The surface sediments of two mud mounds (“Mound 11” and “Mound 12”) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO
3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δ
18O
carbonate values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δ
13C
carbonate values from ?52.2 to ?14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The δ
18O
carbonate values suggest mineral formation from seawater-derived pore fluid (δ
18O
porefluid = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δ
18O
porefluid ≈5 ‰) in Mound 11. A positive correlation between δ
13C
carbonate and δ
18O
carbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δ
13C
porefluid ≈0 ‰) and (2) bicarbonate which formed during the AOM (δ
13C
porefluid ≈?70 ‰). Furthermore, the δ
18O
porefluid composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of
18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δ
13C
CH4 = ?38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).
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