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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. Plain Language Summary: Climate models are widely used to understand warm climates in the geologic past such as the late Eocene (38 million years ago; ∼8°C warmer than today). To determine the quality of these models, simulations are often compared to measured proxies representing the regional environment. Here, we show that a finer‐than‐typical detail in the ocean model causes a profoundly different regional ocean flow and environmental conditions. The improved correspondence to proxy data implies that high‐resolution simulations are required for a meaningful point‐by‐point data‐model comparison. Key Points: Eddying ocean simulations provide a profoundly different local flow compared to noneddying simulationsHeat transport is enhanced in eddying simulations leading to reduced equator‐to‐pole sea surface temperature gradientsEddying simulations reduce model‐data mismatches for sea surface temperature and ocean flow [ABSTRACT FROM AUTHOR]

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 (103–106 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 (∼54.4–51.5 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 δ2H values indicate that the EECO was characterized 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 could have acted as an important negative feedback mechanism during the early Eocene, but also during other warm climate intervals. Plain Language Summary: Organic matter from the terrestrial biosphere can be transferred into rivers and eventually deposited in the ocean. This process helps to remove carbon dioxide from the atmosphere over long (>1,000 years) timescales. However, the importance of this process in warm climates is one of the most poorly understood aspects of the climate system. One way to test the behavior of the Earth in warmer‐than‐present climate states is to examine the geological record. Here we analyzed marine sediments deposited close to the Antarctic shoreline to quantify organic carbon burial during an ancient warm interval (the early Eocene, 56 to 48 million years ago). We analyzed biomolecules from plants and microbes to determine how much organic matter was derived from the terrestrial biosphere. We found evidence for increased terrestrial organic carbon burial in marine sediments during the early Eocene. This process could help to remove carbon dioxide from the atmosphere and is relevant to other episodes of climate change during Earth's history. Key Points: Terrestrial organic carbon is an important CO2 sink when transported via rivers to the ocean and sequestered in coastal marine sedimentsBiomarkers indicate enhanced terrestrial organic carbon burial during early Eocene—this could have acted as a negative feedback mechanismHydrology regulates organic carbon export from the terrestrial biosphere at this site [ABSTRACT FROM AUTHOR]

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 [ABSTRACT FROM AUTHOR]

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. Key Points: We simulate the lateral transport of sinking dinoflagellate cysts in the modern oceanThe transport from encystment to deposition influences the oceanographic signals preserved in sedimentary dinoflagellate cyst assemblagesAccounting for systematic cyst transport could improve paleoceanographic inferrences from sedimentary microplankton distributions [ABSTRACT FROM AUTHOR]

During the Eocene‐Oligocene Transition (EOT; 34–33.5 Ma), Antarctic ice sheets relatively rapidly expanded, leading to the first continent‐scale glaciation of the Cenozoic. Declining atmospheric CO2 concentrations and associated feedbacks have been invoked as underlying mechanisms, but the role of the quasi‐coeval opening of Southern Ocean gateways (Tasman Gateway and Drake Passage) and resulting changes in ocean circulation is as yet poorly understood. Definitive field evidence from EOT sedimentary successions from the Antarctic margin and the Southern Ocean is lacking, also because the few available sequences are often incomplete and poorly dated, hampering detailed paleoceanographic and paleoclimatic analysis. Here we use organic dinoflagellate cysts (dinocysts) to date and correlate critical Southern Ocean EOT successions. We demonstrate that widespread winnowed glauconite‐rich lithological units were deposited ubiquitously and simultaneously in relatively shallow‐marine environments at various Southern Ocean localities, starting in the late Eocene (~35.7 Ma). Based on organic biomarker paleothermometry and quantitative dinocyst distribution patterns, we analyze Southern Ocean paleoceanographic change across the EOT. We obtain strong indications for invigorated surface and bottom water circulation at sites affected by polar westward‐flowing wind‐driven currents, including a westward‐flowing Antarctic Countercurrent, starting at about 35.7 Ma. The mechanism for this oceanographic invigoration remains poorly understood. The circum‐Antarctic expression of the phenomenon suggests that, rather than triggered by tectonic deepening of the Tasman Gateway, progressive pre‐EOT atmospheric cooling played an important role. At localities affected by the Antarctic Countercurrent, sea surface productivity increased and simultaneously circum‐Antarctic surface waters cooled. We surmise that combined, these processes contributed to preconditioning the Antarctic continent for glaciation. Plain Language Summary: The ice sheets of Antarctica are geologically a relatively recent phenomenon. Only by the end of the Eocene Epoch (±34 million years ago), major ice sheets began to develop, likely related to declining greenhouse gas concentrations. We still do not understand what the role—if any—of the tectonic openings of key land bridges (i.e., the present‐day ocean conduits between Antarctica and Tasmania and the southern tip of South America) was in cooling the Antarctic continent and stimulating it to become glaciated. In this study we use organic marine microfossils to date and correlate several marginal marine sediment successions, dispersed throughout the Southern Ocean. We then show that the sediment composition at these sites changed abruptly throughout the Southern Ocean by about 35.7 million years ago, roughly two million years before the ice sheets rapidly expanded. We interpret this change in sediment composition to reflect enhanced surface ocean circulation. We furthermore analyzed chemical fossils to derive changes in past sea‐water temperatures. By combining these data with counts of the marine organic microfossil species, we reconstructed past environmental change across the periods prior, during and after the growth of the Antarctic ice sheets. The results indicate that from about 35.7 million years ago onward, enhanced surface ocean circulation led to sediment winnowing, higher biological productivity in‐ and cooling of the surface waters around Antarctica. Irrespective of deepening of the Tasman Conduit, progressive intensification of ocean currents, probably as a result of stronger atmospheric circulation need to be considered in understanding the conditions that allowed rapid Antarctic ice sheet to expansion. Key Points: Late Eocene accelerated deepening of the Tasman Gateway led to invigorated surface and bottom water circulation in the Southern OceanBiomarker paleothermometry and quantitative dinocyst distribution patterns coevally demonstrate cooling and enhanced productivityInvigoration of a wind‐driven Antarctic counter current had profound effects and aided preconditioning Antarctica for glacial expansion [ABSTRACT FROM AUTHOR]

Abstract: BACKGROUND: Since available arable land is limited and nitrogen fertilizers pollute the environment, cropping systems ought to be developed that do not rely on them. Here we investigate the rapidly growing, N2‐fixing Azolla/Nostoc symbiosis for its potential productivity and chemical composition to determine its potential as protein feed. RESULTS: In a small production system, cultures of Azolla pinnata and Azolla filiculoides were continuously harvested for over 100 days, yielding an average productivity of 90.0–97.2 kg dry weight (DW) ha−1 d−1. Under ambient CO2 levels, N2 fixation by the fern's cyanobacterial symbionts accounted for all nitrogen in the biomass. Proteins made up 176–208 g kg−1 DW (4.9 × total nitrogen), depending on species and CO2 treatment, and contained more essential amino acids than protein from soybean. Elevated atmospheric CO2 concentrations (800 ppm) significantly boosted biomass production by 36–47%, without decreasing protein content. Choice of species and CO2 concentrations further affected the biomass content of lipids (79–100 g kg−1 DW) and (poly)phenols (21–69 g kg−1 DW). CONCLUSIONS: By continuous harvesting, high protein yields can be obtained from Azolla cultures, without the need for nitrogen fertilization. High levels of (poly)phenols likely contribute to limitations in the inclusion rate of Azolla in animal diets and need further investigation. © 2018 The Authors. Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]