Abstract
Alkaline (pH > 8.5) lakes have been common features of Earth’s surface environments throughout its history and are currently among the most biologically productive environments on the planet. The chemistry of alkaline lakes favors the deposition of aluminum-poor magnesian clays (e.g., sepiolite, stevensite, and kerolite) whose chemistry and mineralogy may provide a useful record of the biogeochemistry of the lake waters from which they were precipitated. In this forward-looking review, we present six data-driven, testable hypotheses devoted to furthering our understanding of the biogeochemical conditions in paleolake waters based on the geochemical behavior of Mg and SiO2. In the development of these hypotheses, we bring together a compilation of modern lake water chemistry, recently published and new experimental data, and empirical, thermodynamic, and kinetic relationships developed from these data. We subdivide the hypotheses and supporting evidence into three categories: (1) interpreting paleolake chemistry from mineralogy; (2) interpreting the impact of diatoms on alkaline lake sedimentation; and (3) interpreting depositional mineralogy based on water chemistry. We demonstrate the need for further investigation by discussing evidence both for and against each hypothesis, which, in turn, highlights the gaps in our knowledge and the importance of furthering our understanding of the relevant geological and biological systems. The focused testing of these hypotheses against modern occurrences and the geologic record of alkaline lakes can have profound implications for the interpretation of the paleo-biogeochemistry and paleohabitability of these systems on Earth and beyond.
Highlights
In this forward-looking review, we present a series of testable hypotheses for using the mineralogy of alkaline lake deposits, the presence of various Al-poor Mg-silicate minerals, to make more granular inferences about the paleochemistry of the water from which they were deposited than would be permitted by the chemical divides concept alone
In keeping with the goals of the present study, we focus our introduction on Al-poor Mg-silicates, because aluminous clays are unlikely to provide a directly interpretable record of the chemical conditions of lake waters, given the minimal aqueous Al concentrations in alkaline lakes, which limits their precipitation
Given the preponderance of recent experimental studies examining the role of dissolved Mg and Si in the character of Mg-silicate precipitates, it may be possible to build this qualitative framework into a ity should favor precipitation of stevensite, rather than sepiolite or kerolite
Summary
Alkaline lakes and their mineral deposits are present through much of Earth’s geological record [1,2,3] and may have occurred in the sedimentary record of other terrestrial planets [4,5,6]. Diagnostic morphologies of carbonate minerals precipitated from alkaline lake waters can yield useful inferences about lacustrine pH and SiO2 (aq) concentrations [26] These observations can be complemented with studies of clay mineral geochemistry to yield more specific insight into the lake water chemistry [23]. As pointed out by Eugster and Hardie [24], Mg-silicate clay precipitation is possible for most paths through the chemical divide flow diagram In this forward-looking review, we present a series of testable hypotheses for using the mineralogy of alkaline lake deposits, the presence of various Al-poor Mg-silicate minerals, to make more granular inferences about the paleochemistry of the water from which they were deposited than would be permitted by the chemical divides concept alone. We hope that these hypotheses will help to direct the future of research into magnesian clays and alkaline lake deposits, and their potential for interpreting the biogeochemical evolution of our planet and other bodies in our solar system
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