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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 2; pp. 336 - 341
• Main Authors: Shaheen, Robina, Niles, Paul B., Chong, Kenneth, Corrigan, Catherine M., Thiemens, Mark H.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 13.01.2015; National Acad Sciences
• Subjects: Aqueous chemistry; Dissolution; Isotopes; Minerals; Physical Sciences; aqueous interaction; oxygen isotope anomaly; carbon isotope; photochemistry; Martian meteorite
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1315615112

Significance Martian meteorite ALH 84001 serves as a witness plate to the history of the Martian climate ∼4 Ga ago. This study describes ion microprobe δ ¹⁸O analyses coupled with δ ¹³C, δ ¹⁸O, and Δ ¹⁷O analyses from stepped acid dissolution of the meteorite that identifies a new carbonate phase with distinct isotope compositions. These new measurements of the oxygen isotope composition of carbonates within this meteorite reveal several episodes of aqueous activity that were strongly influenced by atmospheric chemistry. When paired with carbon isotope measurements, these data suggest that the ancient atmosphere of Mars was significantly depleted in ¹³C compared to the present day. This implies substantial enrichment in the δ ¹³C of the atmosphere since the Noachian which may have occurred through extensive atmospheric loss. 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 Δ ¹⁷O, δ ¹⁸O, and δ ¹³C 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 ¹⁸O that formed sometime after the primary aqueous event at 3.9 Ga. The phases showed excess ¹⁷O (0.7‰) that captured the atmosphere–regolith chemical reservoir transfer, as well as CO ₂, O ₃, and H ₂O 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 ¹³C compared with the modern atmosphere.