Bacterially mediated formation of diagenetic aragonite and native sulfur in Zechstein carbonates (Upper Permian, Central Germany)

Abstract

Abstract Neoformed diagenetic aragonite associated with organic-rich aggregations and native sulfur occur within cavities of the Ca2 and Ca3 Zechstein carbonates in Central Germany. The cavities were formerly filled with gypsum. Bacterial sulfate reduction favored the precipitation of the carbonate phase. This can be attributed to an increase in alkalinity accompanied with sulfate reduction. The high Sr concentrations of the neoformed aragonites compared to the low concentrations in the carbonate host rocks point to sulfate dissolution as the cation supplying process for the precipitation of aragonite. Low δ13C values (−10‰ PDB) of the aragonite indicate that some of its carbon is derived from organic matter that has been oxidized by bacterial sulfate reduction. Aragonite inclusions bear rhomb-shaped crystals of calcite, replacing former dolomite. Elevated Mg/Ca ratios due to this dedolomitization may have promoted the precipitation of aragonite instead of calcite. The aragonite precipitated in the near-surface meteoric–vadose zone in recent times. Aragonite crystals display a platy habit. SEM analyses show that two types of micro-rods are associated with these plates. The mineralized micro-rods are interpreted to be fossilized bacteria. Aragonite inclusions, most of which contain organic-rich aggregations, yield a distinctive biomarker pattern. High concentrations of specific unsaturated fatty acids are clearly indicative of newly produced organic matter and reflect the presence of a discrete microbial community being associated with the formation of the aragonite. At one of the studied localities the aragonite is accompanied by native sulfur. The formation of sulfur was mediated by H2S-oxidizing bacteria. This is corroborated by the presence of densely packed curved rods representing permineralized bacterial cells on and within the sulfur.