The Arctic is responding more rapidly to global warming than most other areas on our planet. Northward-flowing Atlantic Water is the major means of heat advection toward the Arctic and strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ~150 years. Here, we present a multidecadal-scale record of ocean temperature variations during the past 2000 years, derived from marine sediments off Western Svalbard (79°N). We find that early-21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming.

Available online xxxx Editor: D. Vance Keywords: sea ice Arctic proxy IP 25 biomarker paleoclimateAnalysis of >100 surface sediments from across the Barents Sea has shown that the relative abundances of the mono-unsaturated sea ice diatom-derived biomarker IP 25 and a tri-unsaturated highly branched isoprenoid (HBI) lipid (HBI III) are characteristic of the overlying surface oceanographic conditions, most notably, the location of the seasonal sea ice edge. Thus, while IP 25 is generally limited to locations experiencing seasonal sea ice, with higher abundances found for locations with longer periods of ice cover, HBI III is found in sediments from all sampling locations, but is significantly enhanced in sediments within the vicinity of the retreating sea ice edge or marginal ice zone (MIZ). The response of HBI III to this well-defined sea ice scenario also appears to be more selective than that of the more generic phytoplankton biomarker, brassicasterol. The potential for the combined analysis of IP 25 and HBI III to provide more detailed assessments of past sea ice conditions than IP 25 alone has been investigated by quantifying both biomarkers in three marine downcore records from locations with contrasting modern sea ice settings. For sediment cores from the western Barents Sea (intermittent seasonal sea ice) and the northern Norwegian Sea (ice-free), high IP 25 and low HBI III during the Younger Dryas (ca. 12.9-11.9 cal. kyr BP) is consistent with extensive sea cover, with relatively short periods of icefree conditions resulting from late summer retreat. Towards the end of the YD (ca. 11.9-11.5 cal. kyr BP), a general amelioration of conditions resulted in a near winter maximum ice edge scenario for both locations, although this was somewhat variable, and the eventual transition to predominantly ice-free conditions was later for the western Barents Sea site (ca. 9.9 cal. kyr BP) compared to NW Norway (ca. 11.5 cal. kyr BP). For both locations, coeval elevated HBI III (but absent IP 25 ) potentially provides further evidence for increased Atlantic Water inflow during the early Holocene, but this interpretation requires further investigation. In contrast, IP 25 and HBI III data obtained from a core from the northern Barents Sea demonstrate that seasonal sea ice prevailed throughout the Holocene, but with a gradual shift from winter ice edge conditions during the early Holocene to more sustained ice cover in the Neoglacial; a directional shift that has undergone a reverse in the last ca. 150 yr according to observational records. Our combined surface and downcore datasets suggest that combined analysis of IP 25 and HBI III can provide information on temporal variations in the position of the maximum (winter) Arctic sea ice extent, together with insights into sea ice seasonality by characterisation of the MIZ. Combining IP 25 with HBI III in the form of the previously proposed PIP 25 index yields similar outcomes to those obtained using brassicasterol as the phytoplankton marker. Importantly, however, some proble...

20Two high-resolution sediment cores from eastern Fram Strait have been investigated 21 for sea subsurface and surface temperature variability during the Holocene (the past ca 12,000 22 years). The transfer function developed by Husum and Hald (2012) has been applied to 23 sediment cores in order to reconstruct fluctuations of sea subsurface temperatures throughout 24 the period. Additional biomarker and foraminiferal proxy data are used to elucidate variability 25 between surface and subsurface water mass conditions, and to conclude on the Holocene 26 climate and oceanographic variability on the West Spitsbergen continental margin. Results 27 consistently reveal warm sea surface to subsurface temperatures of up to 6°C until ca 5 cal ka 28 BP, with maximum seawater temperatures around 10 cal ka BP, likely related to maximum 29 July insolation occurring at that time. Maximum Atlantic Water (AW) advection occurred at 30 surface and subsurface between 10.6 and 8.5 cal ka BP based on both foraminiferal and 31 dinocyst temperature reconstructions. Probably, a less-stratified, ice-free, nutrient-rich surface 32 ocean with strong Atlantic Water advection prevailed in the eastern Fram Strait between 10 33 and 9 cal ka BP. Weakened Atlantic Water contribution is found after ca 5 cal ka BP when 34 © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ subsurface temperatures strongly decrease with minimum values between ca 4 and 3 cal ka 35 BP. Cold conditions during that time are furthermore supported by high planktic foraminifer 36 shell fragmentation and  18 O values of the subpolar planktic foraminifer species T. 37 quinqueloba. While IP 25 -associated indices as well as dinocyst data suggest a sustained 38 cooling due to a decrease in early summer insolation and consequently sea-ice increase since 39 about 7 cal ka BP in surface waters, planktic foraminiferal data including stable isotopes 40 indicate a slight return of stronger subsurface Atlantic Water influx since ca 3 cal ka BP. The 41 observed decoupling of surface and subsurface waters during the later Holocene is most likely 42 attributed to a strong pycnocline layer separating cold sea-ice fed surface waters from 43 enhanced subsurface Atlantic Water advection. This may be related to changes in North 44 Atlantic subpolar versus subtropical gyre activity. 45 46 Husum, K., Hald, M., 2012. Arctic planktic foraminiferal assemblages: Implications for 47 subsurface temperature reconstructions. Marine Micropaleontology 96-97, 38-47. 48 49 world's oceans. Northward flowing warm and saline Atlantic Water via the eastern Fram 60 Strait strongly contributes to the Arctic Ocean's heat budget. 61 Reconstructions of past climate and oceanographic conditions are essential for 62 understanding and modelling of the current and future climate. Extending the record of ocean 63 temperatures beyond the era of instrumental measurements facilitates improved knowledge 64 about the long-term mechanisms of...

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