Organic carbon input, degradation, and preservation in continental margin sediments: an assessment of the role of a strong oxygen deficient zone

Abstract

Rates of organic carbon input, oxidation and burial in marine sediments were studied with the goal of assessing the role of an oxygen deficient water column in the preservation of organic matter. Factors influencing these rates were investigated in two locations: the Washington state and NW Mexican continental margins. The Mexican continental margin is in contact with the oxygen deficient zone of the eastern tropical North Pacific. Carbon budgets for the Washington and Mexican margins indicated a larger percentage of the surface export flux reached the sediments and a greater fraction of the material arrived at the seafloor escaped degradation to be buried permanently. The attenuation in the carbon oxidation rate with water column depth for the Mexican margin was less than is typical of other margin regions. Empirical attenuation coefficients derived from a power function relation were 0.931 and 0.439 for the Washington and Mexican margins, respectively. Multi-G type models suggested that organic matter degraded in Mexican sediments was less reactive than that in Washington sediments. Sediment slurry and whole-core incubations were conducted to assess the effects of mixing and oxygen exposure on the rate and extent of carbon degradation. Sediments from the Mexican oxygen-deficient zone were less susceptible to degradation than sediments from the oxic Mexican shelf and the Washington margin under all incubation conditions, suggesting that the organic matter in the oxygen-deficient zone may be intrinsically less reactive. The isotopic fractionation of oxygen and nitrogen during respiration was measured on both margins. The apparent respiration fractionation factor, A, for oxygen under conditions of carbon-limited degradation was 0.992. It was hypothesized that long oxygen exposure times, respiration under isotopically fractionating conditions and organic carbon:mineral surface-area ratios less than 1.0 are each related to the extent of degradation and imply low carbon burial efficiencies. High carbon burial efficiencies (and low carbon oxidation efficiencies) were found in oxygen-deficient zone sediments which had short oxygen exposure times, little or no fractionation of oxygen and nitrogen and carbon: surface-area ratios >3.0.