SYNCHRONOUS HIGH AND LOW-LATITUDE NORTH ATLANTIC CLIMATE CHANGE DURING THE LAST DEGLACIATION

Abrupt climate shifts during the last deglaciation, similar to the Glacial-Bolling/Allerod-Younger Dryas-Preboreal sequence of the high-latitude North Atlantic region, have been identified in tropical South America, primarily through changes in precipitation and trade wind intensity. Records of vegetation, dustiness, wind strength and river runoff from northern tropical South America reveal dry and windy conditions during the Glacial and Younger Dryas, and non-windy, wet climates during the Bolling/Allerod and Preboreal. This pattern of changes is consistent with a southward shift of the ITCZ during times of cold high-latitude northern climate, with northward ITCZ shifts during northern warming. Such ITCZ variability could be forced by mechanisms originating at high northern latitudes (e.g., cooling following shutdown of North Atlantic Deep Water formation) or possibly in the tropics (e.g., an ENSO-like mechanism operating at longer time scales). These different forcing scenarios might be identifiable through leads and lags between tropical and high-latitude sites. The Cariaco Basin, off the coast of Venezuela, is an ideal location for recovering high-resolution records of both tropical and high-latitude North Atlantic variability.

Cariaco records of the Younger Dryas onset show a dramatic reduction in North Atlantic thermohaline circulation at the same time as low-resolution pollen data from Cariaco and elsewhere in northern South America record a shift from humid rainforest during the Bolling/Allerod to dry grasslands during the Younger Dryas. To investigate the relative timing of these rapid changes, we have constructed high-resolution records of tropical vegetation change from vascular plant biomarkers in Cariaco Basin sediments. Molecular composition and stable isotopic analyses of higher plant leaf waxes were used to distinguish grasslands from rainforest vegetation, and show distinct evidence of drier/cooler and wetter/warmer conditions during the Glacial-Younger Dryas and Bolling/Allerod-Preboreal climatic periods, respectively. At the current decadal sampling resolution, shifts in northern South American vegetation appear synchronous with NADW changes in the high latitude North Atlantic, consistent with either high- or low-latitude forcing of the Younger Dryas.