Biogenic Dolomite and Magnesite Formation at Low Temperatures: Implications for Earth and Mars

This thesis systematically explores the formation of Mg-rich carbonates at low temperatures through experimental and field-based approaches, with particular emphasis on their sedimentological, diagenetic, and astrobiological implications. Chapter 2 reevaluated whether ordered dolomite formed in traditional microbial experiments. Solid-state NMR analyses confirmed that microbial activity promotes the precipitation of poorly ordered dolomite. Structural water and organic occlusions are commonly retained in microbial dolomites, indicating a gradual dehydration and organic degradation process during maturation. Organic residues identified via CPMAS and DPMAS techniques suggest that microbial dolomites may preserve biosignatures. These findings provide valuable insights for identifying early life signatures in the geological record and support the utility of dolomite as a potential biosignature carrier and indictor of water activity for Mars exploration. Chapter 3 investigated the stoichiometry and cation ordering of microbial protodolomite precipitated at 10, 20, 30, and 40 °C. Protodolomite formed under aerobic conditions at 10–30 ℃, whereas ordered dolomite only precipitated at 40° C. XRD patterns lacked ordering peaks of protodolomite at lower temperatures at 10-30 ℃, but HRTEM confirmed cation ordering. The 015 peak was observed in XRD for dolomite formed at 40 °C, supporting a stepwise cation ordering process. This model provides a mechanistic explanation for the formation of sedimentary dolomite in evaporative and warm settings and underscores the importance of early recrystallization in preserving Precambrian dolomite. Chapter 4 demonstrated that magnesite can precipitate at low temperatures via microbial activity by Halomonas venusta, following a non-classical pathway where AMC acts as a precursor. Aerobic microbial metabolism creates microenvironments conducive to magnesite nucleation. The retention of structural water in both AMC highlights water activity’s role in mineral formation. These results suggest a plausible microbial origin and water related for sedimentary magnesite and a terrestrial analogue for astrobiological processes on Mars. Chapter 5 showed that both protodolomite and magnesite can form under aerobic and microaerobic conditions with Halomonas aquamarina at 30° C. This is the first evidence of aerobic bacteria precipitating Mg-carbonates in low-oxygen settings. Under aerobic conditions, higher Mg content in protodolomite and increased magnesite yield were observed, demonstrating the influence of oxygen concentration and evaporation intensity on carbonate stoichiometry. These findings provide a model for Mg-carbonate distribution in stratified environments and support microbial activity under marginal oxygen levels on Mars. Chapter 6 provided a field-based example of dolomite formation and fluid evolution in the Eocene Huanghekou sag. Microrhombic dolomicrite formed during the penecontemporaneous stage, likely mediated by microbes such as cyanobacteria and sulfate-reducing bacteria. Recrystallization transformed proto-dolomite into dolomicrite in early diagenesis. Subsequent isopachous dolomite cement precipitated from Mg-rich fluids in the vadose zone, while Fe-rich saddle dolomite formed during burial under hydrothermal influence. This multi-stage diagenetic history serves as an analogue for interpreting ancient dolomite in similar depositional systems.