Modern lacustrine microbialites: Towards a synthesis of aqueous and carbonate geochemistry and mineralogy

Abstract

Lacustrine microbialites record evidence of their depositional environments in their morphology, mineralogy and geochemistry. This synthesis reviews geochemical data both for microbialites and lake water for 21 modern lacustrine microbialite occurrences younger than 15,000years. Although these data are limited, several trends and associations are identified that provide useful criteria to aid interpretation of the environmental settings of ancient analogues. These microbialites are either thrombolites or thrombolitic stromatolites. They form in diverse settings, including karst, volcanic, coastal and inland (athalassic) lakes. Surveyed lakes are mostly closed basins subject to evaporative processes. Lakes vary from stratified to totally mixed examples. The major hydrochemical types include Na-Cl, Ca-SO4, Ca-HCO3 and Soda Lakes. The salinity and alkalinity of the lakes correlates, as expected, with the mode of mineralization, which in turn is reflected in the microstructure of the microbialites. A correlation between the δ13Ccarbonate and the Ca2+/alkalinity ratio (Ca/Alk) is observed. Generally, lakes with Ca/Alk>1 are subject to carbonate precipitation driven by either uptake of CO2 that results in a broad range of positive and negative δ13Ccarbonate, or mineralization mediated by sulphate reducing-bacteria in saline to hypersaline environments that results in negative δ13Ccarbonate. Ca/Alk<1 correlates with positive δ13Ccarbonate. The mineral make-up of the analysed microbialites trends with lake chemistry and place of deposition. Mg/Ca ratios<0.8 correlate with precipitation of low-Mg calcite (LMC) and low salinity. Extremely high Mg/Ca ratios (>39) are associated with hydromagnesite. Dolomite, high-Mg calcite (HMC) and monohydrocalcite are associated with the total aqueous concentration of Mg (cMg), occurring only in lakes with cMg>75meq/L. Evaporites (e.g., gypsum) are related to high lake water salinities. The absolute concentration of aqueous Si, commonly associated with dissolution of diatom tests, influences precipitation of crystalline and/or amorphous silicates/silica to form microbialites with Si concentrations above 0.54mmol/L, either as amorphous phases or as minerals such as stevensite and kerolite. Combined, the trends observed in this survey demonstrate that modern lacustrine microbialites can preserve information symptomatic of the environments in which they formed. However, in a limited number of examples this is not the case, as subsequent diagenesis may alter the original mineralogy to a point where the geochemical evidence conserved from the depositional environment is lost. Additionally, some geochemical information may be the product of isolated microenvironments within living biofilms, and thus not necessarily directly related to the chemistry of ambient lake water. Further misleading conclusions would result if the hydrochemistry at the time of study had changed from those prevailing when the microbialites were mineralized. Thus, all the correlations developed in this work represent hypotheses to be tested rather than interpretations to be accepted. This compilation demonstrates how microbialites respond to different lacustrine hydrochemical environments, and is intended to provide a guide for future field studies and laboratory simulations. This review also highlights the general scarcity of trace-element data for lake waters and microbialites. However, some recent studies suggest that lacustrine microbialites may fractionate rare earth elements (REE) during uptake from water whereas marine microbialites appear to record the REE distribution from seawater without modification. This possible difference in REE behaviour could have implications for the interpretation of palaeoenvironmental proxies.