A High-Resolution Lipid Biomarker Perspective on North Iceland Shelf Marine Climate over the Last Millennium

Icelandic climate is vulnerable to variations in the dominance of competing Arctic and Atlantic ocean currents. The boundary between these water masses delineates the Polar Front, which today occupies the North Iceland Shelf (NIS). To date, Holocene oceanographic reconstructions along the NIS have employed a variety of proxies including Mg/Ca and δ18O of benthic and planktonic foraminifera, quartz and calcite wt%, the alkenone unsaturation index (Uk'37) and biotic species assemblages. Sea surface temperature (SST) proxies are primarily derived from phytoplankton resulting in a seasonal bias toward spring/summer SST. Furthermore, SST proxies can be influenced by additional confounding variables (e.g. salinity, nutrients, depth habitat of biota) resulting in markedly different Holocene temperature reconstructions between proxy datasets. To evaluate the similarities and discrepancies between various marine proxies, we investigate a high-resolution sediment core collected from the central North Iceland Shelf (B997-316GGC, 658 m depth). Sedimentation covers the last millennium, which captures the transition from the Medieval Warm Period to the Little Ice Age. Age control is constrained by 14C dates and paleomagnetic secular variation. To assess marine surface productivity and sea ice conditions, we analyze quartz and calcite wt% via XRD and a series of highly branched isoprenoid biomarkers. Quantitative paleotemperature estimates are derived from a novel combination of Mg/Ca of foraminifera and two lipid biomarker indices, Uk'37 from Prymnesiophyte alkenones and TEX86 from Thaumarchaeota glycerol dialkyl glycerol tetraethers (GDGTs). The latter TEX86 record is the first paleo application in Icelandic waters, which a recent local calibration study suggests may reflect annual or winter sub-surface (0-200 m) temperatures. Our paleotemperature records are bolstered by the analysis of additional sediment core tops, which expand the established Icelandic calibrations. Finally, we perform statistical analyses in an effort to extract a robust record of paleoceanographic change and to test the applicability of various proxies in high-latitude paleoclimate studies.