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The ocean biological pump is the mechanism by which carbon and nutrients are transported to depth. As such, the biological pump is critical in the partitioning of carbon dioxide between the ocean and atmosphere, and the rate at which that carbon can be sequestered through burial in marine sediments. How the structure and function of planktic ecosystems in the ocean govern the strength and efficiency of the biological pump and its resilience to disruption are poorly understood. The aftermath of the impact at the Cretaceous/Palaeogene (K/Pg) boundary provides an ideal opportunity to address these questions as both the biological pump and marine plankton size and diversity were fundamentally disrupted. The excellent fossil record of planktic foraminifera as indicators of pelagic-biotic recovery combined with carbon isotope records tracing biological pump behaviour, show that the recovery of ecological traits (diversity, size and photosymbiosis) occurred much later (approx. 4.3 Ma) than biological pump recovery (approx. 1.8 Ma). We interpret this decoupling of diversity and the biological pump as an indication that ecosystem function had sufficiently recovered to drive an effective biological pump, at least regionally in the South Atlantic.

The Eocene‐Oligocene transition (EOT) (∼34 Ma) is marked by the rapid development of a semi‐permanent Antarctic ice‐sheet, as indicated by ice‐rafted debris and a 1–1.5‰ increase in deep sea δ18O. Proxy reconstructions indicate a drop in atmospheric CO2 and global cooling. How these changes affected surface ocean temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped‐isotope thermometry to well‐preserved planktonic foraminifera, that are associated with lower mixed‐layer to subthermocline dwelling depths from the drift sediments at international ocean discovery program Site 1411, Newfoundland, across four intervals bracketing the EOT. The thermocline/lower mixed‐layer dwelling foraminifera record a cooling of 1.9 ± 3.5 K (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous sea surface temperature (SST) reconstructions, absolute temperatures (Eocene 20.0 ± 2.9°C, Oligocene 18.0 ± 2.2°C) appear colder than previous organic proxy reconstructions for the northernmost Atlantic extrapolated to this location. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Our subthermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.5°C, Oligocene 13.0 ± 3.2°C, cooling of 5.5 ± 4.6 K) than foraminifera from the thermocline/lower mixed‐layer, consistent with global cooling and an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation. Plain Language Summary: During the Eocene temperatures on Earth were much warmer than today. It is generally believed that the Antarctic ice‐sheet first developed around 34 million years ago, during the Eocene‐Oligocene transition (EOT). How this change occurred is still widely debated, but it was probably caused by decreased CO2 levels and changes to heat distribution through ocean currents. Here, we study how water temperatures at the dwelling depths of planktonic microfossils in the Atlantic Ocean changed across this event. We use clumped isotopes—a way of reconstructing the temperature from fossil shells. We measured shells of planktonic foraminifera, single‐celled microscopic plankton, that lived at two different depth levels in the upper ocean. We find that the foraminifera with a shallower (thermocline/lower mixed‐layer) water dwelling depth in the North Atlantic Ocean cooled by about ∼1.9 K, while those that dwelled deeper cooled by ∼5.5 K across the EOT. The thermocline/lower mixed‐layer cooling is similar in magnitude to reconstructions from organic biomarkers. However, our reconstructed absolute temperatures are colder than previous estimates. We think that our deeper water temperature reconstructions reflect global cooling, while thermocline/lower mixed‐layer temperatures did not cool as much because a warm water current developed, similar to the Gulf Stream. Key Points: Clumped isotopes from well‐preserved planktonic foraminifera imply 1.9 K lower mixed‐layer/thermocline cooling across Eocene‐Oligocene transition (EOT) in North AtlanticOur thermocline temperatures for Eocene (20°C) and Oligocene (18°C) are cooler than organic proxy sea surface temperature estimatesIncreased subthermocline cooling compared to the thermocline indicates increased stratification across the EOT, hinting at intensified Atlantic meridional overturning circulation [ABSTRACT FROM AUTHOR]

The boron isotope (δ11B) proxy for seawater pH is a tried and tested means to reconstruct atmospheric CO2 in the geologic past, but uncertainty remains over how to treat species‐specific calibrations that link foraminiferal δ11B to pH estimates prior to 22 My. In addition, no δ11B‐based reconstructions of atmospheric CO2 exist for wide swaths of the Oligocene (33–23 Ma), and large variability in CO2 reconstructions during this epoch based on other proxy evidence leaves climate evolution during this period relatively unconstrained. To add to our understanding of Oligocene and early Miocene climate, we generated new atmospheric CO2 estimates from new δ11B data from fossil shells of surface‐dwelling planktic foraminifera from the mid‐Oligocene to early Miocene (∼28–18 Ma). We estimate atmospheric CO2 of ∼680 ppm for the mid‐Oligocene, which then evolves to fluctuate between ∼500–570 ppm during the late Oligocene and between ∼420–700 ppm in the early Miocene. These estimates tend to trend higher than Oligo‐Miocene CO2 estimates from other proxies, although we observe good proxy agreement in the late Oligocene. Reconstructions of CO2 fall lower than estimates from paleoclimate model simulations in the early Miocene and mid Oligocene, which indicates that more proxy and/or model refinement is needed for these periods. Our species cross‐calibrations, assessing δ11B, Mg/Ca, δ18O, and δ13C, are able to pinpoint and evaluate small differences in the geochemistry of surface‐dwelling planktic foraminifera, lending confidence to paleoceanographers applying this approach even further back in time. Key Points: We measure δ11B on multiple species of planktic foraminifera to generate new CO2 reconstructions for the late Oligocene and early MioceneUsing a novel cross‐calibration approach, we reconstruct CO2 of 500–680 ppm for the mid‐late Oligocene and 420–700 ppm for the early MioceneMean CO2 values tend to trend higher than other proxy estimates, but generally lower than paleoclimate model simulations [ABSTRACT FROM AUTHOR]

Based on a 7‐member global circulation model ensemble from CMIP6/PMIP4, we compare the regional distribution of Arctic sea ice between a simulation representing the Last Interglacial (LIG) climate, with solar‐forced warming, and an idealized future CO2‐forced simulation with a similar annual sea‐ice volume. The two simulations feature small but robust differences in the Central Arctic and Baffin Bay during summer, and larger differences at the sea‐ice margins in the sub‐Arctic Atlantic and North Pacific sectors during winter. Our results indicate that, under both forcings, sea ice persists north of Greenland until late summer, suggesting that the assumption that this region is the "Last Ice Area" is robust and holds for other climate states. However, we show that processes influencing sea‐ice distribution in winter, such as Atlantification and sea‐ice drift, differ and need to be further investigated. Plain Language Summary: The Last Interglacial (LIG; 129–116 Kyr before present) represents the most recent period with Arctic summer temperatures significantly higher than during the pre‐industrial era. This warming results from higher insolation than today in the Arctic, and is associated with changes in Arctic sea ice that are potentially comparable in magnitude to those projected for the near future. Therefore, the LIG climate represents a good testing ground for investigating processes responsible for sea‐ice loss. Here, we compare the distribution of the Arctic sea‐ice concentration for solar‐forced LIG warming and CO2‐forced future warming in climate simulations. The aim is to determine whether the decline in Arctic sea ice follows a similar spatial pattern for two distinct forcings when focusing on periods with similar sea‐ice volumes. The main differences occur in winter, close to ice margins. They are related to changes in wind intensity over the Greenland and the northern Bering Seas, and differences in Atlantic Water temperatures, which regulate ice melting in the Barents Sea. In summer, sea‐ice cover in Baffin Bay experiences the greatest changes due to differences in circulation regime. As suggested by previous studies, the region north of Greenland is the most resilient to the disappearance of Arctic sea ice. Key Points: The oceanic region north of Greenland is most resilient to sea‐ice loss under both Last Interglacial solar forcing and future CO2 forcingClose to ice margins, wind and ocean heat transport processes impact winter sea‐ice distribution differently depending on the forcing [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of benthic foraminiferal tests (δ18O b) is one of the pre-eminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ18 O b applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ18O b variability compared to open-ocean records. Here we evaluate the controls on Arctic δ18O b by using ostracode Mg/Ca paleothermometry to generate a composite record of the δ18O of seawater (δ18O sw) from 12 sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) that covers the last 600 kyr based on biostratigraphy and orbitally tuned age models. Results show that Arctic δ18O b was generally higher than open-ocean δ18 O b during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial–interglacial Arctic δ18O b range resulted in part from the opposing effect of temperature, with intermediate to deep Arctic warming during glacials counteracting the whole-ocean δ18O sw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ18O b , we find that the intermediate to deep Arctic experienced large (≥1 ‰) variations in local δ18O sw , with generally higher local δ18O sw during interglacials and lower δ18O sw during glacials. Both the magnitude and timing of low local δ18O sw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ18O sw in the intermediate to deep Arctic Ocean during glaciations reflected weaker upper-ocean stratification and more efficient transport of low- δ18O sw Arctic surface waters to depth by mixing and/or brine rejection. [ABSTRACT FROM AUTHOR]