Alkenone paleothermometry in the NW Atlantic: A review and synthesis of surface sediment data and calibrations

English Abstract:

Despite its undisputed importance in the global ocean-atmosphere system, the NW Atlantic is arguably the region of the global ocean where reconstructions of past variations in sea surface temperatures (SST) are fraught with the greatest uncertainty. While alkenone paleothermometry has successfully been applied to reconstruct past SSTs in many parts of the northern Atlantic, including off Iceland and Greenland as well as off Newfoundland and Nova Scotia, previous studies have also highlighted the large scatter of alkenone unsaturation (expressed as the UK37 or UK’37 index) determined from N Atlantic surface sediments around the globally established calibrations of UK37 versus SST.

Here we present a synthesis of 94 previously published and 99 new alkenone unsaturation data from surface sediments in the northern and NW Atlantic. These data will be reviewed in the context of global calibrations of UK37’ versus mean annual, SSTs and of regional calibrations that have been developed specifically in an attempt to improve the calibration between alkenone unsaturation and SSTs in the N Atlantic. Scatter around these calibrations, or residuals, will be discussed in the context of ocean frontal systems with large SST gradients, sea surface salinities, sea ice extent, the possible input of allochthonous alkenones from other areas of the Atlantic and even from land, and potential effects of melt water intrusions from the Greenland ice sheet. Neither of these factors, individually or combined, can fully explain the increased scatter around the low-temperature end of UK37’ - SST calibration. The largest decrease in uncertainty, however, is obtained by considering alkenone unsaturation in terms of bloom and/or spring-summer SSTs, rather than mean annual temperatures.