Although there is no agreement on the nature of glacier-ocean interactions during D-O and Heinrich events, one widely held view maintains that ocean-climate changes were forced by oscillatory internal dynamics of glaciers flowing into the northern North Atlantic. Recurring surges or collapses of those glaciers are thought to have caused repeated and rapid increases in the flux of icebergs (i.e. freshwater) into the ocean. The sudden flow and subsequent melting of the icebergs in convecting regions north and south of Iceland is presumed to have lowered ocean surface salinities enough to force ocean circulation across a threshold, induce a hysteresis behaviour, and abruptly reduce or shut down North Atlantic Deep Water (NADW) formation (e.g. Broecker,
1994; Ganopolski & Rahmstorf, 2001; Rahmstorf, 2002). Results of modelling experiments suggest that an increase in freshwater flux from 0.03 Sv (D-O cycles) to 0.15 Sv (Heinrich events) would have been sufficient to alter deep circulation and lower temperatures in the northern North Atlantic and surrounding regions by a few to several degrees C (Ganopolski & Rahmstorf, 2001; Weaver et al., 2001). The modelled temperatures are broadly consistent with those of palaeoclimate records from the glacial North Atlantic (Cacho et al., 1999; van Kreveld et al., 2000; Landis et al., 2004).
The key pieces of evidence that formed the basis for the glacial instability concept came from North Atlantic deep-sea records of IRD concentrations and surface salinities. Marked increases in IRD concentrations accompany the cold phases of each D-O cycle and even larger increases in IRD coincide with Heinrich events (Fig. 24.2). Petrological and geochemical tracers in IRD indicate that Heinrich icebergs were produced mainly by massive collapses of Laurentide ice in the Hudson Bay region (Grousset et al., 2001; Hemming, 2004; Figs 24.2 & 24.3). For the D-O cycles an Icelandic source is well documented and geochemical fingerprinting may implicate sources in Europe as well.
Large reductions in surface salinity during the IRD maxima in both series are suggested by planktonic and dynocyst census counts, measurements of planktonic Mg/Ca ratios, and measurements of planktonic oxygen isotopes. Although estimating salinity anomalies is notoriously difficult, values of up to -4 per mil have been suggested for Heinrich events and -1 to -2 per mil for cold phases of D-O cycles (Cayre et al., 1999; de Vernal & HillaireMarcel, 2000, van Kreveld et al., 2000). Based on results of modelling experiments, those values would have been sufficient to reduce or shut down NADW formation (e.g. Ganopolanski & Rahmstorf, 2001).
To explain the oscillatory dynamics required by the glacier instability concept, MacAyeal (1993a) proposed a free oscillation mechanism that has come to be known as the binge-purge model. A large glacier or ice sheet frozen to bedrock will slowly build up during the binge phase. The purge phase (i.e. the Heinrich event)
occurs when geothermal heat melts the basal sediment and produces a lubricated discharge pathway. The model produced massive collapses of the ice every 7000 yr, agreeing reasonably well with the observed timing of Heinrich events. In a more elaborate development of the model, Greve & MacAyeal (1996) found that free oscillations from 1000 to 10,000 yr could occur within a large ice sheet, thereby potentially providing an explanation for both Heinrich events and the faster-paced D-O cycles.
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