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American Journal of Science








A steady state, coupled, non-linear model has been developed for early diagenetic processes in pelagic and hemi-pelagic marine sediments. Model results show that the occurrence of oxic and sub-oxic diagenetic processes is significantly affected by variations in parameters such as the sedimentation rate, bioturbation coefficient, sediment porosity, and organic matter flux to the sediments. Increases in the sedimentation rate or the bioturbation coefficient increase organic matter oxidation by sub-oxic processes, whereas an increase in sediment porosity decreases organic matter oxidation by sub-oxic processes. Sediment data from three contrasting MANOP sites are fit reasonably well with the model. The resulting best-fit organic carbon, oxygen, and nitrate fluxes at the sediment-water interface and depth-integrated organic carbon oxidation rates for these sites are also within the range of independent estimates of these quantities. Model results show that the internal redox cycling of manganese in sediments leads either to the formation of a Mn-peak near the sediment redox boundary or to surficial Mn-rich oxic sediments, depending on the depth zonation of manganese oxidation and bioturbation. In sediments with a shallow redox boundary (<5 >cm), upward diffusion of pore water manganese into the oxic sediments dominates over manganese oxidation near the redox boundary. The majority of the manganese oxidation therefore occurs in the surficial, bioturbated sediments, and as a result, manganese-rich oxic sediments are formed. In contrast, in sediments with a deeper redox boundary (>10 cm), manganese oxidation near the sediment redox boundary dominates over pore water manganese diffusion into the overlying oxic sediments. Here, majority of the manganese oxidation occurs below the zone of active bioturbation (assumed to be the upper 8-10 cm of sediment), and in this case, a well developed Mn-peak forms near the sediment redox boundary. Previous models explained the occurrence of this Mn-peak by neglecting bioturbation or suggested that this peak could not occur in bioturbated sediments due to this sediment mixing.

Original Publication Citation

Dhakar, S.P., & Burdige, D.J. (1996). Coupled, non-linear, steady state model for early diagenetic processes in pelagic sediments. American Journal of Science, 296(3), 296-330. doi: 10.2475/ajs.296.3.296