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 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 last glacial period but notably absent from the penultimate glaciation. 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 (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, 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 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. 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 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 (D-O) events, which are best