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Integrated Ocean Drilling Program Expedition 346 “Asian Monsoon” obtained sediment successions at seven sites in the Japan Sea (Sites U1422–U1427 and U1430) and at two closely located sites in the northern East China Sea (Sites U1428 and U1429). The Quaternary sediments of the Japan Sea are characterized by centimeter- to decimeter-scale dark–light alternations at all sites deeper than 500 m water depth. The sedimentary records from these sites allow an investigation of the regional environmental response to global climate change, including changes in the Asian Monsoon and eustatic sea level. However, the discontinuous occurrence of calcareous microfossils in the deep-sea sediments and their distinct isotope signature that deviates from standard marine δ18O records do not permit the development of a detailed stable isotope stratigraphy for Japan Sea sediments. Here, we present the tephrostratigraphy for the two southernmost sites drilled in the Japan Sea (Sites U1426 and U1427) and for one site drilled in the East China Sea (Site U1429) along with the benthic δ18O isotope stratigraphy for the shallower Site U1427 and the East China Sea Site U1429. Eighteen tephra layers can be correlated between sites using the major-element composition and morphology of volcanic glass shards, and the compositions of grains and heavy minerals. Tephra correlations show that negative δ18O peaks in the Japan Sea correspond to positive glacial maxima peaks in the East China Sea. Using this integrated stratigraphic approach, we establish an orbital-scale age model at Site U1427 for the past 1.1 Myr. The correlation of tephra layers between the shallower Site U1427 (330 m below sea level: mbsl) and the deeper Site U1426 (903 mbsl) in the southern Japan Sea provides the opportunity for further age constraints. Our results show that alternations in sediment color at Sites U1426 and U1427 can be correlated for the past 1.1 Myr with minor exceptions. Thus, the stable isotope stratigraphy established at the shallower Site U1427 can be correlated to Site U1426, and in turn to all sites drilled during Expedition 346, based on correlations of dark–light layering.

International Ocean Discovery Program (IODP) Expedition 356 Site U1461 represents one of the few records from the North West Australian shelf that provides information about aridity fluctuations in Australia during the Quaternary. A combination of chronostratigraphic indicators revealed the (partial) preservation of two major glaciations (Marine Isotope Stage (MIS) 2 and MIS 12) in the sedimentary record. The faunal content (mainly benthic foraminifera, corals and bryozoans) was analyzed to estimate paleo-environments and paleo-depths in order to determine if these sediments have been remobilized by reworking processes. Despite the occurrence of a depositional hiatus (including MIS 5d to MIS 9-time interval), the excellent preservation of faunal content suggests that the preserved sediment is in situ. The geochemical composition of the sediments (Nd and major elements) indicates that during MIS 12 riverine input was likely reduced because of enhanced aridity, and the sediment provenance (mainly atmospheric dust) is likely in the central (Lake Eyre) or eastern (Murray Darling Basin) parts of the Australian continent. MIS 2 is confirmed to be one of the driest periods recorded in Australia but with mixed dust sources from the eastern and western parts of the continent. More humid conditions followed the glacial maximum, which might correspond to the peak of the Indian-Australian Summer Monsoon.

We present a new high-resolution reconstruction of annual sea-surface temperatures (SSTa) and net primary productivity (NPP) using novel coccolithophore-based models developed for the Eastern Equatorial Pacific (EEP). We combined published coccolithophore census counts from core-tops in the Eastern Pacific with 32 new samples from the Equatorial region, to derive a new statistical model to reconstruct SSTa. Results show that the addition of the new EEP samples improves existing coccolithophore-based SST-calibrations, and allow reconstructing SSTa in the EEP with higher confidence. We also merged the relative abundance of deep-photic species Florisphaera profunda in the same surface sediment samples with existing calibration datasets for tropical regions, to reconstruct annual NPP. Both temperature and productivity calibrations were successfully applied to fossil coccolith data from Ocean Drilling Project Site 1240, in the EEP. The coccolith-based SSTa estimates show a cooling during the Last Glacial Maximum (LGM) and the Younger Dryas, and warming at the start of the Holocene. This pattern differs in the timing and magnitude of the temperature changes from other available SST-reconstructions based on biogeochemical and faunal proxies. We discuss these discrepancies to be the result of different proxy sensitivities to insolation forcing, seasonal bias, and/or preservation artifacts. Reconstructed annual NPP shows a general decreasing trend from the late last glacial period to recent times, which we relate to the weakening of wind-driven equatorial upwelling towards the Holocene. We also calculated carbon export using our SSTa and NPP reconstructions, and compared to other geochemical-based reconstructions for the same location. Our coupled SSTa-NPP reconstruction provides key data to more fully assess the evolution of primary and export productivity as well as organic carbon burial in the EEP, with implications for its role in global biogeochemical cycles across glacial terminations.

In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry. Here we analyse a sedimentary record from the Agulhas Plateau, located close to the modern position of the subtropical front and use alkenones and coccolith assemblages to reconstruct oceanographic conditions over the past 300,000 years. We identify a strong glacial-interglacial variability in sea surface temperature and productivity associated with subtropical front migration over the Agulhas Plateau, as well as shorter-term high frequency variability aligned with variations in high latitude insolation. Alkenone and coccolith abundances, in combination with diatom and organic carbon records indicate high glacial export productivity. We conclude that the biological pump was more efficient and strengthened during glacial periods, which could partly account for the reported reduction in atmospheric carbon dioxide concentrations.

Coccolithophores are the main type of calcifying phytoplankton in the Southern Ocean (SO) and are key organisms in the production of particulate inorganic carbon (PIC). However, in situ studies of coccolithophores and in particular of their importance for the input of PIC in the SO are sparse in space and time due to the harsh weather conditions in the subantarctic realm. An alternative tool for monitoring PIC is the use of optical remote sensing, as coccolithophores account for most of the optical PIC backscattering in the sea. The aim of the present study is to provide coccolithophore-based estimates of PIC derived from Scanning Electron Microscope coccolith morphometric analyses and MODIS Aqua Level-2 and Level-3 PIC concentration values along two latitudinal transects from New Zealand to Antarctica and across the Drake Passage. In general, the coccolith-estimated PIC and satellite-derived PIC datasets show comparable trends in the Subantarctic and Polar Frontal zones of both transects, with coccolith-derived PIC values being generally lower than satellite PIC values. According to the coccolithophorid data, Emiliania huxleyi type A, over-calcified type A, and other taxa (e.g., Calcidiscus leptoporus) only contribute to coccolithophore PIC in the northernmost sampling locations, whereas E. huxleyi morphogroup B contributes substantially to the PIC content south of the Subantarctic Front in both transects. High satellite-derived PIC concentrations south of the Polar Front are not apparent in the coccolith-based PIC data. We suggest that the high reflectance signal in the Antarctic Zone may instead relate to the presence of small biogenic opal particles (e.g., diatoms, silicoflagellates, and/or small siliceous plankton) or other unknown highly reflective particles (such as Phaeocystis aggregations). Our results highlight the challenges presented by the lack of reliable satellite data in some parts of the SO as well as the importance of in situ measurements and methodological accuracy when estimating PIC values.

The Southern Ocean is experiencing rapid and profound changes in its physical and biogeochemical properties that may influence the distribution and composition of pelagic plankton communities. Coccolithophores are the most prolific carbonate-producing phytoplankton group, playing an important role in Southern Ocean biogeochemical cycles. However, knowledge is scarce about the record of (sub-)fossil coccolith assemblages in the Southern Ocean, which constitute invaluable indicators for palaeoenvironmental reconstructions. This study investigates coccolith assemblages preserved in surface sediments of southernmost Chile and across the Drake Passage that were retrieved during R/V Polarstern expedition PS97. We focused on the coccolith response to steep environmental gradients across the frontal system of the Antarctic Circumpolar Current and to hydrodynamic and post-depositional processes occurring in this region. We used statistical analyses to explore which environmental parameters influenced the coccolith assemblages by means of cluster and redundancy analyses. We specifically assessed the morphological diversity of the dominant taxa, i.e. Emiliania huxleyi, emphasizing biogeographical variability of morphotypes, coccolith sizes and calcite carbonate mass estimations. High coccolith abundances and species diversity compared to studies in the same area and in other sectors of the Southern Ocean occur, with a high species richness especially south of the Polar Front. While the surface sediments offshore Chile and north of the Polar Front provide suitable material to reconstruct overlying surface ocean conditions, further factors such as temporary thriving coccolithophore communities in the surface waters or transport of settling coccoliths via surface and bottom currents and eddies influence the (sub-)fossil coccolith assemblages south of the Polar Front. Additionally, deeper samples in the southern part of the study area are particularly affected by selective carbonate dissolution. We identified five E. huxleyi morphotypes (A, A overcalcified, R, B/C and O) and estimated coccolith carbonate masses on the basis of scanning electron microscope images. E. huxleyi morphologies reflect diverging biogeographical distributions, trending towards smaller and lighter coccoliths to the south and emphasizing the importance of documenting those morphologies in relation to changing environmental conditions to assess their response to projected environmental change in the Southern Ocean.

Coccolithophores are globally distributed microscopic marine algae that exert a major influence on the global carbon cycle through calcification and primary productivity. There is recent interest in coccolithophore polar communities; however field observations regarding their biogeographic distribution are scarce for the Southern Ocean (SO). This study documents the latitudinal, as well as in depth, variability in the coccolithophore assemblage composition and the coccolith mass variation in the ecologically dominant Emiliania huxleyi across the Drake Passage. Ninety-six water samples were taken between 10 and 150 m water depth from 18 stations during POLARSTERN Expedition PS97 (February–April 2016). A minimum of 200 coccospheres per sample were identified in the scanning electron microscope, and coccolith mass was estimated with light microscopy. We find that coccolithophore abundance, diversity and maximum depth habitat decrease southwards, marking different oceanographic fronts as ecological boundaries. We characterize three zones: (1) the Chilean margin, where E. huxleyi type A (normal and overcalcified) and type R are present; (2) the Subantarctic Zone (SAZ), where E. huxleyi reaches maximum values of 212.5×103 cells L−1 and types B/C, C and O are dominant; and (3) the Polar Front Zone (PFZ), where E. huxleyi types B/C and C dominate. We link the decreasing trend in E. huxleyi coccolith mass to the poleward latitudinal succession from the type A to the type B group. Remarkably, we find that coccolith mass is strongly anticorrelated to total alkalinity, total CO2, the bicarbonate ion and pH. We speculate that low temperatures are a greater limiting factor than carbonate chemistry in the Southern Ocean. However, further in situ oceanographic data are needed to verify the proposed relationships. We hypothesize that assemblage composition and calcification modes of E. huxleyi in the Drake Passage will be strongly influenced by the ongoing climate change.

The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability 1 – 3 . Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity 4 . Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial–interglacial cycles 5 – 8 , the long-term evolution of the ACC is poorly known. Here we document changes in ACC strength from sediment cores in the Pacific Southern Ocean. We find no linear long-term trend in ACC flow since 5.3 million years ago (Ma), in contrast to global cooling 9 and increasing global ice volume 10 . Instead, we observe a reversal on a million-year timescale, from increasing ACC strength during Pliocene global cooling to a subsequent decrease with further Early Pleistocene cooling. This shift in the ACC regime coincided with a Southern Ocean reconfiguration that altered the sensitivity of the ACC to atmospheric and oceanic forcings 11 – 13 . We find ACC strength changes to be closely linked to 400,000-year eccentricity cycles, probably originating from modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability 14 . A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO 2 during glacial periods first emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, providing evidence of potentially increasing ACC flow with future climate warming. The strength of the Antarctic Circumpolar Current, as traced in sediment cores from the Pacific Southern Ocean, shows no linear long-term trend over the past 5.3 Myr; instead, the strongest flow occurs consistently in warmer-than-present intervals.

This study examined recent coccolith surface sediment assemblages across the Pacific sector of the Southern Ocean (from Punta Arenas, Chile to Wellington, New Zealand). Twenty five stations located within 44.4°S to 65.4°S and 80.1°W to 174.5°E were selected in order to assess if and how the surface sediment assemblages reflect the present-day coccolithophore community and surface-water océanographie conditions. The highest numbers of coccoliths in the surface sediments are reached near the East Pacific Rise and close to the Subtropical Front, at the New Zealand Margin (>6×10⁹ coccoliths/g of sediment). The dominant taxa are Emiliania huxleyi (including types A, B, B/C and C), Calcidiscus leptoporus, Gephyrocapsa spp. (including G muellerae, G oceanica and G ericsonii), Umbellosphaera tenuis and Coccolithus braarudii. Despite the recognition of species morphotypes being hampered by carbonate dissolution at some locations, we observed that numbers generally decrease southward until almost a monospecific and sporadic record of E. huxleyi (types B/C and C) and C. leptoporus south of the Polar Front occurs. The recent coccolithophore distribution was compared to already published living coccolithophore distributions (i.e., water column samples collected at the same specific locations) showing a fairly similar pattern. Combining the numbers of cells/1 and coccoliths/g of sediment, different coccolithophore assemblages were established coincident with areas bounded by the major surface océanographie fronts, i.e. the Subantarctic Zone and the Polar Front Zone.