COUPLED, NONLINEAR, STEADY-STATE MODEL FOR EARLY DIAGENETIC PROCESSESIN PELAGIC SEDIMENTS

A steady state, coupled, non-linear model has been developed for early diagenetic processes in pelagic and hemi-pelagic marine sediments. Mo del results show that the occurrence of oxic and sub-oxic diagenetic p rocesses 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 sedimentati on rate or the bioturbation coefficient increase organic matter oxidat ion by sub-oxic processes, whereas an increase in sediment porosity de creases organic matter oxidation by sub-oxic processes. Sediment data from three contrasting MANOP sites are fit reasonably well with the mo del. The resulting best-fit organic cat;bon, oxygen, and nitrate fluxe s at the sediment-water interface and depth-integrated organic carbon oxidation rates for these sites are also within the range of independe nt 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 oxidatio n and bioturbation. In sediments with a shallow redox boundary (<5 cm) , upward diffusion of pore water manganese into the oxic sediments dom inates over manganese oxidation near the redox boundary. The majority of the manganese oxidation therefore occurs in the surficial, bioturba ted sediments, and as a result, manganese-rich oxic sediments are form ed. In contrast, in sediments with a deeper redox boundary (>10 cm), m anganese oxidation near the sediment redox boundary dominates over por e water manganese diffusion into the overlying oxic sediments. Here, m ajority of the manganese oxidation occurs below the zone of active bio turbation (assumed to be the upper 8-10 cm of sediment), and in this c ase, 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 bioturbat ed sediments due to this sediment mixing.