Early diagenesis of organic material in equatorial Pacific sediments: stpichiometry and kinetics

Abstract

Benthic incubation chambers and sediment pore water profiles were used to study early diagenesis of organic matter in equatorial Pacific sediments. Replicate measurements with a flux chamber covering 720 cm 2 indicated that the spatial variability of oxygen, TCO 2, alkalinity, nitrate and silica fluxes at a single station did not exceed 10–35%. In contrast, diffusive fluxes of oxygen from replicate cores covering 70 cm 2 at a single station often showed greater variation. In January 1992, benthic oxygen consumption was fairly constant along the equator from 103°W to 140°W at 0.6-0.8 mmol m −2 day −1. In November 1992, consumption was roughly symmetrical across the equator along 140°W, with rates of 0.6-0.8 mmol m −2 day −1 between 2°S and 2°N, declining to rates of 0.1-0.2 mmol m −2 day −1 at 12°S and 9°N. Pore water oxygen profiles were fit with a reaction-diffusion model equation to evaluate reaction kinetics. Most profiles were adequately fit with a model that assumed reaction rates declined exponentially with depth, but at low latitudes better fits often were obtained with a model that assumed decomposing organic matter has two labile components and that each decays with first-order kinetics and decreases exponentially with depth. Results of both fits indicate that at least 70% of the organic matter degradation occurs within the upper 1–2 cm of sediment. At the low-latitude stations fit with the two-component model, 70–90% of the flux is attributable to the more labile component which has an average 1/e penetration depth of 0.4 ± 0.1 cm. The more refractory component at these stations has a penetration depth of 4.4 ± 0.4 cm. From estimates of sediment mixing rates, the mean life of all degrading organic matter at the higher latitude stations is 4–55 years, while at the stations fit with the two-component model, the lifetime of the more labile fraction is weeks to months, and the lifetime of the less labile component is 40–300 years. A third carbon fraction exists at all stations that is far more refractory. The O 2:CO 2 stoichiometry of remineralization is −1.45 ± 0.17, and the C:N ratio is 8 ± 1. Both ratios are in good agreement with those observed from sediment trap and hydrographic studies in the water column, and suggest that degrading organic matter has about 70% of its carbon in -CH 2O-groups and 30% in -CH 2-groups. The C:P atom ratios for benthic remineralization differ by a factor of 3 for the two cruises, showing substantial temporal variability and de-coupling from carbon, although the mean for the two cruises (170 ± 85) is not significantly different than remineralization ratios observed in the water column. The aerally-integrated benthic respiration rate for the equatorial Pacific upwelling region is at least 25% of the integrated respiration rate for the continental margin (slope + rise) areas of the Pacific, emphasizing the importance of the equatorial Pacific sediments as a major site of benthic carbon recycling. Benthic carbon remineralization rates determined during the past decade near the equator and 140°W have varied by a factor of 2, which is not surprising given the short lifetime of the majority of the carbon degrading. The temporal patterns of carbon remineralization rates resemble those of sea-surface temperature, suggesting that benthic carbon oxidation at this site may reflect water column productivity over relatively short timescales.