Heinrich event

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The occurrence of Heinrich events (green lines) plotted against hydrogen / oxygen curves from ice cores .

Heinrich events describe periods of accelerated ice advances ( English Ice flow surges ) and their discharge into the sea. The events were postulated on the basis of observations of increased sediment input of continental origin in the Lower Pleistocene sediment layers on the sea floor. These sediment layers are also known as Heinrich layers or IRD ( English ice rafted debris or ice rafted deposit ). Due to the coarse sediment fraction, transport by ocean currents appears unlikely, so icebergs / floes are more likely to be the transport medium.

Heinrich events were first mentioned in 1988 by Hartmut Heinrich and so far only recorded for the last glacial period. During these events, large numbers of icebergs broke off the advancing glacier masses and drifted across the North Atlantic . The icebergs carried sediments with them that had been eroded and incorporated by glacial activity; When the icebergs melted, this material, which had been carried off by the icebergs, fell to the sea floor.

The melting of the icebergs led to an increased supply of fresh water to the North Atlantic. This influx of cold fresh water likely changed the ocean's density-driven thermohaline circulation patterns . The events often coincide with indications of global climate variability.

Scientists can identify six individual events in drill cores that originate from these areas of the ocean floor; they are referred to as H1 through H6 , with H6 representing the oldest event. There is evidence that events H3 and H6 are different from the rest of the events.

Various mechanisms have been proposed to explain the causes of Heinrich events. Usually the Laurentid ice sheet plays the main role, but other indications point to the unstable West Antarctic ice sheet, which is said to have played a triggering role.

The event

event Age in ka (= 1000 years)
Hemming (2004) Bond & Lotti (1995) Vidal et al. (1999)
H0 ~ 12
H1 16.8 14th
H2 24 23 22nd
H3 ~ 31 29
H4 38 37 35
H5 45 45
H6 ~ 60
H1 and H2 were dated using the radiocarbon method,
H3 to H6 by correlation with the Greenland Ice Sheet Project ( GRIP ).

Heinrich events proceed very quickly according to paleoclimatological standards: They last about 750 years and can start within a few years. The events have so far only been observed from the last icing phase; the low temporal resolution of the sediments before this time makes it impossible to determine whether they also occurred during other glaciation phases in the earth's history.

Heinrich events took place during some, but not all, cold spells that preceded the rapid warming phases known as the Dansgaard-Oeschger events and repeated about every 1500 years. The problems of determining the exact times have led to discussions as to whether all the cases are actually Heinrich events. Some authors (Broecker, Bond & Lotti 1995) see the Younger Dryas as a Heinrich event, which would then be the event H0 .

Diagnosis of the Heinrich events

Heinrich's original observations were six layers in sediment cores of the ocean with an extremely high proportion of material of continental origin, namely minerogenic fragments with a grain size of 180 μm to 3 mm. The coarse sediment fraction could not have been transported by ocean currents. Hence, it was believed to have been incorporated by icebergs or sea ice that had broken off from the great Laurentide ice shelf and fell to the sea floor after melting. The Laurentid Ice Sheet covered large parts of North America at that time. The traces of these events in sediment cores depend significantly on the distance to the region of origin; a ring of iceberg sediments ( IRD ) lines the 50th parallel. It stretches 3,000 kilometers from its North American origin to Europe and its thickness is reduced by an order of magnitude on its way from the Labrador Sea to the end point of the iceberg route in Europe.

Huge amounts of fresh water flowed into the ocean during Heinrich events. According to estimates, 0.29 ± 0.05 Sverdrup were registered for a duration of 250 ± 150 years at the Heinrich event H4  , which corresponds to a volume of 2.3 million km³. Some geological indicators seem to be temporally correlated with Heinrich events, but the impossibility of exact dating does not allow them to be placed before or after the respective event. In some cases it is even difficult to assess whether they have any causal connection at all with the Heinrich events. Heinrich events are usually characterized by the following changes:

Foraminifera are not only ecological indicators in the ocean, but also provide data on the isotope ratios during their lifetime by analyzing their shells
  • Increasing δ 18 O content in the northern seas and in East Asian stalactites , which suggests a decrease in global average temperature (or an increase in ice volume) via climatological proxy data
  • Decreasing salinity of the ocean due to the influx of fresh water
  • Indications of decreasing temperatures of the near-surface seawater west of the African coast, detected by Alkenon biomarkers
  • Changes in sediment mixing ( bioturbation ) caused by burrowing animals
  • Shifts in the isotopic composition of plankton (changes in the δ 13 C proportion, decreasing δ 18 O)
  • Pollen indicates cold-loving pine trees that replaced oaks on mainland North America
  • Decreasing variety of foraminifera ; due to the integrity of many samples, this cannot be explained by measurement errors; the decrease is therefore attributed to the reduced salt content
  • Increased terrigenous runoff on the continents, which can be detected near the mouth of the Amazon
  • Increase in grain size of wind-transported loess in China, indicating stronger winds
  • Changes in 230 th relative abundance suggestive of changes in the speed of oceanic currents
  • Increased sedimentation rates in the North Atlantic, which are noticeable in the increase in rock fragments of continental origin compared to background sedimentation.

The global traceability of these traces shows the dramatic effect of the Heinrich events.

Unusual Heinrich events

The proportion of stone in sediments deposited during the H3 and H6 events is significantly lower than that of other Heinrich events

H3 and H6 do not show the convincing abundance of symptoms typical of Heinrich events, as is the case with H1, H2 , H4 and H5 . This led some researchers to assume that these were not real Heinrich events, which made Bond's thesis that Heinrich events fall within a 7000-year cycle seem false. However, some causal chains suggest that H3 and H6 are different from other events in some ways.

  • Extreme values ​​in the rock fraction: A far lower fraction of rock fragments (3,000 grains per gram) are found in H3 and H6, which compared to normally 6000 grains per gram means that the continents played a lesser role as the source of ocean sediments during these two events.
  • Disintegration of foraminifera: Foramanifer shells appear to have been more heavily eroded during H3 and H6 (Gwiazda et al , 1996). This may have been caused by an influx of nutrient-rich - and therefore corrosive Antarctic bottom water, which in turn could be due to a reorganization of oceanic circulation patterns.
  • Origin of the ice: Icebergs in H1, H2, H4 and H5 seem to have drifted along the Hudson Strait , while H3 and H6 icebergs across it.
  • Distribution of the coarse fraction incorporated by the ice: the glacial sediments extend nowhere near as far to the east as those of the other events during H3 and H6 events. Therefore, some researchers have suggested that at least some H3 and H6 sediments were of European origin.

causes

As with many other problems in climatology, the system is far too complex to be able to reliably identify a cause. There are several possible drives that fall into two categories.

Internal drives - the “ binge purge ” model

This model assumes that internal factors in ice sheets cause the periodic disintegration of large volumes of ice 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. As soon as the shield reached a critical load, the pressure turned the soft, loose sediment under the glacier into a smooth lubricant over which the ice sheet could slide. The purge phase lasted about 750 years. The original model assumed that the geothermal energy started the thawing of the sediment layer under the glacier as soon as the ice volume was large enough to prevent the geothermal energy from escaping into the atmosphere. Mathematical model calculations of such a system are consistent with a 7000-year period - recognizable if H3 and H6 are actually regarded as Heinrich events. However, if H3 and H6 are not Heinrich events, the binge-purge model loses credibility, since the predicted periodicity plays a key role in its assumptions.

The theory seems somewhat unlikely that Heinrich events were not observed during previous ice ages, although this can also be attributed to the lack of high-resolution sediments. In addition, the model predicts that the decreasing size of the ice sheets in the course of the Pleistocene glaciations should have an impact on the size, strength and frequency of the Heinrich events, but this cannot be found in this way.

External drives

Several factors, which cannot be attributed to the ice sheets, could also have caused Heinrich events, but their influences would have to be large enough to be able to overcome their dampening by the large ice volumes.

Gerard Bond suspects that a periodically recurring change in the sun's energy flow every 1500 years is correlated with the Dansgaard-Oeschger events and thus also with the Heinrich events; but the relatively small change in energy makes it seem unlikely that such an extraterrestrial factor would have the necessary power, at least not without large positive feedback processes within the Earth system.

However, it is possible that the warming did not melt ice sheets directly, but rather destabilized the surrounding ice shelf by erosion of its base via a warming-related rise in sea level . Due to its disintegration, ice flows from the ice sheet could suddenly advance to the sea. As soon as a section broke away, the released ice contributed to a further rise in sea level (positive feedback). This theory is supported by the fact that the breaking up of the ice in H1, 2, 4 and 5 did not happen at the same time, whereby it preceded the melting on the European Shield by up to 1,500 years.

current thermohaline circulation . The Gulf Stream on the far left could have been diverted as part of Heinrich events

In the Atlantic Heat Piracy model by Seidov and Maslin (2001) it is assumed that changes in ocean circulation caused warming of one ocean hemisphere at the expense of the other. Currently, the Gulf Stream directs warm, equatorial water towards the North Atlantic. The addition of fresh water to the Arctic Ocean could reduce the strength of the Gulf Stream and also turn it into a southerly current. According to Stocker (1998), this would cause the northern hemisphere to cool while the southern hemisphere warms up, which in turn would result in changes in the ice accumulation and melting rate and possibly lead to the destruction of the ice shelves and Heinrich events.

In Rohling's (2004) bipolar model , it is assumed that a rise in sea level raised floating ice shelves , leading to its destabilization and destruction. Without the support of floating ice shelves, continental ice masses would flow towards the oceans and disintegrate into icebergs and sea ice.

In the coupled ocean / atmosphere model by Ganopolski and Rahmstorf , a freshwater supply was integrated, which showed that both Heinrich events and Dansgaard-Oeschger events show hysteresis behavior. This means that only relatively small changes in the freshwater supply into the Northern Oceans - an increase of 0.15 Sv or a decrease of 0.03 Sv - are sufficient to cause a profound change in the global circulation. The result showed that a Heinrich event did not cause cooling in the area around Greenland, but further south, primarily in the subtropical Atlantic, which is supported by most of the available paleoclimatic data.

This idea was linked to Dansgaard-Oeschger events by Maslin and his co-authors. They suggest that each of the ice sheets had its own conditions of stability, but that when it melted, the influx of freshwater was large enough to divert ocean currents - which in turn triggered melting elsewhere. In other words, Dansgaard-Oeschger events and their associated influx of melt water 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 to increased Heat transfer leads towards the southern hemisphere pole. This warmer water melts the Antarctic ice, reducing the density-driven stratification and strength of the Antarctic Bottom Water Current (AABW ). However, this allows the NADW to return to its old strength, which leads to a melting in the northern hemisphere and another Dansgaard-Oeschger event. If necessary, the melting process reaches a limit value, whereby it raises the sea level enough to corrode the ice shelf belt of the Laurentide ice sheet - thus triggering a Heinrich event and returning the cycle to its original state.

Hunt & Malin (1998) suggested that Heinrich events could also be set in motion by earthquakes , as the rapid de-icing at the edge of the ice sheet suddenly relieves the underlying rock.

See also

Individual evidence

  1. ^ Frank Sirocko: History of the climate. Stuttgart, Theiss 2013, p. 51
  2. ↑ a b S. R. Hemming: Heinrich events: massive late Pleistocene detritus layers of the North Atlantic and their global climate imprint . In: Rev. Geophys . 42, No. 1, 2004. doi : 10.1029 / 2003RG000128 .
  3. ^ GC Bond, R. Lotti: Iceberg Discharges into the North Atlantic on Millennial Time Scales During the Last Glaciation . (abstract) In: Science . 267, No. 5200, February 17, 1995, p. 1005. doi : 10.1126 / science.267.5200.1005 . PMID 17811441 . Retrieved June 28, 2007.
  4. ^ L. Vidal, RR Schneider, O. Marchal, T. Bickert, TF Stocker, G. Wefer: Link between the North and South Atlantic during the Heinrich events of the last glacial period Archived from the original on November 29, 2007. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: Climate Dynamics . 15, No. 12, 1999, pp. 909-919. doi : 10.1007 / s003820050321 . Retrieved June 28, 2007. @1@ 2Template: Webachiv / IABot / www.climate.unibe.ch
  5. ↑ a b c M. Maslin, D. Seidov, J. Lowe: Synthesis of the nature and causes of rapid climate transitions during the Quaternary . In: Geophysical monograph . 126, 2001, pp. 9-52. Retrieved June 6, 2014.
  6. ^ WS Broecker: Massive iceberg discharges as triggers for global climate change . (abstract) In: Nature . 372, 2002, pp. 421-424. doi : 10.1038 / 372421a0 . Retrieved June 28, 2007.
  7. ^ A b H. Heinrich: Origin and consequences of cyclic ice rafting in the Northeast Atlantic Ocean during the past 130,000 years . In: Quaternary Research . 29, 1988, pp. 142-152.
  8. ^ D. Roche, D. Paillard, E. Cortijo: Duration and iceberg volume of Heinrich event 4 from isotope modeling study . In: Nature . 432, 2004, pp. 379-382 .. doi : 10.1038 / nature03059 .
  9. ↑ M. Bar-Matthews, A. Ayalon, A. Kaufman: Late Quaternary paleoclimate in the eastern Mediterranean region from stable isotope analysis of speleothems at Soreq Cave, Israel . (freely accessible reprint in Google Books) In: Quaternary Research . 47, No. 2, 1997, pp. 155-168. doi : 10.1006 / qres.1997.1883 . Retrieved December 21, 2012.
  10. ^ JP Sachs, RF Anderson: Increased productivity in the subantarctic ocean during Heinrich events . In: Nature . 434, No. 7037, 2005, pp. 1118-1121. doi : 10.1038 / nature03544 .
  11. ^ A b F. E. Grousset et al .: Were the North Atlantic Heinrich events triggered by the behavior of the European ice sheets? . (abstract) In: Geology . 28, No. 2, February 1, 2000, pp. 123-126.
  12. ^ EC Grimm et al .: A 50,000-Year Record of Climate Oscillations from Florida and Its Temporal Correlation with the Heinrich Events . (abstract) In: Science . 261, No. 5118, July 9, 1993, p. 198. doi : 10.1126 / science.261.5118.198 . PMID 17829277 . Retrieved May 29, 2007.
  13. ↑ G. Bond et al .: Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period . (abstract) In: Nature . 360, No. 6401, 1992, pp. 245-249. doi : 10.1038 / 360245a0 .
  14. ^ SC Porter, A. Zhisheng: Correlation between climate events in the North Atlantic and China during the last glaciation . (abstract) In: Nature . 375, No. 6529, 1995, pp. 305-308. doi : 10.1038 / 375305a0 . Retrieved May 29, 2007.
  15. ↑ SR Hemming et al .: Provenance of Heinrich layers in core V28-82, northeastern Atlantic: 40 Ar / 39 Ar ages of ice-rafted hornblende, Pb isotopes in feldspar grains, and Nd-Sr-Pb isotopes in the fine sediment fraction . In: Earth and Planetary Science Letters . 164, No. 1-2, 1998, pp. 317-333. doi : 10.1016 / S0012-821X (98) 00224-6 .
  16. ^ REM Rickaby, H. Elderfield: Evidence from the high-latitude North Atlantic for variations in Antarctic Intermediate water flow during the last deglaciation . In: Geochemistry Geophysics Geosystems . 6, No. 5, 2005, p. Q05001. doi : 10.1029 / 2004GC000858 .
  17. ^ ME Kirby, JT Andrews: Mid-Wisconsin Laurentide Ice Sheet growth and decay: Implications for Heinrich events 3 and 4 . (abstract) In: Paleoceanography . 14, No. 2, 1999, pp. 211-223. doi : 10.1029 / 1998PA900019 .
  18. ^ A b D. R. MacAyeal: Binge / purge oscillations of the Laurentide ice sheet as a cause of the North Atlantic's Heinrich events . In: Paleoceanography . 8, No. 6, 1993, pp. 775-784. doi : 10.1029 / 93PA02200 .
  19. ^ M. Sarnthein et al .: Fundamental Modes and Abrupt Changes in North Atlantic Circulation and Climate over the last 60 ky . In: The Northern North Atlantic: a Changing Environment . 2001. Retrieved March 6, 2008.
  20. ↑ D. Seidov, M. Maslin: Atlantic ocean heat piracy and the bipolar climate see-saw during Heinrich and Dansgaard- Oeschger events . In: Journal of Quaternary Science . 16, No. 4, 2001, pp. 321-328. doi : 10.1002 / jqs.595 .
  21. ↑ TF Stocker: The seesaw effect . In: Science . 282, No. 5386, 1998, pp. 61-62. doi : 10.1126 / science.282.5386.61 .
  22. ^ A. Ganopolski, S. Rahmstorf: Rapid changes of glacial climate simulated in a coupled climate model . (full text) In: Nature . 409, 2001, ISSN  0028-0836 , pp. 153-158. doi : 10.1038 / 35051500 .
  23. ^ S. Rahmstorf et al .: Thermohaline circulation hysteresis: A model intercomparison . In: Geophysical Research Letters . 32, No. 23, 2005, p. L23605. doi : 10.1029 / 2005GL023655 . Retrieved May 7, 2007.

literature

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