Highly enriched carbon and oxygen isotopes in carbonate-derived CO2 at Gale crater, Mars

Carbonate minerals are of particular interest in paleoenvironmental research as they form as part of the carbon and water cycles, both of which are relevant to habitability. Given that these cycles are less constrained on Mars than they are on 48 Earth, the identification of carbonates has been a point of emphasis for rover missions. Here we present carbon (δ13C) and oxygen (δ18O) isotope data from four carbonates encountered by the Curiosity rover within Gale crater. The carbon isotope values range from 72 ± 2‰ to 110 ± 3‰ VPDB while the oxygen isotope values span from 59 ± 4‰ to 52 91± 4‰ VSMOW (1 SE uncertainties). Notably, these values are isotopically heavy (13C- and 18O-enriched) relative to nearly every other Martian material. The extreme isotopic difference between the carbonates and other carbon- and oxygen-rich reservoirs on Mars cannot be reconciled by standard equilibrium carbonate-CO2 fractionation, thus requiring 56 an alternative process during or prior to carbonate formation. This paper explores two processes capable of contributing to the isotopic enrichments: 1) evaporative-driven Rayleigh distillation, and 2) kinetic isotope effects related to cryogenic precipitation. None of these processes alone can reproduce the carbonate isotope data; however, each remains a plausible contributor and warrants consideration in future studies.