A 340,000 year record of ice rafting, palaeoclimatic fluctuations, and shelf-crossing glacial advances in the southwestern Labrador Sea

Orphan Basin, southwestern Labrador Sea, is a strategic site for the study of Quaternary palaeoceanography and palaeoclimate. A 31.45-m-long piston co re (MD95-2025) was raised from 2925-m-depth at 49 degrees 47.645' N, 46 deg rees 41.851' W, just beyond the seaward limit of stacked debris-flow tongue s derived from the Northeast Newfoundland Shelf. The core extends to oxygen isotopic stage 9 (similar to 340,000 years), and includes 13 prominent ice -rafted layers (Heinrich events H1-H13), many of which are characterized by abundant detrital Palaeozoic limestone and dolomite. Warm peaks in sea-sur face temperature (SST) show poor correlation with accentuated ice rafting, except for 20-60 ka (H3-H5) when the terminations of meltwater pulses (delt a O-18 minima) lagged warm peaks in SST by similar to 1000 years, and peaks in ice rafting either coincided with peaks in SST (H4, H5), or lagged warm er peaks in SST by similar to 500 years (H3). These lags are attributed to the delayed response of ice sheets (e.g., iceberg and meltwater production rates) to palaeoceanographic and palaeoclimatic forcing factors (e.g., incu rsions of the warm North Atlantic Drift into the Labrador Sea; orbital-indu ced changes in insolarion). The remarkable covariance between SST and ice r afting from 20-60 ka is inconsistent with models for ice-stream surging thr ough Hudson Strait [Marshall, S.J. and G.K.C. Clarke, 1997. A continuum mix ture model of ice stream thermomechanics in the Laurentide Ice Sheet 2: app lication to the Hudson Strait ice stream. J. Geophys, Res., B102, 20615-206 37], and instead suggests that regional changes in ocean circulation played an important role in destabilizing icesheets. Heinrich layers H1, H3-H6, H 9, H11, and H13 formed during times of sharply decreasing delta O-18 values (i.e., ice sheet melting). Heinrich layers H2, H7 and H12 formed at transi tions from interglacial/interstadial to glacial stages, coincident with bot h cool SST and low fluxes of detrital carbonate. They may represent the ini tiation of calving as growing ice sheets readvanced to coastal areas of the Labrador Sea and Baffin Bay. Carbonate-poor H8 and H10 developed during in terglacial stages 5 and 7, and may have been derived mainly from Greenland like the modern ice-rafted sediments of the Labrador Sea. (C) 2001 Elsevier Science B.V. All rights reserved.