EARLY DIAGENESIS OF ORGANIC MATERIAL IN EQUATORIAL PACIFIC SEDIMENTS - STOICHIOMETRY AND KINETICS

Benthic incubation chambers and sediment pore water profiles were used to study early diagenesis of organic matter in equatorial Pacific sed iments. Replicate measurements with a flux chamber covering 720 cml in dicated that the spatial variability of oxygen, TCO2, alkalinity, nitr ate and silica fluxes at a single station did not exceed 10-35%. In co ntrast, diffusive fluxes of oxygen from replicate cores covering 70 cm (2) at a single station often showed greater variation. In January 199 2, benthic oxygen consumption was fairly constant along the equator fr om 103 degrees W to 140 degrees W at 0.6-0.8 mmol m(-2) day(-1). In No vember 1992, consumption was roughly symmetrical across the equator al ong 140 degrees W, with rates of 0.6-0.8 mmol m(-2) day(-1) between 2 degrees S and 2 degrees N, declining to rates of 0.1-0.2 mmol m(-2) da y(-1) at 12 degrees S and 9 degrees N. Pore water oxygen profiles were fit with a reaction-diffusion model equation to evaluate reaction kin etics. Most profiles were adequately fit with a model that assumed rea ction rates declined exponentially with depth, but at low latitudes be tter fits often were obtained with a model that assumed decomposing or ganic matter has two labile components and that each decays with first -order kinetics and decreases exponentially with depth. Results of bot h fits indicate that at least 70% of the organic matter degradation oc curs 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 dept h 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 mixin g rates, the mean life of all degrading organic matter at the higher l atitude 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 refr actory. The O-2:CO2 stoichiometry of remineralization is -1.45+/-0.17, and the C:N ratio is 8+/-1. Both ratios are in good agreement with th ose observed from sediment trap and hydrographic studies in the water column, and suggest that degrading organic matter has about 70% of its carbon in -CH2O- groups and 30% in -CH2- groups. The C:P atom ratios for benthic remineralization differ by a factor of 3 for the two cruis es, showing substantial temporal variability and de-coupling from carb on, although the mean for the two cruises (170+/-85) is not significan tly different than remineralization ratios observed in the water colum n. 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 ma jor site of benthic carbon recycling. Benthic carbon remineralization rates determined during the past decade near the equator and 140 degre es W have varied by a factor of 2, which is not surprising given the s hort lifetime of the majority of the carbon degrading. The temporal pa tterns 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. C opyright (C) 1996 Elsevier Science Ltd.