Heinrich events are episodes during which large numbers of icebergs are released by glaciers into the North Atlantic. More than half a dozen Heinrich events occurred during the last ice age, every 10,000 years or so from 135,000 to 12,000 years ago. Heinrich events add large quantities of freshwater to the ocean, which impacts the climate of the world, but most strongly the Northern Hemisphere.
Some scientists propose that when ocean circulation is destabilized by a Heinrich event, the oceans of the northern and southern hemispheres act like a climate see-saw or pendulum, with warming in the Antarctic accompanied by cooling in the Arctic and vice versa. If so, Heinrich events—or other events that act to push one side of the polar see-saw—may have global climate effects. Although conditions do not exist today for a classic Heinrich event, some scientists speculate that loss of the Arctic sea-ice cap and accelerated glaciation from Greenland due to global warming may act as a significant push on the see-saw. Scientific knowledge of these processes is still forming.
Historical Background and Scientific Foundations
Heinrich events are named after German scientist Hartmut Heinrich, who discovered them in 1988 while analyzing layers in North Atlantic sediments. These sediments are thick beds of muck laid down over many thousands of years by small particles settling down to the ocean floor. These particles include the shells of foraminifera, one-celled organisms that exist by trillions in the oceans. Heinrich also noted layers containing coarse mineral sediment in the core samples. These small pieces of rock, the size of sand grains or smaller, were too large to have been carried by currents out to the middle of the sea where they sank, and so must have been brought there by icebergs. This would have occurred when large, dirty chunks of ice were dumped into the sea by glaciers. The icebergs then floated out across the ocean, melting as they went. The dirt attached to them would have sunk wherever it happened to be released by the melting ice, drizzling down onto the seabed to form the layers we now observe.
These layers of dropstone, as the mineral fragments are called, show several striking features. First, they cover large areas of the Atlantic floor, most thickly near North America. The ocean must have been traversed by legions of the icebergs to supply so much sediment. Second, the sediment appears in distinct, thin layers, so the episodes of ice rafting must have been relatively brief. Third, by examining the characteristics of the dropstone fragments, the source of the rock can be identified. This is found to be primarily the bedrock of Canada, delivered to the ocean by the Laurentide ice sheet that covered most of northern North America during the last Ice Age. (Some rock also came from the Fennoscandian ice sheet, which covered Europe.) Fourth, the density of fossil foraminifera in the sediments greatly decreased during each Heinrich episode, from thousands of shells per gram of sediment to hundreds. This decrease in foraminifera populations extends all across the Atlantic, and shows that the biological productivity of the whole ocean must have been greatly lowered during the Heinrich event.
Other facts are known about the Heinrich events. They occurred at about 12,000, 23,000, 26,500, 29,000, 37,000, 40,000, and 75,000 years ago. All coincided with the coldest parts of the last Ice Age. At about the same time that productivity fell in the Atlantic, it rose near Antarctica, at the opposite pole of the planet. The mechanisms linking the two events are not well-understood, but some scientists have proposed that massive influxes of freshwater to the North Atlantic in the form of melting icebergs dilutes the sea, making it less salty and less dense. This would have the effect of slowing the thermohaline or Great Conveyor Belt circulation of the oceans, which consists of warm surface currents conveying water to the poles and cold deep currents bringing it back to the tropics again. The thermohaline circulation is a crucial part of the global climate system and any changes in its behavior would have many effects.
Impacts and Issues
For years there were uncertainties about the duration and size of the Heinrich events, but in 2004, scientists announced that they had narrowed down the duration of Heinrich event 4 (about 40,000 years ago) to 250 years plus or minus 150 years. They also found that the amount of ice released was enough to raise sea levels 3.3 to 6.6 ft (1 to 2 m). Scientists also announced in 2004 that they had linked ocean tides to the Heinrich events. Although sea levels during the last Ice Age were many meters below today's, the magnitude of high tide varied over time. Advanced computer modeling of tidal patterns shows that the tides were highest at the time of the Heinrich events. It is therefore likely that unusually high tides floated ice free from glacial edges, enabling the Heinrich events. A similar phenomenon is seen today when blocks are broken off the Antarctic ice sheet when tides are highest.
WORDS TO KNOW
CORE SAMPLE: Cylindrical, solid sample of a layered deposit, cut out of the deposit at right angles to its bedding planes. Core samples of lake-bottom and sea-bottom sediments, or of ice layers of the Greenland or Antarctic ice caps, can supply information about past climate variations and changes in atmospheric composition. For example, Antarctic ice cores supply climate data going back 800,000 years.
DROPSTONE: Mineral particles carried out to sea on icebergs and then dropped to the ocean floor when the icebergs melt. Dropstone layers in sea-floor sediments record episodes of strong glacial iceberg activity, for example, Heinrich events. Volcanic dropstones, thrown out to sea by volcanic eruptions, are also found.
FORAMINIFERA: Single-celled marine organisms that produce small shells called tests in order to inhabit and float free in ocean surface waters. There are thousands of species of foraminifera. When the organism dies, its test sinks to the sea bottom as sediment, forming thick deposits over geological time. Because the numbers and types of foraminifera are climate-sensitive, analysis of these sediments gives data on ancient climate changes.
GLACIATION: The formation, movement, and recession of glaciers or ice sheets.
GREAT CONVEYOR BELT: The overturning circulation of the world's seas, driven by temperature and salinity differences between the poles and tropics; also called the thermohaline circulation or meridional overturning circulation. Because the great conveyor belt transports thermal energy from the tropics toward the poles, it is a central component of Earth's climate machine.
ICE AGE: Period of glacial advance.
PALEOCLIMATE RECORD: The history of Earth's climate prior to the beginning of instrumental weather records in the late 1800s. “Paleo” means old or ancient, from the Greek palaiois for old.
THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.
Heinrich events are associated with abrupt climate changes in the paleoclimate (prehistoric climate) record. Some scientists studying Heinrich events have warned that one cannot rule out the possibility of a human-caused shift in ocean circulation that leads to abrupt climate change by related mechanisms.
Cox, John D. Climate Crash: Abrupt Climate Change and What It Means for Our Future. Washington, DC: Joseph Henry Press, 2007.
Broecker, Wallace. “Massive Iceberg Discharges as Triggers for Global Climate Change.” Nature 372 (1994): 421-424.
Heinrich, Hartmut. “Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the Past 130,000 Years.” Quaternary Research 29 (1988): 142-152.
Jennerjah, Tim C. “Asynchronous Terrestrial and Marine Signals of Climate Change During Heinrich Events.” Science 306 (2004): 2236-2239.
Lehman, Scott. “Ice Sheets, Wayward Winds and Sea Change.” Nature 365 (1993): 108-110.
Paillard, D., and L. Labeyrie. “Role of the Thermohaline Circulation in the Abrupt Warming after Heinrich Events.” Nature 372 (1994): 162-164.
Roche, D., et al. “Constraints on the Duration and Freshwater Release of Heinrich Event 4 Through Isotope Modeling.” Nature 432 (2004): 379-382.
Sachs, Julian, and Robert F. Anderson. “Increased Productivity in the Subantarctic Ocean During Heinrich Events.” Journal of Quaternary Science 16 (2001): 321-328.
Seidov, Dan, and Mark Maslin. “Atlantic Ocean Heat Piracy and the Bipolar Climate See-Saw During Heinrich and Dansgaard-Oeschger Events.” Nature 434 (2005): 1118-1121.