Magnesium and calcium isotope fractionation during microbial dolomite formation

Microbial mediation is considered an important process for the formation of primary dolomite at ambient temperature. Yet, no structural, mineralogical, chemical or isotopic means exist to discern this mode of dolomite formation from secondary dolomite. To explore the utility of metal isotopes in allowing this distinction we characterize magnesium and calcium stable isotope ratios in primary (proto)dolomites from a modern hypersaline environment. Samples from the Khor Al-Adaid sabkhas in Qatar show consistent isotopic differences of Ca isotopes (Δ44/40Ca) of -1.1 and -1.8 ‰ between solution and (proto-)dolomite and -0.3 to -0.7 ‰ between solution and organic phases, consistent with a previously postulated two-step fractionation process that enriches microbially mediated dolomite in 40Ca (Krause et al., 2012). Mg isotopes reveal a more complex picture with varying magnitudes of fractionation across different microbial zones in the shallow subsurface in agreement with a wide range of Δ26/24Mg-values previously observed in the sabkahs of Abu Dhabi (Geske et al., 2015) and suggested impact of microbial activity on δ26Mg (Riechelmann et al., 2020). The high variability is moreover modulated by authigenic palygorskite formation close to the water-sediment interface that yields consistently 26Mg-enriched signatures. While clay-free sites suggest simple Mg precipitation from seawater into dolomite (Shalev et al., 2020), our data shows that Mg uptake into clay minerals does not allow a straightforward identification of a characteristic isotopic fingerprint in ancient primary dolomite, but shows potential to obtain a detailed picture of specific microbial involvement.