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Abrupt increase in Greenland snow accumulation
at the end of the Younger Dryas event
R. B. Alley, D. A. Meese, C. A. Shuman, A. J. Gow,
K. C. Taylor, P. M. Grootes, J. W. C. White, M. Ram,
E. D. Waddington, P. A. Mayewski & G. A. Zielinski
Nature 362, 527 - 529 (08 April 1993)
THE warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate, the best-known of which is the Younger Dryas event. Despite much study of the causes of this event and the mechanisms by which it ended, many questions remain unresolved. Oxygen isotope data from Greenland ice cores suggest that the Younger Dryas ended abruptly, over a period of about 50 years; dust concentrations in these cores show an even more rapid transition (20 years). This extremely short timescale places severe constraints on the mechanisms underlying the transition. But dust concentrations can reflect subtle changes in atmospheric circulation, which need not be associated with a large change in climate. Here we present results from a new Greenland ice core (GISP2) showing that snow accumulation doubled rapidly from the Younger Dryas event to the subsequent Preboreal interval, possibly in one to three years. We also find that the accumulation-rate change from the Oldest Dryas to the Bø11ing/Allerød warm period was large and abrupt. The extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the North Atlantic climate system.

The Younger Dryas:
From whence the fresh water?
by T. C. Moore Jr.
Paleoceanography
v 20, PA4021
Oxygen isotopic records of meltwater outflow and records of sea level change do not support the idea that fresh waters derived solely from the melting of Northern Hemisphere ice sheets was likely to have stabilized the upper layers of the North Atlantic Ocean and prevented deep convection during the Younger Dryas. Yet there are paleoceanographic indicators that point to a pause in the formation of North Atlantic Deep Water during the Younger Dryas. This apparent conflict in evidence may be resolved by the existence of large, relatively thick, tabular icebergs that spilled out of the Arctic and into the Norwegian-Greenland Sea and North Atlantic. The melting of large icebergs would have no impact on sea level but combined with meltwater runoff would provide enough fresh water to “cap” the North Atlantic. The timing of the start and the end of the Younger Dryas, however, may not have been directly related to freshwater supply.