Alkenone and coccolith records of the mid-Pleistocene in the south-east Atlantic: Implications for the [formula omitted] index and South African climate

Abstract

Successful application of the alkenone palaeothermometer, the U 37 K ′ index, relies upon the assumption that fossil alkenone synthesisers responded to growth-temperature changes in a similar manner to the modern producers, chiefly the coccolithophores Emiliania huxleyi and Gephyrocapsa oceanica. We compare coccolith and U 37 K ′ data from ODP Site 1087 in the south-east Atlantic between 1500 and 500 ka, and show that evolutionary events and changes in species dominance within the coccolithophore populations had little impact on the U 37 K ′ record. The relative abundances of the C 37 and C 38 alkenones also closely resembled those found in modern populations, and suggest a similar temperature sensitivity of U 37 K ′ during the early and mid-Pleistocene to that found at present. These results support the application of the U 37 K ′ index to reconstruct sea-surface temperatures (SSTs) throughout the Quaternary. The U 37 K ′ record at ODP Site 1087 contains an SST signal that documents the emergence of the 100-kyr cycles that characterise the late Quaternary ice volume records. This is preceded by significant cooling at ODP Site 1087, marked by a negative shift in SSTs and a positive shift in the planktonic δ 18O some 250-kyr earlier, at ca 1150–1000 ka. This results in a permanent fall in average SSTs of around 1.5 °C. The predicted increase in aridity onshore as a result of this cooling can be identified in a number of published records from southern Africa, and may have played a role in some important evolutionary events of the mid-Pleistocene.