A new view on the dolomite problem from Mg, Ca and Sr stable isotopes
Mineralogy, Petrology and Geochemistry
Oceanography
Final Report Abstract
The scarcity of modern environments in which the mineral dolomite (CaMg(CO3)2) forms and the high abundance of dolomite in the rock record is a long-lasting conundrum in the Earth Sciences known as the ‘dolomite problem’. One possible explanation for this apparent contradiction invokes the extent of microbial habitats in which dolomite can form. This project aimed at characterizing metal stable isotope fractionation in microbial dolomite with the ultimate goal of discerning diagenetically formed, secondary dolomite from primary, microbial dolomite based on metal isotope signatures. In this project the first metal isotope measurements on exopolymeric substances (EPS) that are associated with microbial dolomite were performed. One key finding is that the sign and magnitude of fractionation differs in experimental run products and natural microbial mats from a sabkha setting in Qatar. This result documents that isotopic fractionation during formation of EPS and their various organic ligands can alter the δ26Mg of the residual dissolved Mg2+ and hence directly affect the δ26Mg of microbial dolomite. Strongly 26Mg-depleted surface water at the end of hot and dry summer in the summer further attest to 26Mg-enrichment in EPS. This variable modification of the δ26Mg value ought to result in a higher scatter on the cm-scale in microbial dolomite from the geological archive, possibly fingerprinting microbial dolomite. In the sabkha setting, the presence of Mg-bearing clay minerals complicates the assessment of isotopic differences between pore water and the inseparable fine-grained fraction containing dolomite and clay minerals. The measured bulk leached phase has mostly similar or higher δ26Mg values compared to the associated pore water. The dolomites’ δ26Mg-values were thus constructed by a two-component mixing model based on measured δ26Mgclay and Mg/Ca ratios in the leached fraction. Isotopic differences Δ26Mgdolomite-pore water then resulted in the range of ca. -2 to + 0.4 ‰ δ26Mg across a few cm depth where different microbial communities dominate. Preliminary δ88Sr and δ44Ca values show significant depletion in the low-mass isotopes in dolomite relative to pore water. The recently developed approach ‘sample-sample bracketing’ that allows obtaining more precise triple isotope fractionation exponents θ was applied to experimental dolomite. These analyses yielded the first θ-values for microbial dolomite formation with θ= 0.515 ± 0.002 (1SD) and 0.515 ± 0.003 (1SD), values that indicate two possible mechanisms of isotope fractionation, i) during bond-breaking of higher-mass molecules (Mg with hydration shell) or ii) a two-stage fractionation process involving equilibrium and nonequilibrium fractionation. The latter mechanism would be in accordance with incorporation of pre-fractionated Mg from EPS into dolomite, highlighting the importance of EPS in controlling δ26Mg in microbial dolomite.
Publications
- AGU Fall Meeting 2019: Magnesium and strontium stable isotopes from hypersaline microbial mats
Michael Tatzel
- Virtual Goldschmidt conference 2020: Magnesium and strontium isotope fractionation during microbial dolomite formation
Michael Tatzel
(See online at https://doi.org/10.46427/gold2020.2568)
