ASYNCHRONOUS CLIMATE CHANGES BETWEEN JAPAN AND THE NORTH ATLANTIC DURING THE LAST DEGLACIATION: IMPLICATIONS

High-resolution pollen analysis of the annually-laminated lacustrine sediments of Lake Suigetsu, Japan indicates that climatic changes in Japan during the Last termination were not synchronous with those of the North Atlantic. We have analyzed more than 400 horizons from the part of the Suigetsu profile which spans the period ca. 16,000 to 10,000 Suigetsu varve years BP (SG vyr BP). The conventional pollen data were converted into thermal indices (e.g. mean annual temperature; January and July temperatures; etc.) using a modern analogue technique. The results are correlated with the grayscale curve from Cariaco Basin, Venezuela, using the varve chronologies established independently from the two basins.

The results from Lake Suigetsu suggest fluctuations between cold and warm phases. Alhough the overall trends resemble those that characterise North Atlantic records over the same interval, the abrupt transitions are markedly asynchronous between the two regions. A warm phase in the Suigetsu profile (SGPI-1c) that seems to correspond to the Bölling event, started at 15,000 SG vyr BP, about 3 centuries earlier than the onset of the equivalent phase in the Cariaco sequence. Conversely, a Younger Dryas-like cold phase in Japan (SGPS-1) started about 4 centuries later (ca. 12,000 SG vyr BP) than the onset of the YD in the North Atlantic. The amplitude of cooling in Japan was bigger in winter than in summer, though the amplitude of both was smaller than that reconstructed for the North Atlantic region.

We propose that the ocean circulation system in the Pacific responded more quickly (and probably more linearly) to the increase of solar insolation during the last deglaciation, whereas the North Atlantic system could only jump from one circulation pattern to another when external forcing became strong enough to overcome the threshold. This explanation is also supported by results obtained from a longer core sequence from a second lake, adjacent to Lake Suigetsu.

As for the abrupt cooling during the YD, we propose that thermohaline circulation changes in the North Atlantic extended their influence to Japan through an oceanic mechanism which was delayed by several centuries. However, the Asian winter monsoon might have also played an important role in "Connecting" these two distant regions.