Dissolution kinetics of calcium carbonate in equatorial Pacific sediments

Abstract

Benthic chambers were deployed in the equatorial eastern Pacific Ocean on a transect along the equator between 103°W and 140°W and on a transect across the equator at 140°W in order to establish the rate of calcium carbonate dissolution on the seafloor. Dissolution was determined from the rate of alkalinity increase within an incubation chamber, measured over an 80–120 hour incubation period. Dissolution rates were lowest at eastern Pacific sites (0.2‐0.4 mmol CaCO3/m2/d) and highest at the equatorial, 140°W sites (0.5‐0.7 mmol/m2/d). Both oxygen consumption rates and the degree of bottom water saturation govern dissolution rates. Measured dissolution and oxygen consumption rates are used with a numerical model to constrain the value of the dissolution rate constant k, formulated according to the equation developed by Keir [1980]: dissolution rate = kγ(1‐Ω)n. The observed dissolution fluxes are predicted by the model when k = 5 to 100%/d and n = 4.5. This range of k values has important implications regarding the type of carbonate dissolving and its location within the sediment column. At low values of k, organic carbon rain rates to the seafloor become the dominant driving force of carbonate dissolution. At higher values of k, the degree of bottom water undersaturation becomes more important. Dissolution of carbonate within equatorial Pacific sediments can be adequately described with k = 20 ± 10%/d, a rate constant much lower than some previously used values. Dissolution rates do not vary significantly over chamber boundary layer thicknesses between 200 and 800 μm, indicating that dissolution is not controlled by hydrodynamic conditions. Chambers acidified with HCl yield very large dissolution rates, but for a given degree of acidification the dissolution rate was constant for sites ranging from water depths of 3300–4400 m. This implies that there are not more and less easily dissolved forms of CaCO3 arriving on the seafloor between these depths. A budget for alkalinity in the deep Pacific, predicted by the dissolution model and based on the assumption that all carbonate dissolution takes place in the sediments, is within 85% of the input required by a published box model alkalinity budget based on the distribution of nutrients and the water mass transport rates.