Formation of Mg‐silicates in the microbial sediments of a saline, mildly alkaline coastal lake (Lake Clifton, Australia): Environmental versus microbiological drivers

Recent interest in Mg‐rich silicate formation stems from its role as a valuable palaeoclimatic indicator in fluvio‐lacustrine environments and its insights into metal geochemical cycling. Traditionally, Mg‐silicate genesis in lacustrine contexts is linked to alkaline or saline conditions in closed, evaporitic basins. However, the discovery of interparticle amorphous kerolite‐like Mg‐silicates in the sediments of Lake Clifton, a currently hypersaline coastal lagoon in Western Australia with circumneutral pH and moderate alkalinity, challenges existing models. In this study, petrographic, hydrochemical and microbial genomic data from different Lake Clifton sub‐environments (episodically submerged and subaerial settings) and substrates (pustular microbial mats and non‐lithifying microbial sediments) were integrated with geochemical modelling to quantify the mechanisms underlying the formation of Mg‐silicates and aragonite peloids as lake shoreline sediments. Geochemical modelling suggests that neither evaporation‐driven alkalinity fluctuations nor the mixing of lake water with groundwater can solely explain the kerolite‐like/carbonate association observed in lakebed sediments. Kerolite‐like phases nucleate in association with twisted microbial extracellular polymeric substances and organic‐rich bacterial remains; this, combined with the identification of diatom and cyanobacteria‐powered photosynthesis, putative anoxygenic photosynthesis and sulphate‐reducing metabolisms, suggests an intimate link between biologically induced processes and the co‐precipitation of aragonite peloids and interparticle kerolite‐like phases in the lake. Moreover, the contribution of dead diatom frustule dissolution towards kerolite‐like authigenesis was geochemically simulated, revealing that the precipitation of observable amounts of kerolite‐like at pH values measured in Lake Clifton waters would prevent the formation of aragonite, questioning the feasibility of a scenario dominated by large inputs of dissolved biogenic silica. The discovery of kerolite‐like Mg‐silicates in microbial‐bearing sediments of a hypersaline coastal lagoon prompts a holistic re‐evaluation of the environmental and microbiological factors influencing Mg‐silicate–carbonate co‐precipitation in lacustrine‐peri‐marine settings. Studying modern Mg‐silicate‐bearing lacustrine sediments offers the opportunity to better understand the early diagenetic biotic‐abiotic processes that may have had limited petrographic preservation potential in ancient saline lake deposits.