Shaheen, Robina, Niles, Paul B., Chong, Kenneth, Corrigan, Catherine M., and Thiemens, Mark H. 2014. "Carbonate formation events in ALH 84001 trace the evolution of the Martian atmosphere." Proceedings of the National Academy of Sciences of the United States of America 112 (2):336-341. https://doi.org/10.1073/pnas.1315615112
Carbonate minerals provide critical information for defining atmosphere-hydrosphere interactions. Carbonate minerals in the Martian meteorite ALH 84001 have been dated to ~3.9 Ga, and both C and O-triple isotopes can be used to decipher the planet's climate history. Here we report ?(17)O, d(18)O, and d(13)C data of ALH 84001 of at least two varieties of carbonates, using a stepped acid dissolution technique paired with ion microprobe analyses to specifically target carbonates from distinct formation events and constrain the Martian atmosphere-hydrosphere-geosphere interactions and surficial aqueous alterations. These results indicate the presence of a Ca-rich carbonate phase enriched in (18)O that formed sometime after the primary aqueous event at 3.9 Ga. The phases showed excess (17)O (0.7‰) that captured the atmosphere-regolith chemical reservoir transfer, as well as CO2, O3, and H2O isotopic interactions at the time of formation of each specific carbonate. The carbon isotopes preserved in the Ca-rich carbonate phase indicate that the Noachian atmosphere of Mars was substantially depleted in (13)C compared with the modern atmosphere.