The role of bacterial sulphate reduction in carbonate replacement of vanished evaporites: examples from the Holocene, Jurassic and Neoarchaean

Abstract

Vanished evaporites are well documented in the geological record, but many may go unrecognized because of a lack of obvious field or petrographic evidence, particularly in carbonates deposited in restricted settings where bacterial sulphate reduction (BSR) was active. An early stage in evaporite replacement by carbonates may be observed in modern salterns in Eilat, where gypsum crusts are colonized by stratified, endoevaporitic, microbial communities including sulphate-reducing bacteria (SRB). Here, BSR can be associated with gypsum dissolution and replacive carbonate formation. Rapid consumption of aqueous sulphate by SRB is illustrated in the ephemeral Coorong lakes of South Australia, where extremely high concentrations are completely removed by intense BSR during evaporation, so that no solid sulphate precipitates. Here, carbonates form as the consequence of mediation of ambient waters by BSR, with sedimentary pyrite providing the only indication of the former presence of dissolved sulphate in the lake waters.Pyrite patches in the patterned dolomite facies of the largely evaporitic Jurassic Arab Formation have been interpreted as 'birds eyes' or fenestrae, but show no signs of visible porosity. A hostile environment mitigates against their interpretation as burrows, although some form of root pseudomorph is not ruled out. However, it is possible that the pyrite is related to BSR operating in a hypersaline, microbially-dominated environment in which solid sulphate removal and sulphide formation is followed by dolomite formation.Field, petrographic, and stable-isotopic evidence from bedded strata and thin sections of the Neoarchaean Gamohaan and Kogelbeen carbonate formations of South Africa argue for the former existence of evaporites, and suggest deposition in mainly shallow subtidal and marine sabkha environments. On the macroscale, cross-stratified grainstones showing multiple directions of sediment transport, together with abundant microbialites and rare, well-preserved cyanobacterial fossils, indicate shallow-water deposition and microbial growth in the photic zone. Large tepee-like fold structures, intrastratal karst, pseudomorphs after evaporites, replacive calcite after selenite, solution collapse breccias, autobrecciation, corrosion surfaces, nucleation cones, irregular bedding contacts and flowage structures are all indicative of evaporites, their dissolution and their replacement by carbonate.Evaporite replacement in the Neoarchaean carbonates was largely fabric destructive, leaving few obvious clues at the microscale to the former presence of vanished evaporites. Nonetheless, diagenetic recrystallization, deformation, disruption, and crosscutting relationships point to their previous existence, and the occurrence of lengthslow chalcedony in silicified stromatolite heads may suggest the former presence of sulphates. Fold shapes indicative of enterolithic gypsum, picked out by trails of degraded organic matter and dolomitized clusters of folded filaments preserved within replacive calcite, are eviden...

Carbon isotopic composition of lipid biomarkers from an endoevaporitic gypsum crust microbial mat reveals cycling of mineralized organic carbon

“…Thompson and Ferris () have shown that in high‐sulfate environments as pH approaches 7.9, gypsum becomes unstable and dissolution is favored. Dissolution of solid gypsum under similar conditions has been documented in other environments (Douglas, ; Wright & Kirkham, ; Wright & Oren, ), as well as in the Guerrero Negro gypsum crust mats (Vogel et al, , ). While high sulfate reduction rates often facilitate the deposition of CaCO 3 , only trace amounts of carbonate are observed in these crusts (Vogel et al, ).…”

Collapse-Breccia as a Sign of the Pre-Existing Evaporites (Lower Riphean Satka Suite, South Ural)

“…Актуалистическим доказательством замещения сульфата карбонатом является, к примеру, образование диагенетического карбоната по гипсу в гиперсолёных водоёмах Эйлата, Израиль, где гипсовые корки в диагенезе колонизируются стратифицированными, микробиальными сообществами, включая сульфатредуцирующие бактерии. В эфемерных высокомагнезиальных карбонатных осадках лагуны Куронг в Южной Австралии в периоды эвапоритизации гипс замещается пиритом, а карбонат представлен протодоломитом и гидромагнезитом (Wright, Kirkham, 2010).…”

“…Это предполагает ведущую роль предлагаемого механизма удаления твёрдых сульфатов в результате снижения их концентрации в поровом растворе при биосульфатредукции. Для ряда докембрийских формаций, содержащих доломиты с признаками перекристаллизации, брекчирования и гнездовых текстур, напоминающих псевдоморфозы по гипсу, также предполагается былое присутствие эвапоритовых условий, несмотря на отсутствие самих характеристических минералов («исчезнувшие эвапориты (vanished evaporite)» (Wright, Kirkham, 2010)). В то же время некоторые разрезы раннего рифея КБА, к примеру, в сарапульской свите, содержали прослои красноцветных алевролитов с гнездами ангидрита (Мичурин и др., 2009) или эукариотические микрофоссилии (калтасинская свита ), что указывает на эвапоритовое карбонатное осадконакопление в оксидных условиях.…”