Molecular mechanism of metamorphic alteration on traces of early life in banded iron formations

It is well-known that multi-stage metamorphism can result in the alteration of indigenous biological molecules, limiting our understanding of early life on Earth. However, the physiochemical mechanisms involved in these processes are still poorly understood. In this study, we present petrographic observations and micro- to nano-geochemical investigations on the carbonaceous matter (CM) in representative Neoarchean banded iron formations (BIFs) from North China, which have undergone significant alteration during lower amphibolite-facies prograde metamorphism , and subsequent retrograde alteration. The CM is in paragenetic equilibrium with prograde mineral phases, and is often associated with apatite that occurs in Fe-rich bands parallel to layering. This implies that the CM is most likely inherited from syn-depositional biomass, as confirmed by the nanoscale infrared spectroscopy , which shows the presence of C C, C H, and C N/N H bonds. Raman spectroscopic analyses reveal that the maximum metamorphic temperature of CM is 479 ± 50 °C, which is consistent with the metamorphic peak conditions of the host BIFs from petrographic constraints (i.e., garnet-bearing amphibolite-face metamorphism). The BIFs possess average bulk δ 13 C org values of −20.0 ± 0.9‰ ( 1 σ ) and δ 13 C carb values of −12.9 ± 1.8‰ ( 1 σ ), further indicating syngenetic biomass graphitization during prograde metamorphism. This thermal cracking process may have released gaseous hydrocarbons, as shown by secondary CH 4 fluid inclusions in quartz. We further use quantum mechanical simulations to constrain dissociative tendencies for functional groups (C C, C H, C O, and C N) of original organic molecules to assess the stability of organic chemical bonds during prograde metamorphism (0–600 °C, 0–15 kbar). The relatively high thermal durability of C H and the armoring effects of primary organic-phyllosilicate complexes may account for C H preservation in BIFs. Furthermore, the electron microscopy combined with elemental analysis reveals widespread nano-chlorite infiltration into CM during retrograde metamorphism (i.e., partial replacement of garnet by chlorite). The pervasive CM alteration is likely responsible for the absence of C O bonds, where nanopore-scale reactions might have played a key role. Altogether, we suggest that multi-stage metamorphic processes, involving mineral-organic reactions and nanoscale interface interactions, may have governed the preservation of ancient biosignatures in BIFs. Our findings highlight the importance of evaluating metamorphic effects when using molecular signals to reconstruct early life behaviors, and shed new light on the study of primordial microorganisms , particularly those found in iron-rich sediments on early Earth.