Depth-dependent sulfur speciation in modern carbonate microbialites from a monomictic lake

Recently, carbonate microbialites were discovered in Lake Alchichica, Mexico, forming below the oxycline (down to 40 m depth), under seasonally anoxic conditions, while conspicuous microbialites also grow at shallower depths under continuously oxic conditions. Here, we investigated sulfur (S) speciation in these microbialites at submicrometer resolution using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy, complemented by laboratory X-ray fluorescence (XRF) mapping, scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDXS). Our findings revealed that S is pervasive in both shallow and deep microbialites. However, S speciation varied with depth: while carbonate-associated sulfates (CAS) and organic S compounds were present at all depths, more reduced S species were enriched in the aragonitic layers of deep microbialites, particularly in association with remnants of biogenic structures. These variations in S speciation suggest that deep Alchichica microbialites continue to accrete during seasonal anoxia. Moreover, our results indicate that S speciation in modern microbialites is influenced by ambient redox conditions. By documenting the processes affecting S speciation in a modern microbialitic system, this study provides a framework for future investigations into ancient microbialites that formed under sulfidic conditions.