Organic carbon fluxes, degradation, and accumulation in an anoxic basin: Sediment trap results from the Cariaco Basin

Abstract

Organic carbon fluxes were measured in the anoxic water column of the Cariaco Basin under varying levels of primary productivity. Our results indicate that water column degradation of fresh organic matter in this anoxic setting is similar to what has been previously observed in the open ocean. Additionally, the rate at which carbon is consumed increases as primary productivity increases. These findings support the hypothesis that sulfate reduction is as efficient as aerobic respiration in degrading labile organic matter and that an anoxic water column does not necessarily result in an enhanced delivery of organic carbon to the sea floor. We estimate that 1-2% of the organic carbon produced at the surface reaches the seafloor and that approximately 50% of this is preserved in the sediments. This burial efficiency is comparable to that previously reported for both oxic and anoxic settings, and with sedimentation rates similar to that in Cariaco Basin.

Rapid organic matter sulfurization in sinking particles from the Cariaco Basin water column

“…Sinking organic matter in Cariaco Basin is primarily the product of phytoplankton productivity in the surface ocean (Thunell et al, 2000) and contains biosulfur predominantly as cysteine and methionine. The ratio of sulfur to carbon in fresh planktonic biomass is commonly estimated at 1.6% (Wollast et al, 1993) but was also found to range between 0.4% and 1.1% in a variety of diatoms, dinoflagellates, cyanobacteria, algae, and humic acids (Francois et al, 1987 and refs therein).…”

“…In this study, we approach these questions by interrogating the distribution of sulfur and its isotopic composition in sediment trap material from Cariaco Basin in the context of hydrographic profiles and particle composition time-series data (Thunell et al, 2000; http://imars.marine.usf.edu/CAR/). These µg-scale analyses are made possible via the novel use of multicollector inductively-coupled-plasma mass spectrometry to analyze bulk organic fractions (Raven et al, 2016).…”

Sediment record linked to surface processes in the Cariaco Basin

“…However, in 1998, a rate of < 300 gC m 2 y 1 was estimated. The vertical flux of organic carbon measured with the sediment traps follows a regular pat tern; minima occur between September and January and maxima between February and May (Figure 4).The magnitude of the flux tends to decrease with increasing water depth, and similar temporal variability is observed at all four depths (Figure 4).The material reaching the bottom of Cariaco is rich in opal, carbon ate, and organic carbon, and lithogenic materi al accounts for -50% of the total annual particulate flux [Thunell et al, 2000]. trap sample in early July 1997.This flux was associated with a turbidite generated by an earthquake that occurred along the Venezuelan coast.This event was described by Thunell et al [1999].…”

“…This model prescribes the vertical profile of organic carbon flux as an exponential decay function based on surface production and depth. Since this model was developed from observations from an open, oxygenated Pacific Ocean environment, our results suggest that organic matter degradation in the anoxic Cari aco Basin is as efficient as that occurring in well-oxygenated waters [Thunell et a/.,2000].…”

“…In 1995^1996,1998-2000 Surface salinities between August and Octo ber are fresher (< 36.5, coincident with the rainy season. In 1997-1998, upwelling was gen erally inhibited, possibly as a teleconnection to the strong El Nino-Southern Oscillation in the first part of that year, and only one month (March) showed significant upward migration of temperature and salinity isopleths.…”