Description: The Labrador Sea is a particularly suitable high-latitude basin for investigating U and Th behavior in deep-sea sediments. During the late Quaternary, the cyclic development and decay of huge ice sheets on adjacent land masses resulted in large-amplitude changes in sedimentation rates and organic paleoproductivities. The resulting magnification of U and Th response is well illustrated by high-resolution studies on piston-cored sediments from the Greenland continental rise at Ocean Drilling Program Leg 105 Site 646 spanning isotopic stages 8 to 1. Our results show a clear positive correlation of 238U/232Th ratios with organic paleoproductivity indicators (e.g., dinocyst) due to U uptake in the water column and/or during the early early diagenesis of organic matter responding to carbon fluxes and to their climate forcing. 230Th excesses over 234U exceed the theoretical value of the 230Th rain from the overlying water column, indicating lateral input possibly from the Greenland slope and shelf. Because these horizontal fluxes of 230Th may be partly controlled by physical parameters, 230Th excesses cannot be unequivocally correlated with sedimentation rates and/or productivity as reported elsewhere. In this subarctic basin characterized by low overall organic carbon burial, the 238U/232Th ratio appears to be a sensible geochemical indicator of organic activity and paleoproductivity.

Description: Arctic sea ice is a critical component of the climate system, known to influence ocean circulation, earth's albedo, and ocean-atmosphere heat and gas exchange. Current developments in the use of IP25 (a sea ice proxy with 25 carbon atoms only synthesized by Arctic sea ice diatoms) have proven it to be a suitable proxy for palaeo-sea ice reconstructions over hundreds of thousands to even million years timescales. In the NE-Baffin Bay, off NW-Greenland, Melville Bugt is a climate-sensitive region characterized by strong seasonal sea ice variability and strong melt-water discharge from the Greenland Ice Sheet (GIS). Here, we present a centennial-scale resolution Holocene sea ice record, based on IP25 and open-water phytoplankton biomarkers (brassicasterol, dinosterol and HBI III) using core GeoB19927-3 (73°35.26' N, 58°05.66' W). Seasonal to ice-edge conditions near the core site is documented for most of the Holocene period with some significant variability. A cold interval, in the lower-most part, characterized by extensive sea ice cover and very low local productivity is succeeded by an interval (~9.4-8.5 ka BP)) with reduced sea ice cover, enhanced GIS spring melting, and strong influence of the West Greenland Current (WGC) in the earliest part of the record. From ~8.5 untill ~7.8 ka BP, a cooling event is recorded by ice algae and phytoplankton biomarkers. They indicate an enhanced sea ice cover, possibly related to the opening of Nares Strait, which may have led to an increased influx of Polar Water into NE-Baffin Bay. The interval between ~7.8 and ~3.0 ka BP is characterized by generally reduced sea ice cover with millennial-scale variability of the (late winter/early spring) ice-edge limit, increased open-water conditions (polynya-type), and a dominant WGC carrying warm waters at least as far as the Melville Bugt area. During the last ~3.0 ka BP, our biomarker records do not reflect the late Holocene 'Neoglacial cooling' observed elsewhere in the Northern Hemisphere, possibly due to the persistent influence of the WGC and interactions with the adjacent fjords. Peaks in HBI III at about ~2.1 and ~1.3 ka BP, interpreted as persistent ice-edge situations might, correlate with the Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA), respectively, in-phase with the North Atlantic Oscillation (NAO) mode. When integrated with marine and terrestrial records from other circum-Baffin Bay areas (Disko Bay, the Canadian Arctic, the Labrador Sea), the Melville Bugt biomarker records point to close ties with high Arctic and Northern Hemispheric climate conditions, driven by solar and oceanic circulation forcings.