ALBERT

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: Detailed, stratigraphically well-constrained environmental reconstructions are available for Paleocene and Eocene strata at a range of sites in the southwest Pacific Ocean (New Zealand and East Tasman Plateau; ETP) and Integrated Ocean Discovery Program (IODP) Site U1356 in the south of the Australo-Antarctic Gulf (AAG). These reconstructions have revealed a large discrepancy between temperature proxy data and climate models in this region, suggesting a crucial error in model, proxy data or both. To resolve the origin of this discrepancy, detailed reconstructions are needed from both sides of the Tasmanian Gateway. Paleocene–Eocene sedimentary archives from the west of the Tasmanian Gateway have unfortunately remained scarce (only IODP Site U1356), and no well-dated successions are available for the northern sector of the AAG. Here we present new stratigraphic data for upper Paleocene and lower Eocene strata from the Otway Basin, southeast Australia, on the (north)west side of the Tasmanian Gateway. We analyzed sediments recovered from exploration drilling (Latrobe-1 drill core) and outcrop sampling (Point Margaret) and performed high-resolution carbon isotope geochemistry of bulk organic matter and dinoflagellate cyst (dinocyst) and pollen biostratigraphy on sediments from the regional lithostratigraphic units, including the Pebble Point Formation, Pember Mudstone and Dilwyn Formation. Pollen and dinocyst assemblages are assigned to previously established Australian pollen and dinocyst zonations and tied to available zonations for the SW Pacific. Based on our dinocyst stratigraphy and previously published planktic foraminifer biostratigraphy, the Pebble Point Formation at Point Margaret is dated to the latest Paleocene. The globally synchronous negative carbon isotope excursion that marks the Paleocene–Eocene boundary is identified within the top part of the Pember Mudstone in the Latrobe-1 borehole and at Point Margaret. However, the high abundances of the dinocyst Apectodinium prior to this negative carbon isotope excursion prohibit a direct correlation of this regional bio-event with the quasi-global Apectodinium acme at the Paleocene–Eocene Thermal Maximum (PETM; 56 Ma). Therefore, the first occurrence of the pollen species Spinizonocolpites prominatus and the dinocyst species Florentinia reichartii are here designated as regional markers for the PETM. In the Latrobe-1 drill core, dinocyst biostratigraphy further indicates that the early Eocene (∼ 56–51 Ma) sediments are truncated by a ∼ 10 Myr long hiatus overlain by middle Eocene (∼ 40 Ma) strata. These sedimentary archives from southeast Australia may prove key in resolving the model–data discrepancy in this region, and the new stratigraphic data presented here allow for detailed comparisons between paleoclimate records on both sides of the Tasmanian Gateway.

Description: 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.

Description: The timing and role in ocean circulation and climate of the opening of Southern Ocean gateways is as yet elusive. Recent micropalaeontological studies suggest the onset of westward throughflow of surface waters from the SW Pacific into the Australo-Antarctic Gulf through a southern shallow opening of the Tasman Gateway from 49–50 Ma onwards, a direction that is counter to the present-day eastward-flowing Antarctic Circumpolar Current. Here, we present the first model results specific to the early-to-middle Eocene where, in agreement with the field evidence, southerly shallow opening of the Tasman Gateway indeed causes a westward flow across the Tasman Gateway. As a result, modelled estimates of dinoflagellate biogeography are in agreement with the recent findings. Crucially, in this situation where Australia is still situated far south and almost attached to Antarctica, the Drake Passage must be sufficiently restricted to allow the prevailing easterly wind pattern to set up this southerly restricted westward flow. In contrast, an open Drake Passage, up to 517 m deep, leads to an eastward flow, even when the Tasman Gateway and the Australo-Antarctic gulf are entirely contained within the latitudes of easterly wind.

Description: Studies on the palaeoclimate and palaeoceanography using numerical model simulations may be considerably dependent on the implemented geographical reconstruction. Because building the palaeogeographic datasets for these models is often a time-consuming and elaborate exercise, palaeoclimate models frequently use reconstructions in which the latest state-of-the-art plate tectonic reconstructions, palaeotopography and -bathymetry, or vegetation have not yet been incorporated. In this paper, we therefore provide a new method to efficiently generate a global geographical reconstruction for the middle-late Eocene. The generalised procedure is also reusable to create reconstructions for other time slices within the Cenozoic, suitable for palaeoclimate modelling. We use a plate-tectonic model to make global masks containing the distribution of land, continental shelves, shallow basins and deep ocean. The use of depth-age relationships for oceanic crust together with adjusted present-day topography gives a first estimate of the global geography at a chosen time frame. This estimate subsequently needs manual editing of areas where existing geological data indicate that the altimetry has changed significantly over time. Certain generic changes (e.g. lowering mountain ranges) can be made relatively easily by defining a set of masks while other features may require a more specific treatment. Since the discussion regarding many of these regions is still ongoing, it is crucial to make it easy for changes to be incorporated without having to redo the entire procedure. In this manner, a complete reconstruction can be made that suffices as a boundary condition for numerical models with a limited effort. This facilitates the interaction between experts in geology and palaeoclimate modelling, keeping reconstructions up to date and improving the consistency between different studies. Moreover, it facilitates model inter-comparison studies and sensitivity tests regarding certain geographical features as newly generated boundary conditions can more easily be incorporated in different model simulations. The workflow is presented covering a middle-late Eocene reconstruction (38 Ma), using a MatLab script and a complete set of source files that are provided in the supplementary material.