Heinrich Event - Diagnosis of Heinrich Events

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 (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 Sverdrup 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 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.