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Heinrich event

Heinrich event

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Heinrich events, first described by marine geologist Hartmut Heinrich
Hartmut Heinrich
Hartmut Heinrich is a German marine geologist and climatologist. Dr. Heinrich is Head of the Physics Department at the Federal Maritime and Hydrographic Agency in Hamburg. He is actively involved in GOOS...

, occurred during the last glacial
Wisconsin glaciation
The last glacial period was the most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago....

 period, or "ice age". During such events, armadas of iceberg
Iceberg
An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice...

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
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....

 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
Laurentide ice sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and a large portion of the northern United States, multiple times during Quaternary glacial epochs. It last covered most of northern North America between c. 95,000 and...

, but others suggest that the unstable West Antarctic Ice Sheet
West Antarctic Ice Sheet
The West Antarctic Ice Sheet is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains which lies in the Western Hemisphere. The WAIS is classified as a marine-based ice sheet, meaning that its bed lies well...

 played a triggering role.

About the events

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
H1,2 are dated by radiocarbon
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...

; H3-6 by correlation to GISP
Greenland Ice Sheet Project
The Greenland Ice Sheet Project was a decade-long project to drill ice cores in Greenland that involved scientists and funding agencies from Denmark, Switzerland and the United States. Besides the U.S. National Science Foundation, funding was provided by the Swiss National Science Foundation and...

2.

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
Dansgaard-Oeschger event
Dansgaard–Oeschger events are rapid climate fluctuations that occurred 25 times during the last glacial period. Some scientists claim that the events occur quasi-periodically with a recurrence time being a multiple of 1,470 years, but this is debated...

 (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) identify the Younger Dryas
Younger Dryas
The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...

 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 fragment
Lithic fragment (geology)
Lithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. They were first described and named by Bill Dickinson in 1970. Lithic fragments can be derived from sedimentary, igneous or metamorphic rocks)...

s", 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
Laurentide ice sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and a large portion of the northern United States, multiple times during Quaternary glacial epochs. It last covered most of northern North America between c. 95,000 and...

 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
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, and thinning by an order of magnitude from the Labrador Sea
Labrador Sea
The Labrador Sea is an arm of the North Atlantic Ocean between the Labrador Peninsula and Greenland. The sea is flanked by continental shelves to the southwest, northwest, and northeast. It connects to the north with Baffin Bay through the Davis Strait...

 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 Sverdrup
Sverdrup
The sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...

 with a duration of 250±150 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:
  • Decreased δ18O
    Oxygen-18
    Oxygen-18 is a natural, stable isotope of oxygen and one of the environmental isotopes.18O is an important precursor for the production of fluorodeoxyglucose used in positron emission tomography...

     of the northern (Nordic) seas and East Asia
    Asia
    Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

    n stalactites (speleothems), which by proxy
    Proxy (climate)
    In the study of past climates is known as paleoclimatology, climate proxies are preserved physical characteristics of the past that stand in for direct measurements , to enable scientists to reconstruct the climatic conditions that prevailed during much of the Earth's history...

     suggests falling global temperature (or rising ice volume) (Bar-Matthews et al. 1997)

  • Decreased oceanic salinity
    Salinity
    Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

    , due to the influx of fresh water

  • Decreased sea surface temperature
    Sea surface temperature
    Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...

     estimates off the West Africa
    Africa
    Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

    n coast through biochemical indicators known as alkenone
    Alkenone
    Alkenones are highly resistant organic compounds produced by phytoplankton of the class Prymnesiophyceae.The exact function of the alkenones remains under debate....

    s (Sachs 2005)

  • Changes in sedimentary disturbance (bioturbation
    Bioturbation
    In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

    ) caused by burrowing animals (Grousett et al. 2000)

  • Flux in plankton
    Plankton
    Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

    ic isotopic make-up (changes in δ13C, decreased δ18O)

  • Pollen
    Pollen
    Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

     indications of cold-loving pine
    Pine
    Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...

    s replacing oak
    Oak
    An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...

    s 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
    Terrigenous
    In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, that are derived from terrestrial environments. Consisting of sand, mud, and silt carried to sea by rivers, their composition is usually related to their source rocks; deposition of these sediments...

     runoff from the continents, measured near the mouth of the Amazon River
    Amazon River
    The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...


  • Increased grain size in wind-blown loess
    Loess
    Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...

     in China
    China
    Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

    , suggesting stronger winds (Porter & Zhisheng 1995)

  • Changes in relative Thorium-230 abundance, reflecting variations in ocean current
    Ocean current
    An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

     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



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, which means that the role of the continents in providing sediments to the oceans was relatively lower.

  • Foram dissolution: Foraminifera 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
    Antarctic Bottom Water
    The 'Antarctic Bottom Water' is a type of water mass in the seas surrounding Antarctica with temperatures ranging from 0 to -0.8◦ C, salinities from 34.6 to 34.7 psu, and a density near 27.88...

    , due to a reconfiguration of oceanic circulation patterns.

  • 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 (Kirby 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 — 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, 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
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 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
Gerard C. Bond
Gerard Clark Bond was a widely and highly respected American geologist. Bond received his Bachelor of Science degree at Capital University in Columbus, Ohio, where his father Ralph Bond was a Professor of Geology...

 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
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 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 — further destabilizing other ice sheets. In favour of this theory is the non-simultaneity of ice sheet break up in H1, 2, 4, and 5, where European breakup preceded European melting by up to 1,500 years (Maslin et al. 2001).

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
Gulf Stream
The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean...

 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
Stefan Rahmstorf
Stefan Rahmstorf is a German oceanographer and climatologist. Since 2000, he has been a Professor of Physics of the Oceans at Potsdam University. He received his Ph.D. in oceanography from Victoria University of Wellington...

 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 — a 0.15 Sv
Sverdrup
The sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...

 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
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

 but further south, mostly in the subtropical Atlantic, a finding supported by most available 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 — 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 — 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.

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