CaCO 3 dissolution in sediments of the Ceara Rise, western equatorial Atlantic

Abstract

We have used porewater sampling by in situ techniques, including whole-core squeezing, as well as by shipboard sectioning and whole-core squeezing to estimate the rates of sedimentary organic matter oxidation and CaCO 3 dissolution at seven sites on the Ceara Rise in the western equatorial Atlantic Ocean. Porewater NO 3 − profiles at all sites show a pattern indicative of active organic matter oxidation in the upper 15–20 cm of the sediments and in a buried, organic-rich layer. The organic C oxidation rate generally decreases with increasing water depth, from a value of 22 μmol/cm 2/y at the shallowest site (3279 m) to 14 μmol/cm 2/y at the deepest site (4675 m). Over this depth range, the bottomwaters vary from moderately supersaturated with respect to calcite to strongly undersaturated. High-resolution alkalinity profiles, measured in porewaters collected by in situ whole-core squeezing, yield estimated Ca 2+ fluxes of 11 μmol/cm 2/y at a site located at the depth of the calcite saturation horizon, and 7.6 μzmol/cm 2/y at a moderately undersaturated site. Ca 2+ fluxes calculated from profiles in porewaters collected by relatively coarse-resolution in situ sampling methods clearly indicate that there is CaCO 3 dissolution above the calcite saturation horizon. The dissolution of aragonite may contribute to the dissolution flux at the shallowest site. These Ca 2+ fluxes, as well as fluxes estimated from a model of sedimentary organic matter oxidation and calcite dissolution, indicate that 36–66% of the CaCO 3 rain to the seafloor dissolves at sites at and above the calcite saturation horizon, while 52–75% of the rain dissolves at sites below this depth. When these results are incorporated into the oceanic CaCO 3 budget of Milliman 1993, they indicate that 35% of CaCO 3 production is preserved in the deep sea; they suggest a CaCO 3 accumulation rate that is 27% lower than that estimated by Milliman 1993. Our C org oxidation/CaCO 3 dissolution model indicates that a large fraction of the CaCO 3 dissolution that is occurring on the Ceara Rise is attributable to the neutralization of metabolic acids produced during organic matter oxidation. The efficiency with which organic matter oxidation dissolves CaCO 3 (that is, the ratio, CaCO 3 dissolution attributable to organic matter oxidation:organic matter oxidation rate) generally increases as degree of undersaturation of bottomwaters increases. However, there are deviations from the general trend that can be attributed to site-to-site variations in the kinetics of organic matter oxidation and calcite dissolution. This result indicates that the dissolution of CaCO 3 as a result of organic matter oxidation in the deep sea may mask the effects of variations in surface water CaCO 3 productivity and bottomwater chemistry on the accumulation rate of CaCO 3 in deep-sea sediments.