Vertical and temporal variations of particle fluxes in the deep tropical atlantic

Abstract

Results from the study of particle samples collected during two years of sediment trapping experiments at three depths (2000, 200, 10 m above the bottom) in the oligotrophic Cape Verde abyssal plain (4600 m depth) are reported in this paper. Hydrodynamical and sediment data are assessed in order to ascertain their influence on the carbon budget in the deep-sea. The currents affected by tidal, inertial and longer time-scale oscillations (20–30 and 100–120 days) were sufficiently weak (maximum ⩽14 cm s -1) that trap collection was probably unbiased. The average daily flux estimated at 35.2 mg m -2 d -1 at 2000 m a.b. decreased at 28.8 mg m -2 d -1 at 200 m a.b. Organic carbon, nitrogen and carbonate carbon fluxes decreased between these two depths, suggesting organic carbon oxidation and carbonate dissolution during descent or different sources of particles. At 10 m a.b., the total particle flux (41.3 mg m -2 d -1) increased due to near bottom sediment resuspension. The current orientation explains more resuspension recorded in the first data set. A clear seasonal signal was not observed in this study. However, spectral analysis of the data showed a monthly variation, which might have been due to the reproductive cycle of several planktonic species or some hydrodynamical effect. The degree of correlation between fluxes at 2000 and 200 m a.b. suggests that the largest particles sank at rates equal to or greater than 180 m d -1. At 4600 m depth, 0.3% of the surface primary production reached the deep-sea floor. Estimation of the carbon budget in the bioturbated sediment indicated that only 2% of the organic matter carried by the downward flux was buried in the sediment.