Benutzer Diskussion:KattlDerBavaricus

Übersetzung

The timing of Heinrich events inferred from geochemical records of ice cores

Sog. Heinrich Ereignisse, traten vor allem während der letzten Eiszeit auf. Die nach Heinrich Hartmut benannten Ereignisse sind durch den Abbruch sehr voluminöser Eisberge und deren Drift durch den Nordatlantik gekennzeichnet. first described by climatologist 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. Die schmelzenden Eisberge gaben große Mengen Erosionsmaterial frei, sodass es zur Ablagerung dieser "ice rafted debris" am Meeresgrund kommt. Durch wissenschaftliche Bohrungen gewonnene Aufschlüsse über diese Sedimentbildung erlauben die Unterscheidung von 6 disjunkten Heinrich Ereignissen H1-H6 (in zeitlich umgekehrter Reihenfolge); es spricht einiges dafür, dass sich H3 und H6 von den anderen Ereignissen unterscheiden. Auf Grund riesiger Mengen von kaltem Süßwasser, die durch das Abschmelzen der Eisberge in den Nord Atlantik gelangen, ist eine Veränderung der Wasserzirkulation im Ozean durchaus wahrscheinlich; zumal Änderungen der globalen klimatischen Gegebenheiten oftmals mit Heinrich Ereignissen einhergehen.

Es wurden bereits verschiedenste Versuche unternommen, das Auftreten von Heinrich Ereignissen zu erklären. Meist wird dabei der Laurentidische Eisschild als entscheidend erachtet. Allerdings wird auch der instabile West-Antarktische Eisschild als Grund erwogen.

Zu den Ereignissen

Event	Age, Kyr
	Hemming (2004)	Bond & Lotti (1995)	Vidal et al. (1999)
H0	~12
H1	16.8		14
H2	24	23	22
H3	~31	29
H4	38	37	35
H5	45		45
H6	~60
Die Datierung von H1,2 erfolgt durch die Radio-Carbon Methode, die Ereignisse H3 bis H6 durch Korrelation zum 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. 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 - see [1] 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 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 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.

Alley & MacAyeal (1994) estimate the volume of fresh water added to the North Atlantic over each 500 year event at around 3.7±1.2×10¹¹ km³ — which would weigh four hundred million trillion tonnes. It is barely surprising that effects of such a huge influx of cold, fresh water into the oceans were of global extent. Whilst several geological indicators fluctuate approximately in time with these Heinrich events, 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:

As well as indicating oceanic productivity, foramanifera tests also provide valuable isotopic data

• Decreased δ¹⁸O of the northern (Nordic) seas and East Asian stalactites (speleothems), which by 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 African coast through biochemical indicators known as alkenones (Sachs 2005)

• Changes in sedimentary disturbance (bioturbation) caused by burrowing animals (Grousett et al. 2000)

• Flux in planktonic isotopic make-up (changes in δ¹³C, decreased δ¹⁸O)

• Pollen indications of cold-loving pines replacing oaks on the North American mainland (Grimm et al. 1993)

• Decreased foramaniferal abundance - which due to the pristine nature of many samples cannot be attributed to 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

The lithic proportion of sediments deposited during H3 and H6 is substanitally 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 (Grusset 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 - 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, as is 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 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 would predict that the reduced size of ice sheets during the Pleiocene should reduce the size, impact and frequency of Heinrich events, which is not reflected by the evidence.

External forcings

There are several factors external to ice sheets that 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 timescale 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.

Datei:Thermohaline circulation.png

Present day 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 modelling (Ganopolski and Rahmstorf 2001), showing that both Heinrich and Dansgaard-Oeschger events may show hysteresis behaviour. This means that only relatively minor changes in freshwater loading into the Nordic Seas - a 0.15 Sv increase, or 0.03 Sv decrease - would be required 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.

External links

• William C. Calvin, "The great climate flip-flop" adapted from Atlantic Monthly, 281(1):47-64 (January 1998).
• D.L. Hartmann, "Heinrich Events": outline notes and full references (pdf file)
• (Gerald Bond) "Recent, Abrupt Climate-Cooling Cycle Found": Columbia University Press Release, December 11, 1995:
• IPCC TAR section 2.4.3 How Fast did Climate Change during the Glacial Period?

References

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• Bar-Matthews, M., Ayalon, A.; Kaufman, A.: Late Quaternary paleoclimate in the eastern Mediterranean region from stable isotope analysis of speleothems at Soreq Cave, Israel. In: Quaternary Research. 47, Nr. 2, 1997, S. 155-168. Abgerufen am 29. Mai 2007.

• Bond, G., Heinrich, H., Broecker, W., Labeyrie, L., Mcmanus, J., Andrews, J., Huon, S., Jantschik, R., Clasen, S., Simet, C.: Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period. In: Nature. 360, Nr. 6401, 1992, S. 245-249.

• Bond, G.C., Lotti, R.: Iceberg Discharges into the North Atlantic on Millennial Time Scales During the Last Glaciation. In: Science. 267, Nr. 5200, Februar, S. 1005. doi:10.1126/science.267.5200.1005. Abgerufen am 28. Juni 2007.

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• Heinrich, H. (1988) Origin and consequences of cyclic ice rafting in the Northeast Atlantic Ocean during the past 130,000 years, Quaternary Research, 29, 142-152.

• Hemming, S.R., Broecker, W.S.; Sharp, W.D.; Bond, G.C.; Gwiazda, R.H.; McManus, J.F.; Klas, M.; Hajdas, I.: 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, Nr. 1-2, 1998, S. 317-333. doi:10.1016/S0012-821X(98)00224-6. Abgerufen am 26. Mai 2007.

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