%0 Journal Article
%A Sanz Montero, María Esther
%A Rodríguez Aranda, Juan Pablo
%A García del Cura, María Ángeles
%T Dolomite–silica stromatolites in Miocene lacustrine deposits from the Duero Basin, Spain: the role of organotemplates in the precipitation of dolomite
%D 2008
%@ 1365-3091
%U https://hdl.handle.net/20.500.14352/87907
%X This research provides an ancient analogue for biologically mediated dolomiteprecipitation in microbial mats and biofilms, and describes the involvement ofhighly structured extracellular polymeric secretion (EPS) templates indolomite nucleation. The structure of EPS is shown to match the hexagonal–trigonal lattice geometry of dolomite, which favoured the epitaxialcrystallization of dolomite on the organic substrate. This structure of EPSalso matches the arrangement of silica nanospheres in opal, which furtheraccounts for the organically-templated formation of opal enabling the nonreplaciveco-existence of dolomite and silica. The study is focused on a 50 mthick dolomite succession that is exposed in central areas of the Tertiary DueroBasin and was deposited in a mudflat-saline lake sedimentary complex duringthe Middle to Late Miocene (9 to 15 Ma). In the intermediate intervals of thesuccession, poorly indurated dolomite beds pass gradually into silica beds. Onthe basis of sedimentological, compositional, geochemical and petrographicdata, silica and dolomite beds have been interpreted as mineralized microbialmats. The silica beds formed in marginal areas of the lake in response tointense evaporative concentrations; this resulted in the rapid and earlyprecipitation of opal. Silicification accounted for the exceptional preservationof the microbial mat structure, including biofilms, filamentous and coccoidmicrobes, and EPS. Extracellular polymeric secretions have a layeredstructure, each layer being composed of fibres which are arranged inaccordance with a reticular pattern, with frequent intersection angles at 120 and 60 . Therefore, the structure of EPS matches the lattice geometry ofdolomite and the arrangement of silica nanospheres in opal. Additionally, EPSbinds different elements, with preference to Si and Mg. The concurrence ofsuitable composition and surface lattice morphologies in the EPS favoured thecrystallization of dolomite on the substrate. In some cases, dolomite nucleationtook place epicellularly on coccoid micro-organisms, which gave way tospheroid crystals. Organic surfaces enable the inorganic mineral precipitationby lowering the free energy barrier to nucleation. Most of the microbial matsprobably developed on the lake floor, under sub-aqueous conditions, wherethe decomposition of organic matter took place. The subsequent formation ofopenly packed dolomite crystals, with inter-related Si-enriched fibrilsthroughout, is evidence for the pre-existence of fibrillar structures in themats. Miocene dolomite crystals are poorly ordered and non-stoichiometric, with a slight Ca-excess (up to 5%), which is indicative of the low diageneticpotential the microbial dolomite has towards a more ordered andstoichiometric structure; this confirms that microbial imprints can bepreserved in the geological record, and validates their use as biosignatures.
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