Biogenic fluxes in the Cariaco Basin:: a combined study of sinking particulates and underlying sediments

The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO3) and long-chain C-37 alkenones (SigmaAlk(37)) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996-1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r(2) = 0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO3 and SigmaAlk(37) were strongly correlated (0.6 < r(2) < 0.8) with those of OC. The major exception was the lower than expected SigmaAlk(37) fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996-1997, led to elevated opal fluxes (e.g., 35 g m(-2) yr(-1) at 275 m) and diminished SigmaAlk(37) fluxes (e.g., 5 mg m(-2) yr(-1) at 275 m). The opposite trends were evident during the year of weakest upwelling (1998-1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions. The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10-12 g m(-2) yr(-1)) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10-14 g m(-2) yr(-1)) over the 1996-1999 study period. CaCO3 burial fluxes (30-40 g m(-2) yr(-1)), on the other hand, were considerably higher than the fluxes measured in the deep traps (similar to 10 g m(-2) yr(-1)) but comparable to those obtained from the shallowest trap (i.e. 38 g m(-2) yr(-1) at 275 m). In contrast, the burial rates of SigmaAlk(37) (0.4-1 mg m(-2) yr(-1)) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4-6 mg m(-2) yr(-1)), indicating the large attenuation in the flux of these compounds at the sediment-water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (similar to 5 g m(-2) yr(-1)) and elevated SigmaAlk(37) fluxes (similar to 2 mg m(-2) yr(-1)) measured in the sedimentary interval corresponding to 1600-2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day. (C) 2003 Elsevier Science Ltd. All rights reserved.