Ecology and biogeochemistry of alkenone production at Station ALOHA

In our study of Station ALOHA, a site representative of the oligotrophic North Pacific subtropical gyre (NPSG), alkenone content and composition were examined in (1) sinking particulate materials collected in sediment traps moored at 2800 m during 2 year-long deployments (1992-1993 and 2000-2001) and (2) suspended particulate materials collected throughout the euphotic zone in two contrasting seasons (winter and summer). Each year, sediment trap records documented pronounced alkenone export events in both seasons. The winter event appears triggered by eddy diffusive mixing of nutrients into the upper water column and the release of light limitation on alkenone-producing photoautotrophs growing in the deep chlorophyll maximum layer (DCML). The summer event originates apparently from a stratified euphotic depth well above the DCML where others have now documented the occurrence of N2 fixation-based 'new production'. Although our time series record is yet limited, it is likely that the absolute magnitude of alkenone export and its quantitative importance in winter relative to summer is an ecological feature of the NPSG which can oscillate significantly on decadal time scales in response to climatic forcing. Despite the significant seasonal and interannual differences in alkenone export noted in our study, findings suggest that paleotemperature assessments based on analysis of the alkenone unsaturation index U37K' measured in underlying sediments would likely be unimpaired by the higher frequency temporal variability in the surface water ecology of alkenone-producers. When translated using a widely accepted calibration equation (U37K'=0.034 T+0.039), U37K' values in average sediment trap particles are consistent with the mean annual sea-surface temperature at Station ALOHA.