YOUNGER DRYAS-STYLE COOLING EVENTS AT GLACIAL TERMINATIONS I-VI AT ODP SITE 658: ASSOCIATED BENTHIC delta C-13 ANOMALIES CONSTRAIN MELTWATER HYPOTHESIS

High-resolution planktonic and epibenthic stable isotope records from Ocean Drilling Program site 658 off northwest Africa provide a basis for a detailed study of glacial terminations I-VI during the last 650,000 years. T he duration of the terminations was about one half to one quarter of an orbital precession cycle (5800-10,700 years), when its amplitude was high. At low amplitudes, the terminations lasted longer than half an obliquity cycle ( 29,000 years). Marked climatic rebound similar to the Younger Dryas, each with a duration of 1000-2500 years, subdivided all six terminations into distinct steps A, B, and C. Important parts of the deglacial steps were as brief as 700-1000 years. The speed of climatic change suggests that special associations existed between orbital forcing and inherent instability of the ice sheets. In harmony with published models, the more rapid pulses of glacial meltwater incursions to the northern North Atlantic led to one or more brief short-term shut-downs of North Atlantic Deep Water formation. This process is reflected by pronounced benthic delta C-13 minima that precisely coincide, in most cases, with the end of the delta O-18 deglaciation steps and immediately terminate with succeeding Younger Dryas-style cooling events. Thus we conclude that the rebounds resulted from a short-term antecedent estuarine circulation regime in the North Atlantic, except for the Younger Dryas itself, which succeeded the Alleroid with its well ventilated Atlantic deepwater circulation and hence continues to be an enigma.