Coretop Perspective on Alkenone Signatures Preserved Beneath Inland Waters of SE Alaska

In late summer 2004, 27 multicores were collected from coastal and inland water sites throughout SE Alaska (~55° to 61°N). Examination of coretops (0-2cm) revealed long-chain alkenones of haptophyte origin were only present significantly in organic carbon, biogenic silica-rich sediments depositing south of ~58°N, along the outer coastline of the Queen Charlotte and Baranof Islands. Contrary to our prior experience with marine sediment analysis, compositions in the 16 identified sites containing these biomarkers could only be reliably quantified if urea adduction was employed as an additional chemical clean- up step prior to alkenone analysis by gas chromatography with flame ionization detection. Water temperature estimates derived from `clean' U^{K'}37 signatures using a standard calibration equation (U^{K'}37 = 0.034 x T + 0.039) were remarkably uniform, averaging 11.7±0.9°C. The estimates are ~4°C `colder' than actual sea-surface temperatures (SST) measured at the time of core collection in overlying waters at each sampling site. These estimates consistently matched water temperature measured at ~20m depth by CTD casts. Nutrient determinations on collected samples showed surface waters at each of these sites were typically nitrate-depleted to a depth of ~20m, below which a steep nitricline existed. If the U^{K'}37-derived temperature estimates are accurate, at least some fraction of alkenone export to sediments in this region necessarily originates from subsurface production. Formal clarification of how alkenone export production from surface waters acts throughout an annual cycle to `paint' the U^{K'}37-based temperature record in modern sediments for this region will require further oceanographic study. Such effort would benefit the accurate use of U^{K'}37 analysis for purposes of regional paleoceanographic SST reconstruction.