Late Quaternary temperature variability in the Benguela Current System derived from alkenones

Three sediment cores on a transect across the continental slope off Namibia at about 23 degrees S were investigated for alkenone-derived past sea-surf ace temperature (SST) and total organic carbon (TOC) content. These records are used to reconstruct variations of surface circulation, coastal upwelli ng, and paleoproductivity in the northern Benguela Current System for the l ast 150,000 yr. The SST record most distant from the coast resembles a SST pattern typical of the pelagic ocean, with the lowest SST at full-glacial p eriods and the highest SST during the Eemian and the Holocene. In contrast to the modern conditions where annual mean SST decreases toward the coast, the shelf-edge SST record has the most prominent warm anomalies of about 2 degrees C during isotope stages 2 and 6 compared with the open ocean. The g lacial SST minimum in the record close to the shelf is observed between 50, 000 and 35,000 yr B.P., while the record midway along the transect shows in termediate temperature conditions between the offshore and nearshore record s. The causal process for the warm anomalies under full ice-age conditions close to the coast may be similar to that of recent "Benguela Nino events" that originate from perturbations in the tradewind system over the western tropical Atlantic. During these events the Angola-Benguela Front, located a t about 16 degrees S, weakens and intensive southward protrusions of tropic al water masses extend into the nearshore upwelling area as far as 25 degre es S. Thus, the two nearshore records primarily responded to variations in the time-integrated balance between upwelling intensity and southward protr usions of anomalously warm and nutrient-poor Angolan surface waters, as ind icated by the good anticorrelation of SST and TOC content. Accordingly, sur face water cooling off Namibia over the last 150,000 yr was most intense du ring stage 3 due to strong winds that worked in favor of upwelling and a de crease of Angolan warm water influence. (C) 1999 University of Washington.