Dolomite–silica stromatolites in Miocene lacustrine deposits from the Duero Basin, Spain: the role of organotemplates in the precipitation of dolomite

Abstract

This research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms, and describes the involvement of highly structured extracellular polymeric secretion (EPS) templates in dolomite nucleation. The structure of EPS is shown to match the hexagonaltrigonal lattice geometry of dolomite, which favoured the epitaxial crystallization of dolomite on the organic substrate. This structure of EPS also matches the arrangement of silica nanospheres in opal, which further accounts for the organically-templated formation of opal enabling the nonreplacive co-existence of dolomite and silica. The study is focused on a 50 m thick dolomite succession that is exposed in central areas of the Tertiary Duero Basin and was deposited in a mudflat-saline lake sedimentary complex during the Middle to Late Miocene (9 to 15 Ma). In the intermediate intervals of the succession, poorly indurated dolomite beds pass gradually into silica beds. On the basis of sedimentological, compositional, geochemical and petrographic data, silica and dolomite beds have been interpreted as mineralized microbial mats. The silica beds formed in marginal areas of the lake in response to intense evaporative concentrations; this resulted in the rapid and early precipitation of opal. Silicification accounted for the exceptional preservation of the microbial mat structure, including biofilms, filamentous and coccoid microbes, and EPS. Extracellular polymeric secretions have a layered structure, each layer being composed of fibres which are arranged in accordance with a reticular pattern, with frequent intersection angles at 120°a nd 60°. Therefore, the structure of EPS matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal. Additionally, EPS binds different elements, with preference to Si and Mg. The concurrence of suitable composition and surface lattice morphologies in the EPS favoured the crystallization of dolomite on the substrate. In some cases, dolomite nucleation took place epicellularly on coccoid micro-organisms, which gave way to spheroid crystals. Organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation. Most of the microbial mats probably developed on the lake floor, under sub-aqueous conditions, where the decomposition of organic matter took place. The subsequent formation of openly packed dolomite crystals, with inter-related Si-enriched fibrils throughout, is evidence for the pre-existence of fibrillar structures in the mats. Miocene dolomite crystals are poorly ordered and non-stoichiometric, Sedimentology (2008) 55, 729-750 729 with a slight Ca-excess (up to 5%), which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure; this confirms that microbial imprints can be preserved in the geological record, and validates their use as biosignatures.

Bioinduced precipitation of barite and celestite in dolomite microbialites

“…Bedded gypsum accumulated in the lake from a brine of moderate salinity between 20 and 90 g/L NaCl (Ayllón-Quevedo et al, 2007). Intrasedimentary lenticular and rosette-like gypsum crystallised from interstitial brine within dolomite sediments (Sanz-Montero et al, 2008). In contrast, depleted 18 O isotopic values in the dolomite (ranging from an average minimum of −3.3‰ to −0.3‰) exclude evaporation as a main formative process and indicate that the carbonate precipitated from relatively dilute water (Sanz-Montero et al, 2006a).…”

“…On the basis of petrographical, mineralogical, isotope, and facies assemblage analyses, Sanz-Montero et al (2006a) and Sanz-Montero et al (2008) interpreted the dolostone beds as microbialites resulting from the mineralization of microbial mats in saline lake margins. The term microbialite is used in the sense of Burne and Moore (1987), describing an organosedimentary deposit formed from the interaction between benthic microbial communities and detrital or chemical sediments.…”

“…The study succession, located near the village of Cuéllar, was deposited during the Middle-Late Miocene (equivalent to the sedimentary stage "5", defined by Armenteros et al, 2002). The up to 80 m thick stratigraphical section has been described in detail by Sanz-Montero et al (2008). The present study is focused on a 15 m thick interval (S3) that overlies the 35 m thick basal part of the section (Fig.…”

Facies development and sequence stratigraphy of the Ludfordian (Upper Silurian) deposits in the Zbruch River Valley, Podolia, western Ukraine: local facies overprint on the δ13Ccarb record of a global stable carbon isotope excursion

“…Dolomite precipitation from sea water through the activity of microorganisms is a likely mechanism. Their role in precipitation of dolomite has been attributed to the removal of sulphate by sulphate-reducing bacteria (Vasconcelos and McKenzie 1997;Van Lith et al 2003;Sanz-Montero et al 2008), but recent studies point to the organic matrix provided by microbial mats as the crucial factor. The site where microbial dolomite precipitation was originally observed (Lagoa Vermelha, Brazil) closely resembles the environmental model proposed here for the deposition of the Prygorodok Fm.…”

Depositional Depth of Laminated Carbonate Deposits: Insights From the Lower Cretaceous Valdeprado Formation (Cameros Basin, Northern Spain)

“…As the water bodies were as much as a few meters deep, currents and waves affected episodically the sediment, and facilitated formation of thin rippled laminae and deposition of graded laminae in response to sediment resuspension, a process that commonly occurs in modern shallow environments (Bengtsson and Hellströ m 1992;Jin and Sun 2007;Kelderman et al 2012;Evans 1994;Chao et al 2008;Lövsedt and Bengtsson 2008). The areas around these water bodies were occupied by carbonate mudflats disrupted by displacive gypsums, desiccation cracks, and tepees, which are comparable to modern and ancient saline mudflats (Bowler 1986;Salvany et al 1994;Sanz-Montero et al 2008). Considering the very flat bottom of these settings, decimeterscale or few-meters-scale drawdown in water level led to exposure of very broad areas and development of mudcracks in the subaerially exposed sediments, which is not rare in shallow saline lakes (e.g., Smoot and Lowenstein 1991;Schubel and Lowenstein 1997).…”