Microbial precipitation of a strontium calcite phase at a groundwater discharge zone near rock Creek, British Columbia, Canada

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Abstract
Mineralogical, chemical, and microscopic analyses demonstrate an intimate relationship between epilithic cyanobacteria and the precipitation of a strontium calcite phase at a groundwater discharge zone located near Rock Creek, British Columbia, Canada. The groundwater flows out of a serpentinite bedrock outcrop that provides a hard surface for the accretion of a coherent calcareous crust. The mean pH of water samples collected ∼2.0 m above the base of the outcrop was 8.5, whereas a value of 8.8 was recorded for samples taken near the outcrop base. This increase in pH can be attributed to the growth of cyanobacteria that carry out a HCO3 ‐/OH‐ exchange process during photosynthesis. Calcium was present in the water samples at levels of 32–36 ppm, whereas strontium occurred at lower concentrations (5.8–6.6 ppm). In each case, the lowest calcium and strontium values occurred in samples taken near the base of the outcrop, as expected for carbonate mineral precipitation. The crust itself is a porous thrombolitic matrix with intergrowths of calcite and cyanobacterial colonies comprised of filamentous Calothrix, and coccoid Synechococcus and Gleocapsa. Many of the cyanobacteria were completely embedded within fine‐grained crystal aggregates. Examination of thin‐sectioned samples by analytical transmission electron microscopy showed that the cyanobacteria served as nucleation sites for calcite containing up to 1.0 wt% strontium. Bulk whole rock analyses on the crust using x‐ray diffraction and x‐ray fluorescence confirmed the electron microscopic observations. Based on these results, the accretion of the strontium calcite crust is interpreted to be the result of microbial carbonate precipitation in groundwater with an Sr 2+/Ca 2+ ratio that promotes solid solution of SrCO3 in calcite.