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

• Published in: Deep-sea research. Part II, Topical studies in oceanography Topical studies in oceanography, 1996, Vol.43 (4), p.1365-1412
• Main Authors: Hammond, D.E., McManus, J., Berelson, W.M., Kilgore, T.E., Pope, R.H.
• Format: Article
• Language: English
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/0967-0645(96)00027-6

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 variabi