Description: Highlights • We reconstructed variation in nutrient utilization over the Laptev Sea throughout the Holocene. • The Holocene Siberian transgression modulated the water column structure and created unstable conditions until 4 ka. • Oceanographic conditions favourable to the onset of the Laptev Sea ‘sea-ice factory’ were reached around 2 ka. Abstract Understanding the dynamic of freshwater and sea-ice export from the Arctic is crucial to better comprehend the potential near-future climate change consequences. Here, we report nitrogen isotope data of a core from the Laptev Sea to shed light on the impact of the Holocene Siberian transgression on the summer stratification of the Laptev Sea. Our data suggest that the oceanographic setting was less favourable to sea-ice formation in the Laptev Sea during the early to mid-Holocene. It is only after the sea level reached a standstill at around 4 ka that the water column structure in the Laptev Sea became more stable. Modern-day conditions, often described as “sea-ice factory”, were reached about 2 ka ago, after the development of a strong summer stratification. These results are consistent with sea-ice reconstruction along the Transpolar Drift, highlighting the potential contribution of the Laptev Sea to the export of freshwater from the Arctic Ocean.

Description: Water mass exchange between the Arctic Ocean and the Norwegian-Greenland Seas has played an important role for the Atlantic thermohaline circulation and Northern Hemisphere climate. We reconstruct past water mass mixing and erosional inputs from the radiogenic isotope compositions of neodymium (Nd), lead (Pb), and strontium (Sr) at Ocean Drilling Program site 911 (leg 151) from 906 m water depth on Yermak Plateau in the Fram Strait over the past 5.2 Myr. The isotopic compositions of past bottom waters were extracted from authigenic oxyhydroxide coatings of the bulk sediments. Neodymium isotope signatures obtained from surface sediments agree well with present-day deepwater εNd signature of −11.0 ± 0.2. Prior to 2.7 Ma the Nd and Pb isotope compositions of the bottom waters only show small variations indicative of a consistent influence of Atlantic waters. Since the major intensification of the Northern Hemisphere Glaciation at 2.7 Ma the seawater Nd isotope composition has varied more pronouncedly due to changes in weathering inputs related to the waxing and waning of the ice sheets on Svalbard, the Barents Sea, and the Eurasian shelf, due to changes in water mass exchange and due to the increasing supply of ice-rafted debris (IRD) originating from the Arctic Ocean. The seawater Pb isotope record also exhibits a higher short-term variability after 2.7 Ma, but there is also a trend toward more radiogenic values, which reflects a combination of changes in input sources and enhanced incongruent weathering inputs of Pb released from freshly eroded old continental rocks.

Description: During the "RV Polarstem"-Expedition ARK VIII/2 sediment samples were obtained at the continentalslope of NW-Spitsbergen. Detailed sedimentological and geochemical analysis were carriedout at two undisturbed box cores (PS2122-1GKG, PS2123-2GKG) as well as two gravity cores(PS2122-1SL, PS2123-2SL). The following parameters were deterrnined: Organic carbon, nitrogenand carbonate contents, hydrogen index, stable isotopes, ice rafted debris, grain-size distribution andbiogenic opal. The main objective of this study was the reconstruction of paleoenvironmental changes off thenorthwest coast of Spitsbergen during the last glacial/interglacial-cycle, i.e., during the last about128.000 years.The results of the investigations can be summarized as follows:- During isotope stage 1 (Holocene) and 5.5 (Eemian Interglacial), light stable isotopes (8180: 3,4-2%0; 8BC: 0,26-0,5 %0), increased bioturbation, high content of planktonic foraminifera andbiogenic opal and low quantity of ice-rafted material, indicate seasonally ice-free conditions alongthe northwest coast due to the int1uence of the Westspitsbergen Current.- Additionally, the sediment characteristics of the middle of isotope stage 2 (Last Glacial Maximum)and at the end of stage 3 confirms an inflow of warmer Atlantic water. The highest production ofplanktonic and benthic foraminifera (N. pachyderma sin., Cassidulina teretis) (CaC03: 10 %) mayret1ect the expansion of the "Whalers Bay"-Polynya as a result of the int1uence of the WestspitsbergenCurrent. Presumably, occasionally open-ice conditions provide sufficient precipitation to buildupthe Svalbard/Barents Ice Sheet.- The time intervals for the glacier advances on Svalbard given by Mangerud et al. (1992), can becorrelated with increased accumulation of ice-rafted material in the sediments at the northwest coastof Spitsbergen. Especially during isotope stage 4 and at the beginning of the Last Glacial Maximum(isotope stage 2), a drastically increased supply of coarse terrigenous material occurs. The highaccumulation rate (0,18-0,21 g/cm2jky) of terrigenous organic carbon is indicated by high C/Nratios(until 16) and low hydrogen index (50 mg-HC/gC). In constrast to deep sea sediments in theFram-Strait (Hebbeln 1992), the glacier advance between 118.000 and 108.000 years B.P. ist documented in the continental slope sediments.- At the end of the Weichselian ice age, the deglaciation at the northwest coast starts with a typicalmelt-water signal in the stables isotope record (8180: 3,5 %0; 813C: -0,16 %0) and high contents ofgravel (6-13 %). The signal can be assigned to an event at the westcoast of Spitsbergen (core NP9039), dated to 14.500 years B.P. (Andersen et al. 1993).

Description: The development of various combinative methods for Arctic sea ice reconstruction using the sympagic highly-branched isoprenoid IP25 in conjunction with pelagic biomarkers has often facilitated more detailed descriptions of sea ice conditions than using IP25 alone. Here, we investigated the complementary application of the Phytoplankton-IP25 index (PIP25) and a recently proposed Classification Tree (CT) model for describing shifts in sea ice conditions to assess the consistency of both methods. Based on biomarker data from three downcore records from the Barents Sea spanning millennial timescales, we showcase apparent and potential limitations of both approaches, and provide recommendations for their identification or prevention. Both methods provided generally consistent outcomes and, within the studied cores, captured abrupt shifts in sea ice regimes, such as those evident during the Younger Dryas, as well as more gradual trends in sea ice conditions during the Holocene. The most significant discrepancies occurred during periods of highly unstable climate change, such as those characteristic of the Younger Dryas-Holocene transition. Such intervals of increased discrepancy were identifiable by significant changes of HBI distributions and correlations to values not observed in proximal surface sediments. We suggest that periods of highly-fluctuating climate that are not represented in modern settings may hinder the performance and complementary application of PIP25 and CT-based methods, and that data visualisation techniques should be employed to identify such occurrences in downcore records. Additionally, due to the reliance of both methods on biomarker distributions, we emphasise the importance of accurate and consistent biomarker quantification for future investigations.

Description: We investigated active methane seeps in a water depth of 200 m in the Hola area off the coast of Vesteralen, northern Norway, to assess (1) hydrocarbon sources, (2) migration pathways and (3) the influence of hydrocarbon seepage on sediment pore water and water column chemistry. The seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42 nM ca. 5 m above the seafloor. Pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate-methane-transition zone (SMTZ) between 80 cm and 〉 250 cm indicating spatially heterogeneous methane ascent. The isotopic composition of methane (d13C from - 40per mil to - 63per mil and d2H from - 191per mil to - 225per mil) and d13C depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin, consistent with d13C values of C2 to C4 hydrocarbons. Isotope data also indicate considerable biodegradation of propane. Seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying Mesozoic sedimentary rocks. We propose that these act as migration pathways for the hydrocarbons from late Jurassic to early Cretaceous source rocks.