No abstract

Multi-proxy analyses of two sediment cores from Dicksonfjorden were performed to reconstruct Holocene environmental conditions in this northern branch of Isfjorden, the largest fjord system in Svalbard. Factors affecting the depositional processes include shifts in sources of sediments, ice rafting and regional glacio-isostatic rebound. Sediments were derived from Palaeozoic siliciclastics and carbonates occurring at the fjord head and sides, respectively. Their relative contributions were controlled by falling relative sea level and the resulting progradation of the major stream and delta systems closer to the core sites. Deposition of clasts from sea-ice rafting persisted throughout most of the Holocene. Following a period of low, but continuous, clast fluxes (ca. 11 000-7000 calibrated years before the present), ice rafting was most intensive between ca. 7000 and 3000 calibrated years before the present. It can be related to extensive seasonal sea-ice formation caused by regional cooling. The prograding deltas also provided coarse sediments. Reduced ice rafting from ca. 3000 calibrated years before the present suggests enhanced formation of shorefast and/or permanent sea ice, suppressing sea-ice rafting in the fjord, in response to the cool climate and reduced heat flux from Atlantic Water. Episodic inflow of Atlantic Water and low turbidity of surface water can, however, account for a larger amount of marine organic matter produced in the outer fjord. The sedimentary record in Dicksonfjorden, where tidewater glaciers are absent, reflects similar climate and oceanographic variations as reconstructed in fjords on western Spitsbergen that are influenced by tidewater glaciers.

In this study, we aimed to reconstruct spring (April-June) sea ice changes in the western Arctic Ocean over recent centuries (ca. the last 250 years) by measuring biomarker distributions in a multicore (ARA01B-03MUC) retrieved from the Chukchi Shelf region and to evaluate outcomes against known or modelled estimates of sea ice conditions. Specifically, we analyzed for the Arctic sea ice proxy IP 25 and assessed the suitability of a further highly branched isoprenoid (HBI) lipid (HBI III), epibrassicasterol, and dinosterol as complementary biomarkers for use with the so-called phytoplankton marker-IP 25 index (PIP 25 ; P III IP 25 , P B IP 25 , and P D IP 25 , respectively). The presence of IP 25 throughout core ARA01B-03MUC confirms the occurrence of seasonal sea ice at the study site over recent centuries. From a semi-quantitative perspective, all three PIP 25 indices gave different trends, with some dependence on the balance factor c, a term used in the calculation of the PIP 25 index. P III IP 25 -derived spring sea ice concentration (SpSIC) estimates using a c value of 0.63, determined previously from analysis of Barents Sea surface sediments, were likely most reliable, since SpSIC values were high throughout the record (SpSIC >78%), consistent with the modern context for the Chukchi Sea and the mean SpSIC record of the 41 CMIP5 climate models over recent centuries. P B IP 25 -based SpSIC estimates were also high (SpSIC 108%−127%), albeit somewhat over-estimated, when using a c value of 0.023 obtained from a pan-Arctic distribution of surface sediments. In contrast, P D IP 25 values using a pan-Arctic c value of 0.11, and PIP 25 data based on the mean biomarker concentrations from ARA01B-03MUC, largely underestimated sea ice conditions (SpSIC as low as 13%), and exhibited poor agreement with instrumental records or model outputs. On the other hand, P B IP 25 values using a c factor based on mean IP 25 and epi-brassicasterol concentrations exhibited a decline towards the core top, which resembled recent decreasing changes in summer sea ice conditions for the Chukchi Sea; however, further work is needed to test the broader spatial generality of this observation.

No abstract

Mid-Brunhes Event (MBE) occurred at approximately 420 ka between Marine Isotope Stage 11 and 12, and is considered the most pronounced climatic shift during the last ~ 800 kyrs. On the other hand, it is unclear if the MBE was global, despite being observed in the high-latitude Northern Hemispheric cryosphere in terms of climate systems. A 5.35-m long gravity core ARC5-MA01 was obtained from the northern Mendeleev Ridge in the western Arctic Ocean to track the paleoenvironmental changes in terms of the terrigenous sedimentation in response to the glacial-interglacial climate changes across the MBE. Geochemical proxies (biogenic opal, total organic carbon, C/N ratio, carbon isotope of organic matter, and calcium carbonate) of MA01 suggest that the terrigenous input was generally higher during the interglacial periods. Based on a mineralogical examination, most of the terrigenous input was attributed to the abundance of dolomite and the increased kaolinite content from North America. In particular, most paleoceanographic proxies showed that the terrigenous input from North America was enhanced distinctly during the post-MBE interglacial periods. These results suggest that the MBE in the western Arctic Ocean was a global climatic shift closely linked to cryospheric development in North America during the middle Pleistocene.