Carbonate formation events in ALH 84001 trace the evolution of the Martian atmosphere

Carbonate minerals provide critical information for defining atmosphere-hydrosphere interactions. Carbonate minerals in the Martian meteorite ALH 84001 have been dated to similar to 3.9 Ga, and both C and O-triple isotopes can be used to decipher the planet's climate history. Here we report Delta O-17, delta O-18, and delta C-13 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 O-18 that formed sometime after the primary aqueous event at 3.9 Ga. The phases showed excess O-17 (0.7 parts per thousand) that captured the atmosphere-regolith chemical reservoir transfer, as well as CO2, O-3, 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 C-13 compared with the modern atmosphere.