Occurrence and significance of a cold-water carbonate pseudomorph in microbialites from a saline lake

摘要

Micron-scale pseudomorphs of calcite after ikaite were discovered in microbialites from Manito Lake, a large hypersaline lake in the Great Plains of western Canada. Although environmental conditions in the lakes of this region (seasonally cold temperatures, high salinities and elevated productivity) suggest that ikaite should be common, this is the first documentation of lacustrine ikaite in Canada and the Great Plains region of North America. The calcite pseudomorphs form a porous dendritic fabric, comprise the interiors of massive shoreline microbialite mounds and pinnacles, and are encased in centimeter-scale, laminated dolomite-aragonite rinds. Because of the limited thermodynamic stability of ikaite at or near freezing temperatures, the psuedomorphs of this mineral provide evidence of an extended cold episode during the first millennium AD. The Manito deposits also show high δ13C values, indicating elevated productivity and δ18O signatures are consistent with precipitation in a cold, somewhat fresher lake than present.