Organic matter diagenesis in the northeast Pacific: transition from aerobic red clay to suboxic hemipelagic sediments

Abstract

Analyses for dissolved oxygen, nitrate, silicate and manganese in the interstitial water from an in situ sampler and from boxcore sediment samples have been combined with solid phase sediment analyses of carbon and nitrogen to study the transition from aerobic to suboxic diagenesis in the northeast Pacific. The station locations coincide with the VERTEX sediment trap stations. This has enabled us to study diagenesis as a function of the flux of organic carbon to the sediment-water interface. Organic carbon in the sediments decreases with distance from the continental margin. At all stations, except 183-2 on the Monterey deep sea fan, organic carbon decreases rapidly below the sediment-water interface. The organic matter at Stas 183-3, 4, 5 and 6 has a C/N molar ratio of 10. At Sta. 183-2 the organic matter has a relatively nitrogen-poor C/N ratio of 15.8, suggesting terrestrial input. The stoichiometry of the decomposing organic matter at Stas 183-3, 4, 5 and 6 was determined from the porewater oxygen and nitrate, and the resulting C/N ratio was 14. The flux of oxygen into the sediments decreases with increasing water depth. Comparison of the oxygen consumption by the sediments with the rain rate of organic carbon indicates that most of the rain of organic carbon is oxidized within the sediments using oxygen as the electron acceptor. The first order degradation rate constant for organic carbon oxidation calculated from the porewater oxygen profiles was found to correlate strongly with the rain rate of organic carbon. The rate constant increases with the rain rate, suggesting that at higher rain rates the organic matter is more “reactive”. The mean life for sedimentary organic carbon is 12,000 years in the central North Pacific and decreases to 200 years near the continental boundary.