Australia: The Land Where Time Began
Heinrich Events, Massive Detritus Layers from the Late Pleistocene in the North Atlantic -Their Global Climate Imprint
Heinrich Events of the North Atlantic, extreme events that interrupt climate oscillations on a millennial scale, of the glacial interval (1). According to the author1 the near-global footprint of Heinrich Events is testament to the coherent interactions among the atmosphere, oceans and cryosphere on millennial timescales. The region around Hudson Strait appears to have been the source for the Heinrich detritus, having been deposited over a period of about 500 ± 250 years. The mechanisms that have been proposed to have formed the layers include binge-purge cycle of the Laurentide ice sheet, jökulhlaup activity from a Hudson Bay lake and the buildup/collapse of an ice shelf that was fed by Hudson Strait. The author1 suggests several ways of determining Heinrich Event origins - further studies of the timing and duration of these events, further sedimentology study near Hudson Strait and study of the layers and their precursory intervals at greater spatial and temporal resolution, and important constraints may also be provided by studies of previous glacial intervals.
Heinrich Events have been documented in the North Atlantic as anomalous occurrences of ice-rafted detritus (IRD) and are considered to be an important discovery. It has been pointed out that "there is a striking coincidence" between Heinrich Event timing and the climate fluctuations pattern that have been documented from ice cores (Bond et al., 1993; Broecker, 1994). There is good evidence for a footprint that covers the Northern Hemisphere, and it possibly may be global (e.g. Broecker, 1994). Of the studies that were carried out outside the North Atlantic ice-rafting zone most have concluded that there are a number of other events that correlate with Heinrich Events, wind changes: trade winds in the tropics that are stronger (e.g. Arz et al., 1998; see also McIntyre & Molfino, 1996), Stronger winter monsoon winds in China (e.g. Porter & An, 1995; Wang et al., 2001) and the Arabian Sea (e.g. Schulz et al., 1998), and in the western Mediterranean, stronger northerly winds (e.g. Cacho et al., 1999). Evidence has also been found of increased ventilation in the northeast Pacific at intervals that are approximately coincident with Heinrich Events. Compared to the ambient glacial conditions there appears to be a general pattern indicating a tendency for wetter (milder?) conditions along the western margin of the Atlantic during Heinrich Events (e.g. Grimm et al., 1993) and possibly along the eastern margin of the South Atlantic (Little et al., 1997; Kanfoush et al., 2000). On the eastern margin of the North Atlantic and the western Mediterranean (e.g. Cacho et al., 1999; bard et al., 2000). The author1 suggests important clues to the forces driving these abrupt changes of climate may be provided by comparing the pattern of differences in Heinrich Events to ambient glacial, as wellas the distribution of sites sensitive to Heinrich Events compared to Dansgaard-Oeschger (D-O) events.
Layers with lithic fragment percentages that are extremely high, almost 100 %, have been documented by Heinrich by taking the ratio of lithic grains to total entities in the 180 μm to 3 mm (sand is 63 μm to 2 mm) sediment fractions of eastern North Atlantic sediments. Correlative horizon of high lithic percentages or high magnetic susceptibility in a band that coincides approximately with the North Atlantic Current and the ice-rafted detritus (IRD) belt (Ruddiman, 1977). During the last glacial period the layers are generally considered to fall within 6 brief time intervals, labeled "H1" to "H6" from youngest to oldest Bond et al., 1992; Broecker et al., 1992).
The abundance of detrital carbonate (Bond et al., 1992; Broecker et al., 1992) in the Heinrich layers in the North Atlantic make them anomalous. Within intervals that are approximately correlative high concentrations of detrital carbonate have been suggested by parallel observations the Heinrich layers formed by "armadas of icebergs" that were launched from the Hudson Strait (Broecker et al., 1992). The author1 suggests that a likely location for an ice stream that was capable of draining the eastern section of the Laurentide ice sheet is the Hudson Strait that is a major trough. A "binge-purge" mechanism has been proposed (MacAyeal, 1993) which could operate independent of climate change, to account for the large numbers of icebergs, also proposing that the carbonates from the Palaeozoic that line Hudson Bay and Hudson Strait provided a substrate that is relatively soft, that significantly helped foster an ice stream. Other, more recent models, have been proposed, though the binge-purge cycle model remains attractive. It has been pointed out that there is no doubt that glaciological processes are required to account for the enormous amounts of ice discharged during Heinrich Events H1, H2, H4 and H5, that probably involved massive ice surging or collapse in the Hudson Strait ( Bond et al., 1999). They also emphasise that it appears temporal patterns require a climate trigger.
In this paper the author1 reviews the research reported of the many papers on Heinrich layers (events) as a result of the North Atlantic and Labrador Sea correlatives (Andrews & Tedesco, 1992; Bond et al., 1992; Broecker et al., 1992).
Ice-rafted detritus (IRD) is sediment that was deposited on the sea floor when the enclosing ice melted, having been entrained in floating ice, icebergs or sea ice. There is much debate about the quantification of sediment in marine cores. The percentage of IRD - the percentage of mineral or rock fragments compared to all entities, material that is not mineral or rock fragments is almost all foraminifera (Heinrich, 1988). The 180 μm to 3 mm fraction were counted by Heinrich, most subsequent studies have counted from the >150 μm fraction, with some from the Nordic Seas counting from a much larger size fraction. The lithic portion is another IRD indicator, measured by fraction of the coarse fraction of a sample.
When choosing appropriate grain-size intervals in the study of IRD there are 3 complications. In marine sediments the coarse fraction is generally small, and it has been emphasised (Andrews, 2000) that the 63 μm fraction, not the >63 μm fraction, is much more abundant and representative in glacial deposits on land and near marine outlets. In Heinrich layers H1, H2, H4 and H5 in core V28-82 there are about 5,000-6,000 lithic grains (>150 μm)/gram dry bulk sediment, and only about 3,000 in H3 and H6. The grain-size distribution is profoundly influenced by the provenance of sediments eroded by glaciers; therefore different grain-size ranges should arguably be examined to determine different fundamental rock-type contributions. The ultimate grain-size is strongly controlled by the distance the sediment has been transported under the glacier.
As a result of these complications the coarse-grained fractions are inevitably biased toward rocks near the margin of the ocean and towards rocks that tend to fragment into grains of the examined size range. An example is that shale would be underrepresented in the fraction that is >63 μm fraction. In the open ocean a fraction must be used that is coarse enough to not be transported by any mechanism other than ice-rafting, recognising the complexity of glacial sediment that is inevitable (e.g. Dreimanis, 1976; Dowdeswell & Dowdeswell, 1989; Dowdeswell & Murray, 1990; Dowdeswell et al., 1998; Andrews, 1998, 2000).
Grain-size distribution can also be modified by depositional and bottom current processes in the ocean. There is a very large sortable silt fraction in drift deposits (e.g. McCave, 1995), and scouring bottom currents must lead to the quantifiable loss of the fine fraction at some depositional sites. In the eastern North Atlantic core V28-82 is not a on a drift, appearing to be a case in which flux from above matches the accumulation based on 230Thoriumexcess analysis (McManus et al., 1998). As a result it is a good reference core for deep ocean IRD characteristics in the IRD belt. Unlike core V28-82 core V23-14 in the western North Atlantic Heinrich layers are at minima in >63 μm fraction and number of lithic grains per gram. There is a very low sedimentation rate between Heinrich layers of about 3 cm/kyr in core 23-14, the author1 suggesting it is likely the fine fraction is being removed from the site by bottom currents. A place where bottom current processes are adding extra fine fraction is the Feni Drift in the eastern North Atlantic, as indicated by core V23-81, which has a sedimentation rate of about 12 cm/ky, and Heinrich layers are strong maxima in the numbers of lithic grains/gram, though the maximum is only about 1,500 grains/gram.
See Source 1 for more about Heinrich Events.
The author1 describes Heinrich layers as spectacular IRD deposits in the North Atlantic resulting from massive discharges of icebergs through the Hudson Strait from the LaurentideIce Sheet. They have been linked to dramatic shifts of the climate in the Northern Hemisphere, and the author1 suggests they could have correlations around the world. It has been demonstrated by detailed studies that there is a strong connection between Heinrich layer timing and the rate of climate variability in the North Atlantic. The author1 suggests more work needs to be carried out to evaluate the proposal (Denton et al., 1999) that an interhemispheric correlation exists at times of Heinrich Events, also suggesting a good way to start would be with the Antarctic Circumpolar Transect.
Predictions coming from models that have been proposed should be considered in order to resolve the details of the formation of Heinrich layers, i.e. Laurentide Ice Sheet purging through the ice stream in the Hudson Strait, jökulhlaup, and ice shelf. He also suggests important clues about the glacial processes that were operating could be uncovered by careful assessment of the sedimentology of the Labrador Sea near Hudson Strait. More high resolution work is needed across the intervals of Heinrich Events in the IRD belt, including their "precursors", and further constraints on the composition of sources that potentially contribute.
A picture is emerging as a result of a great deal of research into Heinrich Event characterisation in the North Atlantic, the identification of climate events globally that are potentially correlative. It has been found that they occurred in periods of extreme cold in the North Atlantic, and dramatic warming probably followed them, the onset of the warm period being abrupt. In the North Atlantic a large influx of fresh water accompanied these events, apparently a complete shutdown of production of NADW resulted. Whether the correlations are global or restricted to the Northern Hemisphere, and the ultimate driver of these events, remains to be elucidated. The existence of the prominent 100,000 year cycle, and its apparent pacing with northern latitudes insolation appears to need a magnifier such as the ice sheets of the Northern Hemisphere and the impact they have on such things as thermohaline and atmospheric circulation, albedo, greenhouse gases, etc. The author1 suggests that it may be concluded that the presence of very large ice sheets in the Northern Hemisphere might be related to the large, rapid changes of climate that occurred in the last glacial period.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|