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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. Here, we use the current proxy insights to (re-)assess the reliability of the isoGDGT-based SST signal in 69 newly analyzed and 242 reanalyzed sediments at Ocean Drilling Program (ODP) Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques. We compare our results with paleoenvironmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ∼ 130 kyr resolution Maastrichtian–Oligocene SST record based on the TetraEther indeX of tetraethers with 86 carbon atoms (TEX86) 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 Methylation index of Branched Tetraethers with 5-methyl bonds (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 subsidence 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. 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 that most of the generated data are reliable. However, this also implies that 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 without definitive explanation.

Improvements in our capability to reconstruct ancient surface-ocean conditions based on organic-walled dinoflagellate cyst (dinocyst) assemblages from the Southern Ocean provide an opportunity to better establish past position, strength and oceanography of the subtropical front (STF). Here, we aim to reconstruct the late Eocene to early Miocene (37–20 Ma) depositional and palaeoceanographic history of the STF in the context of the evolving Tasmanian Gateway as well as the potential influence of Antarctic circumpolar flow and intense waxing and waning of ice. We approach this by combining information from seismic lines (revisiting existing data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A) in the western Tasmanian continental slope. We apply improved taxonomic insights and palaeoecological models to reconstruct the sea surface palaeoenvironmental evolution. Late Eocene–early Oligocene (37–30.5 Ma) assemblages show a progressive transition from dominant terrestrial palynomorphs and inner-neritic dinocyst taxa as well as cysts produced by heterotrophic dinoflagellates to predominantly outer-neritic/oceanic autotrophic taxa. This transition reflects the progressive deepening of the western Tasmanian continental margin, an interpretation supported by our new seismic investigations. The dominance of autotrophic species like Spiniferites spp. and Operculodinium spp. reflects relatively oligotrophic conditions, like those of regions north of the modern-day STF. The increased abundance in the earliest Miocene of Nematosphaeropsis labyrinthus, typical for modern subantarctic zone (frontal) conditions, indicates a cooling and/or closer proximity of the STF to the site . The absence of major shifts in dinocyst assemblages contrasts with other records in the region and suggests that small changes in surface oceanographic conditions occurred during the Oligocene. Despite the relatively southerly (63–55∘ S) location of Site 1168, the rather stable oceanographic conditions reflect the continued influence of the proto-Leeuwin Current along the southern Australian coast as Australia continued to drift northward. The relatively “warm” dinocyst assemblages at ODP Site 1168, compared with the cold assemblages at Antarctic Integrated Ocean Drilling Program (IODP) Site U1356, testify to the establishment of a pronounced latitudinal temperature gradient in the Oligocene Southern Ocean.

Model simulations of past climates are increasingly found to compare well with proxy data at a global scale, but regional discrepancies remain. A persistent issue in modeling past greenhouse climates has been the temperature difference between equatorial and (sub-)polar regions, which is typically much larger in simulations than proxy data suggest. Particularly in the Eocene, multiple temperature proxies suggest extreme warmth in the southwest Pacific Ocean, where model simulations consistently suggest temperate conditions. Here, we present new global ocean model simulations at 0.1° horizontal resolution for the middle-late Eocene. The eddies in the high-resolution model affect poleward heat transport and local time-mean flow in critical regions compared to the noneddying flow in the standard low-resolution simulations. As a result, the high-resolution simulations produce higher surface temperatures near Antarctica and lower surface temperatures near the equator compared to the low-resolution simulations, leading to better correspondence with proxy reconstructions. Crucially, the high-resolution simulations are also much more consistent with biogeographic patterns in endemic-Antarctic and low-latitude-derived plankton, and thus resolve the long-standing discrepancy of warm subpolar ocean temperatures and isolating polar gyre circulation. The results imply that strongly eddying model simulations are required to reconcile discrepancies between regional proxy data and models, and demonstrate the importance of accurate regional paleobathymetry for proxy-model comparisons.

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 Tasmanian Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasmanian Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on both the physicochemical properties of the water masses and the path of surface-ocean currents. The extent to which climate and tectonics have influenced the distribution and composition of surface currents and thus fossil assemblages has, however, remained unclear. In particular, the contribution of climate change to oceanographic changes, superimposed on long-term and gradual changes induced by tectonics, is still poorly understood. 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: the Middle Eocene Climatic Optimum (MECO; ∼40 Ma). This 500 kyr phase of global warming was unrelated to regional tectonism, and thus provides a test case to investigate the ocean's physicochemical response to climate change alone. We reconstruct changes in surface-water circulation and temperature in and around the Tasmanian Gateway during the MECO through new palynological and organic geochemical records from the central Tasmanian Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia), and the Hampden Beach section (New Zealand). Our results confirm that dinocyst communities track specific surface-ocean currents, yet the variability within the communities can be driven by superimposed temperature change. Together with published results from the east of the Tasmanian Gateway, our new results suggest a shift in surface-ocean circulation during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasmanian 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 coincident with the MECO.

Terrestrial organic carbon (TerrOC) acts as an important CO2 sink when transported via rivers to the ocean and sequestered in coastal marine sediments. This mechanism might help to modulate atmospheric CO2 levels over short- and long timescales (103 to 106 years), but its importance during past warm climates remains unknown. Here we use terrestrial biomarkers preserved in coastal marine sediment samples from Wilkes Land, East Antarctica (~67°S) to quantify TerrOC burial during the early Eocene (~54.4 to 51.5 Ma). Terrestrial biomarker distributions indicate the delivery of plant-, soil- and peat-derived organic carbon (OC) into the marine realm. Mass accumulation rates of plant- (long-chain n-alkane) and soil-derived (hopane) biomarkers dramatically increase between the earliest Eocene (~54 Ma) and the early Eocene Climatic Optimum (EECO; ~53 Ma). This coincides with increased OC mass accumulation rates and indicates enhanced TerrOC burial during the EECO. Leaf wax δ 2H values indicate that the EECO was characterised by wetter conditions relative to the earliest Eocene, suggesting that hydroclimate exerts a first-order control on TerrOC export. Our results indicate that TerrOC burial in coastal marine sediments UOB Open could have acted as an important negative feedback mechanism during the early Eocene, but also during other warm climate intervals.

Antarctica underwent a complex evolution over the course of the Cenozoic, which influenced the history of the Earth’s climate system. The Eocene-Oligocene boundary is a divide of this history when the ice-free ‘greenhouse world’ transitioned to the ‘icehouse’ with the glaciation of Antarctica. Prior to this, Antarctica experienced warm climates, peaking during Early Eocene when tropical-like conditions existed at the margins of the continent where geological evidence is present. Climate signals in the geological record show that the climate then cooled, but not enough to allow the existence of significant ice until the latest Eocene. Glacial deposits from several areas around the continental margin indicate that ice was present by the earliest Oligocene. This matches the major oxygen isotope positive shift captured by marine records. On land, vegetation was able to persist, but the thermophylic plants of the Eocene were replaced by shrubby vegetation with the southern beech Nothofagus, mosses and ferns, which survived in tundra-like conditions. Coupled climate–ice sheet modelling indicates that changing levels of atmospheric CO2 controlled Antarctica’s climate and the onset of glaciation. Factors such as mountain uplift, vegetation changes, ocean gateway opening and orbital forcing all played a part in cooling the polar climate, but only when CO2 levels reached critical thresholds was Antarctica tipped into an icy glacial world.

Nick Thompson received funding from the Natural Environment Research Council from a NERC-funded Doctoral Training Partnership ONE Planet (grant no. NE/S007512/1). Funding for this research was also provided by the Spanish Ministry of Science and Innovation (grant nos. CTM2014-60451-C2- 1/2-P and CTM2017-89711-C2-1/2-P) cofunded by the European Union through FEDER funds. Peter K. Bijl received funding from the European Research Council (OceaNice (grant no. 802835))., This work used Deep Sea Drilling Project archived samples provided by the International Ocean Discovery Program (IODP). We thank the staff at the Gulf Coast core repository (GCR) for assistance in ODP Leg 113 core handling and shipping. We thank CNRS for the salary support of MAS, 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 (SOM) in the Weddell Sea, to investigate changes in terrestrial environments and palaeoclimate 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 12 C and 1802 mm respectively. A phase of latest Eocene terrestrial cooling at 35.5 Ma reveals a decrease in MAT by around 1.4 C possibly linked to the opening of the Powell Basin. This is followed by an increase in reworked Mesozoic sporomorphs together with sedimentological evidence indicating ice expansion to coastal and shelf areas approximately 34.1 Myr 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 taxon diversity and a shift in terrestrial biomarkers reflecting a change from temperate forests to cool temperate forests following 33.5 Ma. This surprising expansion of gymnosperms and cryptogams is suggested to be linked to environmental disturbance caused by repeat glacial expansion and retreat, which facilitated the proliferation of conifers and ferns. The timing of glacial onset at Site 696 is linked to the global cooling at the EOT, yet the latest Eocene regional cooling cannot directly be linked to the observed vegetation changes. Therefore, our vegetation record provides further evidence that the opening of the Drake Passage and Antarctic glaciation were not contemporaneous, although stepwise cooling in response to the opening of ocean gateways surrounding the Antarctic continent may have occurred prior to the EOT., NERC-funded NE/S007512/1, Natural Environment Research Council, European Commission, European Research Council 802835, Ministerio de Ciencia e Innovación CTM2014-60451-C2- 1/2-P, CTM2017-89711-C2-1/2-P, European Regional Development Fund

Fil: Bijl, Peter. Department of Earth Sciences, Laboratory of Palaeobotany and Palynology, Utrecht University. The Netherlands. Fil: Guerstein, Raquel. Departamento de Geología, Universidad Nacional del Sur-Instituto Geológico del Sur, CONICET. Argentina. Fil: Sanmiguel Jaimes, Edgar. Universidad Andres Bello, Facultad de Ingenieria, Autopista Concepción-Talcahuano. Chile. Fil: Sluijs, Appy. Department of Earth Sciences, Laboratory of Palaeobotany and Palynology, Utrecht University. The Netherlands. Fil: Casadio, Silvio. Universidad Nacional de Río Negro, Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina. Fil: Valencia, Víctor. School of the Environment, Washington State University. USA. Fil: Amenábar, Cecilia. Instituto Antártico Argentino-Instituto de Estudios Andinos Don Pablo Groeber-CONICET-Dpto. Ciencias Geológicas, Universidad de Buenos Aires. Argentina. Fil: Encinas, Alfonso. Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad de Concepción. Chile. The tectonic opening of the Tasmanian Gateway and Drake Passage represented crucial geographic requirements for the Cenozoic development of the Antarctic Circumpolar Current (ACC). Particularly the tectonic complexity of Drake Passage has hampered the exact dating of the opening and deepening phases, and the consequential onset of throughflow of the ACC. One of the obstacles is putting key regional tectonic events, recorded in southern Patagonian sediments, in absolute time. For that purpose, we have collected Campanian-Eocene sediment samples from the Chilean sector of Southern Patagonia. Using U-Pb radiometric dating on zircons and dinoflagellate cyst biostratigraphy, we updated age constraints for the sedimentary formations, and the hiatuses in between. Thick sedimentary packages of shallowmarine and continental sediments were deposited in the foreland basin during the early Campanian, mid-Paleocene, the Paleocene-Eocene boundary interval and the middle Eocene, which represent phases of increased foreland subsidence. We interpret regional sedimentary hiatuses spanning the late Campanian, early-to mid-Paleocene, mid-Eocene and latest Eocene-early Oligocene to indicate times of reduced foreland subsidence, relative to sediment supply. We relate these changes to varying subduction rates and Andean orogeny. Dinoflagellate cyst assemblages suggest that the region was under the influence of the Antarctic-derived waters through the western boundary current of the Subpolar Gyre, developed in the southwest Atlantic Ocean and thus argues for limited throughflow through the Drake Passage until at least the latest Eocene. However, the proliferation of dinoflagellate endemism we record in the southwest Atlantic is coeval with that in the southwest Pacific, and on a species level, dinoflagellate cyst assemblages are the same in these two regions. This suggests that both regions were oceanographically connected throughout the early Paleogene, likely through a shallow opening of a restricted Drake Passage. This implies a continuous surface-water connection between the south Pacific and the South Atlantic throughout the late Cretaceous-early Paleogene.

Several studies indicate that North Atlantic Deep Water (NADW) formation might have initiated during the globally warm Eocene (56–34 Ma). However, constraints on Eocene surface ocean conditions in source regions presently conducive to deep water formation are sparse. Here we test whether ocean conditions of the middle Eocene Labrador Sea might have allowed for deep water formation by applying (organic) geochemical and palynological techniques, on sediments from Ocean Drilling Program (ODP) Site 647. We reconstruct a long‐term sea surface temperature (SST) drop from ~30°C to ~27°C between 41.5 to 38.5 Ma, based on TEX86. Superimposed on this trend, we record ~2°C warming in SST associated with the Middle Eocene Climatic Optimum (MECO; ~40 Ma), which is the northernmost MECO record as yet, and another, likely regional, warming phase at ~41.1 Ma, associated with low‐latitude planktic foraminifera and dinoflagellate cyst incursions. Dinoflagellate cyst assemblages together with planktonic foraminiferal stable oxygen isotope ratios overall indicate low surface water salinities and strong stratification. Benthic foraminifer stable carbon and oxygen isotope ratios differ from global deep ocean values by 1–2‰ and 2–4‰, respectively, indicating geographic basin isolation. Our multiproxy reconstructions depict a consistent picture of relatively warm and fresh but also highly variable surface ocean conditions in the middle Eocene Labrador Sea. These conditions were unlikely conducive to deep water formation. This implies either NADW did not yet form during the middle Eocene or it formed in a different source region and subsequently bypassed the southern Labrador Sea., Key Points The middle Eocene Labrador Sea was characterised by warm, low‐salinity surface watersWe find two phases of superimposed sea surface warming and subtropical plankton incursions, including the Middle Eocene Climatic OptimumBottom water stable carbon and oxygen isotope ratios indicate isolated basin

At present, the Southern Ocean plays an important role in the global climate system and in modern Antarctic ice sheet dynamics. Past Southern Ocean configurations are however poorly understood. This information is yet important as it may provide important insights into the climate system and past ice-sheet behavior under warmer than present day climates. Here we study Southern Ocean dynamics during the Oligocene and Miocene when reconstructed atmospheric CO concentrations were similar to those expected during this century. We reconstruct snapshots of late Oligocene to earliest Miocene (~24.2–23 Ma) paleoceanographic conditions in the East Antarctic Wilkes Land abyssal plain. For this, we combine marine sedimentological, geochemical (X-ray fluorescence, TEX,), palynological and isotopic (ε) records from ocean sediments recovered at Deep Sea Drilling Project (DSDP) Site 269. Overall, we find that sediments, delivered to the site by gravity flows and hemipelagic settling during glacial-interglacial cycles, were persistently reworked by a proto-Circumpolar Deep Water (CDW) with varying strengths that result from climatically controlled frontal system migrations. Just prior to 24 Ma, terrigenous input of predominantly fine-grained sediments deposited under weak proto-CDW intensities and poorly ventilated bottom conditions dominates. In comparison, 24 Ma marks the start of episodic events of enhanced proto-CDW current velocities, associated with coarse-grained deposits and better-ventilated bottom conditions. In particular, the dominance of P-cyst and low Calcium (Ca) in the sediments between ~ 24.2 Ma and 23.6 Ma indicate the presence of an active open ocean upwelling associated with high nutrient conditions. This is supported by TEX-derived sea surface temperature (SST) data pointing to cool ocean conditions. From ~ 23.6 to 23.2 Ma, our records reveal an enrichment of Ca in the sediments related to increased calcareous microfossil preservation, high amounts of G-cysts and increasing TEX-SSTs. This implies warmer water masses reaching the Antarctic margin as the polar front migrated southward. Together with the radiogenic Nd isotope data indicating modern-like CDW values, our records suggest a prominent poleward expansion of proto-CDW over our study site and reduced AABW formation during the latest Oligocene (i.e. ~23.2 Ma ago). Our findings support the notion of a fundamentally different Southern Ocean, with a weaker proto-ACC than present during the late Oligocene and the earliest Miocene., This research used samples provided by the International Ocean Discovery Program (IODP). We thank the staff at the Gulf Coast core repository (GCR) for assistance in core handling and shipping. We also thank David Houpt (GCR) for technical support with the XRF core scanning; Katharina Kreissig, Liam Holder, Barry Coles (Imperial College) and Katrina Kerr (Open University) for laboratory and technical support with the Nd isotopes and REE analyses; Emmanuelle Ducassou, Marie-Claire Perello (EPOC) for laboratory and technical support with the grain-size analyses and Hans Nelson for his helpful discussions and improvement of the English in the manuscript. We also thank Laura De Santis and an anonymous reviewer for their constructive comments, which helped to improve this paper. Funding for this research was provided by the Onassis Foundation – Scholarship ID: F ZL 016-1/2015-2016 and the Spanish Ministry of Science and Innovation (grant CTM2017-89711-C2-1-P), co-funded by the European Union through FEDER funds. PB and FH acknowledge funding through the NWO polar programme grant no ALWPP.2016.001 and the European Research Council starting grant no 802835, OceaNice. This paper is a contribution to the SCAR PAIS Programme.

The early and late Eocene have both been the subject of many modelling studies, but few have focused on the middle Eocene. The latter still holds many challenges for climate modellers but is also key to understanding the events leading towards the conditions needed for Antarctic glaciation at the Eocene–Oligocene transition. Here, we present the results of CMIP5-like coupled climate simulations using the Community Earth System Model (CESM) version 1. Using a new detailed 38 Ma geography reconstruction and higher model resolution compared to most previous modelling studies and sufficiently long equilibration times, these simulations will help to further understand the middle to late Eocene climate. At realistic levels of atmospheric greenhouse gases, the model is able to show overall good agreement with proxy records and capture the important aspects of a warm greenhouse climate during the Eocene. With a quadrupling of pre-industrial concentrations of both CO2 and CH4 (i.e. 1120 ppm and ∼2700 ppb, respectively, or 4 × PIC; pre-industrial carbon), sea surface temperatures correspond well to the available late middle Eocene (42–38 Ma; ∼ Bartonian) proxies. Being generally cooler, the simulated climate under 2 × PIC forcing is a good analogue 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 in agreement with the available information. Our simulated middle to late Eocene 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 account for a global average 5–7 ∘C difference between pre-industrial and 38 Ma Eocene boundary conditions, with important contributions from cloud and water vapour feedbacks. This helps to explain Eocene warmth in general, without the need for greenhouse gas levels much higher than indicated by proxy estimates (i.e. ∼500–1200 ppm CO2) or low-latitude regions becoming unreasonably warm. High-latitude warmth supports the idea of mostly ice-free polar regions, even at 2 × PIC, with Antarctica experiencing particularly warm summers. An overall wet climate is seen in the simulated Eocene climate, which has a strongly monsoonal character. Equilibrium climate sensitivity is reduced (0.62 ∘C W−1 m2; 3.21 ∘C warming between 38 Ma 2 × PIC and 4 × PIC) compared to that of the present-day climate (0.80 ∘C W−1 m2; 3.17 ∘C per CO2 doubling). While the actual warming is similar, we see mainly a higher radiative forcing from the second PIC doubling. A more detailed analysis of energy fluxes shows that the regional radiative balance is mainly responsible for sustaining a low meridional temperature gradient in the Eocene climate, as well as the polar amplification seen towards even warmer conditions. These model results may be useful to reconsider the drivers of Eocene warmth and the Eocene–Oligocene transition (EOT) but can also be a base for more detailed comparisons to future proxy estimates.

This research has been supported by the NWO Polar Programme (grant no. ALW.2016.001.); the Spanish Ministry of Economy, Industry and Competitiveness (grant no. CTM2017-89711-C2-1-P/CTM2017-89711-C2-2-P), co-funded by the European Union through FEDER funds; and the Alexander S. Onassis Public Benefit Foundation PhD research grant (grant no. F ZL 016-1/2015-2016)., Antarctic continental ice masses fluctuated considerably during the Oligocene "coolhouse", at elevated atmospheric CO2 concentrations of similar to 600-800 ppm. To assess the role of the ocean in the Oligocene ice sheet variability, reconstruction of past ocean conditions in the proximity of the Antarctic margin is needed. While relatively warm ocean conditions have been reconstructed for the Oligocene offshore of Wilkes Land, the geographical extent of that warmth is unknown. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from Deep Sea Drilling Project (DSDP) Site 274 offshore of the Ross Sea continental margin. This site, located offshore of Cape Adare is ideally situated to characterise Oligocene regional surface ocean conditions, as it is situated between the colder, higher-latitude Ross Sea continental shelf and the warm-temperate Wilkes Land margin in the Oligocene. We first improve the age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse organic walled dinoflagellate cyst assemblages and lipid biomarkers (TEX86, TetraEther indeX of 86 carbon atoms) to reconstruct surface palaeoceanographic conditions during the Oligocene (33.7-24.4 Ma). Both TEX86 based sea surface temperature (SST) and microplankton results show temperate (10-17 degrees C +/- 5.2 degrees C) surface ocean conditions at Site 274 throughout the Oligocene. Oceanographic conditions between the offshore Wilkes Land margin and Cape Adare became increasingly similar towards the late Oligocene (26.5-24.4 Ma); this is inferred to be the consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. Maintaining marine terminations of terrestrial ice sheets in a proto-Ross Sea with offshore SSTs that are as warm as those suggested by our data requires a strong ice flux fed by intensive precipitation in the Antarctic hinterland during colder orbital states but with extensive surface melt of terrestrial ice during warmer orbital states., NWO Polar Programme ALW.2016.001, Spanish Ministry of Economy, Industry and Competitiveness CTM2017-89711-C2-1-P/CTM2017-89711-C2-2-P, European Union through FEDER funds, Alexander S. Onassis Public Benefit Foundation PhD research grant F ZL 016-1/2015-2016

During evolution of the South Sandwich subduction zone, which has consumed South American plate oceanic lithosphere, somehow continental crust of both the South American and Antarctic plates have become incorporated into its upper plate. Continental fragments of both plates are currently separated by small oceanic basins in the upper plate above the South Sandwich subduction zone, in the Scotia Sea region, but how fragments of both continents became incorporated in the same upper plate remains enigmatic. Here we present an updated kinematic reconstruction of the Scotia Sea region using the latest published marine magnetic anomaly constraints, and place this in a South America-Africa-Antarctica plate circuit in which we take intracontinental deformation into account. We show that a change in fracture zone orientation in the Weddell Sea requires that previously inferred initiation of subduction of South American oceanic crust of the northern Weddell below the eastern margin of South Orkney Islands continental crust, then still attached to the Antarctic Peninsula, already occurred around 80 Ma. We propose that subsequently, between ~71-50 Ma, the trench propagated northwards into South America by delamination of South American lithosphere: this resulted in the transfer of delaminated South American continental crust to the overriding plate of the South Sandwich subduction zone. We show continental delamination may have been facilitated by absolute southward motion of South America that was resisted by South Sandwich slab dragging. Pre-drift extension preceding the oceanic Scotia Sea basins led around 50 Ma to opening of the Drake Passage, preconditioning the southern ocean for the Antarctic Circumpolar Current. This 50 Ma extension was concurrent with a strong change in absolute plate motion of the South American Plate that changed from S to WNW, leading to upper plate retreat relative to the more or less mantle stationary South Sandwich Trench that did not partake in the absolute plate motion change. While subduction continued, this mantle-stationary trench setting lasted until ~30 Ma, after which rollback started to contribute to back-arc extension. We find that roll-back and upper plate retreat have contributed more or less equally to the total amount of ~2000 km of extension accommodated in the Scotia Sea basins. We highlight that viewing tectonic motions in a context of absolute plate motion is key for identifying slab motion (e.g. rollback, trench-parallel slab dragging) and consequently mantle-forcing of geological processes.

In the southern high latitudes, dinoflagellate cysts are an important microfossil group for both biostratigraphic and palaeoenvironmental interpretations purposes. In light of this, the peridinioid dinoflagellate cyst Nucicla umbiliphora gen. et sp. nov. from the Antarctic margin is formally described. Nucicla is dorsoventrally compressed, has a rounded pentagonal outline in dorso-ventral view, an epicyst that is only half as high as the hypocyst, an unusual archaeopyle formed by the loss of the three anterior intercalary plates, and a posterior sulcal plate that is positioned at the antapex. The species N. umbiliphora is characterised by a scabrate cyst wall and possesses undulated and/or crenulated folds/ridges. It has been so far exclusively found in Quaternary sediments obtained from the East Antarctic continental shelf and the Ross Sea. Although the dinoflagellate producing this cyst is as yet unknown, its brown color and the lack of autofluorescence suggest that the motile cell is likely a heterotrophic Protoperidinium species. As such, N. umbiliphora might benefit from the phytoplankton blooms occurring close to the Antarctic margin after seasonal sea-ice retreat.

Integrated Ocean Drilling Program (IODP) Expedition 318 recovered a ∼ 170 m long Holocene organic-rich sedimentary sequence at Site U1357. Located within the narrow but deep Adélie Basin close to the Antarctic margin, the site accumulated sediments at exceptionally high sedimentation rates, which resulted in extraordinary preservation of the organic sedimentary component. Here, we present an overview of 74 different mainly marine microfossil taxa and/or types found within the organic component of the sediment, which include the remains of unicellular and higher organisms from three eukaryotic kingdoms (Chromista, Plantae, and Animalia). These remains include phytoplanktonic (phototrophic dinoflagellates and prasinophytes) and very diverse zooplanktonic (heterotrophic dinoflagellates, tintinnids, copepods) organisms. We illustrate each marine microfossil taxon or type identified by providing morphological details and photographic images, which will help with their identification in future studies. We also review their ecological preferences to aid future (palaeo)ecological and (palaeo)environmental studies. The planktonic assemblage shows a high degree of endemism related to the strong influence of the sea-ice system over Site U1357. In addition, we found the remains of various species of detritus feeders and bottom-dwelling scavengers (benthic foraminifers and annelid worms) indicative of high export productivity at Site U1357. This study shows the potential of organic microfossil remains for reconstructing past environmental conditions, such as sea-ice cover and (export) productivity.

Next to atmospheric CO concentrations, ice-proximal oceanographic conditions are a critical factor for the stability of Antarctic marine-terminating ice sheets. The Oligocene and Miocene epochs (∼ 34-5 Myr ago) were time intervals with atmospheric CO concentrations between those of present-day and those expected for the near future. As such, these past analogues may provide insights into ice-sheet volume stability under warmer-than-present-day climates. We present organic-walled dinoflagellate cyst (dinocyst) assemblages from chronostratigraphically well-constrained Oligocene to mid-Miocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356. Situated offshore the Wilkes Land continental margin, East Antarctica, the sediments from Site U1356 have archived the dynamics of an ice sheet that is today mostly grounded below sea level. We interpret dinocyst assemblages in terms of paleoceanographic change on different timescales, i.e. with regard to both glacial-interglacial and long-term variability. Our record shows that a sea-ice-related dinocyst species, Selenopemphix antarctica, occurs only for the first 1.5 Myr of the early Oligocene, following the onset of full continental glaciation on Antarctica, and after the Mid-Miocene Climatic Optimum. Dinocysts suggest a weaker-than-modern sea-ice season for the remainder of the Oligocene and Miocene. 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 those found in the present-day subtropical front. Oligotrophic and temperate surface waters prevailed over the site notably during interglacial times, suggesting that the positions of the (subpolar) oceanic frontal systems have varied in concordance with Oligocene-Miocene glacial-interglacial climate variability., This research used data and samples from the Integrated Ocean Drilling Program (IODP). IODP was sponsoredbytheUSNationalScienceFoundationandparticipating countries under management of Joined Oceanographic Institutions Inc. Peter K. Bijl and Francesca Sangiorgi thank NWO-NNPP grant no. 866.10.110 and NWO-ALW VENI grant no. 863.13.002 for funding and Natasja Welters for technical support. Jörg Pross acknowledges support through the IODP priority program of the German Research Foundation (DFG). Carlota Escutia and Ariadna Salabarnada thank the Spanish Ministerio de Economía y Competitividad for grant CTM2014-60451-C2-1-P. We thank Kasia ´Sliwi´nska, Stephen Gallagher and the anonymous reviewer for their constructive comments that considerably improved our paper.

There is growing interest in the scientific community in reconstructing the paleoceanography of the Southern Ocean during the Oligocene–Miocene because these time intervals experienced atmospheric CO2 concentrations with relevance to our future. However, it has remained notoriously difficult to put the sedimentary archives used in these efforts into a temporal framework. This is at least partially due to the fact that the bio-events recorded in organic-walled dinoflagellate cysts (dinocysts), which often represent the only microfossil group preserved, have not yet been calibrated to the international timescale. Here we present dinocyst ranges from Oligocene–Miocene sediments drilled offshore the Wilkes Land continental margin, East Antarctica (Integrated Ocean Drilling Program (IODP) Hole U1356A). In addition, we apply statistical means to test a priori assumptions about whether the recorded taxa were deposited in situ or were reworked from older strata. Moreover, we describe two new dinocyst species, Selenopemphix brinkhuisii sp. nov. and Lejeunecysta adeliensis sp. nov., which are identified as important markers for regional stratigraphic analysis. Finally, we calibrate all identified dinocyst events to the international timescale using independent age control from calcareous nanoplankton and magnetostratigraphy from IODP Hole U1356A, and we propose a provisional dinoflagellate cyst zonation scheme for the Oligocene–Miocene of the Southern Ocean.

Antarctic ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well resolved. They are however important to understand the influence of high-latitude Southern Hemisphere feedbacks on global climate under CO scenarios (between 400 and 750 ppm) projected by the IPCC for this century, assuming unabated CO emissions. Sediments recovered by the Integrated Ocean Drilling Program (IODP) at Site U1356, offshore of the Wilkes Land margin in East Antarctica, provide an opportunity to study ice sheet and paleoceanographic configurations during the late Oligocene (26-25 Ma). Our study, based on a combination of sediment facies analysis, magnetic susceptibility, density, and X-ray fluorescence geochemical data, 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 as resulting from a northward shift of westerly winds and surface oceanic fronts. Interglacial sediments record more oxygenated and ventilated bottom water conditions and strong current velocities, which suggests enhanced mixing of the water masses as a result of a southward shift of the polar front. Intervals with preserved carbonated nannofossils within some of the interglacial facies are interpreted as forming under warmer paleoclimatic conditions when less corrosive warmer northern component water (e.g., North Atlantic sourced deep water) had a greater influence on the site. Spectral analysis on the late Oligocene sediment interval shows that the glacial-interglacial cyclicity and related displacements of the Southern Ocean frontal systems between 26 and 25Ma were forced mainly by obliquity. The paucity of iceberg-rafted debris (IRD) throughout the studied interval contrasts with earlier Oligocene and post-Miocene Climate Optimum sections from Site U1356 and with late Oligocene strata from the Ross Sea, which contain IRD and evidence for coastal glaciers and sea ice. These observations, supported by elevated sea surface paleotemperatures, the absence of sea ice, and reconstructions of fossil pollen between 26 and 25 Ma at Site U1356, suggest that open-ocean water conditions prevailed. Combined, this evidence suggests that glaciers or ice caps likely occupied the topographic highs and lowlands of the now marine Wilkes Subglacial Basin (WSB). Unlike today, the continental shelf was not overdeepened and thus ice sheets in the WSB were likely land-based, and marine-based ice sheet expansion was likely limited to coastal regions., This research used samples and data provided by the Integrated Ocean Drilling Program, now the International Ocean Discovery Program (IODP). We thank thestaff onboard IODP Exp. 318 and at the Gulf Coast, the Bremen, and the Kochi IODP core repositories for assistance in core handling and shipping. We thank Vera Lukies (MARUM) for technical support with XRF core scanning and Shizu Yanagimoto (KOCHI) fortechnical supportwith CT scans.We alsothank theconstructive comments of an anonymous reviewer and Steven Pekar that have helped to improve this paper. Funding for this research is provided by the Spanish Ministerio de Economía y Competitividad (grants CTM 2011-24079 and CTM2014-60451-C2-1-P), co-funded by the European Union through FEDER funds. Ulrich Salzmann thanks the Deutsche Forschungsgemeinschaft (DFG) (RO 1113/6). Peter K. Bijl, Francesca Sangiorgi, and Julian D. Hartman acknowledge funding through the NWO polar programme grant no 866.10.110. Peter K. Bijl acknowledges funding through NWO-VENI grant no 863.13.002. Ulrich Salzmann acknowledges funding received from the Natural Environment Research Council (NERC grant NE/H000984/1).

Antarctic palaeoclimate evolution and vegetation history after the formation of a continent-scale cryosphere at the Eocene-Oligocene boundary, 33.9 million years ago, has remained a matter of controversy. In particular, the reconstruction of terrestrial climate and vegetation has been strongly hampered by uncertainties in unambiguously identifying non-reworked as opposed to reworked sporomorphs that have been transported into Antarctic marine sedimentary records by waxing and waning ice sheets. Whereas reworked sporomorph grains over longer non-successive geological timescales are easily identifiable within younger sporomorph assemblages (e.g. Permian sporomorphs in Pliocene sediments), distinguishing non-reworked from reworked material in palynological assemblages over successive geological time periods (e.g. Eocene sporomorphs in Oligocene sediments) has remained problematic. This study presents a new quantitative approach to identifying non-reworked pollen assemblages in marine sediment cores from circum-Antarctic waters. We measured the fluorescence colour signature, including red, green, and blue fluorescence; brightness; intensity; and saturation values of selected pollen and spore taxa from Eocene, Oligocene, and Miocene sediments from the Wilkes Land margin Site U1356 (East Antarctica) recovered during Integrated Ocean Drilling Program (IODP) Expedition 318. Our study identified statistically significant differences in red-fluorescence values of non-reworked sporomorph taxa against age. We conclude that red fluorescence is a reliable parameter for identifying the presence of non-reworked pollen and spores in Antarctic marine sediment records from the circum-Antarctic realm that are influenced by glaciation and extensive reworking. Our study provides a new tool to accurately reconstruct Cenozoic terrestrial climate change on Antarctica using fossil pollen and spores., This study was funded by the Faculty of Engineering and Environment Research Studentship from Northumbria University. The samples used for this research were provided by the Integrated Ocean Drilling Program (IODP). Ulrich Salzmann acknowledges funding received from the Natural Environment Research Council (NERC grant NE/H000984/1). Francesca Sangiorgi thanks the Netherlands Organisation for Scientific Research (NWO) for NNPP Polar 866.10.110. Peter K. Bijl acknowledges the Dutch National Organisation for Scientific Research(NWO)for VENI grant no. 863.13.002. Carlota Escutia thanks the MINECO for scientific research grant CTM2014-60451-C2-1-P. Jörg Pross acknowledges support through the German Science Foundation (DFG; grant PR 651/10). We would finally like to thank the anonymous reviewer and Michael Hannah for their useful comments, which helped us to further improve this paper.

Earth's current icehouse phase began ∼34 m.y. ago with the onset of major Antarctic glaciation at the Eocene-Oligocene transition. Changes in ocean circulation and a decline in atmospheric greenhouse gas levels were associated with stepwise cooling and ice growth at southern high latitudes. The Antarctic cryosphere plays a critical role in the ocean-atmosphere system, but its early evolution is still poorly known. With a near-field record from Prydz Bay, Antarctica, we demonstrate that Antarctic ice growth was stepwise and had an earlier onset than previously suggested. Prydz Bay lies downstream of a major East Antarctic Ice Sheet drainage system, and its sedimentary records uniquely constrain the timing of ice-sheet advance onto the continental shelf. We investigated a detrital record extracted from three Ocean Drilling Program drill holes within a new depositional and chronological framework spanning the late Eocene to early Oligocene (ca. 36-33 Ma). The chemical index of alteration (CIA) and the S index, calculated from the major-element geochemistry of bulk samples, yielded estimates of chemical weathering intensities and mean annual temperature on the East Antarctic continent. We document evidence for late Eocene mountain glaciation along with transient warm events at 35.8-34.8 Ma. From 34.4 Ma, associated with the Eocene-Oligocene transition precursor δ18O excursion, glaciers advanced into Prydz Bay, coincident with a decline in chemical weathering and temperature. We conclude that Antarctic continental ice growth commenced with the Eocene-Oligocene transition “precursor” glaciation, during a time of Subantarctic surface ocean cooling and a decline in atmospheric pCO2. These results call for dynamic high-latitude feedbacks that are currently poorly represented in Earth system models and emphasize the need for additional near-field glacio-sedimentological, high-latitude sea-surface temperature and pCO2 records across the Eocene-Oligocene transition.

The taxonomic revision of the dinoflagellate cyst subfamily Wetzelielloideae by Williams et al. (2015) places primary emphasis on the type of archaeopyle, and secondarily on wall ornamentation. Williams et al. (2015) argues that this provides more clarity for taxonomic differentiation within the subfamily of Wetzelielloideae, and adds to the stratigraphical significance of species within. We find, however, that their proposed revision (1) introduces taxonomic criteria that divert drastically from these in other dinoflagellate cyst subfamilies, (2) unnecessarily erects and emends many new genera and species, and (3) poses serious practical limitations, which together (4) lead to profound reduction of the stratigraphical applicability of many marker species. In this contribution, we substantiate our concerns regarding the approach and criteria used by Williams et al. (2015). We propose to retain the generic definitions of Wetzelielloideae that existed prior to the revisions by Williams et al. (2015), until a revision supported by the community is available.

Realistic appraisal of paleoclimatic information obtained from a particular location requires accurate knowledge of its paleolatitude defined relative to the Earth’s spin-axis. This is crucial to, among others, correctly assess the amount of solar energy received at a location at the moment of sediment deposition. The paleolatitude of an arbitrary location can in principle be reconstructed from tectonic plate reconstructions that (1) restore the relative motions between plates based on (marine) magnetic anomalies, and (2) reconstruct all plates relative to the spin axis using a paleomagnetic reference frame based on a global apparent polar wander path. Whereas many studies do employ high-quality relative plate reconstructions, the necessity of using a paleomagnetic reference frame for climate studies rather than a mantle reference frame appears under-appreciated. In this paper, we briefly summarize the theory of plate tectonic reconstructions and their reference frames tailored towards applications of paleoclimate reconstruction, and show that using a mantle reference frame, which defines plate positions relative to the mantle, instead of a paleomagnetic reference frame may introduce errors in paleolatitude of more than 15° (>1500 km). This is because mantle reference frames cannot constrain, or are specifically corrected for the effects of true polar wander. We used the latest, state-of-the-art plate reconstructions to build a global plate circuit, and developed an online, user-friendly paleolatitude calculator for the last 200 million years by placing this plate circuit in three widely used global apparent polar wander paths. As a novelty, this calculator adds error bars to paleolatitude estimates that can be incorporated in climate modeling. The calculator is available at www.paleolatitude.org. We illustrate the use of the paleolatitude calculator by showing how an apparent wide spread in Eocene sea surface temperatures of southern high latitudes may be in part explained by a much wider paleolatitudinal distribution of sites than previously assumed.

Hollis, C.J, Stickley, C.E., Bijl, P.K., Schiøler, P., Clowes, C.D., Li, X, Campbell, H. March 2017. The age of the Takatika Grit, Chatham Islands, New Zealand. Alcheringa 41, xxx–xxx. ISSN 0311-5518. The oldest Paleogene strata on Chatham Islands, east of New Zealand, are the phosphatized conglomerates and sandstones of the Takatika Grit that crops out on the northeastern coast at Tioriori and unconformably overlies the Chatham Schist. An intact Cretaceous–Paleogene boundary transition is not preserved at this locality. New biostratigraphic analysis of dinoflagellate, diatom and radiolarian microfossil assemblages confirms that the Takatika Grit is of late early–middle Paleocene (New Zealand Teurian stage) age but contains reworked microfossils of early Campanian (Early Haumurian) age. Vertebrate fossils found in this unit are inferred to be a mixture of reworked Cretaceous and in situ Paleocene bones and teeth. The overlying Tutuiri Greensand is of middle–late Paleocene age in its lower part and also contains reworked Cretaceous microfossils. Christopher J. Hollis [c.hollis@gns.cri.nz], Chris Clowes [c.clowes@gns.cri.nz], Xun Li [x.li@gns.cri.nz], Hamish Campbell [h.campbell@gns.cri.nz], GNS Science, PO Box 30-368, Lower Hutt 5040, New Zealand; Catherine Stickley, Evolution Applied Limited, 50 Mitchell Way, Upper Rissington, Cheltenham GL54 2PL, UK [catherine.stickley@gmail.com]; Peter Bijl [p.k.bijl@uu.nl], Marine Palynology and Paleoceanography, Department of Earth Sciences, Utrecht University, 3584 CS Utrecht, the Netherlands; Poul Schiøler [poul.schioler@mgpalaeo.com.au], Morgan Goodall Palaeo, Unit 1/5 Arvida St, Malaga, WA 6090, Australia.

Integrated Ocean Drilling Program (IODP) Expedition 318 recovered lower Eocene sediments in Hole U1356A from the continental rise of the Wilkes Land Margin, Antarctica. These sediments yielded a new genus of organic-walled dinoflagellate cysts ( Adeliesphaera gen. nov.). We tentatively place this new genus within the suborder Gonyaulacineae, family Areoligeraceae, based on the obligate apical archeopyle, the asymmetric antapical geometry and the left-side offset sulcal notch. We herein describe the type species of Adeliesphaera gen. nov., Adeliesphaera ohanlonii sp. nov., as well as a new species of Turbiosphaera , Turbiosphaera guersteinae sp. nov. The short stratigraphic range of both species (53.7–53.4 Ma and 53.1–52.9 Ma, respectively) qualifies them as potentially important regional Southern Ocean stratigraphic markers for high-resolution age control.