Mapping mineralogical heterogeneities at the nm-scale by scanning electron microscopy in modern Sardinian stromatolites: Deciphering the origin of their laminations

Stromatolites are found throughout the geological record and have received strong attention because they provide precious information about paleoenvironments microbial paleobiodiversity. However, while this is interpreted based on our knowledge modern analogs, latter remains incomplete. Here, we investigated environmental biological recorded by stromatolites in Mari Ermi, a coastal pond Western Sardinia that experiences severe seasonal evaporation large salinity variations. For purpose, combined variety of analytical tools, allowing to characterize mineralogical composition these from bulk, centimeter-scale nanometer-scale assess spatial distribution mineral phases. In particular, used quantified x-ray elemental maps provided energy dispersive spectrometry analyses coupled with scanning electron microscopy (SEM-EDXS). These were processed using an innovative data treatment (i) phase recognition, (ii) assessment formula each (iii) mapping their resolution down hundred nanometers. Overall, proved as efficient approach unravel heterogeneities detect informative phases overlooked bulk analyses. Ermi mostly composed magnesian calcite average Mg/Ca ~0.1–0.2 indicated observations revealed variable Mg-contents specific distribution, most Mg-enriched calcites (Mg/Ca up ~0.5) being systematically distributed around remnants. Moreover, comprised alternation Mg-richer Mg-poorer laminae, which paralleled varying enrichment remnants alternating laminae. The combination SEM-EDXS, focused ion beam milling transmission allowed several minor phases, hardly or not detected at all analyses, such aragonite, clay minerals well gypsum-like phase. An additional calcium sulfate phase, best S-apatite (cesanite), was SEM-EDXS. assemblage its definite only discrete stages lagoon. microorganisms seem major control Mg substitution calcite. As result, dynamics populations, influenced variations induced lagoons, generates formation laminae stromatolites.