Clumped isotope geochemistry of carbonates formed in association with shallow water continental methane seeps at Prony Bay, New Caledonia and Elba, Italy

Abstract:

The venting of fluids and dissolved gases into shallow seawater is an important component of carbon cycling on Earth and analogous to environments on other planetary bodies that may host hydrothermal vents, now or in the past. In serpentinizing systems, authigenic minerals, such as calcium carbonate and brucite, form when Ca-rich, alkaline fluids encounter bicarbonate and Mg²⁺-rich seawater. These precipitates record the geochemistry and physical conditions of the seep environment in which they formed. Stable isotopes can help constrain the nature of these fluids and surface water interactions, including the contribution of microbial metabolisms. Clumped carbonate isotopes may also be useful in deciphering these processes, as they are sensitive to environmental temperature and kinetic effects, e.g., those associated with rapid precipitation. However, no research to date has reported carbonate clumped isotopes from shallow-sea hydrothermal systems derived from serpentinization. Here we present carbon, oxygen, and clumped isotope data from authigenic carbonates formed at two shallow-sea methane seeps: the Prony Hydrothermal Field (PHF) in southern New Caledonia (samples include submarine vents and travertines and were collected in collaboration with the HYDROPRONY research group) and seeps off the Island of Elba, Italy (aragonitic cements). Most of carbonates from PHF show very low δ¹³C values (-35 to -20 ‰), which suggests a depleted carbon source, possibly atmospheric CO₂ but potentially oxidized CH₄ or plant-derived DIC. Samples from submarine PHF site ‘Aiguille’ have δ¹³C and δ¹⁸O values similar to seawater, which is consistent with reaction of Ca²⁺ with seawater DIC. Typical temperatures of PHF vent fluids range from ~20-40°C, yet temperatures derived from carbonate clumped isotopes are much lower (0-5°C), suggesting isotopic disequilibrium from diffusion or CO₂ hydroxylation/hydration prior to carbonate mineralization. Carbonate cements from submarine methane-seep associated sediments from Elba have δ¹³C values of ~-13‰. However, in contrast to the Prony system, clumped isotope temperatures from Elba are elevated compared with the measured water temperatures. This is empirically similar to deep sea CH₄ derived authigenic carbonates where rapid mineralization is hypothesized to yield apparent clumped isotope disequilibrium. However, we also consider the hypothesis that the anomalous clumped isotope effects are associated with anaerobic oxidation of CH₄.