Fidelity of alkenone paleotemperatures in southern Cape Basin sediment drifts

[1] Multiple geochemical analyses were performed in order to test the fidelity of alkenone paleotemperature reconstructions in southern Cape Basin sediment drifts. Sea surface temperatures (SSTs) derived from the alkenone unsaturation ratio, U-37(k'), in two drift cores (TN057- 21 and Ocean Drilling Program Site 1089) are nearly identical during the interval of overlapping analyses, MIS 4-5c. SSTs in a nearby nondrift core (TN057- 6) covary with those in core TN057- 21 throughout MIS 1-5c. While climatological SSTs overlying TN057- 21 and TN057- 6 are similar and consistent with reconstructed Holocene SSTs in the nondrift core, surface waters overlying TN057-21 are similar to6degreesC colder than Holocene alkenone SSTs in the drift core. Temperatures remain similar to6degreesC warmer in TN057-21 relative to TN057-6 during the similar to100 kyr duration of the records. We hypothesize that alkenone-derived SSTs in TN057- 21 are higher than climatological SSTs in the overlying surface water and those in TN057-6 as a result of the winnowing and focusing of sedimentary alkenones produced in warmer waters to the north. Climatological SSTs in the central and northern Cape Basin are 16degrees-20degreesC. A -1.5parts per thousand offset of fine fraction delta(18)O values relative to planktonic foraminifera in TN057-21 supports this hypothesis. Temporal changes in sediment focusing were evaluated directly using uranium series radioisotopes. While higher sediment focusing factors would be expected to result in higher alkenone-derived SSTs if sediment advection were the primary control on down-core changes in alkenone-derived SSTs, Th-230-derived focusing factors indicate no such correlation exists. We conclude that alkenone paleotemperature reconstructions from southern Cape Basin drifts reliably record regional SST variations in the Cape Basin.