Calcite dissolution driven by benthic mineralization in the deep-sea:: In situ measurements of Ca2+, pH, pCO2 and O2

In situ measured microprofiles of Ca2+, pCO(2) pH and O-2 were performed to quantify the CaCO3 dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m(-2) d(-1) and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m(-2) a(-1). The metabolic CO2, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca2+ microprofiles were 0.6 mmol m(-2) d(-1) for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO2 produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O-2 pH and Ca2+ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw(-1) a(-1) (mol per kg water and year), which equals 95% d(-1). This rate constant is at the upper end of reported in situ values. Copyright (C) 2001 Elsevier Science Ltd.