Rapid fluctuations of the Laurentide Ice Sheet at the mouth of Hudson Strait: New evidence for ocean/ice‐sheet interactions as a control on the Younger Dryas

Ice‐directional features and erratic lithologies of the last glaciation demonstrate that a major outlet glacier of the Laurentide Ice Sheet flowed NNE across outer Hudson Strait rather than SE down the Strait as previously hypothesized. Ice advanced more than 600 km from a Labradorean source, crossing Hudson Strait and outer Frobisher Bay, and advancing onto Hall Peninsula; the highest summits on Loks Land (circa 400 m asl), a large island at the SE tip of Hall Peninsula, were inundated by Labradorean ice. Three distinct Late Wisconsin advances have been recognized. The earliest, most extensive advance crossed Frobisher about 11.5 kyr B.P. and was maintained for circa 1 kyr. Differences in the extent of amino acid racemization in shells from interstadial and deglacial sites on eastern Loks Land limit the duration of the most extensive Late Wisconsin advance(s) to a brief interval of no more than 2 kyr. Radiocarbon dates on in situ shells and on shells incorporated in till indicate that Frobisher Bay was ice‐free from at least 10.5 to 10.2 kyr B.P. after which a second Labradorean advance, the Gold Cove readvance, attained nearly the same extent as the earlier advance. The readvance was short lived, and the Bay was deglaciated before 9.5 kyr B.P. A final readvance occurred during the Cockburn Substage (9 to 8 kyr B.P.), but ice failed to cross Frobisher Bay. The maximum extent of the Labradorean advance coincides with the onset of Younger Dryas cooling. Based on a minimum cross‐sectional area and reasonable estimates of ice velocity, the iceberg flux to the North Atlantic Ocean at this time was between 300 to 2400 km³ year⁻¹, about the same as the freshwater influx from the diversion of Lake Agassiz drainage to the St. Lawrence. The input of a significant volume of ice into the North Atlantic via the Labrador Sea would have cooled sea surface and air temperatures, increased sea‐surface albedo, and diminished the effectiveness of wind mixing of surface waters, thereby reducing the volume of water required to effectively cap the North Atlantic. We suggest that a combination of a massive iceberg flux and increased St. Lawrence discharge may have been required to initiate Younger Dryas cooling. When the outflow of Lake Agassiz was again routed down the St. Lawrence at 9.5 kyr B.P., a second “Younger Dryas” event failed to occur, because by that time Labradorean ice had retreated within Hudson Strait and was unable to generate a substantial (climate‐altering) volume of icebergs. The possibility of an iceberg flood in the North Atlantic could be tested by measuring the concentration of ice‐rafted detritus (IRD; traditionally defined as quartz/feldspar grains >125 µm) in deep‐sea cores. However, sediment entrained by the Labradorean ice is dominated by detrital carbonate, and only a small fraction of the noncarbonate component is >125 µm, hence a revised definition of IRD is required to evaluate the iceberg‐flood hypothesis.