A biomarker perspective on prymnesiophyte productivity in the northeast pacific ocean

Long-chain alkenones derived from prymnesiophyte algae were analysed in 1-year sediment trap time series (September 1987–1988) from three sites along a 630 km offshore transect at ∼42°N in the northeast Pacific Ocean. Biomarker flux monitored at 1000 m water depth was evident throughout the year at all sites and showed a consistent seasonal maximum in late spring which increased in amplitude with distance offshore. The integrated annual biomarker flux was constant along the transect, despite differences in seasonality between sites. Alkenone unsaturation patterns were remarkably uniform throughout the time series, reflecting an algal growth temperature of 10.6 ± 1.1°C. This value corresponds to regional water temterature at the sea-surface in winter. It recurs in seasonal upwelling near the coast and at the depth of the subsurface chlorophyll maximum offshore during seasons of stratification. These biomaker observations, interpreted in view of trap data for total organic (TOC) and inorganic carbon and ancillary hydrographic information, help to clarify seasonal productivity patterns for alkenone-producing prymnesiophytes in the northeast Pacific Ocean. Sediments accumulating with distance offshore along the sampling transect change from suboxic and Mn-reducing at the water-sediment interface to aerobic throughout the depths penetrated by box coring. Comparison of alkenone and TOC accumulation rates in surface (0–1 cm) sediments with corresponding annual fluxes integrated by the trap time series, shows that the fraction of both properties accounted at the seafloor is highest and similar under sub-oxic conditions (∼25%), and declines steeply and disproportionately as aerobic conditions are encountered farther offshore. Only 0.25 and 3.1% of the annual inventory for alkenones and TOC in traps are accountable in surface sediments from the slowest accumulating, most oxidizing site farthest offshore. Despite major loss of biomarker to early diagenesis, surface sediments and trap particles display consistent alkenone unsaturation patterns. Results from this study provide a necessary background for palaeoceanographic reconstruction of the northeast Pacific Ocean from stratigraphic analysis of alkenone abundances, unsaturation patterns and isotopic compositions in sediment cores.