Modern microbialites in the hypersaline Great Salt Lake (Utah, USA) mineralise through a three-step pathway: a poorly crystallized Mg-silicate phase precipitates on the extracellular organic matrix in the zone of active oxygenic photosynthesis; aragonite nucleates at sulfate-reduction hotspots during degradation of cyanobacteria and organic matrix; and aragonite is partially replaced by dolomite, postulated to occur via dissolution-precipitation in neutral to slightly acidic porewater as the most recalcitrant organic matrix is degraded. Pace et al. (2016) established this sequence but left its mechanistic basis and diagenetic fate open: the exact mechanism by which microbial activity promotes Mg-silicate precipitation (direct versus indirect roles, and the influence of individual natural extracellular organic matter components on nucleation and composition) is undetermined; the dolomite-forming step is inferred, never directly observed; and the burial-diagenetic transformation of the microbial Mg-silicate phases, with consequences for biosignature preservation in ancient microbialites, was declared future work. Recent work (2016-2026) has demonstrated cyanobacterial and EPS-promoted Mg-silicate and carbonate precipitation, proposed competing sulfide-mediated versus dissolved-silica-controlled explanations for Great Salt Lake dolomite, and documented diagenetic Mg-silicate formation and clay-mediated biosignature preservation, yet none of the three questions is settled, and the Great Salt Lake dolomite mechanism remains actively contested.

Background

Microbialites are organosedimentary structures built by benthic microbial mats through the trapping of sediment and the precipitation of minerals; because they dominate much of the Precambrian fossil record, interpreting the minerals inside them is a central tool for reconstructing early microbial ecosystems. In modern hypersaline lakes such as the Great Salt Lake (Utah, USA), the mats that build microbialites are dominated by oxygenic photosynthetic cyanobacteria and excrete a gel-like extracellular organic matrix (EOM, also called extracellular polymeric substances or EPS), a chemically heterogeneous mixture of polysaccharides, proteins and other compounds. Mat microbes modify the chemistry of the porewater trapped in the mat: oxygenic photosynthesis consumes CO2 and raises pH during the day, while degradation of EOM and cell material by heterotrophs, including sulfate-reducing bacteria (anaerobes that respire sulfate and produce sulfide), lowers pH and releases ions deeper in the mat. These microenvironmental gradients control which authigenic minerals - minerals precipitated in place - form. Two mineral families matter here. First, poorly crystallized magnesium-silicate phases, precursors of the hydrated Mg-clays stevensite, kerolite and sepiolite, are now known as major authigenic components of microbialites in modern saline and alkaline lakes (Zeyen et al. 2015, Frontiers in Earth Science 3:64); they appear both above and below the bulk-water pH threshold of about 8.6-8.7 calculated for their abiotic precipitation, so physicochemistry alone cannot explain them, and laboratory work shows some organic compounds (e.g., succinic acid) enhance their nucleation while others (e.g., oxalic acid) inhibit it. Second, dolomite (CaMg(CO3)2) epitomizes the long-standing 'dolomite problem': although widespread in ancient rocks, it is notoriously difficult to precipitate at Earth-surface temperatures because of the kinetic barrier posed by the strongly hydrated Mg2+ ion. Both microbial processes (sulfate reduction and specific organic substrates; Van Lith et al. 2003; Krause et al. 2012) and engineered surface chemistry (Roberts et al. 2013) can circumvent this inhibition in the laboratory, but which controls operate in natural lakes remains disputed. Building on this background, Pace et al. (2016) combined mineralogy, microscopy, microelectrode profiling and geochemical modelling (saturation-index modelling in PHREEQC, a widely used aqueous-geochemical equilibrium code, using the Pitzer activity-coefficient formulation suited to hypersaline brines) to show that Great Salt Lake microbialites mineralise in three steps along microbial geochemical gradients: the Mg-silicate phase precipitates on the alveolar (honeycomb-like) EOM framework where photosynthesis raises pH; aragonite (a CaCO3 polymorph) nucleates where sulfate reduction accompanies degradation of cyanobacteria and EOM; and dolomite partially replaces aragonite, postulated to happen via dissolution-precipitation in neutral to slightly acidic porewater (pH 6.5-7.5, values which prevail in the deeper mat layers during most of the day) during degradation of the most recalcitrant EOM. Evidence for this last step is indirect: quasi-stoichiometric dolomite micro-rhombs (about 50.5 mol% MgCO3) in spherical aggregates at aragonite-EOM interfaces, a carbon-isotope shift (dolomite δ13C about 3.06 permil versus 4.98 permil for co-occurring aragonite) consistent with precipitation from porewater modified by organic-matter degradation, and thermodynamic modelling showing aragonite dissolves while dolomite remains oversaturated under the inferred porewater pH. Experiments reproducing burial conditions further show that poorly crystallized Mg-silicate phases recrystallize to kerolite and eventually talc within weeks under hydrothermal conditions (Tosca et al. 2011), and Mg-silicate-talc-dolomite assemblages occur in Neoproterozoic and Cretaceous microbialites - implying that the GSL mineralisation pathway, once understood, could sharpen interpretation of ancient microbialites, but also that its products transform during diagenesis in ways that remain poorly characterized.

Problem Statement

The problem is to give a mechanistic, experimentally and observationally grounded account of the microbially influenced mineralisation pathway of Great Salt Lake microbialites and of the diagenetic fate of its products, so that comparable fabrics in ancient microbialites can be interpreted as records of microbial metabolism. Its three linked components concern successive steps of one paragenetic sequence and one shared objective: (1) Determine the mechanism by which microbial activity promotes precipitation of the poorly crystallized Mg-silicate phase on the extracellular organic matrix - in particular, the respective direct roles of microbes (cell-surface and EOM templating, metabolic uptake and release) versus indirect roles (the pH rise driven by oxygenic photosynthesis), and how individual natural EOM components promote or inhibit Mg-silicate nucleation and control the composition of the phase (e.g., its dioctahedral versus trioctahedral character, that is, whether the octahedral sheet of the layer-silicate structure carries two or three cations per unit cell), noting that bulk lake water pH of ~8.3 sits below the nominal abiotic threshold yet the phase precipitates. (2) Test the postulated final step in which aragonite is partially replaced by dolomite via dissolution-precipitation reactions in neutral to slightly acidic porewater during degradation of the most recalcitrant EOM - establishing whether this pathway actually operates, what biological and chemical controls govern it (e.g., sulfate reduction and sulfide, carboxyl-group cation binding), and how it relates to competing abiotic explanations such as dissolved-silica-driven dolomite formation. (3) Determine how the microbial Mg-silicate phases transform during burial diagenesis (e.g., recrystallization to kerolite, stevensite or talc) and how these transformations preserve or destroy microbial biosignatures (morphological and organic-chemical traces of past microbial life), in order to strengthen the interpretation of ancient microbialites. Resolving these questions requires identifying the mechanisms and controls with evidence sufficient to decide among the competing hypotheses and specifying the conditions under which the pathway's mineral products form, transform and retain biosignatures; the paragenetic sequence in additional modern or fossil examples alone does not settle them.

References

  1. Pace, A., Bourillot, R., Bouton, A., Vennin, E., Galaup, S., Bundeleva, I., Patrier, P., Dupraz, C., Thomazo, C., Sansjofre, P., Yokoyama, Y., Franceschi, M., Anguy, Y., Pigot, L., Virgone, A., & Visscher, P. T. (2016). Microbial and diagenetic steps leading to the mineralisation of Great Salt Lake microbialites. Scientific Reports, 6, 31495. DOI: 10.1038/srep31495. https://doi.org/10.1038/srep31495
  2. Zeyen, N., Benzerara, K., Li, J., Groleau, A., Balan, E., Robert, J.-L., Estève, I., Tavera, R., Moreira, D., & López-García, P. (2015). Formation of low-T hydrated silicates in modern microbialites from Mexico and implications for microbial fossilization. Frontiers in Earth Science, 3, 64. DOI: 10.3389/feart.2015.00064. https://doi.org/10.3389/feart.2015.00064
  3. Tosca, N. J., Macdonald, F. A., Strauss, J. V., Johnston, D. T., & Knoll, A. H. (2011). Sedimentary talc in Neoproterozoic carbonate successions. Earth and Planetary Science Letters, 306, 11-22. DOI: 10.1016/j.epsl.2011.03.041. https://doi.org/10.1016/j.epsl.2011.03.041
  4. Van Lith, Y., Warthmann, R., Vasconcelos, C., & McKenzie, J. A. (2003). Sulphate-reducing bacteria induce low-temperature Ca-dolomite and high Mg-calcite formation. Geobiology, 1, 71-79. DOI: 10.1046/j.1472-4669.2003.00003.x. https://doi.org/10.1046/j.1472-4669.2003.00003.x
  5. Krause, S., Liebetrau, V., Gorb, S., Sánchez-Román, M., McKenzie, J. A., & Treude, T. (2012). Microbial nucleation of Mg-rich dolomite in exopolymeric substances under anoxic modern seawater salinity: New insight into an old enigma. Geology, 40, 587-590. DOI: 10.1130/g32923.1. https://doi.org/10.1130/g32923.1
  6. Roberts, J. A., Kenward, P. A., Fowle, D. A., Goldstein, R. H., González, L. A., & Moore, D. S. (2013). Surface chemistry allows for abiotic precipitation of dolomite at low temperature. Proceedings of the National Academy of Sciences, 110, 14540-14545. DOI: 10.1073/pnas.1305403110. https://doi.org/10.1073/pnas.1305403110
  7. Newell, D. L., Jensen, J. L., Frantz, C. M., & Vanden Berg, M. D. (2017). Great Salt Lake (Utah) microbialite δ13C, δ18O, and δ15N record fluctuations in lake biogeochemistry since the late Pleistocene. Geochemistry, Geophysics, Geosystems, 18, 3631-3645. DOI: 10.1002/2017GC007078. https://doi.org/10.1002/2017GC007078
  8. Dunham, E. C., Fones, E. M., Fang, Y., Lindsay, M. R., Steuer, C., Fox, N., Willis, M., Walsh, A., Colman, D. R., Baxter, B. K., Lageson, D., Mogk, D., Rupke, A., Xu, H., & Boyd, E. S. (2020). An ecological perspective on dolomite formation in Great Salt Lake, Utah. Frontiers in Earth Science, 8, 24. DOI: 10.3389/feart.2020.00024. https://doi.org/10.3389/feart.2020.00024
  9. Kanik, M., Munro-Ehrlich, M., Fernandes-Martins, M. C., Payne, D., Gianoulias, K., Keller, L., Kubacki, A., Lindsay, M. R., Baxter, B. K., Vanden Berg, M. D., Colman, D. R., & Boyd, E. S. (2020). Unexpected abundance and diversity of phototrophs in mats from morphologically variable microbialites in Great Salt Lake, Utah. Applied and Environmental Microbiology, 86, e00165-20. DOI: 10.1128/AEM.00165-20. https://doi.org/10.1128/AEM.00165-20
  10. Lincoln, T. A., Webb, S. M., Present, T. M., Magyar, J. S., & Trower, E. J. (2022). Microbial activity and neomorphism influence the composition and microfabric of ooids from Great Salt Lake, UT. The Sedimentary Record, 20(1). DOI: 10.2110/001c.56183. https://doi.org/10.2110/001c.56183
  11. Suosaari, E. P., Lascu, I., Oehlert, A. M., Parlanti, P., Mugnaioli, E., Gemmi, M., Machabee, P. F., Piggot, A. M., Palma, A. T., & Reid, R. P. (2022). Authigenic clays as precursors to carbonate precipitation in saline lakes of Salar de Llamara, Northern Chile. Communications Earth & Environment, 3, 325. DOI: 10.1038/s43247-022-00658-5. https://doi.org/10.1038/s43247-022-00658-5
  12. Lamérand, C., Shirokova, L. S., Petit, M., Bénézeth, P., Rols, J.-L., & Pokrovsky, O. S. (2022). Kinetics and mechanisms of cyanobacterially induced precipitation of magnesium silicate. Geobiology, 20, 560-574. DOI: 10.1111/gbi.12503. https://doi.org/10.1111/gbi.12503
  13. del Buey, P., Sanz-Montero, M. E., Braissant, O., Cabestrero, Ó., & Visscher, P. T. (2021). The role of microbial extracellular polymeric substances on formation of sulfate minerals and fibrous Mg-clays. Chemical Geology, 581, 120403. DOI: 10.1016/j.chemgeo.2021.120403. https://doi.org/10.1016/j.chemgeo.2021.120403
  14. del Buey, P., Sanz-Montero, M. E., & Sánchez-Román, M. (2023). Bioinduced precipitation of smectites and carbonates in photosynthetic diatom-rich microbial mats: Effect of culture medium. Applied Clay Science, 238, 106932. DOI: 10.1016/j.clay.2023.106932. https://doi.org/10.1016/j.clay.2023.106932
  15. Baskin, R. L., Della Porta, G., & Wright, V. P. (2022). Characteristics and controls on the distribution of sublittoral microbial bioherms in Great Salt Lake, Utah: Implications for understanding microbialite development. The Depositional Record, 8, 39-66. DOI: 10.1002/dep2.159. https://doi.org/10.1002/dep2.159
  16. Fang, Y., Hobbs, F., Yang, Y., & Xu, H. (2023). Dissolved silica-driven dolomite precipitation in the Great Salt Lake, Utah, and its implication for dolomite formation environments. Sedimentology, 70, 1328-1347. DOI: 10.1111/sed.13081. https://doi.org/10.1111/sed.13081
  17. Muller, E., Rapin, W., Caumartin, J., Jézéquel, D., De Wever, A., Thomazo, C., Havas, R., López-García, P., Moreira, D., Tavera, R., & Benzerara, K. (2023). Diagenetic formation of stevensite by replacement of diatom frustules in the sediments of the alkaline Lake Alchichica (Mexico). Sedimentology, 70, 1013-1038. DOI: 10.1111/sed.13069. https://doi.org/10.1111/sed.13069
  18. Mather, C. C., Lampinen, H. M., Tucker, M., Leopold, M., Dogramaci, S., Raven, M., & Gilkes, R. J. (2023). Microbial influence on dolomite and authigenic clay mineralisation in dolocrete profiles of NW Australia. Geobiology, 21, 644-670. DOI: 10.1111/gbi.12555. https://doi.org/10.1111/gbi.12555
  19. Mercedes-Martín, R., Sánchez-Román, M., Ayora, C., Rogerson, M., Thomas, C., Van Spanning, R., Brasier, A., Wacey, D., & Reijmer, J. J. G. (2025). Formation of Mg-silicates in the microbial sediments of a saline, mildly alkaline coastal lake (Lake Clifton, Australia): Environmental versus microbiological drivers. Sedimentology, 72, 1518-1547. DOI: 10.1111/sed.70011. https://doi.org/10.1111/sed.70011
  20. Talon, J., Pellenard, P., Baele, J.-M., Quesnel, F., Briais, J., Iakovleva, A., Dupuis, C., Bruneau, L., & Vennin, E. (2025). A model for the concomitant early formation of dolomite and fibrous clays in coastal bay systems: Evidence from the Eocene (Paris Basin, France). Sedimentology, 72, 1860-1898. DOI: 10.1111/sed.70024. https://doi.org/10.1111/sed.70024
  21. Hickman-Lewis, K., Cuadros, J., Yi, K., Hong, T. E., Byeon, M., Jang, J. H., Choi, M.-Y., Seo, Y., Najorka, J., Montgomery, W., Matlak, K., Wolanin, B., Smith, C. L., & Cavalazzi, B. (2025). Aluminous phyllosilicates promote exceptional nanoscale preservation of biogeochemical heterogeneities in Archaean siliciclastic microbial mats. Nature Communications, 16, 2726. DOI: 10.1038/s41467-025-57727-4. https://doi.org/10.1038/s41467-025-57727-4
  22. Suosaari, E. P., Dupraz, C., Oehlert, A. M., Lascu, I., Vitek, B. E., Piggot, A. M., Palma, A. T., & Reid, R. P. (2026). Coupled authigenic Mg silicate and carbonate precipitation in saline lakes of the Salar de Atacama, Northern Chile. The Depositional Record, 12, e70055. DOI: 10.1002/dep2.70055. https://doi.org/10.1002/dep2.70055
  23. Visscher, P. T., Boussagol, P., Bourillot, R., Dupraz, C., Braissant, O., Decho, A. W., Burns, B. P., White, R. A., Tomko, P., & Vennin, E. (2026). A critical role of heterotrophic bacteria in early diagenesis of carbonates through exopolymer degradation and calcium release. The Depositional Record, 12, e70057. DOI: 10.1002/dep2.70057. https://doi.org/10.1002/dep2.70057
  24. Arizaleta, M. L., Raudsepp, M. J., Zeyen, N., Kenney, J. P. L., Lazowski, C., Alessi, D. S., Konhauser, K. O., & Wilson, S. (2026). Preferential authigenic clay precipitation on microbial mats in alkaline lakes of the Cariboo Plateau, British Columbia, Canada. Applied Geochemistry, 206, 106917. DOI: 10.1016/j.apgeochem.2026.106917. https://doi.org/10.1016/j.apgeochem.2026.106917
  25. Arizaleta, M. L., Raudsepp, M. J., Zeyen, N., Kenney, J. P. L., Snihur, K. N., Alessi, D. S., & Wilson, S. (2026). Microbial and abiotic approaches to carbonating stevensite: what works and what doesn't. Geochimica et Cosmochimica Acta, 414, 202-216. DOI: 10.1016/j.gca.2025.12.052. https://doi.org/10.1016/j.gca.2025.12.052
  26. Li, G., Naim, Z., Gibert, L., Stuut, J.-B., Waajen, A. C., Jimenez-Moreno, G., & Sánchez-Román, M. (2026). Microbial mediation and climatic control on dolomite precipitation in a hypersaline lake: Insights from Salinas Lake, southern Iberia. The Depositional Record, 12, e70058. DOI: 10.1002/dep2.70058. https://doi.org/10.1002/dep2.70058