Impact of diagenesis and acidic alteration on microbial fabrics and trace element distributions in fossilised siliceous hot spring deposits

Siliceous hot spring deposits, or sinters, are important targets in the search for evidence of early life on Earth and possibly Mars, as organic material may be preserved with high fidelity by rapid silicification. Despite a high preservation potential in terrestrial environments, sinters undergo numerous physical, mineralogical, and chemical changes over time, leading to alteration and potential destruction of microfossils and other evidence of past life. In the absence of definitive preservation of organics, trace metals have been suggested as potential biosignatures to support inferences of past biological influence in sinters. However, it is unclear how these trace metals are preserved or modified by diagenesis in the ‘deep time’ geologic record. This study utilises optical, scanning electron and synchrotron x-ray fluorescence microscopy, and electron probe microanalysis to investigate the relationship between trace metals and changes in preservation state of microbial fabrics from the Subrecent fossilised Ohakuri hydrothermal system, Taupō Volcanic Zone, New Zealand. Our results show that trace metals record a complex association with variably preserved microbial fabrics, resulting from multiple diagenetic and hydrothermal alteration overprinting events. Gallium exemplifies this complexity, with distinct spatial correlations to primary microbial fabrics, while also being tied to multiple, late-stage overprinting processes, thereby complicating its utility as a biosignature. Overall, our results suggest that Ga may provide new insights into silicification processes in sinter-forming hydrothermal systems. Understanding the behaviour of Ga in these settings aids in unravelling site specific, hydrothermal fluid histories and interpreting silicification processes. Therefore, Ga may potentially play a limited role in the search for past life in Earth’s earliest rocks or in future returned samples from Mars