Calcium and magnesium‐limited dolomite precipitation at Deep Springs Lake, California

Abstract

Dolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay‐fraction dolomite ooze. Based on δ18O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na‐SO4‐dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg2+ and Ca2+ concentrations are ca 1 and 0·3 mm, respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca2+ (below 0·01 mm). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca2+ and Mg2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ13C values near to equilibrium with atmospheric CO2 (ca 3‰) contrasts the formation of isotopically light (organically derived), hard‐lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C‐rich sediments.

Primary dolomite in the Late Triassic Travenanzes Formation, Dolomites, Northern Italy: Facies control and possible bacterial influence

“…Precipitation of primary dolomite in the Coorong was later associated with BSR (Wright & Wacey, ; Wacey et al ., ), while Meister et al . () suggest that dolomite precipitation in Deep Springs Lake and Coorong is an abiotic process. Zhang et al .…”

Precipitates in landfill leachate mediated by dissolved organic matters

“…A fundamental structural barrier stemming from the lattice limitation on the spatial configuration of CO 3 groups in magnesite crystals, and cation hydration effect that constrains dolomite formation were observed by Xu et al [53]. Ca and Mg are found to be the limiting factor for the dolomite precipitation in a saline lake in California [15].…”

“…In fact, mineral precipitation is not only a chemical process, it also involves with biochemical reactions, such as the methanogenic decomposition of organics into CO 2 and CH 4 , which gives rise to partial pressure of CO 2 and concentration of CO 3 2− in aqueous solution. Latest evidence shows that metals tend to adsorb onto carboxyl surface and then crystallize with much faster reaction rate and much less energy required to overcome obstacles in chemical potentials that may be encountered in a reaction [14][15][16][17][18][19].…”

Formation of fine crystalline dolomites in lacustrine carbonates of the Eocene Sikou Depression, Bohai Bay Basin, East China

“…ions made the very fine dolomite directly precipitate from the evaporating lake waters (Burn et al 2000). Secondly, Meister et al (2011) reported that fine-grained dolomite probably precipitates directly in the well-aerated water column from highly alkaline brine, whereas microbial alkalinity production and pH increases are negligible. So the precipitation of dolomites from saline lake water should be considered in light of the possible origin of very fine rim cement and dolomite lamellae within ooids in the Sikou Sag, and inferring a similar genesis for the origin of dolomite mud should also be explored in dolomitized mudstone.…”

“…Very fine crystalline dolomites are widely formed in varied types of lacustrine systems (Meister et al 2011;Mauger and Compton 2011;Last et al 2012;Casado et al 2014;Meng et al 2014a, b;Köster and Gilg 2015;Lu et al 2015). They may form by replacement (dolomitization) of micrite matrix or allochems, or alternatively, they may occur as intraparticle and interparticle cements (Rosen and Coshell 1992).…”