From the above comparisons of patterns among dolomite rhombs, carbonate particles, and lithified layers, and from the patterns of sedimentation, it is clear that although dolomite formation is somehow tied to high productivity, the important details remain unsolved when using only this field evidence. Presumably, the reduction of sulfate and the concomitant increase in alkalinity are crucial factors (Baker and Kastner, 1981; Compton, 1988; Kastner et al., 1990). Disseminated dolomite can be present at very shallow depths below the sea-floor (e.g., Shimmield and Price, 1984; Kulm et al., 1984; Suess, von Huene, et al., 1988), presumably reflecting highly reducing micro-environments featuring low sulfate and high alkalinity. It is possible that the presence of clay-sized detrital dolomite plays a role in some of these abundance patterns and provides seeds for growth. The availability of dissolved iron might play a similar role in providing the precipitation of seed crystals (cf. Henderson et al., 1984). Dissolved Mg is available for dolomitization from diffusion from overlying sea water (Baker and Burns, 1985; Compton and Siever, 1986). Other sources of Mg are thought to be of minor importance for dolomite formation in continental margin sediments (Baker and Burns, 1985). Removal of calcium from solution (e.g., by precipitation of authigenic calcite in a zone of high alkalinity) might precondition solutions for precipitation of increased proportions of Mg-rich carbonate as a precursor for dolomite (Burns et al., 1988). It is likely that a number of factors have to be balanced precisely to provide the correct environment for dolomite formation, beginning with a sedimentation rate high enough to preserve organic carbon below the sediment/water interface, but low enough to prevent excessive dilution of the reactants (e.g., Baker and Burns, 1985).
At Leg 175 sites, we commonly observed dual depletion of Ca2+ and Mg2+ from the interstitial waters through the same depth range that high alkalinities are generated by degradation of organic matter (Murray et al., Chap. 20, this volume). These pore-water profiles strongly suggest that dolomite is actively precipitating in the uppermost 50–70 mbsf of the sediment column and speaks to the intimate associations between organic matter remineralization, the dissolution of biogenic calcite, and the formation of diagenetic dolomite (Murray et al., Chap. 20, this volume).