HOLOCENE VARIABILITY OF THE ATLANTIC AND INDIAN OCEAN SUBTROPICS

The Holocene was climatically unstable, having been punctuated by a series of cooling events which persisted for centuries and recurred every 1000-2000 years. In the subpolar regions, these events are represented by glacier advances and increases in the concentration of ice-rafted lithic grains. Although large changes in surface ocean currents are implicated, to date there is little evidence for accompanying large changes in deep water circulation.

How were these Holocene climate instabilites recorded at lower latitudes? To evaluate the timing and amplitudes of Holocene climate variability in the subtropics, we examined high accumulation rate sediment cores from the eastern subtropical Atlantic and from the Gulf of Aden. A detailed Holocene record of subtropical Atlantic sea-surface temperature (SST) variations was reconstructed from planktonic foraminiferal assemblages at ODP Site 658 off NW Africa. This detailed record reveals a series of large amplitude (2-4 deg.C) cooling events which are synchonous (within 14C chronologies) with the subpolar SST variations. Also, this record indicates that Holocene variability increased from ca. 5 ka BP toward the present, culminating in the largest amplitude variations associated with the Little Ice Age-Medieval Warm Period oscillation.

The Red Sea is closed inland basin where strong surface evaporation and winter cooling in the north promotes the formation highly saline (40 psu), warm (~18-20 deg.C) intermediate waters (~ca. 700-800m) in the north which are exported southward into the Gulf of Aden. The subsurface export of RSOW is a monitor of regional climate variability and decadal at longer timescales due to the short residence time of these waters in the Red Sea (ca. 25 years). Preliminary benthic oxygen isotope and Mg/Ca data from high sedimentation rate box cores in the Gulf of Aden (western Indian Ocean) document coeval millennial-scale 1-2 deg.C cooling events recorded by RSOW. Collectively, these data argue for large and abrupt variations in subtropical climates which are in phase with high-latitude variability.