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A Heinrich event is a natural phenomenon in which large groups of icebergs break off from the Laurentide ice sheet and traverse the Hudson Strait into the North Atlantic. First described by the marine geologist Hartmut Heinrich, they occurred during five of the last seven glacial periods over the past 640,000 years. Heinrich events are particularly well documented for the Wisconsin glaciation, during the Last Glacial Period, but notably absent from the Penultimate Glacial Period. The icebergs contained rock mass that had been eroded by the glaciers, and as they melted, the material was dropped to the sea floor as ice rafted debris and formed deposits called Heinrich layers.
The icebergs' melting caused vast quantities of fresh water to be added to the North Atlantic. Such inputs of cold and fresh water may well have altered the density-driven, thermohaline circulation patterns of the ocean, and often coincide with indications of global climate fluctuations.
Various mechanisms have been proposed to explain Heinrich events, most of which imply instability of the massive Laurentide Ice Sheet, a continental ice sheet covering most of northeastern North America during the Last Glacial Period. Other Northern Hemisphere ice sheets were potentially involved as well, such as the Fennoscandic and Iceland/Greenland. However, the initial cause of the instability is still debated.
Heinrich events appear related to some but not all of the cold periods preceding the rapid warming events known as Dansgaard–Oeschger events, which are best recorded in the North Greenland Ice Core Project. However, difficulties in synchronising marine sediment cores and Greenland ice cores to the same time scale have raised questions as to the accuracy of that statement.
For events of Laurentide origin, there is a belt of IRD at around 50° N, known as the Ruddiman belt, expanding some from its North American source towards Europe, and thinning by an order of magnitude from the Labrador Sea to the European end of the present iceberg route (Grousset et al., 1993). During Heinrich events, huge volumes of fresh water flow into the ocean. For Heinrich event 4, based on a model study reproducing the isotopic anomaly of oceanic oxygen-18, the fresh water flux has been estimated to 0.29±0.05 Sverdrup with a duration of 250±150 years, equivalent to a fresh water volume of about , or a sea-level rise.
Several geological indicators fluctuate approximately in time with those Heinrich events, but difficulties in precise dating and correlation make it difficult to tell whether the indicators precede or lag Heinrich events or, in some cases, whether they are related at all. Heinrich events are often marked by the following changes:
The global extent of those records illustrates the dramatic impact of Heinrich events.
Several lines of evidence suggest that H3 and H6 were somehow different from the other events.
The gradual accumulation of ice on the Laurentide Ice Sheet led to a gradual increase in its mass, as the "binge phase". Once the sheet reached a critical mass, the soft, unconsolidated sub-glacial sediment formed a "slippery lubricant" over which the ice sheet slid, in the "purge phase", lasting around 750 years. The original model proposed that geothermal heat caused the subglacial sediment to thaw once the ice volume was large enough to prevent the escape of heat into the atmosphere.
The mathematics of the system are consistent with a 7,000-year periodicity, similar to that observed if H3 and H6 are indeed Heinrich events. However, if H3 and H6 are not Heinrich events, the binge-purge model loses credibility, as the predicted periodicity is key to its assumptions.
It may appear suspect also that similar events are not observed in other ice ages, although this may be due to the lack of high-resolution sediments.
In addition, the model predicts that the reduced size of ice sheets during the Pleistocene should reduce the size, impact, and frequency of Heinrich events, which is not reflected by the evidence.
Gerard C. Bond suggests that changes in the flux of solar energy on a 1,500-year scale may be correlated to the Dansgaard-Oeschger cycles and in turn the Heinrich events, but the small magnitude of the change in energy makes such an extraterrestrial factor unlikely to have the required large effects, at least without huge positive feedback processes acting within the Earth system. However, rather than the warming itself melting the ice, sea-level change associated with the warming destabilised ice shelves. A rise in sea level could begin to corrode the bottom of an ice sheet, undercutting it; when one ice sheet failed and surged, the ice released would further raise sea levels, and further destabilizing other ice sheets. In favour of this theory is the non-simultaneity of ice sheet break-up in H1, H2, H4, and H5, where European breakup preceded European melting by up to 1,500 years. The Atlantic Heat Piracy model suggests that changes in oceanic circulation cause one hemisphere's oceans to become warmer at the other's expense. Currently, the Gulf Stream redirects warm, equatorial waters towards the northern Nordic Seas. The addition of fresh water to northern oceans may reduce the strength of the Gulf stream and allow a southwards current to develop instead. This would cause the cooling of the northern hemisphere, and the warming of the southern, causing changes in ice accumulation and melting rates and possibly triggering shelf destruction and Heinrich events.
Rohling's 2004 Bipolar model suggests that sea level rise lifted buoyant ice shelves, causing their destabilisation and destruction. Without a floating ice shelf to support them, continental ice sheets would flow out towards the oceans and disintegrate into icebergs and sea ice.
Freshwater addition has been implicated by coupled ocean and atmosphere climate modeling, showing that both Heinrich and Dansgaard–Oeschger events may show hysteresis behaviour. This means that relatively minor changes in freshwater loading into the Nordic Seas, such as a 0.15 Sverdrup increase or 0.03 Sv decrease, would suffice to cause profound shifts in global circulation. The results show that a Heinrich event does not cause a cooling around Greenland but further south, mostly in the subtropical Atlantic, a finding supported by most available paleoclimate data. This idea was connected to D-O events by Maslin et al. (2001). They suggested that each ice sheet had its own conditions of stability, but that on melting, the influx of freshwater was enough to reconfigure ocean currents, and cause melting elsewhere. More specifically, D-O cold events, and their associated influx of meltwater, reduce the strength of the North Atlantic Deep Water current (NADW), weakening the northern-hemisphere circulation and therefore resulting in an increased transfer of heat polewards in the southern hemisphere. This warmer water results in melting of Antarctic ice, thereby reducing density stratification and the strength of the Antarctic Bottom Water current (AABW). This allows the NADW to return to its previous strength, driving northern hemisphere melting and another D-O cold event. Eventually, the accumulation of melting reaches a threshold, whereby it raises sea level enough to undercut the Laurentide Ice Sheet, thereby causing a Heinrich event and resetting the cycle.
Hunt & Malin (1998) proposed that Heinrich events are caused by earthquakes triggered near the ice margin by rapid deglaciation.Hunt, A.G. and P.E. Malin. 1998. The possible triggering of Heinrich Events by iceload-induced earthquakes. Nature 393: 155–158
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