Climatic and hydrographic variability in the late Holocene Skagerrak as deduced from benthic foraminiferal proxies

Abstract

Two Holocene sediment cores from the southern flank of the Skagerrak are investigated for the stable oxygen and carbon isotopic composition of benthic foraminiferal tests and faunal assemblages. Core 225514 was recovered from 420 m and Core 225510 from 285 m water depth.The stable oxygen isotopic composition of Bulimina marginata tests in Core 225514 is demonstrated to indicate Skagerrak deep-water renewal during the last 1200 years. Since deep-water renewal is characterized by sudden dropsin temperature and salinity, and since δ18O values reflect both temperature and salinity changes, the influences of the two parameters have to be evaluated separately. By comparing the measured δ18O variability with a salinity-δ18O mixing line valid for marine to brackish Scandinavian waters, it was shown that salinity changes are responsible for maximal 9 % of the total δ18O variability. Correlationof temperature monitoring data with the North Atlantic Oscillation (NAO) index reveals that Skagerrak deep-water renewal is triggered by the negative phase of the NAO. During highly negative index phases very cold and calm conditions prevail over the North Sea. Central North Sea water masses are cooled down strongly and hence reach densities, which are higher than those of the deep Skagerrak water masses. Occasionally, these dense water masses start to cascade into the Skagerrak.The stable carbon isotopic composition of benthic foraminiferal tests is used to investigate the organic matter flux to the seafloor and the oxygen availabilitywithin the sediments as well as to approach microhabitat-corrected vital effects of four species. The δ13C values of Uvigerina mediterranea indicate that the flux of organic matter to the seafloor was relatively constant between AD 1500 and 1950 and increased after AD 1950. We suggest that this increase in organic matter flux to the seafloor results from hydro-climatic variability within the North Sea region. A persistently high NAO index during the 1980s and 1990s enhanced the influx of nutrient-rich water masses through the English Channel. In concurrence with high temperatures, these nutrient-rich water masses allowed for increased primary productivity within the North Sea and presumably also within the Skagerrak-Kattegat region.The comparison of reconstructed δ13C gradients of dissolved inorganic carbon (DIC) within the two investigated cores indicates that organic matterremineralization due to respiration was generally enhanced in Core 225514 compared to Core 225510. Since the flux of organic matter to the seafloor was similar at both core sites and both sites were bathed by the same water mass, it is suggested that oxygen availability within the sediments is responsible for the difference. Higher sedimentation rates at Site 225510 result in enhanced carbon burial due to lower oxygen exposure times. The influence of oxygen exposure times on δ13C values should be especially important in shelf environments because sedimentation rates might be very variable there. Minimum estimates of microhabitat-corrected vital effects with reference to Globobulimina turgida are determined for Hyalinea balthica (> 1.3 �), Cassidulina laevigata (> 0.7 �), and Melonis barleeanus (> 0.7 �). Melonis zaandami seems to calcify its test close to pore water δ13CDIC.Faunal investigations produced three clearly distinguishable assemblages for each investigated core. In Core 225514 these assemblages occur consecutively, whereas the assemblages in Core 225510 intermittently change. The chronological order of dominant species in Core 225514 reflects the lateral succession of dominant species in modern surface sediments from the basin margin to the deepSkagerrak. For each dominant seasonal-phytophagous species a Gaussian-like relationship between species frequencies and specific sedimentary total organic carbon (TOC) contents is demonstrated. We propose that the chronologicalspecies succession in Core 225514 is the result of increasing sedimentary TOC contents with time, whereas the intermittently changing assemblages in Core 225510 are attributed to the vertically changing position of the Northern Jutland Current.