Showing papers by "Sidney R. Hemming published in 1996"

Provenance of icebergs during Heinrich Event 3 and the contrast to their sources during other Heinrich episodes

Abstract: The sources of ice-rafted debris (IRD) from Heinrich layer 3 (H 3) in the North Atlantic were studied by means of Pb isotopic analyses of single- and multiple-grain samples of ice-rafted feldspars from core V28-82 (49° 27′N, 22° 16′W). Pb ratios of H 3 grains point to multiple continental sources and are similar to the isotopic composition of the background sediment. This similarity, as well as previous observations about the age and the Sr and Nd isotopic composition of the fine fraction of H 3 in cores of the same area, are all consistent with the hypothesis that in the eastern North Atlantic along the latitude band of maximum IRD deposition (40°–50°N), H 3 is a product of foraminifera dissolution and/or low productivity. In contrast, analyses of composite samples of H 1, H 2, H 4, and H 5 plot in 206Pb/204Pb versus 206Pb/204Pb coordinates along the Churchill province line, supporting the premise that the Hudson Strait was the major pathway of iceberg drainage during these four events. Since an advance of the Laurentide ice sheet during H 3 is demonstrated by increases in detrital carbonate at the level of H 3 in cores from the western North Atlantic, the results obtained here suggest that if indeed that advance took place, the iceberg discharge melted mostly in the western basin.

Tracking the sources of icebergs with lead isotopes: The provenance of ice‐rafted debris in Heinrich layer 2

Abstract: The provenance of ice-rafted debris (IRD) deposited in the North Atlantic before, during, and after Heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple-grain composites from the larger than 150-µm size fraction, from cores from the eastern and western North Atlantic and from the Labrador Sea. Single-grain analyses are used to identify the specific continental sources of the IRD, whereas composite samples are used to assess the relative IRD contributions from different sources. All single grains from Heinrich layer 2 (H 2) as well as H 2 composites plot along a correlation line on a 207Pb/204Pb versus 206Pb/204Pb diagram characteristic of the Churchill province of the Canadian shield. This is yet another strong piece of evidence that this Heinrich event was dominated by a massive iceberg discharge of the Laurentide ice sheet lobe located over Hudson Bay. In contrast, single grains from the ambient glacial sediment (above and below H 2) have multiple sources: many of them also lie along the correlation line with H 2 grains, but many others have Pb signatures consistent with derivation from the Grenville province and the Appalachian range in North America and possibly from Scandinavia and Greenland. Composites from the ambient sediment generally lie well to the right of the H 2 reference line in agreement with the results of the single-grain analyses. The evidence provided by lead isotopes regarding the dominant role played by the Hudson Bay lobe of the Laurentide ice sheet in the development of the Heinrich events lends support to the binge/purge model advanced by MacAyeal [1993a, b] that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the Heinrich events.

Evidence from 40Ar/39Ar Ages for a Churchill province source of ice-rafted amphiboles in Heinrich layer 2

Abstract: 40Ar/ 39 Ar ages of most single ice-rafted amphiboles from Heinrich layer 2 (H2) from a core in the Labrador Sea, a core in the eastern North Atlantic and a core in the western North Atlantic range from 1600 to 2000 Ma. This range is identical to that for K/Ar ages from the Churchill province of the Canadian Shield that outcrops at Hudson Strait and forms the basement of the northern part of Hudson Bay. The ambient glacial sediment includes some younger and older grains derived from Paleozoic, Mesoproterozoic and Archean sources, but still the majority of the amphiboles have ages in the 1600-2000 Ma interval. The Ca/K ratios of these 1600-2000 Ma old amphiboles, however, have a bimodal distribution in contrast with the uniformity of the Ca/K ratios of H2 amphiboles. This indicates that 1600-2000 Ma old amphiboles of the ambient sediment were derived from an additional Early Proterozoic source besides Churchill province. In H2, Churchill-derived grains constitute 20-40% of the ice-rafted debris (IRD). The fraction in the ambient glacial sediment is 65-80%. Results presented here are consistent with the hypothesis that Heinrich events were produced by a sudden intensification of the iceberg discharge through Hudson Strait that mixed, in the North Atlantic, with icebergs that continued to calve from other ice sheets. The shift from mixed sources in the background sediment to a large dominance of Churchill province grains in H2 indicates that, even if calving of other ice sheets intensified during the Heinrich episode, the increase in the iceberg discharge via Hudson Strait from the Hudson Bay drainage basin of the Laurentide ice sheet was by far the largest.