Arsenic-cycling metabolism recorded in hot spring silica deposits

Identifying early-life remnants is challenging due to the alteration of biosignatures over billions of years. Siliceous sinter deposits provide a unique window into the ancient biosphere due to their widespread occurrence in the ancient rock record and their excellent potential for preserving biosignatures. Indeed, hot spring silica deposits often preserve microbial traces, including microfossils, microbial mats, and stromatolites. In this study, we use synchrotron X-ray Absorption Near Edge Spectroscopy to investigate the distribution and speciation of Arsenic, a key element in microbial life, within organic matter-bearing silica sinters along an outflow channel of a modern hot spring in the Atacama Desert (El Tatio, Chile) and within organic matter-bearing, high-temperature facies of an extinct Jurassic geyser mound (Claudia, Argentina). We demonstrate that the coexistence of Arsenic(III) and Arsenic(V) is preserved within organic matter, and that the co-occurrence of Arsenic(III) and Arsenic(V) in hydrothermal sinter deposits can be used as an indicator of microbial activity in the fossil record. This finding serves as an additional tool in the search for fossil biosignatures in the early Earth rock record.