Department of Geology, University of Kansas, Lawrence, KS 66045, USA.
Proc Natl Acad Sci U S A. 2013 Sep 3;110(36):14540-5. doi: 10.1073/pnas.1305403110. Epub 2013 Aug 20.
Although the mineral dolomite is abundant in ancient low-temperature 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 pCO2, 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.
尽管白云石矿物质在古代低温沉积系统中很丰富，但在 50°C 以下的现代系统中却很稀缺。促进其形成的化学机制仍然是个谜，因为在实验室环境中，低温条件下的非生物合成白云石一直具有挑战性。已经有报道称微生物可以促进低温下白云石的沉淀，但微生物如何影响反应动力学仍不清楚。在这里，我们记录了在实验室实验中非生物合成低温白云石的过程，并限制了白云石形成的可能机制。使用具有相同 pH 值和 pCO2 的古代和现代海水溶液成分，在 30°C 下仅需 20 天即可沉淀出有序的、化学计量的白云石相。富镁相仅在碳酸钙的基础上优先在羧基化聚苯乙烯球上成核，而文石则通过均相成核在溶液中形成。我们推断镁离子与表面结合的羧基配位并脱水，从而降低了碳化所需的能量。这些结果表明，包括有机物和微生物生物量在内的天然表面，其羧基密度较高，可能是有序白云石核形成的一种机制。尽管富含有机物的环境可能很有意义，但我们的数据表明，促进微生物死亡的尖锐生物地球化学界面以及高盐度环境可能部分控制有机碳表面上羧基的密度，这与白云石起源于微生物生物膜以及高盐和混合区环境相一致。
