Heinrich events describe periods of accelerated ice advances (English Ice flow surges ) and their outflow into the sea. The events have been postulated due to continental observations of increased sediment input origin in upper pleistocene sediment layers on the sea floor. These sedimentary layers are referred to as Heinrich layers or IRD ( ice rafted debris abbreviation or ice rafted deposit). Due to the coarse sediment fraction transport by ocean currents seems unlikely, likely as a transport medium, therefore, rather icebergs / floes come into question.
Heinrich events were first mentioned in 1988 by Hartmut Heinrich and so far only demonstrated for the last glacial period. These events broke off large amounts of icebergs by the advancing glaciers and drifted across the North Atlantic. The icebergs led sediment with it, had been removed and incorporated by the glacier activity; by the melting of icebergs fell this displaced material of icebergs on the ocean floor.
The melting of icebergs led to an increased inflow of fresh water into the North Atlantic. This influx of cold fresh water has probably changed the density -driven thermohaline circulation pattern of the ocean. The events often fall together with references to global climate fluctuations.
In cores that come from these areas of the ocean floor, scientists can recognize six individual events; they are called H1 to H6 referred, where H6 is the oldest event. There are indications that the events differ H3 and H6 of the other events.
There have been proposed several mechanisms to explain the causes of Heinrich events. Most of the Laurentide ice sheet in this case plays the main role, however, suggest other references to the West Antarctic ice sheet unstable way, is said to have played a causative role.
Heinrich events run by paläoklimatologischen scales very quickly: They last about 750 years and may use within a few years. The events were observed only from the last glacial period; the low temporal resolution of the sediment before this time makes it impossible to determine whether they occurred during other phases of glaciation in Earth history.
Heinrich events found for some, but not all cold spells instead, which preceded the rapid warming phases, which are known as Dansgaard - Oeschger events and repeated approximately every 1500 years. The problems to determine the exact times, have led to discussions about whether it is in fact also to Heinrich events in all cases. Some authors ( Broecker, Bond & Lotti 1995) see the Younger Dryas as a Heinrich event, then the event which H0 would be.
Diagnosis of Heinrich events
Henry's original observations were six layers in sediment cores of the ocean with an extremely high proportion of material of continental origin, namely mine neous fragments with a particle size of 180 microns to 3 mm. The coarse sediment fraction could not have been transported by ocean currents. Therefore, it was assumed that they had been incorporated by icebergs or sea ice, which was canceled by the large Laurentide ice shelf and after the melting fell to the ocean floor. The Laurentide ice sheet covered much of North America at this time. The traces of these events in sediment cores depends significantly on the distance to the region of origin; a ring on iceberg sediments (IRD ) lines the 50th parallel. It extends 3000 km from its North American origins to Europe and its thickness is reduced on its way from the Labrador Sea to the end point of the iceberg route in Europe an order of magnitude.
During Heinrich events flowed huge amounts of fresh water into the ocean. It is estimated that during Heinrich event H4 0:29 ± 0.05 Sverdrup for a period were entered by 250 ± 150 years, which corresponds to a volume of 2.3 million km ³. Some geological time indicators seem to correlate with Heinrich events, but the impossibility of an exact date does not allow it to classify time before or after the respective event. In some cases it is even difficult to judge whether they are at all with the Heinrich events in a causal context. Heinrich events are usually characterized by the following changes:
- Increasing δ18O content in the northern seas and in East Asian stalactites that suggest so climatological proxy data a decrease in the global average temperature ( or an increase in the volume of ice )
- The influx of freshwater -related, decreasing salinity of the ocean
- Evidence of decreasing temperatures of the near-surface seawater west of the African coast, detected by alkenone biomarkers
- Changes in the sediment mixing ( bioturbation ), which is caused by grave animals
- Shifts in the isotopic composition of plankton ( changes in the δ13C - share, decreasing δ18O )
- Pollen have cold-loving pine groves, replace the oak trees on the North American mainland
- Decreasing diversity of foraminifera; due to the integrity of many samples, this can not be explained by measurement error; the decrease is therefore attributed to the reduced salinity
- Increased terrigenous runoff on the continents, which can be detected near the mouth of the Amazon
- Grain size increase of the transported by winds loess in China, suggesting stronger winds
- Changes in the relative frequency of 230Th, suggest changes in the speed of oceanic currents
- Increased sedimentation in the Northern Atlantic, which is manifested in the rise of rock fragments of continental origin compared to Hintergrundsedimentation.
The global detectability of these tracks shows the dramatic effect of the Heinrich events.
Unusual Heinrich events
H3 and H6 do not show convincing fullness of the typical symptoms for Heinrich events, as is the case with H1, H2, H4 and H5. This has led some researchers to believe that it was this is not real Heinrich events, which Bonds theory, Heinrich events would fall into a 7000 - year cycle, would appear to be wrong. Some causal chains, however, suggest that H3 and H6 differ from other events in certain respects.
- Extreme values in the rock portion: A far smaller proportion of rock fragments ( 3,000 grains per gram) are found in H3 and H6. Which, compared with 6000 grains per gram usually means that the continents have played a minor role as a source of ocean sediments during these two events.
- Resolution of foraminifera: Foramaniferen case seem to have been eroded more during H3 and H6 to be ( Gwiazda et al, 1996). This can be an influx of nutrient rich - have been caused and therefore corrosive Antarctic Bottom Water, which could be due to a reorganization of oceanic circulation patterns in turn.
- Origin of the ice: Icebergs in H1, H2, H4 and H5 appear to have drifted along the Hudson Strait, H3 and H6 icebergs contrast across it.
- Distribution of the incorporated by ice coarse fraction: the glacial sediments extend while H3 and H6 events not nearly as far east as those of the other events. Therefore, some researchers have begun to suspect that at least some of H3 and H6 sediments European origin.
As with many other problems in climatology also here the system is too complex to safely identify a cause can by far. There are several possible drives that fall into two categories.
Internal drives - the " binge - purge " model
In this model it is assumed that internal factors in ice sheets, the periodic disintegration of large ice volume cause that is the trigger for Heinrich events.
The gradual accumulation of ice on the Laurentide ice sheet, the binge phase, led to a gradual increase in its mass. Once the plate had reached a critical applied load, the soft, loose sediment was transformed beneath the glacier by the pressure in a smooth lubricant over which the ice sheet could slide off. The purge phase ( emptying phase ) lasted about 750 years. In the original model, it was assumed that geothermal heat put the thawing of glaciers located below the sediment layer in motion as soon as the ice volume was large enough to prevent the escape of the earth's heat in the atmosphere. Mathematical modeling of such a system are consistent with a 7000 - year period - seen when H3 and H6 are actually regarded as Heinrich events. However, if H3 and H6 no Heinrich events, thus losing the binge - purge model of credibility, since the predicted periodicity for its assumptions plays a key role.
The fact that Heinrich events are not observed during previous ice ages, makes the theory seem somewhat unlikely, although this can also be attributed to the lack of high-resolution sediments. In addition, the model predicts that the decline during the Pleistocene glaciations size of the ice sheets should have an influence on size, strength and frequency of the Heinrich events, but this is so not to be found.
Several factors that are causally not allocated to the ice sheets, may also have caused Heinrich events, but their influences would have to be large enough to overcome the attenuation through the large Eisvolumina can.
Gerard Bond suspects that a periodically every 1500 years recurring change in the energy flux of the sun was correlated with the Dansgaard -Oeschger events and thus also with the Heinrich events; However, the relatively small change in energy makes it unlikely that such an extraterrestrial factor has the necessary power, at least not without great positive feedback processes within the Earth system.
However, it is possible that the warming ice sheets melted directly, but rather destabilized via a warming -related sea level rise the surrounding ice shelf belt through an erosion of its base. Whose decay then ice streams from the ice sheet could suddenly getting to the sea. Once a section broke away, carrying the liberated ice to a further rise in sea level at ( positive feedback ). For this theory is the fact that the break-up of ice in H1, 2, 4 and 5 are not at the same time happened, with it being the European shield the melting by up to 1,500 years ahead of rushed.
In Atlantic Heat Piracy model of Seidov and Maslin (2001) it is assumed that changes in ocean circulation heating an ocean - hemisphere at the expense of others will. Currently, the Gulf Stream directs warm equatorial water towards the North Atlantic. The supply of fresh water to the North Sea could reduce the strength of the Gulf Stream and also let him turn in a southerly flow. According Stocker ( 1998), this would cause a cooling of the northern hemisphere with simultaneous heating of the southern hemisphere, which in turn changes in the Eisakkumulations and melting rate would result and could possibly lead to ice shelf destruction and Heinrich events.
In the bipolar model of blank ( 2004) it is assumed that a rise in sea level raised floating ice shelf, leading to its destabilization and destruction. Without the support of floating ice shelf continental ice sheets were flowed in the direction of the oceans and disintegrate into icebergs and sea ice.
In the coupled ocean / atmosphere model of Ganopolski and Rahmstorf, a fresh water supply was integrated, which showed that both Heinrich events as well as Dansgaard - Oeschger events show a hystereses behavior. This means that only relatively small changes in the supply of fresh water into the northern oceans - an increase of 0.15 Sv or a decrease of 0.03 Sv - enough to cause a profound change in the global circulation. The results showed that a Heinrich event did not have a cooling in the area around Greenland, but further south, mainly in the subtropical Atlantic result, which is supported by most of the available paleoclimatic data.
This idea was linked by Maslin and his co-authors with Dansgaard - Oeschger events. They suggest that each of the ice sheets have its own stability conditions, but that the influx of fresh water was large enough by a melting to divert the ocean currents - which in turn elsewhere triggered melting. In other words, Dansgaard -Oeschger events and their associated influx of meltwater reduce the strength of the North Atlantic Deep flow ( NADW, North Atlantic Deep Water), which in turn the meridional circulation ( AMOC, Atlantic meridional overturning circulation ) weakens and thus elevated to a heat transfer leads towards the southern hemisphere poles. This warmer water leads to melting of the Antarctic ice, whereby the density driven stratification and the strength of the Antarctic bottom water flow ( AABW, Antarctic Bottom Water current) were reduced. However, this allowed the NADW return to their former strength, which causes melting in the northern hemisphere and a renewed Dansgaard - Oeschger event by itself. If necessary, the fusion process reaches a threshold, where he raises the sea level enough to corrode the ice shelf belt of Laurentide Ice Sheet - and so triggers a Heinrich event and returned to the cycle to its original condition.
Hunt & Malin (1998) suggested that Heinrich events could also be brought by earthquakes in transition, as the fast de-icing abruptly relieved the underlying rock on Eisschildrand.