Trace metal and organic biosignatures in digitate stromatolites from terrestrial siliceous hot spring deposits: Implications for the exploration of martian life

Novel biosignatures of laminated, microbial, digitate structures – stromatolites – from modern geothermal fields of the Taupō Volcanic Zone, New Zealand, and from El Tatio, Chile, provide an opportunity to investigate evidence of extremophile life preserved in siliceous hot spring deposits, or sinters, interpreted as analogs for early life on Earth and possibly Mars. Synchrotron-μXRF, electron microprobe analysis, Raman spectroscopy, and optical microscopy are used in a coordinated approach to identify corroborating textural, chemical and organic evidence of life in these modern siliceous materials. Fluid mobile elements, such as As and Sr, track the growth history of digitate structures. Trace element enrichments of Ca, Al, Ga, +/- Fe, Mn, As, Rb, Cs, and Sr, are identified in silicified sheaths of microbial filaments embedded within the sinter. In contrast, silicified diatoms in some sinter samples show no trace element enrichment. Gallium enrichments have also been observed in 16 ka and Jurassic (150 Ma) microbial palisade sinter textures, suggesting the potential for preservation through geologic time, even after recrystallization to quartz. For the first time, scytonemin and β-carotene, pigments for UV protection in cyanobacteria, are identified by Raman analysis in silicified sheaths around microbial filaments and co-located with trace metal enrichments in digitate structures. The combined analytical approach outlined here provides a robust means to assess the validity of novel biosignatures, with application to the exploration of Mars, where preservation of opaline silica has the potential to preserve a range of microbial biosignatures.