Valence state fossils in Proterozoic stromatolites by L‐edge X‐ray absorption spectroscopy

Paleoproterozioc (ca. 1.9 Ga) stromatolites from the Biwabik Iron‐Formation (Minnesota, United States) contain fossil forms which in the literature have been attributed to ancient microbes including iron‐oxidising bacteria and cyanobacteria. To search for valence state fossils, we measured Fe 3+ /ΣFe (ΣFe = Fe 2+ + Fe 3+ ) transverse to the laminae of a Biwabik, probably biogenic stromatolite and compared the results with a scan across the enclosing, likely abiotic sedimentary rock (or interstromatolite fill). To obtain Fe 3+ /ΣFe and information about the site symmetry and crystal field strength, we used L 3 (2p 3/2 → 3d) soft X‐ray absorption spectroscopy and interpreted the measurements using our calculated spectra. We found that Fe 3+ /ΣFe is approximately constant at 0.3 within the stromatolite, increases steeply to around 0.6 in the region of the stromatolite‐fill boundary, and then reduces significantly in the interstromatolite fill. The crystal field strength for octahedral Fe 3+ increased and became more irregular as the scan moved from the stromatolite into the fill. Our results tentatively suggest that a combination of ancient biological and later diagenetic processes can produce steep gradients in Fe 3+ /ΣFe and alterations in the crystal field strength across a probable abiotic‐biotic boundary. To describe the L 3 absorption spectra in the stromatolite‐fill region, it was necessary to include a significant Fe 3+ tetrahedral component, which suggests the presence of magnetite. At other positions, fits using only octahedral Fe 2+ and Fe 3+ theoretical curves satisfactorily reproduce the spectra. The search for a possible new type of spatial biosignature in the valence state record for ancient stromatolites might help differentiate between the biogenic stromatolites and abiogenic stromatolite‐like structures and could even be relevant to the search for fossil evidence of life in extraterrestrial rocks.