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During the Pliocene, atmospheric CO2 concentrations (p CO2) were similar to today's and global average temperature was ~3 °C higher. However, the relationships and phasing between variability in climate and p CO2 on orbital time scales are not well understood. Specifically, questions remain about the nature of a lag of p CO2 relative to benthic foraminiferal δ18O in the late-Pliocene Marine Isotope Stage M2 (3300 kiloannum ago, ka), which was longer than during the Pleistocene. Here, we present a multi-proxy paleoceanographic reconstruction of the late-Pliocene subantarctic zone, which is today one of the major ocean sinks of atmospheric CO2. New dinoflagellate cyst assemblage data is combined with previously published sea surface temperature reconstructions, to reveal past surface conditions, including latitudinal migrations of the subtropical front (STF) over the late-Pliocene at ODP Site 1168, offshore west Tasmania. We observe strong oceanographic variability at the STF over glacial-interglacial timescales, especially across the M2 (3320–3260 ka). By providing tight and independent age constraints from benthic foraminiferal δ18O, we find that, much more than benthic δ18O or local SST, latitudinal migrations of the STF are tightly coupled to p CO2 variations across the M2. Specifically, a northerly position of the STF during M2 deglaciation coincides with generally low p CO2. We postulate that the efficiency of the Southern Ocean carbon outgassing varied strongly with migrations of the STF, and that is in part accounted for the variability in p CO2 across M2. [ABSTRACT FROM AUTHOR]

Climate variability is typically amplified towards polar regions. The underlying causes, notably albedo and humidity changes, are challenging to accurately quantify with observations or models, hampering projections of future polar amplification. Polar amplification reconstructions from the ice-free early Eocene (~56-48 million years ago) can exclude ice albedo effects, but the required tropical temperature records for resolving timescales shorter than multi-million years are lacking. Here, we reconstruct early Eocene tropical sea surface temperature variability by presenting an up to ~4 kyr-resolution biomarker-based temperature record from Ocean Drilling Program Site 959, located in the tropical Atlantic Ocean. This record shows warming across multiple orbitally paced carbon cycle perturbations, coeval with high-latitude-derived deep-ocean bottom waters, showing that these events represent transient global warming events (hyperthermals). This implies that orbital forcing caused global temperature variability through carbon cycle feedbacks. Importantly, deep-ocean temperature variability was amplified by a factor 1.7-2.3 compared to the tropical surface ocean, corroborating available long-term estimates. This implies that fast atmospheric feedback processes controlled meridional temperature gradients on multi-million year, as well as orbital timescales during the early Eocene. Our combined records have several other implications. First, our amplification factor is somewhat larger than the same metric in fully-coupled simulations of the early Eocene (1.1-1.3), suggesting that models slightly underestimate the non-ice related -- notably hydrological -- feedbacks that cause polar amplification of climate change. Second, even outside the hyperthermals, we find synchronous eccentricity-forced temperature variability in the tropics and deep ocean that represent global mean sea surface temperature variability of up to 0.7 °C, and requires significant variability in atmospheric pCO2. We hypothesize that the responsible carbon cycle feedbacks that are independent of ice, snow and frost-related processes might play an important role in Phanerozoic orbital-scale climate variability throughout geological time, including Pleistocene glacial-interglacial climate variability. [ABSTRACT FROM AUTHOR]

At present, a strong latitudinal sea surface temperature (SST) gradient of ~16°C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC-system, its development towards modern-day strength, and the consequences for e.g., the latitudinal SST gradient around the southern Atlantic Ocean, are still uncertain. Here we present new TEX86-biomarker records, calibrated to SST, from two sites located east of Drake Passage (southern South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: The Late Eocene-Early Oligocene (ODP Site 696) and Middle-Late Miocene (IODP Site U1536) transitions. Our results overall show rather temperate conditions (20-11°C) during the Late Eocene to Early Oligocene interval, with a weaker latitudinal SST gradient (~8°C) across the Atlantic sector of the Southern Ocean compared to present day (~16°C). We ascribe the regional similarity in SSTs across the Late Eocene-Early Oligocene South Atlantic to a persistent, strong Subpolar Gyre circulation, connecting all sites, which can only exist in absence of a strong throughflow across the Drake Passage. Surprisingly, the southern South Atlantic records show comparable SSTs (~12-14°C) during both the Earliest Oligocene Oxygen Isotope Step (EOIS, ~33.65 Ma) and the Miocene Climate Optimum (MCO, ~16.5 Ma). Apparently, maximum Oligocene Antarctic ice volume could coexist with warm ice-proximal surface ocean conditions, while at similar ocean temperatures, the Middle Miocene Antarctic ice sheet was strongly reduced. Southern South Atlantic SSTs cooled to ~5°C at the onset of the Middle Miocene Climate Transition (MMCT, 14 Ma), making it the coldest oceanic region recorded around Antarctica and the likely main location for deep water formation. The already cold southern South Atlantic conditions at MMCT meant it experienced little cooling during the latter part of the Miocene, which contrasts the profound cooling due to northward expansion of the Southern Ocean frontal systems in the lower latitudes and other sectors of the Southern Ocean. [ABSTRACT FROM AUTHOR]

The Neogene (23.04-2.58 Ma) is characterized by progressive buildup of Antarctic and Northern Hemisphere ice volume and climate cooling. Heat/moisture delivery to Antarctica is to a large extent regulated by the strength of meridional temperature gradients. However, the evolution of the Southern Ocean frontal systems remains scarcely studied in the Neogene. Here we present the first long-term continuous sea (sub)surface temperature (SST) record of the subtropical front area in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 off western Tasmania. This site is at present located near the subtropical front (STF), as it was during the Neogene, despite a 10 degree northward tectonic drift of Tasmania during the Neogene. We analyzed glycerol dialkyl glycerol tetraethers (GDGTs, on 433 samples) and alkenones (on 163 samples) and reconstructed the paleotemperature evolution using TEX86 and Uk'37 as two independent quantitative proxies. Both proxies indicate that Site 1168 experienced a temperate ~25°C during early Miocene (23-17 Ma), reaching ~29 °C during the mid-Miocene Climatic Optimum. The stepwise ~10°C cooling (20-10°C) in the mid-to-late Miocene (12.5-5.0 Ma) is larger than observed in records from lower and higher latitudes. From the Pliocene to modern (5.3-0 Ma), STF SST first plateaus at ~15°C (3 Ma), then decreases to ~6°C (1.3 Ma), and eventually increases to the modern levels around ~16 °C (0 Ma), with a higher variability of 5 degrees compared to the Miocene. Our results imply that the latitudinal temperature gradient between the Pacific equator and STF during late Miocene cooling increased from 4°C to 14°C. Meanwhile, the SST gradient between the STF and the Antarctic margin decreased due to amplified STF cooling compared to the Antarctic Margin. This implies a narrowing SST gradient in the Neogene, with contraction of warm SSTs and northward expansion of subpolar conditions. [ABSTRACT FROM AUTHOR]

Considered as one of the most significant climate reorganisations of the Cenozoic period, the Eocene-Oligocene Transition (EOT; ca. 34.44-33.65) is characterised by global cooling and the first major glacial advance on Antarctica. While in the southern high-latitudes, the EOT cooling is primarily recorded in the marine realm, the extent and effect on terrestrial climate and vegetation is poorly documented. Here, we present a new, well-dated, continuous, high-resolution palynological (sporomorph) data and quantitative sporomorph-based climate estimates recovered from the East Tasman Plateau (ODP Site 1172) to reconstruct climate and vegetation dynamics from the late Eocene (37.97 Ma) to early Oligocene (33.06 Ma). Our results indicate three major climate transitions and four vegetation communities occupying Tasmania under different precipitation and temperature regimes: (i) a warm-temperate Nothofagus-Podocarpaceae dominated rainforest with paratropical elements from 37.97-37.52 Ma; (ii) cool-temperate Nothofagus dominated rainforest with secondary Podocarpaceae rapidly expanding and taking over regions previously occupied by the warmer taxa between 37.306-35.60 Ma; (iii) fluctuation between warm temperate - paratropical taxa and cool temperate forest from 35.50-34.49 Ma, followed by a cool phase across the EOT (34.30-33.82 Ma); (iv) post-EOT (earliest Oligocene) recovery characterised by a warm-temperate forest association from 33.55-33.06 Ma. Coincident with changes in stratification of water masses and sequestration of carbon from surface water in the Southern Ocean, our sporomorph-based temperature estimates between 37.52 Ma and 35.60 Ma (phase ii) showed 2-3 ºC terrestrial cooling. The unusual fluctuation between warm and cold temperate forest between 35.50 to 34.59 Ma is suggested to be linked to the initial deepening of the Tasmanian Gateway allowing eastern Tasmania to come under the influence of warm water associated with the proto-Leeuwin Current (PLC). Further to the above, our terrestrial data show mean annual temperature declining by about 2 ºC across the EOT before recovering in the earliest Oligocene. This phenomenon is synchronous with regional and global cooling during the EOT and linked to declining 푝CO2. However, the earliest Oligocene climate rebound along eastern Tasmania is linked to transient recovery of atmospheric 푝CO2 and sustained deepening of the Tasmanian Gateway, promoting PLC throughflow. The three main climate transitional events across the studied interval (late Eocene-earliest Oligocene) in the Tasmanian Gateway region suggest that changes in ocean circulation due to accelerated deepening of the Tasmanian Gateway may not have been solely responsible for the changes in terrestrial climate and vegetation dynamics, but a series of regional and global events, including a change in stratification of water masses, sequestration of carbon from surface waters, and changes in 푝CO2 may have played vital roles. [ABSTRACT FROM AUTHOR]

The role and climatic impact of the opening of the Drake Passage and how it affected both marine and terrestrial environments across the Eocene-Oligocene Transition (EOT ~ 34 Ma) period remains poorly understood. Here we present new terrestrial palynomorph data compared with recently compiled lipid biomarker (n-alkane) data from Ocean Drilling Program (ODP) Leg 113 Site 696 drilled on the margin of the South Orkney Microcontinent in the Weddell Sea, to investigate changes in terrestrial environments and paleoclimate across the late Eocene and early Oligocene (~ 37.6-32.2 Ma). Early late Eocene floras and sporomorph-based climate estimates reveal Nothofagus-dominated forests growing under wet temperate conditions, with mean annual temperature (MAT) and precipitation (MAP) around 13 °C and 1660 mm, respectively. A phase of latest Eocene terrestrial cooling at 35.5 Ma reveals a decrease in MAT by around 2 °C possibly linked to the opening of the Powell Basin. This is followed by an increase in Mesozoic sporomorphs together with a shift in terrestrial biomarkers and sedimentological evidence indicating ice expansion to coastal and shelf areas approximately 34.1 million years ago. However, major changes to the terrestrial vegetation at Site 696 did not take place until the early Oligocene, where there is a distinct expansion of gymnosperms and cryptogams accompanied by a rapid increase in taxa diversity following 33.5 Ma. This unusual expansion of gymnosperms and cryptogams is suggested to be linked to environmental disturbance caused by repeat glacial expansion and retreat, which facilitated the expansion of conifer and ferns. We conclude that the timing of glacial onset rather suggests that the event at site 696 is linked to the global cooling at the EOT and that latest Eocene regional cooling cannot directly be linked. Therefore, confirming that the opening of ocean gateways alone did not trigger Antarctic glaciation, even if ocean gateways may have played a role in stepwise cooling before the EOT. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (sw) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be affected by contributions of additional isoGDGT sources. Methods now exist to identify and possibly correct for overprinting effects on the isoGDGT distribution in marine sediments. We here use the current proxy insights to assess the reliability of the isoGDGT-based SST signal in 69 newly analysed and 242 re-analysed sediments ODP Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques, in context of paleo-environmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ~130 kyr-resolution Maastrichtian-Oligocene TEX86-based SST record confirms previous conclusions of anomalous warmth in the early Eocene sw Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and assemblages show a strong response to the local SST evolution, supporting the robustness of the TEX86 record. Soil-derived branched GDGTs stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land using the MBT'5me proxy. MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen. General trends in SST and MAAT are similar, except for: 1) an enigmatic absence of MAAT rise during the Paleocene-Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and 2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocysts and GDGT signals suggest a mid-shelf depositional environment with strong river-runoff during the Paleocene-early Eocene, progressively becoming more marine thereafter. This trend reflects gradual drying and more pronounced wet/dry seasons in the northward drifting Australian hinterland, which may also explain the subdued middle Eocene MAAT cooling relative to that of SST. The overall correlation between dinocyst assemblages, marine biodiversity and SST changes suggests that temperature exerted a strong influence on the surface-water ecosystem, probably in part through sea level changes caused by steric effects. Finally, we find support for a potential temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in marine sediments. It is encouraging that a critical evaluation of the GDGT signals confirms the vast majority of the generated data is reliable. However, this also implies the high TEX86-based SSTs for the Eocene sw Pacific, and the systematic offset between absolute TEX86-based SST and MBT'5me-based MAAT estimates remain unexplained. [ABSTRACT FROM AUTHOR]

Antarctic continental ice masses fluctuated considerably in size during the elevated atmospheric CO2 concentrations (~ 600-800 ppm) of the Oligocene "coolhouse”. To evaluate the role of ocean conditions to the Oligocene ice sheet variability requires understanding of past ocean conditions around the ice sheet. While warm ocean conditions have been reconstructed for the Oligocene Wilkes Land region, questions arise on the geographical extent of that warmth. Currently, we lack data on surface ocean conditions from circum-Antarctic locations, and ice-proximal to ice-distal temperature gradients are poorly documented. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from the Deep Sea Drilling Project (DSDP) Site 274, offshore the Ross Sea continental margin. This site offshore Cape Adare is ideally located to characterise the Oligocene regional surface ocean conditions, as it is situated between the colder, ice-proximal Ross Sea continental shelf, and the warm-temperate Wilkes Land Margin in the Oligocene. We improve the existing age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse dinoflagellate cyst assemblages and lipid biomarkers (TEX86) to reconstruct surface paleoceanographic conditions during the Oligocene (33.7-25.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10-17 °C ± 5.2 °C) surface ocean conditions at Site 274 throughout the Oligocene. Increasingly similar oceanographic conditions between offshore Wilkes Land margin and Cape Adare developed towards the late Oligocene (26.5-25.4 Ma), likely in consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. To maintain marine terminations of terrestrial ice sheets in a proto-Ross Sea with as warm offshore SST as our data suggests, requires a strong ice flux fed by intensive precipitation during colder orbital states in the Antarctic hinterland, but with extensive surface melt of terrestrial ice during warmer orbital states. [ABSTRACT FROM AUTHOR]

While the early Eocene has been considered in many modelling studies, detailed simulations of the middle and late Eocene climate are currently scarce. To get a better understanding of both Antarctic glaciation at the Eocene-Oligocene transition (~34 Ma) and late middle Eocene warmth, it is vital to have an adequate reconstruction of the middle-to-late Eocene climate. Results of higher (CMIP5-like) resolution coupled climate simulations are represented here using the Community Earth System Model (CESM) version 1. Two middle-to-late Eocene cases are considered with the same general boundary conditions but a different radiative forcing, using a new detailed 38 Ma geography reconstruction. Under 4x pre-industrial concentrations (PIC) of both CO2 (i.e. 1120 ppm) and CH4 (~2700 ppb), equilibrium sea surface temperatures correspond well to the available late middle Eocene (42-38 Ma; ~Bartonian) proxies. Being generally cooler, the simulated climate under 2x PIC forcing is a good analog for that of the late Eocene (38-34 Ma; ~Priabonian). Terrestrial temperature proxies, although their geographical coverage is sparse, also indicate that the results presented here are realistic. The reconstructed 38 Ma climate has a reduced equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. The collective effects of geography, vegetation and ice accounts for a global mean 5-7 °C difference between pre-industrial and 38 Ma Eocene boundary conditions, with important contributions from cloud and water vapour feedbacks. These simulations effectively show that a realistic middle-to-late Eocene climate can be reconstructed without the need for greenhouse gas concentrations much higher than proxy estimates (i.e. ~500-1200 ppm CO2). Equilibrium climate sensitivity is reduced (0.62 °C/W m²; 3.2 °C warming between 38 Ma 2x PIC and 4x PIC) compared to that of the present-day climate (0.79 °C/W m²; 3.1 °C per CO2 doubling). Despite very limited sea ice and snow cover in both 38Ma cases, the model still shows a factor ~2 polar amplification in response to a further increase of atmospheric greenhouse gas concentrations. High latitudes in the modelled Eocene climate are mainly kept warm by an altered radiative balance in combination with global changes in geography and the absence of polar ice sheets compared to the pre-industrial reference. [ABSTRACT FROM AUTHOR]

Global climate cooled from the early Eocene hothouse (~52-50 Ma) to the latest Eocene (~34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasman Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasman Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on physico-chemical properties of the water masses in which these organisms thrived. The degree to which the geographic path of surface currents (primarily controlled by tectonism) or their physico-chemical properties (significantly impacted by climate) have controlled the composition of the fossil assemblages has, however, remained unclear. In fact, it is yet poorly understood to what extent oceanographic response as a whole was dictated by climate change, independent of tectonics-induced oceanographic changes that operate on longer time scales. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend, a 500 thousand year long global warming phase termed the Middle Eocene Climatic Optimum (MECO; ~40 Ma). The MECO warming is unrelated to regional tectonism, and thus provides a test case to investigate the oceans physiochemical response to climate change only. We reconstruct changes in surface-water circulation and temperature in and around the Tasman Gateway during the MECO through new palynological and organic geochemical records from the central Tasman Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia) and the Hampden Section (New Zealand). Our results confirm that dinocyst communities track tectonically driven circulation patterns, yet the variability within these communities can be driven by superimposed temperature change. Together with published results from the east of the Tasman Gateway, our results suggest that as surface-ocean temperatures rose, the East Australian Current extended further southward during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasman Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints we suggest that a regional southeast Australian transgression might have been caused by sea-level rise during MECO. [ABSTRACT FROM AUTHOR]

While the early Eocene has been considered in many modelling studies, detailed simulations of the middle and late Eocene climate are currently scarce. To understand Antarctic glaciation at the Eocene-Oligocene Transition (~ 34 Ma) as well as middle Eocene warmth, it is vital to have an adequate reconstruction of the middle-to-late Eocene climate. Here, we present a set of high resolution coupled climate simulations using the Community Earth System Model (CESM) version 1. Two middle-to-late Eocene cases are considered with new detailed 38 Ma geographical boundary conditions with a different radiative forcing. With 4 x pre-industrial concentrations of CO2 (i.e. 1120 ppm) and CH4 (~ 2700 ppb), the equilibrium sea surface temperatures correspond well to available late middle Eocene (42-38 Ma) proxies. Being generally cooler, the simulated climate with 2 x pre-industrial values is a good analog for that of the late Eocene (38-34 Ma). Deep water formation occurs in the South Pacific Ocean, while the North Atlantic is strongly stratified and virtually stagnant. A shallow and weak circumpolar current is present in the Southern Ocean with only minor effects on southward oceanic heat transport within wind-driven gyres. Terrestrial temperature proxies, although limited in coverage, also indicate that the results presented here are realistic. The reconstructed 38 Ma climate has a reduced equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. Climate sensitivity is similar (~ 0.7 °C/Wm²) to that of the present-day climate (~ 0.8 °C/Wm²; 3 °C per CO2 doubling), with significant polar amplification despite very limited sea ice and snow cover. High latitudes are mainly kept warm by albedo and cloud feedbacks in combination with global changes in geography and the absence of polar ice sheets. The integrated effect of geography, vegetation and ice accounts for a 6-7 °C offset between pre-industrial and 38 Ma Eocene boundary conditions. These 38 Ma simulations effectively show that a realistic middle-to-late Eocene climate can be reconstructed without the need for greenhouse gas concentrations much higher than proxy estimates. The general circulation and radiative budget allow for mild high-latitude regions and little to no snow and ice cover, without making equatorial regions extremely warm. [ABSTRACT FROM AUTHOR]

The late Oligocene experienced atmospheric concentrations of CO2 between 400 and 750 ppm, which are within the IPCC projections for this century, assuming unabated CO2 emissions. However, Antarctic ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well resolved, but are important to understand the influence of high-latitude Southern Hemisphere feedbacks on global climate under such CO2 scenarios. Here, we present late Oligocene (26-25 Ma) ice sheet and paleoceanographic reconstructions recorded in sediments recovered by IODP Site U1356, offshore of the Wilkes Land margin in East Antarctica. Our study, based on a combination of sediment facies analysis, physical properties, and geochemical parameters, shows that glacial and interglacial sediments are continuously reworked by bottom-currents, with maximum velocities occurring during the interglacial periods. Glacial sediments record poorly ventilated, low-oxygenation bottom water conditions, interpreted to represent a northward shift of westerly winds and surface oceanic fronts. During interglacial times, more oxygenated and ventilated conditions prevailed, which suggests enhanced mixing of the water masses with enhanced current velocities. Micritic limestone intervals within some of the interglacial facies represent warmer paleoclimatic conditions when less corrosive warmer northern component water (e.g. North Atlantic sourced deep water) had a greater influence on the site. The lack of iceberg rafted debris (IRD) throughout the studied interval contrasts with early Oligocene and post-Oligocene sections from Site U1356 and with late Oligocene strata from the Ross Sea (CRP and DSDP 270), which contain IRD and evidence for coastal sea ice and glaciers. These observations, supported by elevated paleotemperatures and the absence of sea-ice, suggest that between 26 and 25 Ma reduced glaciers or ice caps occupied the terrestrial lowlands of the Wilkes Land margin. Unlike today, the continental shelf was not over-deepened, and thus marine-based ice sheet expansion was likely limited to coastal regions. Combined, these data suggest that ice sheets in the Wilkes Subglacial Basin were largely land-based, and therefore retreated as a consequence of surface melt during late Oligocene, rather than direct ocean forcing and marine ice sheet instability processes as it did in younger past warm intervals. Spectral analysis on late Oligocene sediments from the eastern Wilkes Land margin show that the glacial-interglacial cyclicity and resulting displacements of the Southern Ocean frontal systems between 26-25 Ma were forced by obliquity. [ABSTRACT FROM AUTHOR]

Today, the temperature of the surface waters near the Antarctic coast is a determining factor in the formation of Antarctic Bottom Water (AABW) through sea-ice production, sea-ice extent, and the extent of the ice shelf. For the Oligocene, deep-sea benthic foraminiferal oxygen isotope (δ18O) reconstructions suggest that the volume of the Antarctic continental ice sheet(s) varied substantially both on million-year and on orbital timescales after its inception in the early Oligocene, and even reached larger than modern-day volumes. Replication of such dynamicity through physical modeling remains problematic, suggesting the existence of complex feedbacks between the cryosphere, the ocean and the atmosphere. To assess the relation between cryosphere, ocean and atmosphere, knowledge of sea surface conditions close to the Antarctic margin is essential. We present a TEX86-based surface water paleotemperature record measured on Oligocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356, offshore Wilkes Land, Antarctica. This record allows us to reconstruct the magnitude of seawater temperature variability and trends on both million-year and on glacial-interglacial timescales. TEX86 index values suggest surface temperatures between 10 and 21 °C during the Oligocene, which is on the upper end of the few available reconstructions. Sea surface temperature (SST) maxima occur around 30.5 and 25 Ma, irrespective of the calibration equation chosen. Based on glacial-interglacial lithological alternations we have established that SST variability between glacial intervals and their successive interglacials ranged between 1.8-3.2 °C. As benthic foraminiferal δ18O data incorporate both an ice-volume and a temperature component, our reconstructed Oligocene temperature variability could have implications for current Oligocene ice-volume estimates. If the long-term ad orbital SST variability is representative of that of the nearby region of deep-water formation, we can assess the impact of this temperature record on the volume and dynamics of the Antarctic ice sheet(s) by comparing it with the δ18O trends and variability. From this comparison, we argue that a significant portion of the variability and trends contained in long-term δ18O records can be explained by variability in Southern high-latitude temperature. If indeed a large part of the δ18O variability is due to large glacial-interglacial bottom-water temperature shifts, the Oligocene Antarctic ice volume was less sensitive to climate change than previously assumed. [ABSTRACT FROM AUTHOR]

Next to atmospheric CO2 concentrations, oceanographic conditions are a critical factor determining the stability of Antarctic marine-terminating ice sheets. The Oligocene and Miocene epochs (~ 34-5 Ma) were time intervals with atmospheric CO2 concentrations between those of present-day and those expected for the near future. As such, these time intervals may bear information to resolve the uncertainties that still exist in the projection of future ice-sheet volume decline. We present organic-walled dinoflagellate cyst (dinocyst) assemblages from chronostratigraphically well-constrained Oligocene to mid-Miocene sediments from Integrated Ocean Drilling Program Expedition (IODP) Site U1356. Situated offshore the Wilkes Land continental margin, East Antarctica, the sediment core has archived past dynamics of an ice sheet that is today mostly grounded below sea level. We interpret dinocyst assemblages in terms of paleoceanographic change on different time scales, i.e., on glacial-interglacial and long-term variability. Sea-ice indicators occur only for the first 1.5 Ma following the full Antarctic continental glaciation during the early Oligocene, and after the Middle Miocene Climatic Optimum. During the remainder of the Oligocene and Miocene dinocysts suggest a weaker-than-modern sea-ice season. The assemblages generally bear strong similarity to present-day open-ocean, high-nutrient settings north of the sea ice edge, with episodic dominance of temperate species similar to the present-day subtropical front. Oligotrophic and temperate surface waters prevailed over the site notably during interglacial time intervals, suggesting that the position of the (subpolar) oceanic frontal systems have varied in concordance with Oligocene-Miocene glacial-interglacial climate variability. [ABSTRACT FROM AUTHOR]