Tracking phosphorus redox speciation in microbial carbonates through Earth’s history and beyond

Phosphorous availability is required for biological productivity, nutrient cycling and oxygenation. Over recent years, reduced phosphorous (phosphite) has moved into focus as a potentially new proxy that can provide information about environmental conditions and biogeochemical cycles in deep time. Phosphite can be generated by a range of biological and abiotic processes, but its distribution and implications are so far poorly understood.To address this knowledge gap, we investigated phosphate and phosphite concentrations in stromatolites spanning from the Archean to the modern. Stromatolites are among the oldest life forms found on Earth, preserved in the fossil record, dating back to 3500 million years ago. They are formed in shallow water, mostly by the metabolic activity of a diverse microbial ecosystem. They are composed of carbonate minerals, which can trap both phosphate and phosphite in their crystal lattice. We measured phosphorus speciation with Ion Chromatography and Inductively coupled plasma mass spectrometry. The data reveal that carbonate-associated phosphate and phosphite date back to the early Precambrian, presenting the first record of phosphite in carbonate rocks of low metamorphic grade. The phosphite may be of biogenic origin, but also non-biological sources such as meteorite impacts, hydrothermal activity or weathering of high-grade metamorphic rocks are plausible. These abiotic sources could potentially be more important on Mars, whose mantle has a lower oxygen fugacity, and where impact debris is well-preserved near the surface. Our study reveals that carbonate records can be used to reconstruct the history of phosphorus redox speciation on Earth and perhaps early Mars.