CACO3 DISSOLUTION IN SEDIMENTS OF THE CEARA RISE, WESTERN EQUATORIAL ATLANTIC

We have used porewater sampling by in situ techniques, including whole -core squeezing, as well as by shipboard sectioning and whole-core squ eezing to estimate the rates of sedimentary organic matter oxidation a nd CaCO3 dissolution at seven sites on the Ceara Rise in the western e quatorial Atlantic Ocean. Porewater NO3- 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 organi c C oxidation rate generally decreases with increasing water depth, fr om a value of 22 mu mol/cm(2)/y at the shallowest site (3279 m) to 14 mu mol/cm(2)/y at the deepest site (4675 m). Over this depth range, th e bottomwaters vary from moderately supersaturated with respect to cal cite to strongly undersaturated. High-resolution alkalinity profiles, measured in porewaters collected by in situ whole-core squeezing, yiel d estimated Ca2+ fluxes of 11 mu mol/cm(2)/y at a site located at the depth of the calcite saturation horizon, and 7.6 mu mol/cm(2)/y at a m oderately undersaturated site, Ca2+ fluxes calculated from profiles in porewaters collected by relatively coarse-resolution in situ sampling methods clearly indicate that there is CaCO3 dissolution above the ca lcite saturation horizon. The dissolution of aragonite may contribute to the dissolution flux at the shallowest site. These Ca2+ fluxes, as well as fluxes estimated from a model of sedimentary organic matter ox idation and calcite dissolution, indicate that 36-66% of the CaCO3 rai n to the seafloor dissolves at sites at and above the calcite saturati on horizon, while 52-75% of the rain dissolves at sites below this dep th. When these results are incorporated into the oceanic CaCO3 budget of Milliman (1993), they indicate that 35% of CaCO3 production is pres erved in the deep sea,hey suggest a CaCO3 accumulation rate that is 27 % lower than that estimated by Milliman (1993). Our C-org oxidation/Ca CO3 dissolution model indicates that a large fraction of the CaCO3 dis solution that is occurring on the Ceara Rise is attributable to the ne utralization of metabolic acids produced during organic matter oxidati on. The efficiency with which organic matter oxidation dissolves CaCO3 (that is, the ratio, CaCO3 dissolution attributable to organic matter oxidation:organic matter oxidation rate) generally increases as degre e of undersaturation of bottomwaters increases, However, there are dev iations from the general trend that can be attributed to site-to-site variations in the kinetics of organic matter oxidation and calcite dis solution. This result indicates that the dissolution of CaCO3 as a res ult of organic matter oxidation in the deep sea may mask the effects o f variations in surface water CaCO3 productivity and bottomwater chemi stry on the accumulation rate of CaCO3 in deep-sea sediments.