Phosphorus Cycling and Bioavailability in Archean Carbonates: Analogues for Martian Paleoenvironments

Phosphorus is a key element in all forms of life. However, P on Earth is scarce as it is locked in low solubility phosphate minerals, raising the fundamental and long-lasting question about where early life got its P supply. Hydrothermal vents or extremely acidic hot springs have been commonly investigated as favourable environments for the emergence and evolution of primitive organisms. More recently, carbonate-rich lakes have been proposed to concentrate enough dissolved P to favour the formation of life (Toner & Catling 2020), thus providing a plausible solution to phosphate supply. 

In this study, we investigate P recycling and bioavailability in carbonate and iron-rich Archaean deposits (~3 Ga) to expand our understanding of the possible origin of life on Earth, and to test the astrobiological implications on other terrestrial planets, like Mars. Unlike Earth, carbonates on Mars are scarce (Bultel et al., 2019). Interestingly, the Jezero crater, where the rover Perseverance is currently searching for sights of ancient life, is one of the few locations showing abundant carbonates on Mars (Horgan et al., 2020). Thus, these Archaean deposits serve as a Jezero crater analogue for early life on Earth. 

Phosphorus recycling is driven by the redox chemistry of the water column. Accordingly, this study will explore the prevailing redox conditions (oxic-dysoxic, ferruginous and/or euxinic), and their fluctuation, in the water column at the time of deposition. This is important because ferruginous conditions may promote P fixation in the sediments due to the drawdown of iron particles and organic matter. By contrast, euxinic conditions may enhance the flux of bioavailable P in the water column due to the degradation of organic matter and the dissolution of Fe oxides, both effective P carriers to the sediment. 

To better approach P bioavailability in these carbonate and iron-rich deposits, combined geochemical analyses and biogeochemical modelling are used. On one hand, coupling iron and phosphorus speciation geochemical techniques allow for the sequential extraction of different forms of Fe and P and serve to quantify their concentrations (Poulton & Canfield, 2005; Thompson et al., 2019). On the other hand, biogeochemical models allow for the understanding of Ca-Fe-S-P interactions within variable redox conditions. 

Iron speciation results will determine whether the studied carbonates were deposited under euxinic or ferruginous conditions, relative to total iron (Fe_T), and will track the redox evolution upward stratigraphy. Phosphorus speciation results will quantify different operationally defined P sedimentary pools, including iron associated (P_Fe), authigenic (P_auth), detrital (P_det), organic (P_org) and total (P_T). Subsequently, Fe and P results will be modeled to address two fundamental research questions: a) shifts from ferruginous to euxinic conditions promoted the accumulation of dissolved phosphate, and b) iron reduction (microbial or abiotic) favoured the increase of alkalinity, contributing to carbonate precipitation. 

Paleoenvironmental reconstructions of early Earth play a key role in unravelling the co-evolution of life and the Earth system. Our understanding of the biogeochemical evolution of the P cycle during the Archean may shed light on whether carbonates on early Mars could have accumulated enough dissolved P to become favourable environments for the emergence and evolution of life. 

References: 

Bultel, B. et al. (2019). Journal of Geophysical Research: Planets, 124, 989–1007. 

Horgan, B. et al. (2020). Icarus 339, 113526. 

Poulton, S. W. & Canfield, D. E. (2005). Chemical Geology, 214, 209–221. 

Thompson, J. et al. (2019). Chemical Geology, 524, 383–393. 

Toner, J. D. & Catling, D. C. A. (2020). PNAS. 117, 883–888.