Preferential authigenic clay precipitation on microbial mats in alkaline lakes of the Cariboo Plateau, British Columbia, Canada

Alkaline lakes commonly precipitate authigenic Mg-phyllosilicates, yet the controls on formation of these clay minerals remain poorly understood. This study examines the geochemical and microbial controls on precipitation of Mg-rich clays and carbonates in four alkaline lakes of the Cariboo Plateau located in British Columbia, Canada. Mineralogical analyses reveal that lake sediments and microbial mats contain abundant stevensite [(Na 2 y · n H 2 O)(Mg 3– y □ y )Si 4 O 10 (OH) 2 ], a Mg-clay phase typically associated with high-alkalinity, closed-basin systems. Mg-carbonates are also present, magnesite (MgCO 3 ) and very high Mg-calcite [also known as disordered dolomite, (Ca 0.5 Mg 0.5 CO 3 )]. Despite extremely low aqueous Mg and Si concentrations, we observe that Mg-clay precipitation is favoured over Mg-carbonates within microbial mats compared to sediment from the same lake. Our findings suggest that microbial processes, including cyanobacterial activity and diatom frustule dissolution, play a key role in facilitating Mg-clay formation by influencing local geochemical conditions and providing nucleation sites. These findings highlight the importance of considering microbial activity when using lacustrine sediments as paleoenvironmental proxies, as they can significantly impact the mineralogical composition of the sediment associated with the microbial mat.