recorded in the NGRIP Greenland ice core. However, difficulties in synchronising marine sediment cores and Greenland ice cores to the same time scale 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 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 which 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 then covering 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 (1,865 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 (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 or a sea-level rise. 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: Increased δ18O of the northern (Nordic) seas and East Asian stalactites (speleothems), which by proxy suggests falling global temperature (or rising ice volume) 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) Warming of the subsurface ocean in the subpolar North Atlantic Changes in sedimentary disturbance (bioturbation)