Holocene changes in climate, environment, and ocean reservoir age on the Iceland Shelf: magnesium/calcium, delta-oxygen-18, and tephrochronology of core MD99-2269

The focus of this dissertation is a high-resolution Holocene record from marine sediment core MD99-2269 (66.37.53 N, 23.51.16 W). The core's overall chronology is constrained by 24 radiocarbon dates, augmented by 20 dates from core MD99-2322 (E Greenland shelf) correlated in depth by paleomagnetic secular variation. Tephra-grain counting, morphology and geochemistry on the 150-1000 mum fraction of core MD99-2269 were used to identify 22 cryptotephras, some reported here for the first time. The cryptotephras and one visible tephra layer (Saksunarvatn) were intercalibrated with the MD99-2269 paleomagnetic secular variation record and radiocarbon chronology, providing a fundamental chrono-stratigraphic template for the Icelandic region and beyond. Holocene changes in ocean reservoir age, related to water mass variations, are calculated from the age difference between contemporaneous marine and terrestrial tephra-markers and a new age model, which takes into account varying reservoir age during the Holocene, was constructed. Two calibration studies of the modern environment, using data collected during the B997 cruise, produced three new Mg/Ca-temperature calibrations and new delta 18Ocalcite-temperature relationships. Using the above chronology, calibrations and relations, the Holocene paleotemperature record of core MD99-2269 was reconstructed. Atlantic bottom-water dominates from after the deposition of the Saksunarvatn tephra until Neoglaciation, with the exception of a return to Arctic bottom-water conditions between 8240 and 7980 cal yr BP. At the onset of Neoglaciation Arctic water (coinciding with older reservoir ages) floods the seafloor but Atlantic water is still observed as an intermediate water mass between c. 4000-2000 cal yr BP. Arctic water then dominates after 2000 cal yr BP. The multiproxy comparison in core MD99-2269 indicates (1) the presence of a stratified watercolumn over the site (2) that chemical proxies record seasonal influx of warm Atlantic water (calcifying and thus recording during peak warmth) whereas assemblage proxies are more affected by annual conditions, controlled by cold winters (constant presence of sea ice diatoms) and formation of cold winter water on the shelf prohibiting full boreal benthic fauna to thrive. This dissertation will contribute to the Holocene research initiative of NSF/ESH (Earth System History) focusing on the nature and significance of natural oscillations in climate during the last 10,000 years.