Effects of Ca-enrichment on authigenic carbonate formation at cold vent sites off Costa Rica: a numerical model approach

THE Costa Rican forearc is characterized by active fluid venting related to mud diapirism and volcanism. The geochemical compositions of fluids obtained from dewatering sites indicate that mineral precipitation or dehydration is the major driver of fluid mobilization and upward migration. The peculiar situation at the latter sites is that Ca concentrations in the upward migrating fluids are well above seawater levels. In turn, these Ca-enriched fluids could offer a potential reason for widespread carbonate caps on top of the mounds. Here, a reactive-transport numerical model is applied to investigate the main biogeochemical processes at cold vents off Costa Rica (Fig. 1). It is used to determine the ventdriven CaCO3 (calcium carbonate for solid phase) precipitation, quantification of methane turnover and Anaerobic Oxidation of Methane (AOM) at five different stations with varying Ca concentrations (Table 1). We also analysed the control parameters such as Ca concentrations and saturation state affecting CaCO3 precipitation rates. Fluids from two of the five sites (Culebra Fault, and Mudpie) are enriched in Ca. In contrast, fluids at the other two sites (Mound 11 and Quepos Slide) have relatively small amounts of Ca. At the active-vent location of Mound 11, ~98% of the CH4 is released into the bottom waters due to the high advection rates (200 cm y ). The smaller CH4 turnover by AOM at Mound 11 also