Carbon Sequestration on Mars: Constraints from the Nili Fossae Carbonate Plains

Abstract:

Martian carbonates have been observed telescopically, from orbit, in situ and in Martian meteorites; however, a long-postulated geologic reservoir that accounts for proposed thinning of a multi-bar early Mars atmosphere by CO₂ sequestration has not yet been identified. One striking aspect of the Martian geologic record is the presence of valley networks and open basin lakes last active around the Noachian/Hesperian boundary, at ca. 3.5 Ga. If surface waters were supported by a thicker atmosphere, hundreds of millibars to bars of CO₂ would need to be lost to space during the Hesperian/Amazonian, inconsistent with current atmospheric models. Was this late CO₂ sequestered in the Martian crust? We consider the role of diffuse and localized CO₂ sequestration and constrain the timing and explore implications for late Noachian atmospheric conditions via examination of the age and composition of the largest contiguous exposure of carbonate-bearing rock on Mars, the Nili Fossae carbonate plains (21.5°N, 78.5°E). Morphological, spectral and thermophysical data sets from the Thermal Emission Spectrometer, Compact Reconnaissance Imaging Spectrometer for Mars, Thermal Emission Imaging System, and High Resolution Imaging Science Experiment are considered in the context of past atmospheric drawdown.

We find the olivine-enriched (~20%–25%) basalts of the Nili Fossae plains have been altered, by low-temperature, in-situ carbonation processes, to at most ~20% Fe-Mg carbonate, thus limiting carbon sequestration in the Nili Fossae region to ~0.25–12 mbar of CO₂ during the late Noachian/early Hesperian, before or concurrent with valley network formation. While large compared to modern-day CO₂ reservoirs, the lack of additional, comparable-sized post-Late Noachian carbonate-bearing deposits on Mars indicates ineffective carbon sequestration in rock units over the past ~3.7 Ga. This implies a thin atmosphere (≲500 mbar) during valley network formation, extensive post-Noachian atmospheric loss to space or diffuse, and deep sequestration by a yet-to-be understood process. In stark contrast to Earth’s biologically mediated crust:atmosphere carbon reservoir ratio of ~10⁴–10⁵, Mars’ ratio is a mere 10–10³, even if buried pre-Noachian crust holds multiple bars.