Carbonates and Other Salts in the Atacama Desert and on Mars, and the Implications for the Role of Life in Carbonate Formation

Abstract

The scarcity of carbonate on Mars has been difficult to reconcile with the morphologic evidence for a wet epoch in Martian history, and has weakened early interpretations of a water-rich Noachian. Limited soil carbonate from pre-Silurian Earth has created a similar conundrum, and in both instances this paradox has likely led to overreaching interpretations about past climates. To better understand the formation of carbonate on Mars, early Earth, and in present day hyperarid climates, we examined the distribution of carbonate in the Atacama Desert—a region that spans multiple climate regimes and allows us to isolate the effects of precipitation and plant cover on soil mineralogy. To better quantify the influences of vegetation on carbonate we utilized a simple one-dimensional precipitation model and simulated carbonate formation with or without plant cover under a range of relevant climatic conditions and soil morphologies. In the Atacama we found two distinct zones with only trace (<5%) carbonate: the "absolute desert" with precipitation too low to sustain plant life, and the high Andes where precipitation was significantly higher, but where the low mean annual temperature (MAT) inhibits plants. The fog-supported, low-elevation coastal "lomas" below approximately 800 meters above sea level (masl) and the higher elevations between approximately 2500-4500 masl are variably vegetated and contain abundant carbonate within the soils. Plants increase total evapotranspiration and its distribution with depth, weathering rates, and total pCO₂. Our model results show that all of these factors increase the formation of pedogenic soil carbonate. Without the influence of vegetation the diminished carbonate that is produced is flushed through the shallow soil, where it eventually precipitates in the deep vadose zone or is entrained by groundwater.