What Mixed Silica-Carbonate Hot Springs Can Tell us About the Role of Mineralogy in Biosignature Preservation and Retention at Lýsuhóll, Iceland

On Earth, hot spring environments host highly diverse microbial communities. The precipitation of silica in these environments enables the capture and preservation of these communities over geologic timescales. Therefore, hot spring deposits are important targets for studying the presence of life on early Earth and other planets, such as Mars. While it has been established that siliceous hot springs are some of the best environments for biosignature preservation, further research is needed to better understand how these biosignatures are modified during fossilization and subsequent diagenesis. Silica-carbonate mixed hot springs are uncommon, but important end-members because they allow us to explore the role of mineralogy in the preservation and retention of biosignatures within the same system. Lýsuhóll is a Holocene, silica-carbonate mixed system located on the Snæfellnes Peninsula in Western Iceland. In this study, we reconstructed the depositional history of Lýsuhóll, documented the different styles of microfossil preservation, and tracked how microfossils are modified during fossilization and subsequent diagenesis within each mineral component. This study is the first to show exceptional preservation of microfossils in both the calcite and silica (opal-A). We show that there is a range of preservation styles from cellular-scale permineralization to encrustation of microorganisms by opal-A and opal-A+calcite. Shared preservational styles across these mineralogies suggest that the taphonomic history (burial, fossilization, and diagenesis) strongly influences the retention of microfossils, regardless of the mineralogy. This study also highlights that while systems with complex mineralogy and depositional histories may be difficult to interpret, they can provide more information to reconstruct the paleoenvironmental and paleobiologic settings. Understanding these processes of preservation and diagenetic modifications are crucial for detecting and interpreting the role of life in deep time and space. This will be especially important as we begin to explore the history and biogenecity of the carbonate deposits at Jezero Crater.