Geochemistry of Sedimentary Processes on Mars
Mars has an extensive, long-lived sedimentary record that is complimentary to the terrestrial record, bearing both first-order similarities and first-order differences. The igneous record is composed of basaltic rocks, in fundamental contrast to the granodioritic upper continental crust of the Earth, which in turn dominates the provenance of clastic and chemical sedimentary rocks. The crust and sedimentary mass of Mars on average are older than the terrestrial records, and Mars provides exceptional potential for understanding processes that were active during the earliest history (>3.5 Gyr) of the solar system. Numerous sedimentary minerals have been identified both from orbit and by rovers/landers and include a variety of clays, sulfates, amorphous silica, minor carbonates, and possibly chlorides. The Martian sedimentary mineralogical record is Fe- and Mg-enriched and Na- and K-depleted compared to the terrestrial record, reflecting differing crustal compositions and differing aqueous surficial environments. There is evidence for three distinct sedimentary mineralogical epochs: an early clay-rich era, intermediate sulfate-rich era, and a younger era dominated by secondary iron oxides. This mineralogical evolution likely records desiccation, acidification, and oxidation of the surface over geological time. There is also evidence that surficial processes were controlled by a sulfur cycle, rather than the carbon cycle, over much of Martian geological time, leading to low-pH aqueous conditions. The nature of this S cycle changed over time as volcanic sulfur sources and amounts of near-surface water diminished. There is a linkage between the S cycle and iron/oxygen cycles related to diagenetic oxidation of iron sulfates to form iron oxides. Where studied in detail, weathering is dominated by low pH, with mobility of ferric iron being common. Lack of evidence for expected aluminum mobility indicates that low water-rock ratio conditions prevailed. In Noachian terrains, where clay minerals are common, it is more likely that aqueous conditions were closer to circum-neutral, but detailed study awaits future landed missions. Numerous depositional environments are recognized, including fluvial, deltaic, lacustrine, eolian, and glacial settings. Evaporitic rocks appear common and are characterized by distinctive suites of Mg-, Ca-, and Fe-sulfates and possibly chlorides. A system of chemical divides can be constructed and indicates that the range of observed evaporite minerals can be explained by typical water compositions derived from acidic weathering of Martian crust, and with variable initial pH controlled by HCO−3/SO2−4 ratios. Several diagenetic processes have also been identified, including complex groundwater diagenetic histories. One process, consistent with experimental studies, that explains the correlation between sulfate and iron oxide minerals seen from orbit, as well as formation of hematitic concretions in the Burns Formation on Meridiani Planum, is oxidation of iron sulfates to form iron oxides. In general, the diagenetic record that has been identified, including incomplete iron sulfate oxidation, limited clay mineral transformations, and absence of amorphous silica recrystallization, indicates highly water-limited postdepositional conditions. Among the most important outstanding questions for sedimentary geochemistry are those related to the quantification of the size and lithological distribution of the sedimentary record, the detailed history of near-surface water, and the origin and history of acidity in the aqueous environment.