Size-dependent isotopic composition of planktic foraminifers from Chukchi Sea vs. NW Atlantic sediments - implications for the Holocene paleoceanography of the western Arctic

In Arctic and sub Arctic seas, shell growth and/or secondary calcite overgrowth of Neogloboquadrina pachyderma (left coiled)Npl-occur along the pycnocline, and their delta(13)C and delta(18)O-values are size and weight dependent. However, whereas the Npl O-18 data from the NW Atlantic indicate near-equilibrium conditions with ambient waters and a positive relationship between shell weight and O-18-content, assemblages from box-cored sediments of the Chukchi Sea (western Arctic) are depleted by similar to2%. with respect to equilibrium values with modern conditions, and depict a negative relationship between shell weight and its delta(18)O-value (-0.15+/-0.03parts per thousand/mug on VPDB scale). A similar feature is also depicted by the dextral form of N. pachyderma (Npd). We associate the reverse shell-size or weight vs. delta(18)O relationship to the reverse temperature gradient observed along the thermocline between the surface cold and dilute water layer, and the underlying near 3degreesC-warmer saline North Atlantic water mass. The analysis of two late to post-glacial sedimentary sequences from the Chukchi Sea indicates that such a water mass stratification with a reverse thermocline persisted throughout the Holocene, thus reflecting an early onset of the modern-like linkage between the Arctic Ocean and the North Atlantic. Moreover, lower delta(18)O-values in both Npl and Npd together with larger delta(18)O-gradients between the different shell sizes at ca 9-7 ka BP suggest similar to3degreesC higher temperatures in the upper North Atlantic water mass, in comparison with the present (approximately + 1degreesC, at the study site), thus likely a higher inflow rate of this water mass during the early Holocene. (C) 2003 Elsevier Ltd. All rights reserved.