Heinrich event

A Heinrich event is a natural phenomenon in which large groups of icebergs break off from the Laurentide ice sheet and traverse the Hudson Strait into the North Atlantic. First described by the marine geologist Hartmut Heinrich, they occurred during five of the last seven glacial periods over the past 640,000 years. Heinrich events are particularly well documented for the Wisconsin glaciation, during the Last Glacial Period, but notably absent from the Penultimate Glacial Period. The icebergs contained rock mass that had been eroded by the glaciers, and as they melted, the material was dropped to the sea floor as ice rafted debris and formed 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 Heinrich events, most of which imply instability of the massive Laurentide Ice Sheet, a continental ice sheet covering most of 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 the instability is still debated.

Description

The strict definition of a Heinrich event is the climatic event causing the ice rafted debris (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 can refer also to the associated climatic anomalies registered at other places around the globe at approximately the same time periods. The events are rapid and last probably less than a millennium, a duration varying from one event to the next, and their abrupt onset may occur in mere years. 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 then makes it more difficult to deduce whether they occurred during other glacial periods in the Earth's history. Some researchers identify the Younger Dryas 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 events, which are best recorded in the North Greenland Ice Core Project. However, difficulties in synchronising marine sediment cores and Greenland ice cores to the same time scale have 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 3 mm (1⁄8 in) size range. The larger-size fractions cannot be transported by ocean currents and are thus interpreted as having been carried by icebergs or sea ice that 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, which covered 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 (1,900 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 (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, equivalent to a fresh water volume of about 2.3 million cubic kilometres (0.55 million cubic miles), or a 2 ± 1 m (6 ft 7 in ± 3 ft 3 in) sea-level rise. Several geological indicators fluctuate approximately in time with those 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:

Increased δ18O of the northern (Nordic) seas and East Asian stalactites (speleothems), which by proxy suggests a falling global temperature (or a rising ice volume) Decreased oceanic salinity from the influx of fresh water Decreased sea surface temperature estimates off the West African coast through biochemical indicators known as alkenones (Sachs 2005) Warming of the subsurface ocean in the subpolar North Atlantic Changes in sedimentary disturbance (bioturbation) caused by burrowing animals 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 foraminiferal abundance, which the pristine nature of many samples does not allow to be attributed to preservational bias and has been related to reduced salinity 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 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 Expansion of grass and shrubland across large areas of Europe The global extent of those records illustrates the dramatic impact of Heinrich events.

Unusual Heinrich events

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