Episodic particle flux in the deep Sargasso Sea: an organic geochemical assessment

Since 1978, the Oceanic Flux Program (OFP) time-series sediment trap study has continuously measured particle fluxes in the deep Sargasso Sea (31 degr ees 50'N, 64 degrees 10'W). One feature of this 19 + year record has been t he episodic occurrence of large, short-lived flux maxima that are not assoc iated with the annual spring bloom. These maxima generally occur during the Dec.-Jan. period, but not necessarily every year. They have also occurred in other seasons. In January 1996, OFF traps located at 3200 and 3400 m dep ths intercepted a major flux "event" in which there was an abrupt, threefol d increase in mass flux at both depths. Mass flux measured at 3200 m during the event (87 mg m(-2) d(-1)) was the highest recorded since biweekly reso lved sampling was begun in 1989. Organic biomarker analyses of material col lected prior to, and during this high flux event determined that there was an abrupt change in material composition associated with the sudden flux in crease. Prior to the event, cholesterol, a single bacteria-derived C-27 hop anone (22,29,30-trisnorhopan-21-one), and saturated and odd/branched fatty acids predominated: these compounds indicated that the sedimenting material was extensively degraded. During the event, organic material was greatly e nriched in C-26-C-29 phytosterols, haptophyte algae-derived C-37-C-39 alken ones, labile polyunsaturated acids, degradation products such as steroidal ketones, and also in bacteria-derived compounds such as C-27-C-34 hopanoids and beta and omega - 1 hydroxy acids. These compounds indicated the organi c fraction contained a large amount of relatively fresh phytoplankton-deriv ed debris and tracers of bacterial biomass and metabolism, which suggested that the sinking material was undergoing active bacterial decomposition. Th us, the flux "event" appears to have resulted from a short-lived bloom in t he overlying surface waters which, for reasons not currently apparent, was inefficiently remineralized in the upper ocean and rapidly settled to depth . These findings are the first direct documentation of episodic delivery of labile phytoplankton-derived detritus to the deep ocean in an oligotrophic mid-gyre region. Such transient productivity/flux events may account for a significant fraction of the export flux of biologically available carbon a nd easily remineralized elements, not only in highly productive areas, but throughout the deep oceans. (C) 1998 Elsevier Science Ltd. All rights reser ved.