![Approximate chronology of Heinrich events vs Dansgaard-Oeschger events and Antarctic Isotope Maxima](https://upload.wikimedia.org/wikipedia/commons/f/f7/Approximate_chronology_of_Heinrich_events_vs_Dansgaard-Oeschger_events_and_Antarctic_Isotope_Maxima.png)
A Heinrich event is a natural phenomenon in which large groups of
iceberg
An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s break off from glaciers and traverse the North Atlantic. First described by marine geologist
Hartmut Heinrich
Hartmut Heinrich (born 5 March 1952 in Northeim, Lower Saxony) is a German marine geologist and climatologist. Heinrich was Head of the Marine Physics Department at the Federal Maritime and Hydrographic Agency (''BSH'') in Hamburg until September ...
(Heinrich, H., 1988), they occurred during five of the last seven
glacial periods over the past 640,000 years (Hodell, et al., 2008). Heinrich events are particularly well documented for the
last glacial period but notably absent from the
penultimate glaciation (Obrochta et al., 2014). The icebergs contained rock mass that had been eroded by the glaciers, and as they melted, this material was dropped to the sea floor as
ice rafted debris
Ice rafting is the transport of various materials by ice. Various objects deposited on ice may eventually become embedded in the ice. When the ice melts after a certain amount of drifting, these objects are deposited onto the bottom of the water ...
(abbreviated to "IRD") forming deposits called Heinrich layers.
The icebergs' melting caused vast quantities of fresh water to be added to the North Atlantic. Such inputs of cold and 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 of which imply instability of the massive
Laurentide Ice Sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the Northern United States, multiple times during the Quaternary glacial epochs, from 2.58 million years a ...
, a continental glacier covering northeastern North America during the last glacial period. Other northern hemisphere ice sheets were potentially involved as well, such as the (
Fennoscandic and
Iceland/Greenland). However, the initial cause of this instability is still debated.
Description
The strict definition of Heinrich events is the climatic event causing the IRD layer observed in marine sediment cores from the North Atlantic: a massive collapse of northern hemisphere ice shelves and the consequent release of a prodigious volume of icebergs. By extension, the name "Heinrich event" can also refer to the associated climatic anomalies registered at other places around the globe, at approximately the same time periods. The events are rapid: they last probably less than a millennium, a duration varying from one event to the next, and their
abrupt onset may occur in mere years (Maslin ''et al''. 2001). Heinrich events are clearly observed in many North Atlantic marine sediment cores covering the last glacial period; the lower resolution of the sedimentary record before this point makes it more difficult to deduce whether they occurred during other glacial periods in the Earth's history. Some (Broecker 1994, Bond & Lotti 1995) identify the
Younger Dryas
The Younger Dryas (c. 12,900 to 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stage ...
event as a Heinrich event, which would make it event H0 (''table, right'').
Heinrich events appear related to some, but not all, of the cold periods preceding the rapid warming events known as
Dansgaard-Oeschger (D-O) events, which are best recorded in the
NGRIP Greenland ice core. However, difficulties in synchronising marine sediment cores and Greenland ice cores to the same time scale raised questions as to the accuracy of that statement.
Potential climatic fingerprint 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 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 glaciers or ice shelves, and dumped debris onto the sea floor as the icebergs melted. Geochemical analyses of the IRD can provide information about the origin of these debris: mostly the large
Laurentide Ice Sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the Northern United States, multiple times during the Quaternary glacial epochs, from 2.58 million years a ...
then covering North America for Heinrich events 1, 2, 4 and 5, and on the contrary, European ice sheets for the minor events 3 and 6. The signature of the events in sediment cores varies considerably with distance from the source region. For events of Laurentide origin, there is a belt of IRD at around 50° N, known as the Ruddiman belt, expanding some 3,000 km (1,865 mi) from its North American source towards
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, and thinning by an order of magnitude from the
Labrador Sea
The Labrador Sea (French: ''mer du Labrador'', Danish: ''Labradorhavet'') is an arm of the North Atlantic Ocean between the Labrador Peninsula and Greenland. The sea is flanked by continental shelf, continental shelves to the southwest, northwest, ...
to the European end of the present iceberg route (Grousset ''et al''., 1993). During Heinrich events, huge volumes of fresh water flow into the ocean. For Heinrich event 4, based on a model study reproducing the isotopic anomaly of oceanic oxygen 18, 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 or a sea-level rise.
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:
![Elphidium excavatum clavatum](https://upload.wikimedia.org/wikipedia/commons/7/73/Elphidium_excavatum_clavatum.jpg)
* Increased
δ18O of the northern (Nordic) seas and East
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
n stalactites (
speleothems
A speleothem (; ) is a geological formation by mineral deposition (geology), deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety ...
), which by
proxy
Proxy may refer to:
* Proxy or agent (law), a substitute authorized to act for another entity or a document which authorizes the agent so to act
* Proxy (climate), a measured variable used to infer the value of a variable of interest in climate re ...
suggests falling global temperature (or rising ice volume) (Bar-Matthews ''et al.'' 1997)
* Decreased oceanic
salinity
Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...
, due to the influx of fresh water
* Decreased
sea surface temperature
Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mass ...
estimates off the West
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
n coast through biochemical indicators known as
alkenone
Alkenones are long-chain unsaturated methyl and ethyl ''n''-ketones produced by a few phytoplankton species of the class Prymnesiophyceae.Marlowe, I.T., Green, J.C., Neal, A.C., Brassell, S.C., Eglinton, G. and Course, P.A. (1984) "Long-chain (''n ...
s (Sachs 2005)
* Changes in sedimentary disturbance (
bioturbation
Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
) caused by burrowing animals (Grousett ''et al.'' 2000)
* Flux in
plankton
Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
ic isotopic make-up (changes in δ
13C, decreased δ
18O)
*
Pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
indications of cold-loving
pine
A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accep ...
s replacing
oak
An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...
s on the North American mainland (Grimm ''et al.'' 1993)
* Decreased
foramanifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly a ...
l 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
In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, they are derived from ''terrestrial'' (as opposed to marine) environments. Consisting of sand, mud, and silt carried to sea by rivers, their ...
runoff from the continents, measured near the mouth of the
Amazon River
The Amazon River (, ; es, RÃo Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile.
The headwaters of t ...
* Increased grain size in wind-blown
loess
Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits.
Loess is a periglacial or aeolian ...
in
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
, suggesting stronger winds (Porter & Zhisheng 1995)
* Changes in relative
Thorium-230 abundance, reflecting variations in
ocean current
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
velocity
* Increased deposition rates in the northern Atlantic, reflected by an increase in continentally derived sediments (lithics) relative to background sedimentation (Heinrich 1988)
* Expansion of grass and shrubland across large areas of Europe (e.g., Harrison and Sánchez Goñi, 2010)
The global extent of these records illustrates the dramatic impact of Heinrich events.
Unusual Heinrich events
![Sand bei 200 fach 2](https://upload.wikimedia.org/wikipedia/commons/b/ba/Sand_bei_200_fach_2.jpg)
H3 and H6 do not share such a convincing suite of Heinrich event symptoms as events H1, H2, H4, and H5, which has led some researchers to suggest that they are not true Heinrich events. That would make
Gerard C. Bond
Gerard Clark Bond (May 20, 1940 – June 29, 2005) was an American geologist.
Biography
Bond received his Bachelor of Science degree at Capital University in Columbus, Ohio, where his father Ralph Bond was a Professor of Geology. He worked at t ...
's suggestion of Heinrich events fitting into a 7,000-year cycle ("
Bond events") suspect.
Several lines of evidence suggest that H3 and H6 were somehow different from the other events.
* Lithic peaks: a far smaller proportion of lithics (3,000 ''vs.'' 6,000 grains per gram) is observed in H3 and H6, which means that the role of the continents in providing sediments to the oceans was relatively lower.
* Foram dissolution:
Foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...
tests appear to be more eroded during H3 and H6 (Gwiazda ''et al.'', 1996). That may indicate an influx of nutrient-rich, hence corrosive,
Antarctic Bottom Water by a reconfiguration of oceanic circulation patterns.
* Ice provenance: Icebergs in H1, H2, H4, and H5 are relatively enriched in
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
"detrital carbonate" originating from the
Hudson Strait
Hudson Strait (french: Détroit d'Hudson) links the Atlantic Ocean and Labrador Sea to Hudson Bay in Canada. This strait lies between Baffin Island and Nunavik, with its eastern entrance marked by Cape Chidley in Newfoundland and Labrador and ...
region; while H3 and H6 icebergs carried less of this distinctive material (Kirby and Andrews, 1999; Hemming et al., 2004).
* 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
![H-events](https://upload.wikimedia.org/wikipedia/commons/e/e0/H-events.png)
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, as 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, in 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
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
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 Bond suggests that changes in the flux of solar energy on a 1,500-year scale may be correlated to the Dansgaard-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, and further destabilizing other ice sheets. In favour of this theory is the non-simultaneity of ice sheet break-up in H1, H2, H4, and H5, where European breakup preceded European melting by up to 1,500 years (Maslin ''et al.'' 2001).
![Thermohaline circulation](https://upload.wikimedia.org/wikipedia/commons/b/b0/Thermohaline_circulation.png)
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
The Gulf Stream, together with its northern extension the North Atlantic Current, North Atlantic Drift, is a warm and swift Atlantic Ocean, Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida a ...
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
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
behaviour. This means that relatively minor changes in freshwater loading into the Nordic Seas, such as 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
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
but further south, mostly in the
subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Geographical z ...
Atlantic, a finding supported by most available
paleoclimatic
Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...
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, and cause 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, thereby 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.
See also
*
Ice sheet dynamics
Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: ...
*
Bond event
References
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* Hunt, A.G. and P.E. Malin. 1998. ''The possible triggering of Heinrich Events by iceload-induced earthquakes.'' Nature 393: 155–158
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Further reading
* 2011 summary of recent work:
External links
William C. Calvin, "The great climate flip-flop"adapted from ''Atlantic Monthly,'' 281(1):47–64 (January 1998).
Columbia University Press Release, December 11, 1995:
{{Good article
Paleoclimatology
Historical geology
Pleistocene events
Geology of the Atlantic Ocean
Icebergs
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