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Heinrich events, first described by marine geologist Hartmut Heinrich, occurred during the last glacial period, or "ice age". During such events, armadas of icebergs broke off from glaciers and traversed the North Atlantic. The icebergs contained rock mass eroded by the glaciers, and as they melted, this matter was dropped onto the sea floor as "ice rafted debris". Scientists drilling through marine sediments can distinguish six distinct events in cores of mud retrieved from the sea floor, which are labelled H1-H6 going back in time; there is some evidence that H3 and H6 differ from other events.
The icebergs' melting caused prodigious amounts of fresh water to be added to the North Atlantic. Such inputs of cold, 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 the cause of Heinrich events. Most centre around the activity of the Laurentide ice sheet, but others suggest that the unstable West Antarctic Ice Sheet played a triggering role.
|Hemming (2004)||Bond & Lotti (1995)||Vidal et al.. (1999)|
|H1,2 are dated by radiocarbon; H3-6 by correlation to GISP2.|
Heinrich events are global climate fluctuations which coincide with the destruction of northern hemisphere ice shelves, and the consequent release of a prodigious volume of sea ice and icebergs. The events are rapid: they last around 750 years, and their abrupt onset may occur in mere years (Maslin et al.. 2001). Heinrich events are observed during the last glacial period; the low resolution of the sedimentary record before this point makes it impossible to deduce whether they occurred during other glacial periods in the Earth's history.
Heinrich events occur during some, but not all, of the periodic cold spells preceding the rapid warming events known as Dansgaard-Oeschger (D-O) events, which repeat around every 1,500 years. However, difficulties in establishing exact dates cast aspersions on the accuracy—or indeed the veracity—of this statement. Some (Broecker 1994, Bond & Lotti 1995) identify the Younger Dryas event as a Heinrich event, which would make it H0.
Heinrich's original observations were of six layers in ocean sediment cores with extremely high proportions of rocks of continental origin, "lithic fragments", in the 180 μm to 3 mm size range (Heinrich 1988). The larger size fractions cannot be transported by ocean currents, and are thus interpreted as having been carried by icebergs or sea ice which broke off from the large Laurentide ice sheet then covering North America, and dumped on the sea floor as the icebergs melted. The signature of the events in sediment cores varies considerably with distance from the source region—there is a belt of ice rafted debris (sometimes abbreviated to "IRD") at around 50° N, expanding some 3,000 km (1,865 mi) 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.
During Heinrich events, huge volumes of fresh water flow into the ocean. For Heinrich event 4, the fresh water flux has been estimated to 0.29±0.05 Sverdrup with a duration of 250±150 years (Roche et al., 2004), equivalent to a fresh water volume of about 2.3 million km³. Several geological indicators fluctuate approximately in time with these 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 these records illustrates the dramatic impact of Heinrich events.
H3 and H6 do not share such a convincing suite of Heinrich event symptoms as events H1, H2, H4, and H5. This has led some researchers to suggest that they are not true Heinrich events, which would make Bond's suggestion of Heinrich events fitting into a 7,000-year cycle suspect. Several lines of evidence do suggest that H3 and H6 were somehow different from the other events.
As with so many climate related issues, the system is far too complex to be confidently assigned to a single cause. There are several possible drivers, which fall into two categories.
This model suggests that factors internal to ice sheets cause the periodic disintegration of major ice volumes, responsible for Heinrich events.
The gradual accumulation of ice on the Laurentide ice sheet led to a gradual increase in its mass — 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 — the "purge phase", lasting around 750 years. The original model (MacAyeal, 1993) proposed that geothermal heat caused the sub-glacial 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 (Sarnthein et al.. 2001). 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 also appear suspect because similar events are not observed in other ice ages (Hemming 2004), 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.
Several factors external to ice sheets may cause Heinrich events, but such factors would have to be large to overcome attenuation by the huge volumes of ice involved (MacAyeal 1993).
Gerard Bond suggests that changes in the flux of solar energy on a 1,500-year scale may be correlated to the Daansgard-Oeschger cycles, and in turn the Heinrich events; however the small magnitude of the change in energy makes such an exo-terrestrial 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, it is possible that 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 — further destabilizing other ice sheets. In favour of this theory is the non-simultaneity of ice sheet break up in H1, 2, 4, and 5, where European breakup preceded European melting by up to 1,500 years (Maslin et al. 2001).
The Atlantic Heat Piracy model suggests that changes in oceanic circulation cause one hemisphere's oceans to become warmer at the other's expense (Seidov and Maslin 2001). 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 (Stocker 1998).
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 (Ganopolski and Rahmstorf 2001), 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 — a 0.15 Sv increase, or 0.03 Sv decrease — would suffice to cause profound shifts in global circulation (Rahmstorf et al. 2005). 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 paleoclimatic 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 — causing 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 — 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.