for deeper waters, indicating that these waters were ∼4°C and ∼2°C cooler, respectively, during glacial time. The glacial temperature gradient (dT/dz) was similar to today, while the base of the thermocline was ∼100 m shallower. These results differ significantly from our earlier results from multiple shell δ18O analyses, which implied upper thermocline waters were only ∼1°C cooler and dT/dz was greater during the glacial maximum. The difference occurs because bioturbation adversely affects multiple shell analyses of glacial-aged samples from shallow water depths. At all depths above 1500 m, foraminiferal δ13C are greater during the glacial maximum than the late Holocene by at least 0.1 to 0.2‰ (as much as 0.6‰ in the lower thermocline), indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum. This suggests greater, more uniform ventilation of the thermocline. Results of single and multiple shell δ13C analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial-interglacial change of δ13C was small. Cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal surfaces outcrop, including large areas of the subtropical ocean. A shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water production. Enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of Ekman downwelling.

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO 2 and CH 4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.

THE suggestion1 that changes in North Atlantic Deep Water (NADW) production are linked through surface heat flux to the atmospheric temperature over Greenland is supported by earlier indications2,3 that NADW production decreased during glacial times, and by the subsequent finding4–6 that it declined during the Younger Dryas cool period at the end of the last glaciation. Changes in North Atlantic surface temperatures have been found7 to mirror high-frequency temperature changes recorded in Greenland ice cores over the past 80 kyr, but the connection to abyssal circulation has yet to be established, except for one or two isolated oscillations8,9. Here we present carbon and oxygen isotope analyses of benthic foraminifera in a high-resolution North Atlantic deep-sea sediment core for the period 70–130 kyr ago. These data allow us to reconstruct the history of NADW production, which shows a close correlation with Greenland climate variability for much of this time interval, suggesting that the climate influence of NADW variability was widespread. We see no evidence, however, for changes in NADW production during substage 5e (the Eemian interglacial period), in contrast with recent ice-core data10 which suggest severe climate instability in Greenland during this time period. Our results may support suggestions, based on data from a second ice core, that this apparent instability is an artefact caused by ice flow11. Alternatively, the Eemian climate instability may have had a different origin from the subsequent climate events.

[1] The waters passing through the Florida Straits today reflect both the western portion of the wind‐driven subtropical gyre and the northward flow of the upper waters which cross the equator, compensating North Atlantic Deep Water export as part of the large‐scale Atlantic meridional overturning circulation. It has been postulated from various lines of evidence that the overturning circulation was weaker during the Younger Dryas cold event of the last deglaciation. We show here that the contrast in the oxygen isotopic composition of benthic foraminiferal tests across the Florida Current is reduced during the Younger Dryas. This most likely reflects a decrease in the density gradient across the channel and a decrease in the vertical shear of the Florida Current. This reduced shear is consistent with the postulated reduction in the Atlantic meridional overturning circulation. We find that the onset of this change in density structure and flow at the start of the Younger Dryas is very abrupt, occurring in less than 70 years.

Long-term sediment trap studies have been carried out since 1986 at three locations in the western, central and eastern Arabian Sea. Here we present total and bulk component fluxes measured for 3 years at the central station and for 4 years at the western and eastern stations. Particulate fluxes to the deep sea are controlled by the monsoons with generally higher fluxes during the SW and NE monsoons and lower fluxes during the intermonsoon periods. The increase of particle fluxes occurs simultaneously with a drop in surface water temperature, induced by wind-or convective-mixing and an associated entrainment of nutrients into the euphotic zone. More than 50% of the annual particle fluxes to the deep sea occurs during the SW monsoon at the western location due to the prolonged influence of the monsoonal upwelling as indicated by increased biogenic carbonate and opal fluxes. However, the opal fluxes peak a month later than the carbonate fluxes. The delayed onset of opal flux peak appears to be controlled by the observed premonsoon silica distribution in the Arabian Sea, where the subsurface waters are silica depleted down to the thermocline at 150 m. At the central location particle fluxes are of similar magnitude during the SW and NE monsoons. The interannual variability of particle fluxes at the eastern location is determined by the NE monsoon. At the western and central locations, on the other hand, maximum interannual variability of fluxes occurs during the SW monsoon and particle fluxes were higher during years of stronger SW monsoon. The results further suggest that, apart from monsoon strength, geographic shifts of the area of maximum wind-stress may produce significant variabilities in particle fluxes to the deep ocean at the western Arabian Sea site.

[1] Paleoceanographic studies using benthic foraminiferal Cd as a nutrient tracer have provided a robust means of reconstructing glacial Atlantic Ocean water mass geometry, but a paucity of data from the South Atlantic above 1200 m has limited investigation of Antarctic Intermediate Water (AAIW) configuration and formation. A new Cd depth profile from Brazil margin sediments suggests that AAIW penetrated northward at 1100 m to at least 27°S in the glacial Atlantic. It exhibited substantially reduced δ13Cas values, confirming preliminary evidence that this AAIW was unique to the glacial Atlantic and that it formed differently than today, with less atmospheric contact.

THE ocean's intermediate1–6 and deep7,8 circulation are both known to have differed during the last glaciation from those of today, but little is known about the history of the subtropical gyres. Variations in gyre processes should be closely linked to variations in global climate, as gyre circulation is driven by air–sea interactions and reflects the climate at the ocean surface. The gyres are also a significant reservoir of carbon and nutrients, so that gyre processes affect the distribution of carbon and nutrients in the oceans and thus atmospheric CO2. Here we use measurements of δ13C and δ18O in foraminifera from the Bahamas to produce a detailed reconstruction of nutrient and temperature profiles in the thermocline during the last glaciation. The thermocline of the glacial North Atlantic subtropical gyre lacked the oxygen minimum that is characteristic of the modern ocean, and was depleted in nutrients, indicating greater, more uniform thermocline ventilation at that time. Thus not only intermediate waters1–5 but the entire upper water column within the glacial North Atlantic was depleted of nutrients. Moreover, glacial thermocline waters were cooler, had a steeper temperature gradient and a shallower base. Both the ventilation and the thermal structure of the glacial thermocline are consistent with what is known of the glacial climate9–14.

[1] The density structure across the Florida Straits is reconstructed for the last 8000 years from oxygen isotope measurements on foraminifera in sediment cores. The oxygen isotope measurements suggest that the density contrast across the Florida Current increased over this time period. The magnitude of this change corresponds to an increase in the geostrophic transport referenced to 800 m water depth of 4 sverdrups (Sv) over the last 8000 years. The spatial and seasonal distribution of incoming solar radiation due to changes in the Earth's orbit has caused systematic changes in the atmospheric circulation, including a southward migration of the Intertropical Convergence Zone over the last 8000 years. These changes in atmospheric circulation and the associated wind-driven currents of the upper ocean could readily account for a 4 Sv increase in the strength of the Florida Current. We see no evidence in our data for dramatic changes in the strength of the Atlantic Meridional Overturning Circulation over this time period.

We propose a method to determine past sea levels based on the record of 230Th accumulating in seafloor sediments. The decay of uranium in seawater produces this nuclide, which then adsorbs rapidly onto particles settling to the seafloor. Since the concentration of uranium is similar at all depths in the water column, the flux of 230Th to the seafloor at any location is proportional to water column height. Consequently, changes in sea level must cause changes in these fluxes. At shallow depths (

High-resolution δ18O records from the equatorial Pacific (site 503B), equatorial Atlantic (site 665A), and North Atlantic (site 606A) based on the benthic foraminifera Cibicidoides wuellerstorfi show the 2.4 Ma onset of major northern hemispheric glaciation to be a package of three events occurring at 2.39, 2.35, and 2.31 Ma in which a periodicity of about 40 kyr is evident. The amplitude of the signals at the three sites indicates that these events were 1/2 to 2/3 the size of the latest Quaternary glaciation and also indicates cooling of northern source bottom water by 2.7°–4.1°C relative to southern source water during glaciations. Carbon isotopes indicate that southern source waters were less oxygenated than in the Quaternary and that there was reduced production of northern source water during glacial intervals. The dominant presence of southern source water in the eastern basin of the equatorial Atlantic, regardless of climatic cycles, throughout the late Pliocene indicates a greater influence of these waters relative to northern source waters in the late Pliocene ocean.

[1] We use geochemical and isotope measurements on a 225-year old brain coral (Diploria labyrinthiformis) from the south shore of Bermuda (64°W, 32°N) to construct a record of decadal-to-centennial-scale climate variability. The coral was collected alive, and annual density bands visible in X radiographs delineate cold and warm seasons allowing for precise dating. Coral skeletons incorporate strontium (Sr) and calcium (Ca) in relative proportions inversely to the sea surface temperature (SST) in which the skeleton is secreted. Previous studies on this and other coral colonies from this region document the ability to reconstruct mean annual and wintertime SST using Sr/Ca measurements (Goodkin et al., 2007, 2005). The coral-based records of SST for the past 2 centuries show abrupt shifts at both decadal and centennial timescales and suggest that SST at the end of the Little Ice Age (between 1840 and 1860) was 1.5° ± 0.4°C colder than today (1990s). Coral-reconstructed SST has a greater magnitude change than does a gridded instrumental SST record from this region. This may result from several physical processes including high rates of mesoscale eddy propagation in this region. Oxygen isotope values (δ18O) of the coral skeleton reflect changes in both temperature and the δ18O of seawater (δOw), where δOw is proportional to sea surface salinity (SSS). We show in this study that mean annual and wintertime δ18O of the carbonate (δOc) are correlated to both SST and SSS, but a robust, quantitative measure of SSS is not found with present calibration data. In combination, however, the Sr/Ca and δOc qualitatively reconstruct lower salinities at the end of the Little Ice Age relative to modern day. Temperature changes agree with other records from the Bermuda region. Radiative and atmospheric forcing may explain some of the SST variability, but the scales of implied changes in SST and SSS indicate large-scale ocean circulation impacts as well.

An evaluation of C. pachyderma Mg/Ca using a new suite of warm water multicores from the Florida Straits shows that the slope of Mg/Ca with temperature is shallower than previously thought. Using secondary ionization mass spectrometry, we have documented that the distribution of magnesium within the polished walls of foraminiferal tests is Gaussian, suggesting that the Mg/Ca in these samples is not affected by the addition of a secondary high-magnesium calcite in the walls. The Mg/Ca within a typical C. pachyderma test varies by about ±20% (1σ/μ · 100), and the variability increases slightly in tests with higher Mg/Ca. The regression of C. pachyderma Mg/Ca with temperature has a slope of 0.13 ± 0.05 mmol mol−1 per °C, indistinguishable from the slope observed in inductively coupled plasma–mass spectrometry measurements from a different subset of the same multicores, but about one half the slope of previously published calibrations. The largest differences between the calibrations comes at the warm water end of the regression, where previously published C. pachyderma Mg/Ca values from Little Bahama Bank are at least 3 mmol mol−1 higher than observed in these new cores. The reasons for this difference are not fully known but are most likely related to diagenesis at Little Bahama Bank.

The factors driving glacial changes in ocean overturning circulation are not well understood. On the basis of a comparison of 20 climate variables over the past four glacial cycles, the SPECMAP project proposed that summer insolation at high northern latitudes (that is, Milankovitch forcing) drives the same sequence of ocean circulation and other climate responses over 100-kyr eccentricity cycles, 41-kyr obliquity cycles and 23-kyr precession cycles. SPECMAP analysed the circulation response at only a few sites in the Atlantic Ocean, however, and the phase of circulation response has been shown to vary by site and orbital band. Here we test the SPECMAP hypothesis by measuring the phase of orbital responses in benthic delta(13)C (a proxy indicator of ocean nutrient content) at 24 sites throughout the Atlantic over the past 425 kyr. On the basis of delta(13)C responses at 3,000-4,010 m water depth, we find that maxima in Milankovitch forcing are associated with greater mid-depth overturning in the obliquity band but less overturning in the precession band. This suggests that Atlantic overturning is strongly sensitive to factors beyond ice volume and summer insolation at high northern latitudes. A better understanding of these processes could lead to improvements in model estimates of overturning rates, which range from a 40 per cent increase to a 40 per cent decrease at the Last Glacial Maximum and a 10-50 per cent decrease over the next 140 yr in response to projected increases in atmospheric CO(2) (ref. 4).

The circulation of the deep Atlantic Ocean during the height of the last ice age appears to have been quite different from today. We review observations implying that Atlantic meridional overturning circulation during the Last Glacial Maximum was neither extremely sluggish nor an enhanced version of present-day circulation. The distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the Atlantic only slightly greater than today. However, evidence from multiple water-mass tracers supports a different distribution of deep-water properties, including density, which is dynamically linked to circulation.

The geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin. Here we examine how the density difference across 30°S in the upper 2 km of the Atlantic Ocean (and thus the magnitude of the shear associated with the overturning circulation) has changed between the last glacial maximum and the present. We use oxygen isotope measurements on benthic foraminifera to reconstruct density. Today, the density in upper and intermediate waters along the eastern margin in the South Atlantic is greater than along the western margin, reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing North Atlantic Deep Waters below. The greater density along the eastern margin is reflected in the higher δ18O values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km. For the last glacial maximum the available data indicate that the eastern margin foraminifera had similar δ18O to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher δ18O values in the western margin benthic foraminifera above 1 km. If this reversal in benthic foraminifera δ18O gradient reflects a reversal in seawater density gradient, these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the Atlantic basin are balanced by the southward export of Glacial North Atlantic Intermediate Water.

surface water (d 18 Ow) near Dry Tortugas increased 0.4% during the course of the Little Ice Age (LIA) (� 1200– 1850 A.D.), equivalent to a salinity increase of 0.8–1.5. On the Great Bahama Bank, where surface waters are influenced by the North Atlantic subtropical gyre, d 18 Ow increased by 0.3% during the last 200 years. Although a portion (� 0.1%) of this shift may be an artifact of anthropogenically driven changes in surface water SCO2, the remaining d 18 Ow signal implies a 0.4–1 increase in salinity after 200 years B.P. The simplest explanation of the d 18 Ow data is southward migration of the Atlantic Hadley circulation during the LIA. Scaling of the d 18 Ow records to salinity using the modern low-latitude d 18 Ow-S slope produces an unrealistic reversal in the salinity gradient between the two sites. Only if d 18 Ow is scaled to salinity using a high-latitude d 18 Ow-S slope can the records be reconciled. Variable atmospheric 14 C paralleled Dry Tortugas d 18 Ow, suggesting that solar irradiance paced centennial-scale migration of the Inter-Tropical Convergence Zone and changes in Florida Current salinity during the last millennium. Citation: Lund, D. C., and W. Curry (2006), Florida Current surface temperature and salinity variability during the last millennium, Paleoceanography, 21, PA2009, doi:10.1029/2005PA001218.

[1] Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin. The cores span water depths from about 400 to 3000 m and intersect the major water masses in this region. These new data fill a critical gap in the South Atlantic Ocean and provide the motivation for updating the classic glacial western Atlantic δ13C transect of Duplessy et al. (1988). The distribution of δ13C of ΣCO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (∼1000 m), southern source water mass with an end-member δ13C value of about 0.3–0.5‰ VPDB, a middepth (∼1500 m), northern source water mass with an end-member value of about 1.5‰, and a deep (>2000 m), southern source water with an end-member value of less than −0.2‰, and perhaps as low as the −0.9‰ values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal δ18O. A revised glacial section of deep water δ13C documents the positions and gradients among these end-member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.

[1] Planktonic foraminiferal δ18O time series from three well-dated, high sedimentation rate cores near the Florida Keys (24.4°N, 83.3°W) exhibit repeated centennial to millennial-scale oscillations during the late Holocene. Isotopic shifts of 0.2–0.3‰ over the past 5200 years represent changes in sea-surface temperature (SST) of 1.0–1.5°C or salinity variability of 1–2 psu. The largest significant isotopic events are centered at approximately 200, 2000, 3200, and prior to 4000 calendar years BP. High Florida Current δ18O during the Little Ice Age (LIA) correlates with published records of high δ18O in the Sargasso Sea and low SST off the coast of west Africa. An interval of generally low δ18O in the Florida Straits from 1800 to 500 years BP is synchronous with the Medieval Warm Period off west Africa but leads low δ18O in the Sargasso Sea by several hundred years. Synchronous cooling across the subtropical gyre during the LIA is difficult to explain using interannual North Atlantic Oscillation patterns but may be consistent with the simulated effects of reduced solar irradiance. At frequencies between 1/1000 and 1/300 years during the Late Holocene, Florida Current δ18O is coherent with a published estimate of 14C production rate. Radiocarbon production seems to lead δ18O at these frequencies, but uncertainty in the phase calculation precludes a clear lead-lag relationship. At frequencies lower than 1/300 years, Florida Current δ18O is coherent and in phase with atmospheric Δ14C. The coherence of Δ14C and δ18O at periods >1000 years implies oceanic circulation may play a role in modulating atmospheric radiocarbon on millennial timescales.

Author Posting. © American Geophysical Union, 2003. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 18 (2003): 1086, doi:10.1029/2003PA000888.

Offsets from isotopic equilibrium in biogenic carbonates have complicated paleoclimate recon- structions for decades. A new archive of climate, deep-sea corals, is used to evaluate the calcification processes, independent of photosynthesis, that contribute to these offsets. Carbon and oxygen stable isotope data from six modern deep-sea corals show strong linear trends between 13 C and 18 O. Slopes of these trends between samples are similar and range between 1.9 to 2.6 for 13 C/ 18 O. Linear trends intersect isotopic equilibrium for 18 O and are slightly depleted for 13 C. Variations in the isotopic ratios are strongly correlated with the density banding structure. Isotopically depleted aragonite is associated with light, quickly precipi- tating bands, whereas isotopically enriched points correspond to slowly accumulating, less dense aragonite. The densest white band at the trabecular center is furthest from isotopic equilibrium for both carbon and oxygen. Data from this region fall off the linear trend between 18 O and 13 C. This deviation, where 13 C remains constant while the 18 O continues to decrease, does not support "vital effect" mechanisms that call upon kinetic fractionation to explain offsets from isotopic equilibrium. We propose a new mechanism for vital effects in these deep-sea corals that is based on a thermodynamic response to a biologically induced pH gradient in the calcifying region. Copyright © 2003 Elsevier Science Ltd

[1] Mid-Pleistocene benthic δ18O and δ13C time series from the North Atlantic site 983 and Ceara Rise site 928 are compared to an array of existing isotopic records spanning the Atlantic basin and the geographic extremes of the North Atlantic Deep Water/Southern Ocean Water interface during both glacial and interglacial periods. This comparison allows the persistent millennial-scale intermediate depth North Atlantic ventilation changes recorded at site 983 to be placed within the context of the longer period water mass reorganizations taking place throughout the mid-Pleistocene. Our benthic δ13C results suggest that the intermediate depth North Atlantic experienced millennial-scale changes in ventilation throughout the mid-Pleistocene climate shift. The times of poorest ventilation (low benthic δ13C) persisted for only a few millennia and were associated with rapid decreases in benthic δ18O, suggesting that ice sheet decay and melt water induced salinity changes were effective at throttling deep water production in the North Atlantic throughout the mid-Pleistocene. Similar but less pronounced decreases in the δ13C of the middepth waters also punctuated interglacials, suggesting that large ice sheet fluctuations do not explain all of the observed thermohaline circulation mode shifts in the North Atlantic. Meanwhile, on orbital timescales, glacial deep to intermediate water δ13C gradients evolved after ∼0.95 Ma. Taken together, these observations provide a number of new constraints for understanding the timing and evolution of deep water circulation changes across the mid-Pleistocene.