SMECTITE–CARBONATE–MICROBE INTERACTIONS IN THE CARBON CYCLE

Transformations between clay minerals, such as smectites or serpentines, and carbonate minerals play an important role in regulating Earth’s climate. Both types of minerals have also preserved fossil, isotopic and molecular records of Earth’s biosphere throughout geological time. There is an old and intimate connection between magnesium-rich clay minerals, carbonate minerals and microorganisms. Today, such interactions are best studied in alkaline lakes such as the Coorong Lakes, South Australia and the Cariboo Plateau Lakes, British Columbia, Canada. Here, authigenic stevensite [(M⁺_2x)(Mg_3-x☐_x)Si₄O₁₀(OH)₂⋅nH₂O] co-occurs with anhydrous magnesium-bearing carbonate minerals such as magnesite (MgCO₃) and very high magnesium calcite (VHMC, also known as protodolomite, Ca_0.5Mg_0.5CO₃). These lakes also host cyanobacteria-dominated microbial mats and some lakes support large populations of diatoms. We have conducted two sets of experiments to examine carbonation and silicification reactions in lacustrine environments. Our carbonation experiments were done using (1) filtered water and the microbial consortium from a lake in the Cariboo Plateau plus synthetic stevensite, (2) filtered water and microbes alone, and (3) filtered water and synthetic stevensite without the addition of microbes. Our results show that carbonate minerals only form in the presence of microbes and that magnesium-bearing carbonate minerals can only form from stevensite where diatoms act as a sink for silica. Our silicification experiments were done using synthetic hydrated magnesium carbonate minerals in contact with glass, diatomaceous earth and quartz under water limited conditions. These experiments produced stevensite and all of the carbonate minerals were consumed within 7–18 months at temperatures between 23 and 75 ºC. Thus, carbonation of stevensite at Earth’s surface (which sequesters CO₂) requires a delicate geochemical and geomicrobial balance whereas silicification of magnesium carbonates to form stevensite (which releases CO₂) requires only contact with a source of silica and thin films of water. Our results highlight the need to understand how coupling amongst the Mg, Si and C cycles influences the long-term stability of mineral stores for CO₂ and the role of clay minerals in the climate system.