Systematic pattern in U37K′ – Temperature residuals for surface sediments from high latitude and other oceanographic settings

Twenty-seven multicores, collected in late summer 2004 from coastal and inland marine sites throughout SE Alaska (∼55 to 61°N), were analyzed for C37 to C39 alkenones of specific haptophyte origin. This biomarker signal, detected in core-top sediments from 20 of the sites, was quantitatively most significant in organic carbon and biogenic silica-rich sediments deposited south of Icy Strait (∼58°N), along the outer coastline of Baranof and Prince of Wales islands. U37K′-based water temperature estimates derived using a standard calibration equation (U37K′ = 0.034T + 0.039) were uniform throughout the study area, averaging 11.9 ± 1.0 °C, a value ∼4 °C warmer than mean annual SST (maSST) for the region. Epoxides, indicative of selective alkenone degradation by an aerobic bacterial process, were also detected and quantified in all sediments containing alkenones. Semi-quantitative accounting of the impact that compound-selective degradation can impose on alkenone unsaturation patterns indicates the standard U37K′-based temperature estimates could be too warm by a variable amount (0.7–2.4 °C) depending upon the site, with an overall average of ∼1.4 °C. Even after adjustment for this potential diagenetic ‘warming’ effect, estimates still best match summer and not maSST in this subarctic region. Re-examination of alkenone signatures preserved in sediment from the North Atlantic and on the continental margin located south of ∼45°S along the Chilean coast indicates the apparent summer signal in U37K′-derived SST estimates is not unique to SE Alaskan sediments but rather a common finding when alkenone thermometry is applied to sediments from high latitude environments. Examination of model residuals from analysis of surface sediment samples collected at lower latitude settings along the west coast of the Americas also identifies other sites where significant biogeographic patterns seem apparent and viable oceanographic explanations can be offered.