a Cation data from Martin and Meybeck (1979); HCO3- from Meybeck (1982).
1988, Walsh et al., 1988), but CaCO3 is the most slowly cycled of the three biogenic sediment components.
The order of importance of early diagenesis to the internal cycles of the elements is, then, Ca > Si > C. We have estimated the involvement of early diagenesis by comparing its rate to the rate of formation of the solid phase in the surface ocean (Table 10.6). For carbon, we have calculated the early diagenetic flux as the difference between the rain rate to the sea floor and the burial rate, with the rain rate estimated from surface water productivity and compilations of benthic flux measurements for the deep sea (Smith and Hinga, 1983; Jahnke and Jackson, 1987), and a water column particle degradation model for coastal regions (Henrichs and Reeburgh, 1987). For Ca and Si, we show two estimates: in each case, the lower of the two is derived from compilations of pore water profile based benthic flux measurements, and the higher value is derived from steady-state ocean models. Both estimates are uncertain. Those based on benthic fluxes are lower limits, and are likely to be underestimates. This is especially true for Ca, since the pore water sampling was by coarse-resolution, in situ methods (Sayles, 1979, 1981), whereas the CaCO3 dissolution rate in surficial sediments is very rapid (Keir, 1983). Our estimates for the rate of benthic degradation of solid phase organic carbon, CaCO3, and SiO2 relative to the rates of formation of the solids: about 4 percent of the organic carbon formed annually degrades in the sediments, 5 to 40 percent of the SiO2, and 10 to 80 percent of the CaCO3. For the latter two, and especially for CaCO3, the lower estimate is probably unrealistic. Early diagenesis is a minor player in the cycling within the ocean of organic carbon, but is quantitatively significant both for CaCO3 and SiO2.
Early diagenetic reactions are clearly important determinants of the loss rates of all three elements from the oceans. Over 90 percent of the organic carbon rain to the sea floor degrades during early diagenesis, as do 30 to 80 percent of the CaCO3 rain and 50 to 90 percent of the SiO2 rain. Thus, the rates of early diagenetic reactions are generally larger than burial rates. If other parts of the oceanic cycles remain constant, changes in the rates of early diagenetic reactions must be reflected in changing burial rates.