Determination of sediment provenance at drift sites using hydrogen isotopes and unsaturation ratios in alkenones

The large (20‰) hydrogen isotopic gradient in surface waters of the northwest Atlantic Ocean is exploited to track changes in the source of alkenones to the Bermuda Rise sediment drift. Cultures of the predominant alkenone-producing coccolithophorid, E. huxleyi, were grown in deuterium-enriched seawater and shown to possess alkenones with a D/H ratio that closely tracked the water D/H ratio (r 2 0.999, n 5 isotopic enrichments) with a fractionation factor () between 0.732 and 0.775. A hydrogen isotopic depletion of -193 3‰ (n 9) was measured in alkenones from suspended particles relative to seawater in the subpolar and subtropical northwest Atlantic Ocean. This value was used to calculate the water D values in which alkenones from Bermuda Rise sediment were synthesized, and by extension, the water mass in which they were produced. Applying this technique we find that 60% to 100% of the alkenones in late Holocene Bermuda Rise sediment were produced in deuterium-depleted subpolar water to the northwest of the drift. To reconcile values of the alkenone unsaturation ratio (U k= 37 ), a widely used proxy for sea surface temperature, with the D values of alkenones in late Holocene sediments from the Bermuda Rise at least three sources of sediment must be invoked: a cold, very isotopically depleted source, almost certain to be the Scotian Margin; a warm, moderately isotopically-depleted source, likely to be the northwestern edge of the subtropical gyre; and a cold, isotopically enriched source, which we hypothesize to be the subpolar waters overlying the main branch of North Atlantic Deep Water flowing southwest from the Nordic Seas. Copyright © 2005 Elsevier Ltd Paleoclimate records with sufficient length and temporal resolution to study the occurrence and causal mechanisms of abrupt climate change are exceedingly rare. Rapidly deposited ocean sediments provide the best archive for studying these events through geologic time, but such sites in the open ocean are limited to sediment drift deposits such as the Bermuda Rise in the northwest Atlantic. Using multiple climate proxies in a single core is becoming more common in high-resolution pa- leoclimate investigations, but a major potential concern for this approach arises from the possibility that the fine fraction of sediment (e.g., 63 m), and the climate proxies within it, may represent conditions far from the deposition site. Here we propose and test the hypothesis that hydrogen isotope ratios of alkenones, a class of lipids from phytoplankton, may provide insight into the source of fine-grained sediment. Because of their restricted sources, broad geographic distri- bution, and excellent preservation properties, alkenones are of particular interest in the emerging field of compound-specific hydrogen isotopic analysis. The sedimentary abundances, ex- tents of unsaturations, and isotopic compositions of alkenones provide quantitative and near-continuous records. We isolated alkenones from cultured unicellular algae (haptophyte Emil- iania huxleyi), surface ocean particulate material, and open ocean sediments to determine the extent and variability of hydrogen isotopic fractionation in the di-, tri-, and tetraunsat- urated C37 compounds. We then compared the D values of the alkenones in surface sediments between the Bermuda Rise and the Scotian Margin above which a large (20‰) D gradient exists. We determined the fractionation between alkenones from suspended particulate samples and the water in which the phytoplanton lived, and examined the variability of alkenone D values during the Medieval Warm Period-Little Ice Age climate transition (cf. Keigwin, 1996) at the Bermuda Rise.