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.

Calcareous nannofossils provide biostratigraphic age-markers for Pleistocene Arctic Ocean sediments. However, Pleistocene Arctic sediments are dominated by fine-grained terrigenous material, and commonly contain rare and poorly preserved coccolith specimens that can be difficult to identify under the light microscope (LM). Using paired observations of the same specimens under LM and a scanning electron microscope (SEM), we recently discovered that poorly preserved Noelaerhabdaceae specimens that cannot be identified at the species level were previously classified as Gephyrocapsa huxleyi using LM observations alone. Moreover, the visual resemblance under LM between G. huxleyi and another Quaternary marker species, Pseudoemiliania lacunosa, also led to occasional misdiagnosis. Given the importance of G. huxleyi and P. lacunosa for stratigraphic age control, this has potentially profound implications for our understanding of the paleoceanographic history of the Arctic, and also other ocean basins. This study focuses on challenges met in improving and applying the paired LM-SEM technique for observation of the same nannofossil specimens, and on its subsequent adjustments in the case of Quaternary Arctic sediments, which often contain low abundances of calcareous micro- and nannofossils. Moreover, we review morphological aspects and discuss potential difficulties in unambiguously identifying G. huxleyi, P. lacunosa and Gephyrocapsa under LM and illustrate the need of integrating SEM images – which can be difficult to obtain in low-diversity assemblage sediments dominated by silt and clay.

The extent and seasonality of Arctic sea ice during the Last Interglacial (129,000 to 115,000 years before present) is poorly known. Sediment-based reconstructions have suggested extensive ice cover in summer, while climate model outputs indicate year-round conditions in the Arctic Ocean ranging from ice free to fully ice covered. Here we use microfossil records from across the central Arctic Ocean to show that sea-ice extent was substantially reduced and summers were probably ice free. The evidence comes from high abundances of the subpolar planktic foraminifera Turborotalita quinqueloba in five newly analysed cores. The northern occurrence of this species is incompatible with perennial sea ice, which would be associated with a thick, low-salinity surface water. Instead, T. quinqueloba's ecological preference implies largely ice-free surface waters with seasonally elevated levels of primary productivity. In the modern ocean, this species thrives in the Fram Strait-Barents Sea 'Arctic-Atlantic gateway' region, implying that the necessary Atlantic Ocean-sourced water masses shoaled towards the surface during the Last Interglacial. This process reflects the ongoing Atlantification of the Arctic Ocean, currently restricted to the Eurasian Basin. Our results establish the Last Interglacial as a prime analogue for studying a seasonally ice-free Arctic Ocean, expected to occur this century. The warm Last Interglacial led to a seasonally ice-free Arctic Ocean and a transformation to Atlantic conditions, according to planktic foraminifera records from central Arctic Ocean sediment cores.

Despite extensive chronological studies, the relationship between the age and sub-seafloor depth of Arctic Ocean sediments remains ambiguous. This prevents confident identification of paleoceanographic changes in the Arctic during the Quaternary. Currently, age-depth models derived from uranium-series decay in Arctic sediments diverge by hundreds of thousands of years compared to those built on known evolutionary appearances and extinctions of calcareous nannoplankton, a group of globally valuable age-markers. Here we report on highresolution biostratigraphic analysis of late Quaternary sediments in six cores from the central Arctic Ocean (CAO). We applied paired light microscope (LM) and scanning electron microscope (SEM) imaging to improve nannofossil diagnosis. We argue that low abundances and poor preservation have led to misidentification of the true stratigraphic depth of the critical Pleistocene nannofossil bio-events that have underpinned age models for many Arctic sedimentary records for decades. The revised calcareous nannofossil biochronology provides a radically different geochronological framework for CAO sediments - indicating that what had previously been identified as Marine Isotope Stage (MIS) 7 (191-243 ka) in many sedimentary records is older than MIS 12 (424-478 ka). Furthermore, it suggests that previously inferred sub-stages of MIS 5 could represent full interglacial periods rather than interstadials. The results help reconcile the different dating approaches and provide a transformative step towards resolving the disparity in Quaternary Arctic age-depth models, bringing us one step closer to accurate paleoceanographic reconstructions based on sediment cores.

A 1000 m-thick sequence of Upper Cretaceous sediments outcropping in the Isabena Valley (Tremp-Graus Basin, Spain) has been studied to explore the evolution of environmental conditions that prevailed in this basin. A biostratigraphic study based on calcareous nannofossils was carried out to better constraint the age of the deposits, supplemented by carbon isotope stratigraphy on bulk carbonates. Clay mineral assemblages were identified by X-Ray diffraction combined with organic matter (OM) characterisation by Rock–Eval pyrolysis. The Late Campanian Event and Campanian Maastrichtian Boundary Event are clearly identified from the new δ¹³C_carb dataset. The clay assemblage is composed of a complex mixture of chlorite, illite, kaolinite and mixed-layers including illite–smectite and chlorite–smectite. A progressive illitisation of smectite is recorded from the top to the base of the section due to the increasing burial depth. This evolution is consistent with increasing Tmax values of OM evolving from 425 (immature OM) to 449 °C (mature OM) from the top to the base of the section. Thus, detrital minerals are preserved only in the upper part of the section. The clay sedimentation is dominated by smectites likely originating from the Ebro massif, while increasing proportions of kaolinite are recorded from the uppermost Campanian and during the Maastrichtian. This evolution of the clay mineral assemblage is interpreted as a result from a change of source from south to northeast, with contributions from kaolinite-rich weathering profiles (including bauxites) to the northeast of the study area, reflecting a more hydrolysing climate.