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a b s t r a c tStudies of spatial and temporal changes in modern and past sea-ice occurrence may help to understand the processes controlling the recent decrease in Arctic sea-ice cover. Here, we determined concentrations of IP 25 , a novel biomarker proxy for sea ice developed in recent years, phytoplankton-derived biomarkers (brassicasterol and dinosterol) and terrigenous biomarkers (campesterol and b-sitosterol) in the surface sediments from the Kara and Laptev seas to estimate modern spatial (seasonal) sea-ice variability and organic-matter sources. C 25 -HBI dienes and trienes were determined as additional palaeoenvironmental proxies in the study area. Furthermore, a combined phytoplankton-IP 25 biomarker approach (PIP 25 index) is used to reconstruct the modern sea-ice distribution more quantitatively. The terrigenous biomarkers reach maximum concentrations in the coastal zones and estuaries, reflecting the huge discharge by the major rivers Ob, Yenisei and Lena. Maxima in phytoplankton biomarkers indicating increased primary productivity were found in the seasonally ice-free central part of the Kara and Laptev seas. Neither IP 25 nor PIP 25 , however, shows a clear and simple correlation with satellite sea-ice distribution in our study area due to the complex environmental conditions in our study area and the transportation process of sea-ice diatom in the water column. Differences in the diene/IP 25 and triene/IP 25 ratios point to different sources of these HBIs and different environmental conditions. The diene/IP 25 ratio seems to correlate positively with sea-surface temperature, while negatively with salinity distributions.

To evaluate the present sea ice changes in a longer-term perspective, the knowledge of sea ice variability on preindustrial and geological time scales is essential. For the interpretation of proxy reconstructions it is necessary to understand the recent signals of different sea ice proxies from various regions. We present 260 new sediment surface samples collected in the (sub-)Arctic Oceans that were analyzed for specific sea ice (IP 25) and open-water phytoplankton biomarkers (brassicasterol, dinosterol, and highly branched isoprenoid [HBI] III). This new biomarker data set was combined with 615 previously published biomarker surface samples into a pan-Arctic database. The resulting pan-Arctic biomarker and sea ice index (PIP 25) database shows a spatial distribution correlating well with the diverse modern sea ice concentrations. We find correlations of P B IP 25 , P D IP 25 , and P III IP 25 with spring and autumn sea ice concentrations. Similar correlations with modern sea ice concentrations are observed in Baffin Bay. However, the correlations of the PIP 25 indices with modern sea ice concentrations differ in Fram Strait from those of the (sub-)Arctic data set, which is likely caused by region-specific differences in sea ice variability, nutrient availability, and other environmental conditions. The extended (sea ice) biomarker database strengthens the validity of biomarker sea ice reconstructions in different Arctic regions and shows how different sea ice proxies combined may resolve specific seasonal sea ice conditions.

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Using the sea ice proxy IP 25 and phytoplankton-derived biomarkers (brassicasterol and dinosterol), Arctic sea ice conditions were reconstructed for Marine Isotope Stage (MIS) 3 to 1-with special emphasis on the Last Glacial Maximum (LGM)-in sediment cores from the northern Barents Sea continental margin across the central Arctic Ocean to the southern Mendeleev Ridge. Our results suggest more extensive sea ice cover than present day during the latter part of MIS 3, increasing sea ice growth during MIS 2 and decreased sea ice cover during the last deglacial. The summer ice edge remained north of the Barents Sea even during extremely cold (i.e., Last Glacial Maximum (LGM)) as well as warm periods (i.e., Bølling-Allerød). During the LGM, the western Svalbard margin and the northern Barents Sea margin areas were characterized by high concentrations of both IP 25 and phytoplankton biomarkers, interpreted as a productive ice edge situation caused by the inflow of warm Atlantic water. In contrast, the LGM central Arctic Ocean (north of 84°N) was covered by thick permanent sea ice throughout the year with rare breakup, indicated by zero or near-zero biomarker concentrations. The spring/summer sea ice margin significantly extended southward to the Laptev Sea shelf (southern Lomonosov Ridge) and southern Mendeleev Ridge during the LGM. Our proxy reconstructions are very consistent with published model results based on the North Atlantic/Arctic Ocean Sea Ice Model.

The Holocene environment evolution in the East China Sea (ECS) is characterized by the gradual establishment and strengthening of its shelf circulation system, but knowledge about temperature responses in temporal and spatial scales is limited due to the lack of continuous high-resolution records. Here, we compare ′ U K 37 and TEX 86 temperature records for three cores from the ECS shelf, which provide the temporal and spatial patterns of Holocene temperature structure variations. These temperature records revealed broadly consistent temporal trends with three intervals characterized by two distinct shifts. During the early Holocene (10.0-6.0 ka), the modern-type circulation system was not established, which resulted in strong water column stratification; and the higher sea surface temperature (SST) might be associated with the Holocene Thermal Maximum (HTM). The interval of 6.0 to 1.0/2.0 ka displayed a weaker stratification caused by the intrusion of the Yellow Sea Warm Current (YSWC) and the initiation of the circulation system. A decreasing SST trend was related to the formation of the cold eddy generated by the circulation system in the ECS. During 1.0/2.0 to 0 ka, temperatures were characterized by much weaker stratification and an abrupt decrease of SST caused by the enhanced circulation system and stronger cold eddy, respectively. Thus, the temperature structure in the shelf of ECS was closely related with circulation system changes during the mid-late Holocene, which was most likely driven by the intrusion of Kuroshio Current (KC). The significant asynchrony of temperature decreases in the three locations during the late Holocene was likely caused by the gradual expansion of the ECS cold eddy area.

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