A DELTA-C-13 RECORD OF UPPER NORTH-ATLANTIC DEEP-WATER DURING THE PAST 2.6-MILLION YEARS

Benthic foraminiferal delta(13)C data from site 502 in the Caribbean Sea (sill depth similar to 1800 m) indicate that throughout the past 2.6 m.y., glacial delta(13)C values in the middepth Atlantic were higher during glaciations than interglaciations. This is interpreted as indicating a greater proportion of Upper North Atlantic Deep Water (UNADW) relative to southern source waters during glaciations. The contribution of UNADW during interglaciations to the middepth Atlantic remained approximately constant, and the contribution during glaciations may have been as much as 10% higher in the late Pleistocene than in the late Pliocene. This small increase is in striking contrast to the much larger decrease in glacial Lower North Atlantic Deep Water (LNADW) contribution relative to southern sources, from about 80% to about 20%, that occurred over the past 2.6 m.y. Glacial intensification over the past 2.6 m.y. was probably coupled with a decrease in northward heat transport by the upper limb of the North Atlantic circulation cell, as was previously suggested on the basis of a LNADW record alone. Late Pleistocene (1 Ma-present) delta(13)C values in the Caribbean Sea were approximately 0.2 parts per thousand higher than they were from 2.6 to 2.0 Ma. The delta(13)C rise is not due to an increase in the mean ocean delta(13)C value, nor can it be entirely attributed to an increase in the proportion of high-delta(13)C source waters. An increase in the delta(13)C value of the surface source waters must have contributed to the delta(13)C rise.