Alkenones as Proxies of Sea Surface Temperatures: Seasonal Production of Alkenones in Guaymas, Carmen and Santa Barbara Basins

Abstract

Use of molecular proxies in palaeoceanography began with the realization (Brassell et el., 1986) that the biosynthesis of unsaturation in alkenones (codified as U~7) by haptophyte algae is temperaturedependent and can survive in sediments to provide a record of sea-surface temperatures (SSTs). A combination of culture experiments, field collections and sediment analyses (e.g. Prahl et el., 1988; RosellMel6 et el., 1995) has established the veracity of U3K7 as a measure of SSTs and subsequent investigations have further confirmed, developed and calibrated the application of alkenones as palaeoclimatic indicators. They also prompt continuing efforts to examine and critically evaluate environmental controls on the timing and intensity of seasonal production of alkenones in the modern ocean and concurrent assessment of the transport of these alkenone SST records through the water column to the underlying sediments. Here, sediment trap studies help provide a critical link between the production of alkenones in surface waters and their sedimentary records that form the basis for molecular pa laeocl imat ic studies. Specifically, exploration of U3K7-temperature relationships in sediment trap particulates recovered during long-term, high-frequency, time series affords the opportunity to monitor seasonal changes in alkenone production and to determine the temporal character of the temperature signal recorded by the annual flux of alkenone sedimentation. The locations chosen in this study provide characteristics well-suited to addressing these goals. The Gulf of California (GoC) experiences a sharp seasonal contrast in SSTs as seasonal shifts in winds cause changes in surface water masses which produce an annual temperature range of >15~ The recent historical record of alkenones in the Santa Barbara basin (SBB) is well documented (e.g. Kennedy and Brassell, 1992) enabling direct comparison of current sedimentation with past events. Furthermore alkenoneBiogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA