In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean 1-7 have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations 8-10 is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1-2 • C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming. Our study of deep Arctic Ocean temperature variability during the last glacial-interglacial cycle focuses on sediment cores from Arctic submarine ridges (Lomonosov, Gakkel and Mendeleev), Nansen and Makarov abyssal plains, Yermak Plateau and Morris Jesup Rise, Laptev Sea Slope, Chukchi Shelf and the Iceland Plateau in the central Nordic seas (Greenland, Norwegian and Iceland seas; Fig. 1a and Supplementary Information). Modern Arctic water masses in these regions (Fig. 1b) consist of cold, low-salinity water from the Polar Mixed Layer that is separated from the underlying warm Atlantic Layer by a strong halocline. Atlantic water enters the Arctic Basin in two branches, one through the Fram Strait and the other through the Barents Sea 11,12. The inflowing Atlantic water is entrained with water formed along the margins to form Arctic Intermediate Water (AIW), which lies above Eurasian and Amerasian Basin Deep Water (Fig. 1b). The results presented here show that the central Arctic Basin at depths occupied by today's AIW and upper Eurasian Basin Deep Water and Amerasian Basin Deep Water experienced temperature variability during the last glacial period and, to a lesser degree, the Holocene interglacial, signifying large changes in circulation in Arctic and subarctic seas and variability in halocline depth.

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-This work is the first detailed description of the Late Pleistocene-Holocene and Recent Ostracoda of the Laptev Sea. A total of 45 species in 22 genera and 13 families have been identified. All these species are described monographically. Three different ecological assemblages of ostracodes corresponding to different combinations of environmental parameters have been established; they are restricted to three regions of the sea: western-central, eastern, and southern. The recent ostracode assemblages of the Laptev Sea have been compared with those from other Arctic areas and are most similar to those of the Beaufort and Kara seas. Data on recent Ostracoda are used for paleoenvironmental reconstructions on the eastern shelf and western continental slope of the Laptev Sea. For this purpose, ostracodes from five sections obtained from these parts of the sea have been examined. The oldest sediments, which are of Late Pleistocene age (15.8 cal. ka BP), have been recovered in a core from the western continental slope. These yielded five ostracode assemblages, which correspond to different paleoenvironments and replaced each other in the course of the rapid postglacial sea-level rise, thus showing variations in the Atlantic water inflow from the west and freshwater discharge from the subaerially exposed shelf. On the outer shelf of the eastern part of the sea, the rapid sea-level rise in the Early Holocene (lowermost dating 11.3 cal. ka BP) led to a rapid transition from assemblages of brackish-water nearshore environments to those of modernlike normal marine environments; modern environments were established about 8.2 cal. ka ago. Since the core sections from the inner shelf correspond to the time when the level of the sea had already reached its modern values, the changes in the taxonomic composition of ostracode assemblages primarily mirror variations in river runoff. STEPANOVA Holocene the sedimentation rates remained quite stable, varying from 1.2 mm/yr in the Lena River valley to 0.1 mm/yr in the Khatanga River valley (Bauch et al., 2001b). DOI:In marine sediments, the changes caused by sealevel rise and variations in river discharge and water parameters are reflected in downcore variations of fossil benthic assemblages, such as ostracodes, mollusks, and foraminifers; among these, ostracodes are the most sensitive to environmental changes, water depth and salinity. Analysis of ostracode assemblages allows reconstructing paleoenvironments in great detail. Since this group is so important for paleoreconstructions, we have undertaken a comprehensive study of recent and fossil ostracodes from the Laptev Sea, data on which still remain quite scarce. This work is based on new materials obtained in the framework of the cooperative RussianGerman projects "Laptev Sea System" and "Laptev Sea System-2000." CHAPTER 1. MATERIALS AND METHODS In this study we investigated samples obtained by Taldenkova and, partly, by the author from sediment core sections PS51/138-12(10), PS51/135-4, PS51/080-13(11), and PS51/92-12(11...

Abstract. Sediment records recovered from the Baltic Sea during Integrated Ocean Drilling Program Expedition 347 provide a unique opportunity to study paleoenvironmental and climate change in central and northern Europe. Such studies contribute to a better understanding of how environmental parameters change in continental shelf seas and enclosed basins. Here we present a multi-proxy-based reconstruction of paleotemperature (both marine and terrestrial), paleosalinity, and paleoecosystem changes from the Little Belt (Site M0059) over the past ∼ 8000 years and evaluate the applicability of inorganic-and organic-based proxies in this particular setting.All salinity proxies (diatoms, aquatic palynomorphs, ostracods, diol index) show that lacustrine conditions occurred in the Little Belt until ∼ 7400 cal yr BP. A connection to the Kattegat at this time can thus be excluded, but a direct connection to the Baltic Proper may have existed. The transition to the brackish-marine conditions of the Littorina Sea stage (more saline and warmer) occurred within ∼ 200 years when the connection to the Kattegat became established after ∼ 7400 cal yr BP. The different salinity proxies used here generally show similar trends in relative changes in salinity, but often do not allow quantitative estimates of salinity.Published by Copernicus Publications on behalf of the European Geosciences Union. U. Kotthoff et al.: Little Belt multi-proxy comparisonThe reconstruction of water temperatures is associated with particularly large uncertainties and variations in absolute values by up to 8 • C for bottom waters and up to 16 • C for surface waters. Concerning the reconstruction of temperature using foraminiferal Mg / Ca ratios, contamination by authigenic coatings in the deeper intervals may have led to an overestimation of temperatures. Differences in results based on the lipid paleothermometers (long chain diol index and TEX L 86 ) can partly be explained by the application of modern-day proxy calibrations to intervals that experienced significant changes in depositional settings: in the case of our study, the change from freshwater to marine conditions. Our study shows that particular caution has to be taken when applying and interpreting proxies in coastal environments and marginal seas, where water mass conditions can experience more rapid and larger changes than in open ocean settings. Approaches using a multitude of independent proxies may thus allow a more robust paleoenvironmental assessment.

Integrated Ocean Drilling Program Expedition 347 aimed to retrieve sediments from different settings of the Baltic Sea, encompassing the last interglacial-glacial cycle to address scientific questions along four main research themes: 1. Climate and sea level dynamics of marine isotope Stage (MIS) 5, including onsets and terminations; 2. Complexities of the latest glacial, MIS 4-MIS 2; 3. Glacial and Holocene (MIS 2-MIS 1) climate forcing; and 4. Deep biosphere in Baltic Sea Basin (BSB) sediments. These objectives were accomplished by drilling in six subbasins: (1) the gateway of the BSB (Anholt), where we focused on sediments from MIS 6-5 and MIS 2-1; (2) a subbasin in the southwestern BSB (Little Belt) that possibly holds a unique MIS 5 record; (3, 4) two subbasins in the south (Bornholm Basin and Hanö Bay) that may hold long complete records from MIS 4-2; (5) a 450 m deep subbasin in the central Baltic (Landsort Deep) that promises to contain a thick and continuous record of the last ~14,000 y; and (6) a subbasin in the very north (Ångermanälven River estuary) that contains a uniquely varved (annually deposited) sediment record of the last 10,000 y. These six areas were expected to contain sediment sequences representative of the last ~140,000 y, with paleoenvironmental information relevant on a semicontinental scale because the Baltic Sea drains an area four times as large as the basin itself. The location of the BSB in the heartland of a recurrently waning and waxing ice sheet, the Scandinavian Ice Sheet, has resulted in a complex development: repeated glaciations of different magnitudes, sensitive responses to sea level and gateway threshold changes, large shifts in sedimentation patterns, and high sedimentation rates. Its position also makes it a unique link between Eurasian and northwest European terrestrial records. Therefore, the sediments of this largest European intracontinental basin form a rare archive of climate evolution over the latest glacial cycle. High sedimentation rates provide an excellent opportunity to reconstruct climatic variability of global importance at a unique resolution from a marine-brackish setting. Comparable sequences cannot be retrieved anywhere in the surrounding onshore regions. Furthermore, and crucially, the large variability (salinity, climate, sedimentation, and oxygenation) that the BSB has under

A new Arctic Ostracode Database-2015 (AOD-2015 provides census data for 96 species of benthic marine Ostracoda from 1340 modern surface sediments from the Arctic Ocean and subarctic seas. Ostracoda is a meiofaunal, Crustacea group that secretes a bivalved calcareous (CaCO 3 ) shell commonly preserved in sediments. Arctic and subarctic ostracode species have ecological limits controlled by temperature, salinity, oxygen, sea ice, food, and other habitat-related factors. Unique species ecology, shell chemistry (Mg/Ca ratios, stable isotopes), and limited stratigraphic ranges make them a useful tool for paleoceanographic reconstructions and biostratigraphy. The database, described here, will facilitate the investigation of modern ostracode biogeography, regional community structure, and ecology. These data, when compared to downcore faunal data from sediment cores, will provide a better understanding of how the Arctic has been affected by climatic and oceanographic change during the Quaternary. Images of all species and biogeographic distribution maps for selected species are presented, with brief discussion of representative species' biogeographic and ecological significance. Publication of AOD-2015 is opensourced and will be available online at several public Guest editors: Alison J. Smith, Emi Ito, B. Brandon Curry & Patrick De Deckker / Multidisciplinary aspects of aquatic science: the legacy of Rick Forester Electronic supplementary material The online version of this article (websites with latitude, longitude, water depth, and bottom water temperature for most samples. It includes material from Arctic abyssal plains and submarine ridges, continental slopes, and shelves of the Kara, Laptev, East Siberian, Chukchi, Beaufort Seas, and several subarctic regions.