Early diagenesis of the coupled dissolved silica-opal system in bioturbated sediments may be explained by one of three possible models of increasing mathematical complexity, i.e. the simple, but unused constant-opal model (abbreviated C.O.), the Schink et al. (1975) model (abbreviated SG&F) for which Wong and Grosch (1978) have supplied an analytical solution (designated as W&G), and the improved but much more sophisticated model proposed by Schinck and Guinasso (1980) that must be solved numerically (S&G). Scaling analysis and computational comparisons show that the C.O. model and the SG&F model, as calculated via the W&G solution, are asymptotically valid forms of more complete S&G model for the limits of ''large'' and ''small'' opal concentrations, respectively. Specifically, the C.O. model is found to provide an excellent approximation to the vastly complicated S&G model if the amount of opal preserved at depth in the sediment, b(∞), satisfies the inquality, b(∞)≥{<0.1(1-&phgr;)DB><&phgr;&ggr;DS(CS-CW)>-1+ &rgr;-1b}-1, where &phgr; is the porosity, DB is the mixing coefficient, DS is the tortuosity-corrected molecular diffusivity of silica, CS is th esolubility of opal, CW is the silica concentration in the overlying waters, &rgr;b is the intrinsic density of opal and &ggr; is a unit conversion constant if b and C are in different units.

Schink and Guinasso (1980) have criticized Wong and Grosch (1978) for utilizing their solution of the earlier SG&F model to describe opal accumulation, a situation for which they believed the SG&F model was invalid. This study has found, however, that for conditions characteristic of the deep F model and so the W&G solution remain reasonably accurate even if small amounts of opal escape dissolution and collect, but that radical divergence from the S&G model can be expected if the flux of silica is sufficient to create a siliceous ooze. This reflects the minor role played by the divergence of the advective flux in the balance of terms in the diagenetic conservation equations when little opal is preserved. These findings should resolve any uncertainty and controversy over the use of the W&G solution. Opal diagenesis in bioturbated shelf-like sediments appears to be adquately described by the C.O. model alone. The C.O. model, coupled where necessary to G solution, consitiute an attractive alternative to the S&G model because relatively simple analytical methods of solution may be employed rather than advanced numerical techniques. ¿ American Geophysical Union 1990