Kathy Tedesco

Detrital carbonate-rich sediments, northwestern Labrador Sea: Implications for ice-sheet dynamics and iceberg rafting (Heinrich) events in the North Atlantic

Much of the bed of the central and eastern sectors of the Laurentide Ice Sheet was underlain by Paleozoic carbonates. We propose that pulses of detrital carbonate-rich sediments in two cores from the northwestern Labrador Sea reflect episodes when an ice stream from the Hudson Strait extended to the shelf break and delivered sediment onto the slope and deep-sea plain. Atomic mass spectroscopy 14 C dating of planktonic foraminifera in cores HU75-009-IV-55 (2.4 km water depth) and HU87-033-009 (1.4 km water depth and 150 km north-northwest), indicates that the youngest event occurred between ca.14 and 15 ka and another occurred between 19 and 21 ka. The two carbonate intervals in northwestern Labrador Sea cores are coeval with Heinrich events 1 and 2 in the eastern North Atlantic (lat 45°-50°N, long 20°W), where they are associated with an increase in lithic fragments and a drastic reduction in the numbers of foraminifera. These changes have been linked with the massive production of icebergs associated with ice-sheet surges. Our evidence indicates that Heinrich events 1 and 2 are associated with the dynamics of the Hudson Strait ice stream and denote considerable glaciological instability.

Shelf erosion and glacial ice proximity in the Labrador Sea during and after Heinrich events (H-3 or 4 to H-0) as shown by foraminifera

Abstract Collapses of the marine-based Laurentide Ice Sheet during the last glaciation are documented by large, abrupt influxes of detrital carbonate into the northwest Labrador Sea, known as detrital carbonate or Heinrich events. The extent of grounded ice on the continental shelf is poorly constrained for these events, but has been assumed to be at the shelf edge. Possible grounding line positions of ice streams in Hudson Strait and Cumberland Sound over the last c. 34 ka are delimited using the abundance of shelf-dwelling benthic foraminifera, sediment colour and calcite to dolomite ratios in two Labrador Sea cores (87033-009 LCF and 75-009-IV-55 PC). Heinrich events 1 and 2 contain no evidence of glacial erosion of the continental shelf, suggesting that the Hudson Strait ice stream may have grounded on the Hudson Strait sill or on the inner shelf rather than at the shelf edge during these events. Dark grey, dolomite-rich sediments with abundant shelf-dwelling foraminifera document an advance of Cumberland Sound ice onto the shelf after H-3 or H-4 (c. 34 ka), but before H-2 (c. 21 ka). Cumberland Sound ice readvanced to a position at or near the shelf edge at c. 11.1 ka, during the Younger Dryas chron. This advance is nearly synchronous with an advance of Labradorean ice on the southeastern Baffin Shelf and with a detrital carbonate event in Sunneshine Fjord, to the north, suggesting climatic forcing of separate ice dispersal centres during the Younger Dryas.

HEINRICH EVENTS: CHRONOLOGY AND PROCESSES, EAST-CENTRAL LAURENTIDE ICE SHEET AND NW LABRADOR SEA

Over the last two decades, studies of deep-ocean sediment records have clearly demonstrated changes in the global climate on Milankovitch time-scales (see Imbrie, this volume). Research over the last few years shows that the climate system also changes at much higher frequencies. Examples of rapid climate changes include the Younger Dryas cold period of NW Europe (Mangerud, 1987; Rind, et al., 1986; Wright, 1989), the “Dansgaard-Oeschger” oscillations in the Greenland ice cores (Broecker, et al., 1988; Dansgaard, et al., 1989; Johnsen, et al., 1992), and the“Heinrich” layers in North Atlantic sediment cores (Heinrich, 1988; Broecker, et al., 1992; Andrews and Tedesco, in the press). The latter two types of changes have durations of circa 1 ± ka with recurrence intervals of a few thousand years. All of these phenomena have been associated with the sensitivity of the thermohaline circulation to influx of fresh water and ice (Broecker and Denton, 1989; Broecker, et al., 1989; Stocker and Wright, 1991).