ABSTRACT: Late Holocene paleoceanography and climate variability over the Mackenzie Slope (Beaufort Sea, Canadian Arctic).

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Abstract

Late Holocene paleoceanography and climate variability of the Southeastern Beaufort Sea (Canadian Arctic) have been investigated on the basis of sedimentary cores collected over the Mackenzie Slope. Piston, trigger and box cores were sampled at station 803 in 2004 aboard the CCGS Amundsen at 218 m water depth. The chronology of the piston core is constrained by 4 AMS¬14C dates, and the sedimentation rate in the box core is assessed from 210Pb data. We obtain a continuous composite sequence covering the last 4600 years, with a sedimentation rate of ~140 cm/kyr. Palynological data reveal that dinocyst assemblages are dominated by Operculodinium centrocarpum (mean of 43.3%), with the accompanying taxa Brigantedinium spp. (19.6%), Islandinium minutum (15.6%) and cysts of Pentapharsodinium dalei (13.7%). Four zones have been established on the basis of dinocyst relative abundances. Dinocyst assemblage zone 1 (D1), from 4600 to 2700 cal BP, is dominated by O. centrocarpum (mean of 49,0%). In zone D2 (2700-1500 cal BP), the relative abundance of O. centrocarpum decreases (34,4%) in favour of that of the opportunistic, heterotrophic taxa Brigantedinium spp. (28,8%) and cysts of Polykrikos sp. var. arctic/ quadratus (2,8%). This shift is also illustrated by the decrease of the ratio Gonyaulacales versus Peridiniales, a trend observed in both zones D2 and D3. In fact, dinocyst zone D3 (1500-30 cal BP or 450-1920 AD) is characterized by the high relative abundance of the peridinioid taxa I. minutum (19,9%). The uppermost zone (D4), spanning from 1920 to 2004 AD, is again dominated by O. centrocarpum (44,5%), and shows low relative abundances of Brigantedinium spp. and cf. Echinidinium karaense.Quantitative reconstructions of past sea-surface parameters (August sea-surface temperature: SST, August sea-surface salinity: SSS, and duration of sea-ice cover) have been assessed on the basis of dinocyst assemblages using transfer functions (modern analogue technique). They indicate relatively stable conditions over the last 4,6 ka, with episodic cooling events (SST of ~ 1,5°C below the modern value of 6°C) that took place between 700 and 1820 AD. We associate the last and the longest of these cooling events (1560 - 1820 AD) with the Little Ice Age. Reconstructed SSS shows decadal oscillations since 1920 AD that we tentatively associate with the accumulation of freshwater by the Beaufort Gyre and the subsequent Great Salinity Anomalies. Our data suggest that similar salinity anomalies could have occurred around ~1860 and ~1790 AD. Stable isotopic data (d13C and d15N) suggest that the rate of relative sea-level rise in the southern Beaufort Sea increased since 1820 AD. This constitutes a major concern with regard to present and future coastal erosion. The d15N pro?le might also have recorded variations in Paci?c water in?uence from 4600 to ~ 1300 cal BP, probably associated with centennial-scale shifts of the Arctic Oscillation phases.

Bibliography of Canadian Geomorphology