Bioinduced precipitation of barite and celestite in dolomite microbialites

Abstract

This paper provides an ancient analogue for biologically mediated celestite and barite formation in dolomite precipitating microbial mats developed in lacustrine environments during the Miocene. Barite and celestite occurrences were studied in three temporally and spatially separated sedimentary successions: S1 and S2 in the Madrid Basin and S3 in the Duero Basin. In S1, macrocrystalline selenite gypsum occurs as laterally continuous beds; in the two other successions (S2 and S3), calcite pseudomorphs of lenticular gypsum aggregates are hosted in dolomite beds as evidence for the former presence of this evaporite. In S1, only celestite is associated with dolomite. Celestite crystals occur as both intergrown clusters, concentrated in pockets likely created by the dissolution of intrasedimentary anhydrite precursors, and as single precipitates associated with dolomite masses that replace selenite gypsum. Celestite crystals are nucleated commonly on organic substances that are pervasively associated with them. In S2 and S3, scarce single celestite crystals are restricted to calcite pseudomorphs after gypsum, whereas barite is the sulphate precipitated in the pseudomorphs' surroundings. Barite is commonly present as patchy poikilotopic crystals which include microbial structures and is embedded in organic matter. Additionally, barite is found as a secondary precipitate within Ba-bearing feldspars. Feldspar weathering is, thus, envisaged as a major source of barium at these sites. Petrographical, isotopic and compositional observations point out that the barite and celestite formation was not caused by abiological processes only. Rather, the patchy distribution of the sulphates, close links to organic matter with biogenic isotope signatures, and inclusion of microbial structures, such as biologically mediated dolomite, provides evidence for the involvement of microbes in the formation of the sulphates. The coprecipitation of barite and celestite with dolomite entails complex interactions between different microorganisms and reinforces the biological formation of dolomite in saline lakes.

Controls on Barite Crystal Morphology during Abiotic Precipitation

“…Early Earth's marine environments were anoxic and sulfate deposits prevailed, resulting in barite as the dominant or the only sulfate mineral within bedded sulfate deposits older than 2.4 Ga [2,[5][6][7][8][9][10]. Diverse barite occurrences included what have been described as massive beds, laminations, rosettes, and nodules [2,[5][6][7]9]. Barite could form through diagenetic replacement or hydrothermal exhalation, which were both common in the Archean environments [5].…”

“…Barite has been recognized as a useful paleoproductivity and paleoalkalinity proxy to understand past and present ocean productivity [14][15][16]. However, further studies of barite formation are needed to better understand the genesis of ancient barite deposits and potentially distinguish biologically maintained from purely abiotically precipitated barite [2,[5][6][7]9].…”

Biomineralization of ordered dolomite and magnesian calcite by the green alga Spirogyra

“…Rather, these results suggest that microbial Mg‐carbonates, including dolomicrite, have potential for preservation in the rock record. Good examples of lacustrine rocks containing this type of association have been reported in Cenozoic successions (García del Cura et al ., 2001; Sanz‐Montero et al ., 2009). Considering that green algae related to Spirogyra could date back to the Palaeozoic, and that Spirogyra were especially abundant in the Cenozoic fossil record (Martín‐Closas, 2003), these findings add a new perspective to the study of ancient lacustrine dolomites.…”

“…Baryte can also form as biominerals in green algae such as Spirogyra (Kreger & Boere, 1969) and Desmids (Krejci et al ., 2011). The association of dolomite with accessory, but ubiquitous, baryte had been reported in lacustrine sedimentary rocks, where the dolomite and baryte could be primary minerals and biotic in origin (Sanz‐Montero et al ., 2008, 2009). In contrast to ancient examples, the natural occurrences of the mineral assemblage comprised of Mg‐rich carbonates, especially dolomite, and baryte in modern alkaline lakes are much less common.…”

Morphology of Barite Synthesized by In-Situ Mixing of Na2SO4 and BaCl2 Solutions at 200 °C

“…Barite (BaSO 4 ) is a ubiquitous mineral in the earth's crust [1,2]. Early Earth's marine environments were anoxic and sulfate deposits prevailed, resulting in barite as the dominant or the only sulfate mineral within bedded sulfate deposits older than 2.4 Ga [3][4][5][6][7]. Due to its high density and strong resistance to chemical weathering, barite is present throughout Earth's history and has the potential to preserve a record of conditions attending formation, which could be useful for interpretations of Earth's ancient rocks and paleoenvironments [8].…”