The perennially sea-ice covered Central Arctic Ocean (CAO) hosts a single planktonic foraminifera species, Neogloboquadrina pachyderma, a polar specialist that predominantly exhibits sinistral-coiling. Widely used as a palaeoceanographic proxy for polar conditions, it displays a range of morphologies, including an uncommon dextral form which resembles its subpolar relative, Neogloboquadrina incompta. The biological significance of dextral coiling in N. pachyderma remains unclear, complicating climate reconstructions and interpretations of its reproduction in the CAO. While culture studies link coiling direction to a biphasic life cycle involving an asexual stage producing both coiling types, supporting field data are lacking. This study analysed N. pachyderma collected from eight plankton net and four box core stations in the CAO beneath permanent sea ice. Morphometric and genetic analyses identified six N. pachyderma morphotypes concentrated in the upper 100 m, dominated by relatively small specimens (80–125 μm). Unusually high proportions of dextral coilers (up to 32 %) were observed in the water column, compared to ∼6 % in the underlying sediment. Proloculus (first chamber) size-analysis and Gaussian Mixture Modelling revealed three proloculus-size means in the water column, suggesting the presence of an asexual clonal schizont generation alongside the typical sexual-asexual cycle. These observations provide the first in situ evidence of schizont reproduction in natural N. pachyderma populations, a strategy that may facilitate rapid population growth and adaptability in the CAO. These results clarify the biological significance of coiling direction in N. pachyderma's life cycle, and reduce the risk of misidentifying N. incompta in Arctic palaeoclimatic studies.

Data on marine microfossil assemblage composition have multiple applications. Initially, they were primarily used for (chrono)stratigraphy and palaeoecology, but these data are now also widely used to study evolutionary and ecological processes, such as past biodiversity and its links with environmental dynamics, or to provide a basis for conservation efforts and biomonitoring. The large range of potential applications renders microfossil abundance data ideal for reuse. However, the complexity inherent in taxonomic data, which encompass extant and extinct species, coupled with the inherent intricacies of information on biological communities extracted from sedimentary archives, poses considerable hurdles in reusing marine microfossil data, even when they are publicly available. Here, we present guidelines derived from an online survey conducted within the marine micropalaeontological community, aimed at improving the reusability of microfossil assemblage data. These guidelines advocate for clarity and transparency in the documentation of the methods and the outcome, and we outline the data attributes required for effective reuse of micropalaeontological data. These guidelines are intended for researchers who generate microfossil abundance datasets and for reviewers, editors, and data curators at repositories.

A total of 113 researchers evaluated the relevance of about 50 data attributes that might be needed to enable and maximise the reuse of marine microfossil abundance datasets. Each property is ranked based on the survey results. All information is, in principle, considered “desired”. Information that improves the reusability is ranked as “recommended”, and information that is required for reuse is ranked as “essential”. Analysis of a selection of datasets available online reveals a rather large gap between data properties deemed essential by survey participants and what is actually contained in publicly available microfossil assemblage datasets. While the survey indicates that the micropalaeontological community values good data stewardship, improving data reusability still requires new efforts to incorporate all the essential information. The guidelines presented here are intended as a step in that direction. Determining the optimal forms and formats for data sharing are obvious next steps the community needs to take.

The Miocene (∼23–5 Ma) is a past warm epoch when global surface temperatures varied between ∼5 and 8°C warmer than today, and CO₂ concentration was ∼400–800 ppm. The narrowing/closing of the tropical ocean gateways and widening of high-latitude gateways throughout the Miocene is likely responsible for the evolution of the ocean's overturning circulation to its modern structure, though the mechanisms remain unclear. Here, we investigate early and middle Miocene ocean circulation in an opportunistic climate model intercomparison (MioMIP1), using 14 simulations with different paleogeography, CO₂, and vegetation. The strength of the Southern Ocean-driven Meridional Overturning Circulation (SOMOC) bottom cell is similar in the Miocene and Pre-Industrial (PI) but dominates the Miocene global MOC due to weaker Northern Hemisphere overturning. The Miocene Atlantic MOC (AMOC) is weaker than PI in all the simulations (by 2–21 Sv), possibly due to its connection with an Arctic that is considerably fresher than today. Deep overturning in the North Pacific (PMOC) is present in three simulations (∼5–10 Sv), of which two have a weaker AMOC, and one has a stronger AMOC (compared to its PMOC). Surface freshwater fluxes control northern overturning such that the basin with the least freshwater gain has stronger overturning. While the orography, which impacts runoff direction (Pacific vs. Atlantic), has an inconsistent impact on northern overturning across simulations, overall, features associated with the early Miocene—such as a lower Tibetan Plateau, the Rocky Mountains, and a deeper Panama Seaway—seem to favor PMOC over AMOC.

Planktonic foraminifera are calcifying protists that represent a minor but important part of the pelagic microzooplankton. They are found in all of Earth's ocean basins and are widely studied in sediment records to reconstruct climatic and environmental changes throughout geological time. The Arctic Ocean is currently being transformed in response to modern climate change; however, the effect on planktonic foraminiferal populations is virtually unknown. Here, we provide the first systematic sampling of planktonic foraminifera communities in the "high"Arctic Ocean - defined in this work as areas north of 80° N - specifically in the broad region located between northern Greenland (the Lincoln Sea with its adjoining fjords and the Morris Jesup Rise), the Yermak Plateau, and the North Pole. Stratified depth tows down to 1000 m using a multinet were performed to reveal the species composition and spatial variability in these communities below the summer sea ice. The average abundance in the top 200 m ranged between 15 and 65 individuals m-3 in the central Arctic Ocean and was 0.3 individuals m-3 in the shelf area of the Lincoln Sea. At all stations, except one site at the Yermak Plateau, assemblages consisted solely of the polar specialist Neogloboquadrina pachyderma. It predominated in the top 100 m, where it was likely feeding on phytoplankton below the ice. Near the Yermak Plateau, at the outer edge of the pack ice, rare specimens of Turborotalita quinqueloba occurred that appeared to be associated with the inflowing Atlantic Water layer. Our results would suggest that the anticipated turnover from polar to subpolar planktonic species in the perennially ice-covered part of the central Arctic Ocean has not yet occurred, in agreement with a recent meta-analysis from the Fram Strait which suggested that the increased export of sea ice is blocking the influx of Atlantic-sourced species. The presented data set will be a valuable reference for continued monitoring of the abundance and composition of planktonic foraminifera communities as they respond to the ongoing sea-ice decline and the "Atlantification"of the Arctic Ocean basin. Additionally, the results can be used to assist paleoceanographic interpretations, based on sedimented foraminifera assemblages.

Deep Sea Drilling Project (DSDP) Site 407, located near the Reykjanes Ridge (southwest of Iceland) offers a rare and extensive record of Late Cenozoic planktonic foraminifera evolution spanning the Neogene and Quaternary periods. This ca. 300 m sequence provides a nearly continuous record of planktonic foraminifera with mostly good preservation quality, aiding the study of pelagic diversity changes over the past 25 million years as the modern North Atlantic Ocean system evolved. Initially investigated in 1979 by Poore, this study presents a taxonomic reassessment of upper Oligocene to Pleistocene planktonic foraminifera at Site 407, including species range documentation, assemblage analysis, biostratigraphic zonation, and age modelling based on planktonic foraminifera, calcareous nannofossils, and scanning electron microscopy. This study employs modern taxonomic perspectives that integrate morphological and stratophenetic frameworks for fossil species with genetic data for taxa having living representatives. Systematic species counts enable quantitative diversity analysis, with a particular focus on the genus Neogloboquadrina, which becomes increasingly prevalent at Site 407 from the late Neogene to Quaternary. The planktonic foraminifera assemblages at Site 407 exhibit a contraction in diversity and a shift in species dominance, notably around 160 m b.s.f. (metres below seafloor) (ca. 8.9–16.5 Ma) and 56 m b.s.f. (ca. 2–3.4 Ma). The upper Oligocene and lower Miocene include species belonging to the genera Catapsydrax, Globoturborotalita, Dentoglobigerina, and Paragloborotalia. An acme of “Ciperoella” pseudociperoensis (lower and middle Miocene), still of uncertain generic affiliation, may have biostratigraphic use. Well-preserved Turborotalita quinqueloba are relatively common throughout the sequence. In Oligocene and Miocene material, T. quinqueloba is accompanied by Tenuitella spp. From the upper Miocene onwards, neogloboquadrinids including Neogloboquadrina praeatlantica, N. atlantica, N. incompta, and N. pachyderma become increasingly common and dominate Pliocene assemblages, together with Globigerina bulloides. Assemblages with an increasingly high-latitude nature, i.e. where N. pachyderma dominates, take over in the lower Pleistocene. Multiple hiatuses are recorded, of which the largest is ca. 8 million years long, separating the middle and upper Miocene (8.9–16.5 Ma; 158.56–160.06 m b.s.f.). Continuous biozonation at Site 407 is challenged by limited species diversity and the absence of standard low-latitude biozone markers, rendering standard schemes ineffective. Recognizable biozones include the low-latitude O7 and M1 Zones in the late Oligocene and early Miocene, respectively; the high-latitude Neogloboquadrina atlantica sinistral Zone in the late Miocene and Pliocene; the Globoconella inflata Zone in the late Pliocene; and the Neogloboquadrina pachyderma Zone in the Pleistocene. The nannofossil biozonation faces similar challenges. A revised biostratigraphic age model integrates calibrated planktonic foraminifera and nannofossil events, incorporating abundant species like “C.” pseudociperoensis, N. atlantica dextral and sinistral, Globoconella puncticulata, G. inflata, and N. pachyderma. These findings are expected to contribute to the Neogene–Quaternary Middle Atlas of planktonic foraminifera and potentially improve the use of neogloboquadrinids in palaeoceanography and biostratigraphy.

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.

Climate tipping elements are large-scale subsystems of the Earth that may transgress critical thresholds (tipping points) under ongoing global warming, with substantial impacts on the biosphere and human societies. Frequently studied examples of such tipping elements include the Greenland Ice Sheet, the Atlantic Meridional Overturning Circulation (AMOC), permafrost, monsoon systems, and the Amazon rainforest. While recent scientific efforts have improved our knowledge about individual tipping elements, the interactions between them are less well understood. Also, the potential of individual tipping events to induce additional tipping elsewhere or stabilize other tipping elements is largely unknown. Here, we map out the current state of the literature on the interactions between climate tipping elements and review the influences between them. To do so, we gathered evidence from model simulations, observations, and conceptual understanding, as well as examples of paleoclimate reconstructions where multi-component or spatially propagating transitions were potentially at play. While uncertainties are large, we find indications that many of the interactions between tipping elements are destabilizing. Therefore, we conclude that tipping elements should not only be studied in isolation, but also more emphasis has to be put on potential interactions. This means that tipping cascades cannot be ruled out on centennial to millennial timescales at global warming levels between 1.5 and 2.0 ∘C or on shorter timescales if global warming surpassed 2.0 ∘C. At these higher levels of global warming, tipping cascades may then include fast tipping elements such as the AMOC or the Amazon rainforest. To address crucial knowledge gaps in tipping element interactions, we propose four strategies combining observation-based approaches, Earth system modeling expertise, computational advances, and expert knowledge.

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.

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 δ¹⁸O_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  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 δ¹⁸O_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.