Surface chemistry allows for abiotic precipitation of dolomite at low temperature

Abstract

Although the mineral dolomite is abundant in ancient lowtemperature sedimentary systems, it is scarce in modern systems below 50°C. Chemical mechanism(s) enhancing its formation remain an enigma because abiotic dolomite has been challenging to synthesize at low temperature in laboratory settings. Microbial enhancement of dolomite precipitation at low temperature has been reported; however, it is still unclear exactly how microorganisms influence reaction kinetics. Here we document the abiotic synthesis of low-temperature dolomite in laboratory experiments and constrain possible mechanisms for dolomite formation. Ancient and modern seawater solution compositions, with identical pH and pCO 2 , were used to precipitate an ordered, stoichiometric dolomite phase at 30°C in as few as 20 d. Mg-rich phases nucleate exclusively on carboxylated polystyrene spheres along with calcite, whereas aragonite forms in solution via homogeneous nucleation. We infer that Mg ions are complexed and dewatered by surface-bound carboxyl groups, thus decreasing the energy required for carbonation. These results indicate that natural surfaces, including organic matter and microbial biomass, possessing a high density of carboxyl groups may be a mechanism by which ordered dolomite nuclei form. Although environments rich in organic matter may be of interest, our data suggest that sharp biogeochemical interfaces that promote microbial death, as well as those with high salinity may, in part, control carboxyl-group density on organic carbon surfaces, consistent with origin of dolomites from microbial biofilms, as well as hypersaline and mixing zone environments.biomineralization | carbonates A lthough synthesis of dolomite in laboratory settings at high temperature (80-250°C) has yielded valuable information regarding dolomite formation (1, 2), the validity of extrapolating kinetic data at 250°C down to 25°C is questionable. Synthesis of low-temperature dolomite is hindered by slow reaction kinetics (2). Kinetic inhibition is attributed to lack of solution supersaturation (3), sulfate inhibition (1), cation desolvation (4), and lack of nucleation sites (5). Laboratory precipitation at low temperature has only been successful in producing disordered dolomite: from solutions with high salinity (6); through intermittent (7) or complete dehydration (8); by using organic or inorganic compounds that effectively dewater Mg 2+ ions (9-11); or in the presence of microorganisms, their exudates, or surfaces (12, 13).Microbial dolomite has been produced in the presence of several different metabolic pathways including sulfate reduction, methanogenesis, methanotrophy, sulfide oxidation, and aerobic respiration (12-16), which may drive precipitation through the supersaturation of solutions with respect to dolomite. Recent work, however, has focused on the role of microbial cells and exopolymeric substances (EPS) as surfaces for dolomite nucleation (17). Whereas these studies clearly demonstrate that these surfaces are involved in dolomite formation, specific ...

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“…), thereby affecting their concentration in solution (Dupraz and Visscher, 2005;Visscher and Stolz, 2005;Dupraz et al, 2009;Arp et al, 2010;Roberts et al, 2013;Zhu and Dittrich, 2016). They may as well glue particles together, such as certain filamentous microbes in caves, or retain water in their framework (Barton et al, 2001;Barton and Northup, 2007;Jones, 2010).…”

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