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

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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 (δ18O 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 δ18O 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.

Introduction

Compositionally distinctive ice-rafted deposits (Heinrich layers), widespread in the North Atlantic basin, have been ascribed to surges of the Laurentide Ice Sheet each ∼7000–10,000 years through Hudson Strait Heinrich, 1988, Andrews and Tedesco, 1992, Broecker et al., 1992, Bond et al., 1993, Andrews et al., 1994, Hillaire-Marcel et al., 1994, Bond and Lotti, 1995, Dowdeswell et al., 1995, Johnson and Lauritzen, 1995, Gwiazda et al., 1996a, Gwiazda et al., 1996b, Gwiazda et al., 1996c, Hiscott and Aksu, 1996, van Kreveld et al., 1996. The Labrador Sea is strategically located for the study of abrupt climatic and palaeoceanographic changes associated with growth and decay of Quaternary continental ice sheets because ice-rafted Heinrich layers are well-developed there (Hiscott and Aksu, 1996). Sea-surface palaeotemperatures and proxy climatic indicators show significant excursions that track the interplay of the cold Labrador Current and the warm North Atlantic Drift (Aksu et al., 1992), and debris-flow wedges along segments of the continental margin (e.g., Orphan Basin — Aksu and Hiscott, 1992) record times when ice tongues reached the edge of the shelf.

Orphan Basin is located in the southern Labrador Sea, north of the Grand Banks of Newfoundland (Fig. 1). The western slope and rise of the basin are underlain by as much as 1 km of alternating sheet-like acoustically stratified units of hemipelagic sediments (∼1 s two-way travel in seismic profiles), including ice-rafted deposits, and wedge-shaped units formed of debris-flow deposits Aksu and Hiscott, 1992, Hiscott and Aksu, 1996. Hiscott and Aksu (1996) described stratified glaciomarine sediments just beyond the seaward limit of debris-flow wedges, and identified nine Heinrich layers deposited since oxygen-isotopic substage 5e in 11.7-m-long piston core 92045-11P. In an effort to better understand the record of glacial–interglacial climate changes in this strategic area, a 31.45-m-long piston core (MD95-2025) was collected from the research vessel Marion Dufresne II at position 49°47.645′N, 46°41.851′W (260 m from coresite 92045-11P; 2925 m of water). Most 1995 Marion Dufresne II cores were stretched by 50–100% during coring, apparently because of enormous piston-suction in the Calypso corer (Hillaire-Marcel et al., 1999, Section 5.8.2). As a result, 11-m-depth in core 92045-11P correlates with ∼16.5-m-depth in core MD95-2025. This thickness difference is a coring artifact, not a primary feature.

Section snippets

Methods

Immediately after core recovery, P-wave velocity was measured each 2 cm using Ocean Drilling Program procedures. On shore, the split core was described and then systematically sampled each 10 cm. Procedures outlined by Aksu et al. (1992) were followed for textural, foraminiferal, palynological and stable isotopic analyses, and for sea-surface-temperature determinations. Following Heinrich (1988), a split of 200 loose grains in the size range 180–3000 μm was counted using a binocular microscope

Core analysis

The core consists mainly of burrowed, slightly to moderately calcareous and/or dolomitic silty mud (Fig. 2a) with widely scattered plutonic, metamorphic and carbonate pebbles. There are several slightly burrowed intervals of sandy mud with gradational tops, sharp unburrowed bases, and high carbonate contents (H1, H3–H6, H9, H11, H13, Fig. 2c and e). Carbonate abundance is tracked by peak values of P-wave velocity (Fig. 2b and e). Calcite and dolomite abundances are similar (Fig. 2e), except in

Abrupt palaeoclimatic and glaciogenic events

Key palaeoclimatic and palaeoceanographic variables are plotted in a time domain in Fig. 4. The transformation to time is based on four AMS radiocarbon dates (Table 1) and the ages of Imbrie et al. (1984) for boundaries of oxygen-isotopic stages and substages, derived from orbital theory. The stage boundaries are placed, by convention, at the midpoint between adjacent positive and negative peaks in δ18O. Sharp increases or decreases in δ18O at stage boundaries ensure little error in the

Conclusions

Piston core MD95-2025 provides a wealth of new data on short-duration pulses of ice-rafting (Heinrich layers H1–H13), water–mass interactions (SST estimates) and local ice advances on the island of Newfoundland (debris-flow wedges). There are complex and temporally varying relationships between the intensity of ice-rafting (Heinrich variable), global ice volume or meltwater discharge (fine structure of the δ18O curve), and the incursion of warm water from the North Atlantic Drift into the

Acknowledgments

Funding for this study was provided by a Natural Sciences and Engineering Research Council (NSERC) Research Networks Grant in support of the Climate System History and Dynamics (CHSD) and IMAGES projects, and by A-base funding for Geological Survey of Canada-Altantic (GSCA) Project 920063. We thank the captain, crew and technical staff on Marion Dufresne II for acquiring samples and data at sea. Shore-based laboratory assistance was provided by H. Gillespie, P. King, and K. Jarrett. We also

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