From atoms to landscapes through time: the chemical controls on carbonate precipitation and phosphate concentration in alkaline lakes
Keywords: Pitzer ion activity coefficient interaction model
Precambrian
Cretaceous
crystal growth
self-organisation
prebiotic chemistry
nutrients
carbonate geochemistry
biogeochemistry
thermodynamics
nucleation
aqueous geochemistry

As some of the most biologically productive environments on the modern Earth, alkaline lakes may have figured prominently in the evolution of the global carbon cycle and in the chemical origins of life. Significant CaCO3 supersaturation is a ubiquitous trait of alkaline lakes, controlling the geochemical evolution of lake waters as well as the mineralogy and sedimentary expression of their deposits. By definition, CaCO3 supersaturation requires compounds that interfere or inhibit precipitation, yet the processes controlling CaCO3 saturation in many alkaline lakes are poorly understood. In the pursuit to understand the essential physicochemical processes influencing non-skeletal CaCO3 precipitation, this study focusses on the roles of aqueous phosphate and magnesium in influencing the dissolved carbonate system. It also dissects the roles of alkalinity and pH, addressing a comprehensive compilation of published water chemistry data of alkaline lakes from 16 countries in six continents, implementing a Pitzer ion interaction activity coefficient model to explore carbonate geochemistry and the potential role of these ions in maintaining CaCO3 supersaturation. This survey reveals that ∼80% of the investigated lakes are supersaturated with respect to calcite (0 < SI_cal < 2.9). In fact, as pointed out by another study (Fukushi and Matsumiya 2018), alkaline lake waters are predominantly poised between monohydrocalcite and amorphous Ca-Mg carbonate saturations, being a strong indication that these phases control the water chemistry of alkaline lakes that have not yet become depleted in Ca2+ (i.e., barring those alkaline lakes comprising soda lakes). This provides additional evidence indicating that the crystallisation of more stable CaCO3 phases in these settings occurs preferentially via the formation of more soluble precursors that will undergo recrystallisation likely via dissolution-reprecipitation, as also observed experimentally in a different study (Rodriguez-Navarro et al. 2016). Furthermore, the present comprehensive dataset is complemented with experimental observations demonstrating that small concentrations of total phosphate (ΣPO4 ≥25 μmol/kg) inhibit specific CaCO3 phases such as aragonite and allow supersaturations (Ω_calcite) above 70–100 (or SI_cal ∼1.85–2) to be maintained, whilst overall rates of CaCO3 precipitation decrease with proportionally higher ΣPO4. Together, these observations support previous suggestions that CaCO3 supersaturation in alkaline settings must be sustained at least in part by kinetic inhibition of CaCO3 nucleation and/or growth. Importantly, the natural alkaline lake water data paired with geochemical and thermodynamic calculations stemming from kinetic theory suggest that monohydrocalcite should be the predominant primary CaCO3 phase forming in these lakes. The implications for the geological record of the precipitation via metastable intermediate phases, concerning the incorporation of trace metals and isotopic fractionation in the final, non-stoichiometric products forming in disequilibrium with their surroundings are also briefly discussed. The complete alkaline lake water chemistry database compiled in the present investigation, the modified implementation of the code “thermo_phrqpitz.tdat” (Plummer et al. 1988; Bethke et al. 2020), comprising the thermodynamic and geochemical database for equilibrium calculations used to assess the water chemistry dataset, as well as a MATLAB code developed in this study for determination of other geochemical relations investigated in this research are being stored in the online repository under the name “raphapietzsch/alkaline_lakes” on github.com. In a related direction but concerning environments for a plausible prebiotic chemistry, the study also shows that high phosphate concentrations, i.e., ΣPO4 ≈ 2–3 mol/kg at [Ca2+] ≈ 4–5 mmol/kg and pH ≈ 9 are theoretically possible, carrying implications for scenarios of the emergence of life requiring high soluble phosphate levels. In contrast, even though some present-day alkaline lakes can reach ΣPO4 = ∼100 mmol/kg and above, their maximum ΣPO4 tends to be even lower, at ∼40 mmol/kg. This is likely the result of the high primary productivity taking place in these modern environments, in part due to the large availability of carbon as dissolved inorganic carbon (DIC) but also of other limiting nutrients such as nitrogen and phosphate itself. Conversely, ammonia volatilisation, a phenomenon that may occur as a result of high alkalinity in some lakes, may limit productivity, thereby allowing phosphate levels to increase without this constrain. Moving a step further, the knowledge from theory, geochemical modelling and experimental observations is combined with a detailed investigation of the ancient carbonate sediments from the Lower Cretaceous Barra Velha Formation, Santos Basin, offshore Brazil, in the South Atlantic ocean. These carbonate rocks are generally composed of two intriguing non-skeletal CaCO3 textures, broadly referred to as calcite “shrubs” and “spherulites”. This latter investigation rests on multiple microanalytical techniques addressing the mineralogical, isotopic and geochemical composition of these carbonate rocks. It proceeds employing computational statistics to evaluate minor and trace element (including phosphorus) distribution between different carbonate mineral textures and phases. This permits an assessment of mineral formation in the context of fluid evolution and degree of diagenesis. These tasks ultimately underpin the goal to reconstruct a plausible scenario for the original environment of deposition of the Barra Velha Fm, including calcite saturation state, alkalinity and phosphorus concentrations and cycling. Building on this assessment and on the experimental and theoretical work above, the two main CaCO3 morphologies bearing the hallmark of this Formation, the “shrubs” and the “spherulites” may be used at least as rough indicators of growth rate and saturation state, both increasing from shrubs to spherulites. The average P/(Ca + Mg) molar ratio values based on electron micro-probe analysis (EPMA) data for carbonate crusts, “shrubs” and “spherulites” in the upper part of the analysed interval are 1.45 ± 0.98, 1.13 ± 0.97 and 1.09 ± 0.90 mmol/mol, respectively. These values may suggest the waters from which these calcite minerals precipitated had [Ca2+]/ALK_T<0.75. From these and additional data, it is inferred that lake waters had Ω_calcite > 14–45 and possibly within the range of 100–180. Nonetheless, it is pointed out in the present study that possible antagonistic effects of dissolved SiO2 and phosphate on carbonate precipitation and growth are presently not well constrained and should be investigated in future studies. Another relevant contribution of potentially wider implications for global budgets at the time of their formation is the maximum estimated volume of water implied in the deposition of these rocks, calculated to be in the order of 6 × 1019 L, ∼three orders of magnitude greater than the present volume of water in the Caspian Sea, the largest modern endorheic lake in the world. Finally, the picture emerging from the combination of theory, experimentation and geological investigation provided in this study establishes a robust framework for our understanding of the chemical controls on non-skeletal carbonate minerals deposited in sedimentary basins elsewhere. In turn, this extended knowledge may allow us to place better constraints on the chemical evolution of Earth’s surface conditions dating as far as the existing sedimentary carbonate rock record goes back in time.