BENTHIC ORGANIC-CARBON DEGRADATION AND BIOGENIC SILICA DISSOLUTION IN THE CENTRAL EQUATORIAL PACIFIC

Shipboard whole-core squeezing was used to measure pore water concentration vs depth profiles of NO3-, O2 and SiO2 at 12 stations in the equatorial Pacific along a transect from 15-degrees-S to 11-degrees-N at 135-degrees-W. The NO3- and SiO2 profiles were combined with fine-scale resistivity and porosity measurements to calculate benthic fluxes. After using O2 profiles, coupled with the NO3-profiles, to constrain the C:N of the degrading organic matter, the NO3- fluxes were converted to benthic organic carbon degradation rates. The range in benthic organic carbon degradation rates is 7-30-mu-mol cm-2 y-1, with maximum values at the equator and minimum values at the southern end of the transect. The zonal trend of benthic degradation rates, with its equatorial maximum and with elevated values skewed to the north of the equator, is similar to the pattern of primary production observed in the region. Benthic organic carbon degradation is 1-2% of primary production. The range of benthic biogenic silica dissolution rates is 6.9-20-mu-mol cm-2 y-1, representing 2.5-5% of silicon fixation in the surface ocean of the region. Its zonal pattern is distinctly different from that of organic carbon degradation: the range in the ratio of silica dissolution to carbon degradation along the transect is 0.44-1.7 mol Si mol C-1, with maximum values occurring between 12-degrees-S and 2-degrees-S, and with fairly constant values of 0.5-0.7 north of the equator. A box model calculation of the average lifetime of the organic carbon in the upper 1 cm of the sediments, where 80 +/- 11% of benthic organic carbon degradation occurs, indicates that it is short: from 3.1 years at high flux stations to 11 years at low flux stations. The reactive component of the organic matter must have a shorter lifetime than this average value. In contrast, the average lifetime of biogenic silica in the upper centimeter of these sediments is 55 +/- 28 years, and shows no systematic variations with benthic flux.