title: Carbonate formation events in ALH 84001 trace the evolution of the Martian atmosphere.
abstract: 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(17)O, δ(18)O, and δ(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.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92122;
authors.forename: Robina
authors.initials: R
authors.lastname: Shaheen
authors.affiliation: Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058; paul.b.niles@nasa.gov.
authors.forename: Paul B
authors.initials: PB
authors.lastname: Niles
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92122; Department of Chemistry, California State Polytechnic University, Pomona, CA 91768; and.
authors.forename: Kenneth
authors.initials: K
authors.lastname: Chong
authors.affiliation: Smithsonian Institution, Washington, DC 20004.
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Corrigan
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92122;
authors.forename: Mark H
authors.initials: MH
authors.lastname: Thiemens
keywords: Martian meteorite; aqueous interaction; carbon isotope; oxygen isotope anomaly; photochemistry