Heinrich event 1944832 225763506 2008-07-15T08:16:00Z Smith609 846901 Undo removal of reference and addition of uncited references: suspect that editor may be related to Harunur Rashid. References are welcome if supporting facts in the article. [[Image:heinrich-events.png|thumb|400px|right|The timing of Heinrich events inferred from geochemical records of ice cores]] '''Heinrich events''', first described by marine geologist Hartmut Heinrich, occurred during the [[Wisconsin glaciation|last glacial]] period, or "ice age". During such events, armadas of [[iceberg]]s 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. == About the events == {| border="1" cellspacing="0" cellpadding="3" style="background: #FFFFFF; border-collapse: collapse; margin: 0 0 1em 1em; text-align:center; float:right;" |- !rowspan="2" style="width:5em; background:#aaeecc"|Event !colspan="3" style=" background:#aaeecc"|Age, Kyr |- |style="width:8em; background:#bdfadd"|Hemming (2004) |style="width:8em; background:#bdfadd"|Bond & Lotti (1995) |style="width:8em; background:#bdfadd"|Vidal ''et al''. (1999) |- |style="background:#dfe"|H0 |~12 | | |- |style="background:#dfe"|H1 |16.8 | |14 |- |style="background:#dfe"|H2 |24 |23 |22 |- |style="background:#dfe"|H3 |~31 |29 | |- |style="background:#dfe"|H4 |38 |37 |35 |- |style="background:#dfe"|H5 |45 | |45 |- |style="background:#dfe"|H6 |~60 | | |- |colspan=4|<small>H1,2 are dated by [[carbon-14|radiocarbon]]; H3-6 by correlation to [[Greenland Ice Sheet Project|GISP]]2.</small> |} 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 event|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 - see [http://wikyonos.seos.uvic.ca/people/afanning/Glaciation/Bond.html] for overview) identify the [[Younger Dryas]] event as a Heinrich event, which would make it H0. == Diagnosis of Heinrich events == 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&nbsp;μm to 3&nbsp;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&nbsp;[[Sverdrup]] with a duration of 250±150&nbsp;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: [[Image:Elphidium excavatum clavatum.jpg|thumb|right|As well as indicating oceanic productivity, foramanifera tests also provide valuable [[Isotope geochemistry|isotopic]] data]] * Decreased [[Oxygen-18|&delta;<sup>18</sup>O]] of the northern (Nordic) seas and East [[Asia]]n stalactites ([[speleothems]]), which by [[proxy (climate)|proxy]] suggests falling global temperature (or rising ice volume) (Bar-Matthews ''et al.'' 1997) * Decreased oceanic [[salinity]], due to the influx of fresh water * Decreased [[sea surface temperature]] estimates off the West [[Africa]]n coast through biochemical indicators known as [[alkenone]]s (Sachs 2005) * Changes in sedimentary disturbance ([[bioturbation]]) caused by burrowing animals (Grousett ''et al.'' 2000) * Flux in [[plankton]]ic isotopic make-up (changes in δ<sup>13</sup>C, decreased δ<sup>18</sup>O) * [[Pollen]] indications of cold-loving [[pine]]s replacing [[oak]]s on the North American mainland (Grimm ''et al.'' 1993) * Decreased [[foramanifera]]l abundance - which due to the pristine nature of many samples cannot be attributed to [[sampling bias|preservational bias]] and has been related to reduced salinity (Bond 1992) * Increased [[terrigenous]] runoff from the continents, measured near the mouth of the [[Amazon River]] * Increased grain size in wind-blown [[loess]] in [[China]], suggesting stronger winds (Porter & Zhisheng 1995) * Changes in relative [[Thorium-230]] abundance, reflecting variations in [[ocean current]] velocity * Increased deposition rates in the northern Atlantic, reflected by an increase in continentally derived sediments (lithics) relative to background sedimentation (Heinrich 1988) The global extent of these records illustrates the dramatic impact of Heinrich events. == Unusual Heinrich events == [[Image:Sand bei 200 fach 2.jpg|thumb|right|The lithic proportion of sediments deposited during H3 and H6 is substantially below that of other 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. * Lithic peaks: a far smaller proportion of lithics (3000 ''vs.'' 6000 grains per gram) is observed in H3 and H6 (e.g. Hemming ''et al.'' 1998), which means that the role of the continents in providing sediments to the oceans was relatively lower. * Foram dissolution: Foramanifera tests appear to be more eroded during H3 and H6 (Gwiazda ''et al'', 1996). This may indicate an influx of nutrient-rich — hence corrosive — [[Antarctic Bottom Water]], due to a reconfiguration of oceanic circulation patterns (see Rickaby and Elderfield, 2005). * Ice provenance: Icebergs in H1, H2, H4, and H5 appear to have flowed along the Hudson Strait; H3 and H6 icebergs appear to have flowed across it (Kirkby and Andrews, 1999). * Ice rafted debris distribution: Sediment transported by ice does not extend as far East during H3/6. Hence some researchers have been moved to suggest a European origin for at least some H3/6 clasts: America and Europe were originally adjacent to one another; hence the rocks on each continent are difficult to distinguish and the source is open to interpretation (Grousset ''et al.'' 2000). == Causes == 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. === Internal forcings — the "Binge - Purge" model === 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 &mdash; 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 &mdash; the "purge phase", lasting around 750 years. The original model (MacAyeal, 1993) proposed that [[Geothermal (geology)|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. === External forcings === 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 C. Bond|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 &mdash; further destabilizing other ice sheets. In favour of this theory is the non-simeltaneouity of ice sheet break up in H1, 2, 4, and 5, where European breakup preceded European melting by uo to 1,500&nbsp;years (Maslin ''et al'' 2001). [[Image:Thermohaline circulation.png|thumb|right|350px|Present day [[thermohaline circulation|ocean circulation]]. The Gulf stream, far left, may be redirected during Heinrich events]] 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 [[Stefan Rahmstorf|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 &mdash; a 0.15 [[Sverdrup|Sv]] increase, or 0.03 Sv decrease &mdash; 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 [[paleoclimate|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 &mdash; 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 &mdash; causing a Heinrich event and resetting the cycle. ==See also== *[[Ice sheet dynamics]] == References == <div class="references-small" style="-moz-column-count:2; column-count:2;"> *{{cite journal | author = Alley, R.B. | coauthors = MacAyeal, D.R. | year = 1994 | title = Ice-rafted debris associated with binge/purge oscillations of the Laurentide Ice Sheet | journal = Paleoceanography | volume = 9 | issue = 4 | pages = 503–512 | doi = 10.1029/94PA01008 | url = http://www.agu.org/journals/pa/pa9404/94PA01008.pdf | accessdate = 2007-05-07 }} *{{cite journal | author = Bar-Matthews, M. | coauthors = Ayalon, A.; Kaufman, A. | year = 1997 | title = Late Quaternary paleoclimate in the eastern Mediterranean region from stable isotope analysis of speleothems at Soreq Cave, Israel | journal = Quaternary Research | volume = 47 | issue = 2 | pages = 155–168 | url = http://epsc.wustl.edu/courses/epsc484/barmatthewsetal97.pdf | accessdate = 2007-05-29 | doi = 10.1006/qres.1997.1883 | format = {{Dead link|date=May 2008}} }} *{{cite journal | author = Bond, G. | coauthors = Heinrich, H., Broecker, W., Labeyrie, L., Mcmanus, J., Andrews, J., Huon, S., Jantschik, R., Clasen, S., Simet, C. | year = 1992 | title = Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period | journal = Nature | volume = 360 | issue = 6401 | pages = 245–249 | url = http://www.nature.com/nature/journal/v360/n6401/abs/360245a0.html | doi = 10.1038/360245a0 | format = abstract }} *{{cite journal | author = Bond, G.C. | coauthors = Lotti, R. | date = [[1995-02-17]] | title = Iceberg Discharges into the North Atlantic on Millennial Time Scales During the Last Glaciation | journal = Science | volume = 267 | issue = 5200 | pages = 1005 | doi = 10.1126/science.267.5200.1005 | url = http://www.sciencemag.org/cgi/content/abstract/267/5200/1005 | accessdate = 2007-06-28 | format = abstract | pmid = 17811441 }} *{{cite journal | author = Broecker, W.S. | year = 2002 | title = Massive iceberg discharges as triggers for global climate change | journal = Nature | volume = 372 | pages = 421–424 | doi = 10.1038/372421a0 | url = http://www.nature.com/nature/journal/v372/n6505/abs/372421a0.html | accessdate = 2007-06-28 | format = abstract }} *Heinrich, H. 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Erlenkeuser, H.; Schulz, M.; Seidov, D.; Simstich, J.; Van Kreveld, S. | year = 2001 | title = Fundamental Modes and Abrupt Changes in North Atlantic Circulation and Climate over the last 60 ky | journal = The Northern North Atlantic: A Changing Environment | url = http://books.google.com/books?hl=en&lr=&ie=UTF-8&id=iVAzudOPt1kC&oi=fnd&pg=PA365&dq=%22fundamental+modes+and+abrupt%22&ots=EPiv7TWvfh&sig=vP2L5OMmOl0YZwubzWnBj9noP6M | accessdate = 2008-03-06 }} * {{cite journal | author = Stocker, T.F. | year = 1998 | title = The seesaw effect | journal = Science | volume = 282 | issue = 5386 | pages = 61–62 | issn = | doi = 10.1126/science.282.5386.61 | url = http://cat.inist.fr/?aModele=afficheN&cpsidt=1674017 | accessdate = 2007-05-26 }} *{{cite journal | author = Vidal, L. | coauthors = Schneider, R.R.; Marchal, O.; Bickert, T.; Stocker, T.F.; Wefer, G. | year = 1999 | title = Link between the North and South Atlantic during the Heinrich events of the last glacial period | journal = Climate Dynamics | volume = 15 | issue = 12 | pages = 909–919 | url=http://www.climate.unibe.ch/~stocker/papers/vidal99cd.pdf | accessdate = 2007-06-28 | doi = 10.1007/s003820050321 }} </div> == External links == * [http://williamcalvin.com/1990s/1998AtlanticClimate.htm William C. Calvin, "The great climate flip-flop"] adapted from ''Atlantic Monthly,'' 281(1):47-64 (January 1998). * [http://www.atmos.washington.edu/~dennis/571_Lecture_4_Heinrich_Note.pdf D.L. Hartmann, "Heinrich Events"]: outline notes and full references (pdf file) * [http://www.columbia.edu/cu/pr/95/18779.html (Gerald Bond) "Recent, Abrupt Climate-Cooling Cycle Found"]: Columbia University Press Release, December 11, 1995: * [http://www.grida.no/climate/ipcc_tar/wg1/074.htm IPCC TAR section 2.4.3 How Fast did Climate Change during the Glacial Period?] [[Category:Paleoclimatology]] [[Category:Historical geology]] [[Category:History of climate]] [[Category:Pleistocene]] [[fr:Événement de Heinrich]] [[is:Heinrich-atburður]] [[fi:Heinrichin tapahtuma]]