Mg/Ca ratios in freshwater microbial carbonates: Thermodynamic, kinetic and vital effects

Abstract

The ratio of magnesium to calcium (Mg/Ca) in carbonate minerals in an abiotic setting is conventionally assumed to be predominantly controlled by (Mg/Ca) solution and a temperature dependant partition coefficient. This temperature dependence suggests that both marine (e.g. foraminiferal calcite and corals) and freshwater (e.g. speleothems and surface freshwater deposits, "tufas") carbonate deposits may be important archives of palaeotemperature data. However, there is considerable uncertainty in all these settings. In surface freshwater deposits this uncertainty is focussed on the influence of microbial biofilms. Biogenic or "vital" effects may arise from microbial metabolic activity and/or the presence of extracellular polymeric substances (EPS). This study addresses this key question for the first time, via a series of unique through-flow microcosm and agitated flask experiments where freshwater calcite was precipitated under controlled conditions. These experiments reveal there is no strong relationship between (Mg/Ca) calcite and temperature, so the assumption of thermodynamic fractionation is not viable. However, there is a pronounced influence on (Mg/Ca) calcite from precipitation rate, so that rapidly forming precipitates develop with very low magnesium content indicating kinetic control on fractionation. Calcite precipitation rate in these experiments (where the solution is only moderately supersaturated) is controlled by biofilm growth rate, but occurs even when light is excluded indicating that photosynthetic influences are not critical. Our results thus suggest the apparent kinetic fractionation arises from the electrochemical activity of EPS molecules, and are therefore likely to occur wherever these molecules occur, including stromatolites, soil and lake carbonates and (via colloidal EPS) speleothems.

A microbial role in the construction of Mono Lake carbonate chimneys?

“…Drews & Weckesser, 1982;Saunders, Rogerson, Wadhawan, Greenway, & Pedley, 2014), cannot be ruled out, the mechanism that best explains the morphologies, compositions and distribution of silicates in the Mono Lake chimneys is via trapping and binding by microbial EOM (cf. Drews & Weckesser, 1982;Saunders, Rogerson, Wadhawan, Greenway, & Pedley, 2014), cannot be ruled out, the mechanism that best explains the morphologies, compositions and distribution of silicates in the Mono Lake chimneys is via trapping and binding by microbial EOM (cf.…”

“…While some authigenic chemical control of silicate precipitation by microbes, for example by attraction of cations including Al, K and Si to negatively charged compounds including uronic acids within the EOM (cf. Drews & Weckesser, 1982;Saunders, Rogerson, Wadhawan, Greenway, & Pedley, 2014), cannot be ruled out, the mechanism that best explains the morphologies, compositions and distribution of silicates in the Mono Lake chimneys is via trapping and binding by microbial EOM (cf. Reid et al, 2000).…”

Stable isotopic and elemental characteristics of recent tufa from a karstic Krka River (south‐east Slovenia): useful environmental proxies?

“…The distribution of trace elements in the water-calcite system has been proven, in rather simple laboratory-controlled experiments, to be temperature-controlled in the case of Mg and in part also growth-controlled in the case of Sr (Mucci & Morse, 1983;Huang & Fairchild, 2001). However, geochemical proxies can also be biased because of anthropogenic pollution of rivers (Lojen et al, 2009) or photochemical activity suppressing incorporation of alien cations such as Mg into the calcite crystal lattice (Rogerson et al, 2008;Saunders et al, 2014).…”

Sedimentology and geochemistry of a human‐induced tufa deposit: Implications for palaeoclimatic research

“…This is apparently opposite to that described by Saunders et al . (), where biofilm acts as an ionic sieve producing low Mg contents within the biofilm and in the calcite precipitated within it. The presence of Mg in all analysed samples may also suggest a relatively high Mg/Ca ratio in waters from which calcite precipitated in the El Calabozo deposit.…”