Late Holocene Ikaite Pseudomorphs In a Saline Lake In the Northern Great Plains, Canada

Ikaite (CaCO₃ 6H₂O), is a rare carbonate mineral phase with a limited stability field at or near freezing temperatures. Because of this limited thermodynamic stability, most field studies have been on pseudomorphs after ikaite. The vast majority of reported occurrences of ikaite and its pseudomorphs have been from marine settings. Notable exceptions are the thinolites in the Lahontan basin of southwestern United States (e.g., Pyramid and Walker Lakes), ikaite from Mono Lake, California, and spring discharge deposits in the Canadian Arctic and Japan.

We hereby report a new occurrence of late Holocene calcite after ikaite from Manito Lake, Canada. Although environmental conditions in the lakes of this region (seasonally cold temperatures, high salinities and elevated productivity) suggest that ikaite should be a common occurrence, this is the first documentation of lacustrine ikaite in Canada. Manito Lake is a large permanent, hypersaline (~45ppt TDS) lake located in the northern Great Plains of western Canada. The calcite pseudomorphs, which form an open porous dendritic and shrub-like fabric, comprise the interiors of massive shoreline microbialite mounds and pinnacles. These ikaite pseudomorphs are encased in centimeter-scale laminated dolomite-aragonite rinds. The calcite after ikaite crystals are similar in morphology, although not in size, to the thinolites of the Lahontan basin, and SEM images clearly indicate microcrystalline forms that are comparable to ikaite with respect to cell refinement parameters. Because the ¹⁴C reservoir effect is negligible in this lake, the chronology of ikaite formation can be firmly established at ~2000-1400 yBP. The Manito deposits show the highest δ¹³C values published for ikaite pseudomorphs, indicating very high productivity. δ¹⁸O values are consistant with precipitation in a cold relatively fresher lake than present.