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°50′N, 64°10′W). One feature of this 19+ year record has been the episodic occurrence of large, short-lived flux maxima that are not associated 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, OFP traps located at 3200 and 3400 m depths intercepted a major flux “event” in which there was an abrupt, threefold 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 resolved sampling was begun in 1989. Organic biomarker analyses of material collected prior to, and during, this high flux event determined that there was an abrupt change in material composition associated with the sudden flux increase. Prior to the event, cholesterol, a single bacteriaderived C27 hopanone (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 enriched in C26–C29 phytosterols, haptophyte algae-derived C37–C39 alkenones, labile polyunsaturated acids, degradation products such as steroidal ketones, and also in bacteria-derived compounds such as C27–C34 hopanoids and β and ω−1 hydroxy acids. These compounds indicated the organic fraction contained a large amount of relatively fresh phytoplankton-derived debris and tracers of bacterial biomass and metabolism, which suggested that the sinking material was undergoing active bacterial decomposition. Thus, the flux “event” appears to have resulted from a shortlived bloom in the 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 and easily remineralized elements, not only in highly productive areas, but throughout the deep oceans.