Early diagenesis of biogenic opal: Dissolution rates, kinetics, and paleoceanographic implications

A study was undertaken to measure the rate of biogenic opal dissolution in equatorial Pacific sediments along the equator between 103 and 140°W, and across the equator between 12°S and 9°N. Along the equator, benthic incubation chamber measurements indicate a gradient in the opal dissolution rate, with rates decreasing from ∼0.7 mmol m−2 day−1 at 103°W to 0.4 mol m−2 day−1 at 140°W. Across the equator at 140°W, the pattern of opal dissolution is symmetrical, with dissolution rates of ∼0.4 mmol m−2 day−1 from 2°S to 2°N, decreasing to ∼0.1 mmol m−2 day−1 at the ends of the transect. Benthic fluxes calculated from pore water profiles of silicic acid are in good agreement with incubation chamber measurements. Each pore water profile fits with a function that exponentially approaches a constant value with depth (Cd), and Cd co-varies with the dissolution flux. At least three previously published models can explain this relationship: one in which Cd is regulated by the solubility of the opal present in the sediments; a second in which Cd depends on the availability of easily dissolvable opal; and the sediment mixing rate and a third rate in which Cd 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 the opal rain rate, although the rate at which pore waters become saturated varies little among the stations between 5°N and 5°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 sediment mixing rate indicate that at least three types of solid phase opal must be present in the equatorial Pacific region, one that is essentially 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 × 10−4 year−1. The more reactive phase dominates the dissolution flux between 5°S and 5°N, whereas the less reactive phase dominates the flux at the high latitude extremes 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, surface coating development and opal preservation may depend upon the kinetics of the opal surface aging process or on the concentration of the coating material within the sediments. Storage experiments suggest that this third model may be the most realistic, but the implications of this model cannot be explored until the factors regulating coating growth are identified.