Potential influence of sulphur bacteria on Palaeoproterozoic phosphogenesis

摘要

All known forms of life require phosphorus, and biological processes strongly influence the global phosphorus cycle~1. Although the record of life on Earth extends back to 3.8 billion years ago~2 and the advent of biological phosphate processing can be tracked to at least 3.5 billion years ago~3, the earliest known P-rich deposits appeared only 2 billion years ago~(4,5). The onset of P deposition has been attributed to the rise of atmospheric oxygen 2.4–2.3 billion years ago and the related profound biogeochemical shifts~(6–9), which increased the riverine input of phosphate to the ocean and boosted biological productivity and phosphogenesis~(5,10). However, the P-rich deposits post-date the rise of oxygen by about 300 million years. Here we use microfabric, trace element and carbon isotope analyses to assess the environmental setting and redox conditions of the 2-billion-year-old P-rich deposits of the vent- or seep-influenced Zaonega Formation, northwest Russia. We identify phosphatized microorganism fossils that resemble modern methanotrophic archaea and sulphur-oxidizing bacteria, analogous to organisms found in modern seep settings and upwelling zones with a sharp redoxcline~(11,12). We therefore propose that the P-rich deposits in the Zaonega Formation were formed by phosphogenesis mediated by sulphur bacteria, similar to modern sites~(13), and by the precipitation of calcium phosphate minerals on microbial templates during early diagenesis.