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

Abstract

Orphan Basin, southwestern Labrador Sea, is a strategic site for the study of Quaternary palaeoceanography and palaeoclimate. A 31.45-m-long piston core (MD95-2025) was raised from 2925-m-depth at 49°47.645′N, 46°41.851′W, just beyond the seaward limit of stacked debris-flow tongues derived from the Northeast Newfoundland Shelf. The core extends to oxygen isotopic stage 9 (∼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-surface temperature (SST) show poor correlation with accentuated ice rafting, except for 20–60 ka (H3–H5) when the terminations of meltwater pulses ( δ 18O minima) lagged warm peaks in SST by ∼1000 years, and peaks in ice rafting either coincided with peaks in SST (H4, H5), or lagged warmer peaks in SST by ∼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., incursions of the warm North Atlantic Drift into the Labrador Sea; orbital-induced changes in insolation). The remarkable covariance between SST and ice rafting from 20–60 ka is inconsistent with models for ice-stream surging through Hudson Strait [Marshall, S.J. and G.K.C. Clarke, 1997. A continuum mixture model of ice stream thermomechanics in the Laurentide Ice Sheet 2: application to the Hudson Strait ice stream. J. Geophys. Res., B102, 20615–20637], and instead suggests that regional changes in ocean circulation played an important role in destabilizing icesheets. Heinrich layers H1, H3–H6, H9, H11, and H13 formed during times of sharply decreasing δ 18O values (i.e., ice sheet melting). Heinrich layers H2, H7 and H12 formed at transitions from interglacial/interstadial to glacial stages, coincident with both cool SST and low fluxes of detrital carbonate. They may represent the initiation of calving as growing ice sheets readvanced to coastal areas of the Labrador Sea and Baffin Bay. Carbonate-poor H8 and H10 developed during interglacial stages 5 and 7, and may have been derived mainly from Greenland like the modern ice-rafted sediments of the Labrador Sea.