EARLY DIAGENESIS OF BIOGENIC OPAL - DISSOLUTION RATES, KINETICS, AND PALEOCEANOGRAPHIC IMPLICATIONS

A study was undertaken to measure the rate of biogenic opal dissolutio n in equatorial Pacific sediments along the equator between 103 and 14 0 degrees W, and across the equator between 12 degrees S and 9 degrees N. Along the equator, benthic incubation chamber measurements indicat e a gradient in the opal dissolution rate, with rates decreasing from similar to 0.7mmolm(-2) day(-1) at 103 degrees W to 0.4 mol m(-1) day( -1) at 140 degrees W. Across the equator at 140 degrees W, the pattern of opal dissolution is symmetrical, with dissolution rates of similar to 0.4mmolm(-2) day(-1) from 2 degrees S to 2 degrees N, decreasing t o similar to 0.1 mmol m(-2) day(-1) at the ends of the transect. Benth ic fluxes calculated from porewater profiles of silicic acid are in go od agreement with incubation chamber measurements. Each pore water pro file fits with a function that exponentially approaches a constant val ue with depth (C-d), and C-d co-varies with the dissolution flux. At l east three previously published models can explain this relationship: one in which C-d is regulated by the solubility of the opal present in the sediments; a second in which C-d depends on the availability of e asily dissolvable opal; and the sediment mixing rate and a third rate in which C-d is controlled by the development of surface coatings. If the first model is correct, the data demonstrate that opal solubility varies spatially and: that solubility is positively correlated with th e opal rain rate, although the rate at which pore waters become satura ted varies little among the stations between 5 degrees N and 5 degrees S. The implication of this model is that the opal burial rate depends on dissolution kinetics and sediment accumulation rate. If the second model is correct, fits to the pore water data and knowing the sedimen t mixing rate;indicate that at least three types of solid phase opal m ust be present in the equatorial Pacific region, one that is essential ly unreactive, one that has a dissolution rate constant between 0.27 /- 0.09 and 0.05 +/- 0.02 year(-1), and another that has a dissolution rate constant of 6 +/- 4 x 10(-4) year(-1). The more reactive phase d ominates the dissolution flux between 5 degrees S and 5 degrees N, whe reas the less reactive phase dominates the flux at the high latitude e xtremes of the transect. The implication of this second model is that sedimentary opal in equatorial Pacific sediments provides a record of only the non-reactive opal supply. If the third model is correct, surf ace coating development and opal preservation may depend upon the kine tics of the opal surface aging process or on the concentration of the coating material within the sediments, Storage experiments suggest tha t this third model may be the most realistic, but the implications of this model cannot be explored until the factors regulating coating gro wth are identified.