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Any type of non-buoyant material in the ocean is transported horizontally by currents during its sinking journey. This lateral transport can be far from negligible for small sinking velocities. To estimate its magnitude and direction, the material is often modelled as a set of Lagrangian particles advected by current velocities that are obtained from Ocean General Circulation Models (OGCMs). State-of-the-art OGCMs are strongly eddying, similar to the real ocean, providing results with a spatial resolution on the order of 10 km on a daily frequency. While the importance of eddies in OGCMs is well-appreciated in the physical oceanographic community, other marine research communities may not. Further, many long term climate modelling simulations (e.g. in paleoclimate) rely on lower spatial resolution models that do not capture mesoscale features. To demonstrate how much the absence of mesoscale features in low-resolution models influences the Lagrangian particle transport, we simulate the transport of sinking Lagrangian particles using low- and high-resolution global OGCMs, and assess the lateral transport differences resulting from the difference in spatial and temporal model resolution. We find major differences between the transport in the non-eddying OGCM and in the eddying OGCM. Addition of stochastic noise to the particle trajectories in the non-eddying OGCM parameterises the effect of eddies well in some cases (e.g. in the North Pacific gyre). The effect of a coarser temporal resolution (once every 5 days versus monthly) is smaller compared to a coarser spatial resolution (0.1° versus 1° horizontally). We recommend to use sinking Lagrangian particles, representing e.g. marine snow, microplankton or sinking plastic, only with velocity fields from eddying Eulerian OGCMs, requiring high-resolution models in e.g. paleoceanographic studies. To increase the accessibility of our particle trace simulations, we launch planktondrift.science.uu.nl, an online tool to reconstruct the surface origin of sedimentary particles in a specific location.

Microfossils from plankton are used for paleoceanographic reconstructions. An often‐made assumption in quantitative microplankton‐based paleoceanographic reconstructions is that sedimentary assemblages represent conditions of the directly overlying surface water. However, any immobile particle sinking down the water column is subjected to transport by three‐dimensional currents, which results in a lateral relocation along transport. We model dinoflagellate cyst (dinocyst) transport in a high‐resolution (0.1° horizontally) global model of the present‐day ocean and compare ocean conditions in the simulated origin of sedimentary particles to that in the directly overlying water. We find that the assumption that sedimentary particles represent the overlying surface waters is in most regions not valid. The bias induced by dinocyst transport depends on ocean current strength and direction, aggregation of particles which could increase the sinking speed, and the sediment sample depth. By using realistic sinking speeds of dinocysts and aggregates, extreme biases up to approximately ±16 °C warmer or ±4 PSU saltier are found, while other regions show lower bias from particle transport. Our model results provide a way to mechanistically and statistically explain the unexpected occurrences of some dinocyst species outside of their “normal” occurrence region, such as the northerly occurrence of the allegedly sea‐ice‐affiliated dinocyst Selenopemphix antarctica. Exclusion of such outlier occurrences will yield better constrained ecological affinites for dinocyst species, which has implications for microfossil‐based quantitative and qualitative proxies for paleoceanographic conditions. We recommend paleoceanographers to a priori evaluate the (paleo)water depth, oceanographic setting, current strength, and particle aggregation probability for their sedimentary microplankton assemblages.

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.

Reconstructing the early Palaeogene climate dynamics of terrestrial settings in the high southern latitudes is important to assess the role of high-latitude physical and biogeochemical processes in the global climate system. However, whereas a number of high-quality Palaeogene climate records has become available for the marine realm of the high southern latitudes over the recent past, the long-term evolution of coeval terrestrial climates and ecosystems is yet poorly known. We here explore the climate and vegetation dynamics on Tasmania from the middle Palaeocene to the early Eocene (60.7–54.2 Ma) based on a sporomorph record from Ocean Drilling Program (ODP) Site 1172 on the East Tasman Plateau. Our results show that three distinctly different vegetation types thrived on Tasmania under a high-precipitation regime during the middle Palaeocene to early Eocene, with each type representing different temperature conditions: (i) warm-temperate forests dominated by gymnosperms that were dominant during the middle and late Palaeocene (excluding the middle/late Palaeocene transition); (ii) cool-temperate forests dominated by southern beech (Nothofagus) and araucarians that transiently prevailed across the middle/late Palaeocene transition interval (~ 59.5 to ~ 59.0 Ma); and (iii) paratropical forests rich in ferns that were established during and in the wake of the Palaeocene–Eocene Thermal Maximum (PETM). The transient establishment of cool-temperate forests lacking any frost-sensitive elements (i.e. palms and cycads) across the middle/late Palaeocene transition interval indicates markedly cooler conditions, with the occurrence of frosts in winter, on Tasmania during that time. The integration of our sporomorph data with previously published TEX86-based sea-surface temperatures from ODP Site 1172 documents that the vegetation dynamics on Tasmania were closely linked with the temperature evolution in the Tasman sector of the Southwest Pacific region. Moreover, the comparison of our season-specific climate estimates for the sporomorph assemblages from ODP Site 1172 with the TEX86L- and TEX86H-based temperature data suggests a warm bias of both calibrations for the early Palaeogene of the high southern latitudes.

Investigation of the early Palaeogene palaeoecological and palaeoclimatological evolution of the Polar Regions is hindered by the absence of calcite microfossils in sedimentary archives, which are conventionally the main dating tool. To overcome this problem, we have generated large datasets of organic dinoflagellate cyst (dinocyst) assemblages from Southern Ocean shelf sediments over the past decade, and we here calibrate these to the Geomagnetic Polarity Time Scale (GPTS) using magnetostratigraphy and stable isotope stratigraphy. This now for the first time allows a high-resolution Southern Pacific Ocean dinocyst zonation for the late Palaeocene to late Eocene (58–36 million years ago; Ma). We compile published dinocyst chronologies from Ocean Drilling Program (ODP) Hole 1171D on the South Tasman Rise, Hole 1172A/D on the East Tasman Plateau and Integrated Ocean Drilling Program (IODP) Hole U1356A on the Wilkes Land margin. Correlation to dinocyst zonations from New Zealand lead to revisions of the magnetostratigraphic age model at Holes 1171D and 1172A/D. Stable carbon and oxygen isotope records reveal the stratigraphic location of the Palaeocene–Eocene Thermal Maximum (~ 56 Ma) and the Middle Eocene Climatic Optimum (~ 40 Ma), respectively. The resulting zonation consists of thirteen dinocyst zones, calibrated to the Geomagnetic Polarity Time Scale (GPTS) of Vandenberghe et al. (2012) , which can likely be applied to the entire Southern Ocean. Finally, we apply the revised stratigraphy to all published TEX86 data, a biomarker-based proxy for sea surface temperature (SST), from ODP Site 1172 to assess long-term climate evolution. This shows that Southwest Pacific SST trends mimic the global compilation of benthic foraminiferal oxygen isotopes even better than previously appreciated.

The Middle Eocene Climatic Optimum (MECO) was an approximately 500,000-year-long episode of widespread ocean-atmosphere warming about 40 million years ago, superimposed on a long-term middle Eocene cooling trend. It was marked by a rise in atmospheric CO2 concentrations, biotic changes and prolonged carbonate dissolution in the deep ocean. However, based on carbon cycle theory, a rise in atmospheric CO2 and warming should have enhanced continental weathering on timescales of the MECO. This should have in turn increased ocean carbonate mineral saturation state and carbonate burial in deep-sea sediments, rather than the recorded dissolution. We explore several scenarios using a carbon cycle model in an attempt to reconcile the data with theory, but these simulations confirm the problem. The model only produces critical MECO features when we invoke a sea-level rise, which redistributes carbonate burial from deep oceans to continental shelves and decreases shelf sediment weathering. Sufficient field data to assess this scenario is currently lacking. We call for an integrated approach to unravel Earth system dynamics during carbon cycle variations that are of intermediate timescales (several hundreds of thousands of years), such as the MECO.

Despite warm polar climates and low meridional temperature gradients, a number of different high-latitude plankton assemblages were, to varying extents, dominated by endemic species during most of the Paleogene. To better understand the evolution of Paleogene plankton endemism in the high southern latitudes, we investigate the spatiotemporal distribution of the fossil remains of dinoflagellates, i.e., organic-walled cysts (dinocysts), and their response to changes in regional sea surface temperature (SST). We show that Paleocene and early Eocene (∼65-50 Ma) Southern Ocean dinocyst assemblages were largely cosmopolitan in nature but that a distinct switch from cosmopolitan-dominated to endemic-dominated assemblages (the so-called "transantarctic flora") occurred around the early-middle Eocene boundary (∼50 Ma). The spatial distribution and relative abundance patterns of this transantarctic flora correspond well with surface water circulation patterns as reconstructed through general circulation model experiments throughout the Eocene. We quantitatively compare dinocyst assemblages with previously published TEX86-based SST reconstructions through the early and middle Eocene from a key locality in the southwest Pacific Ocean, ODP Leg 189 Site 1172 on the East Tasman Plateau. We conclude that the middle Eocene onset of the proliferation of the transantarctic flora is not linearly correlated with regional SST records and that only after the transantarctic flora became fully established later in the middle Eocene, possibly triggered by large-scale changes in surface-ocean nutrient availability, were abundances of endemic dinocysts modulated by regional SST variations. Fil: Bijl, Peter K.. Utrecht University; Países Bajos Fil: Pross, Jörg. Goethe Universitat Frankfurt; Alemania Fil: Warnaar, Jeroen. Utrecht University; Países Bajos Fil: Stickley, Catherine E.. University Of Norway; Noruega Fil: Huber, Matthew. Purdue University; Estados Unidos Fil: Guerstein, Gladys Raquel. Universidad Nacional del Sur; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentina Fil: Houben, Alexander J. P.. University of Utrecht; Países Bajos Fil: Sluijs, Appy. Utrecht University; Países Bajos Fil: Visscher, Henk. Utrecht University; Países Bajos Fil: Brinkhuis, Henk. Utrecht University; Países Bajos

The Eocene-Oligocene Transition (EOT, ~. 34. Ma) represents the culmination of Eocene cooling by the initiation of large-scale Antarctic glaciation. Recognition and correlation of the EOT in Southern Ocean sedimentary successions have been difficult as a result of the general lack of well-calibrated biostratigraphic markers. Here we describe an unusual hypnozygotic organic walled dinoflagellate cyst (dinocyst) that originated in the Southern Ocean in conjunction with the onset of major Antarctic glaciation as reflected by 'oxygen isotope event 1' (Oi-1).We restudied samples from Deep Sea Drilling Program (DSDP) Site 511 using Light Microscopy and Scanning Electron Microscopy and found that a taxon previously known as Forma T constitutes a new protoperidinioid dinocyst genus and species that we name Malvinia escutiana. Published and re-evaluated data from other Southern Ocean sites show that M. escutiana did not occur before the Oi-1 (33.7. Ma), emphasizing its potential as a useful biostratigraphic marker for this key interval in the Southern Ocean's Cenozoic climate history. Fil: Houben, Alexander J. P.. Utrecht University; Países Bajos Fil: Bijl, Peter K.. Utrecht University; Países Bajos Fil: Guerstein, Gladys Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentina Fil: Sluijs, Appy. Utrecht University; Países Bajos Fil: Brinkhuis, Henk. Utrecht University; Países Bajos

The Dependable Warmer During the middle of the Eocene, about 40 million years ago, a transient warming event interrupted the long-term cooling trend that had been in progress for the previous 10 million years. Bijl et al. (p. 819 ; see the Perspective by Pearson ) constructed records of sea surface temperature and atmospheric CO 2 concentrations across the warming period. It appears that vast amounts of CO 2 were injected into the atmosphere, and a sea surface temperature increase of as much a 6°C accompanied the atmospheric CO 2 rise.

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.