Based on petrographic, mineralogical, isotope, and facies assemblage analysis, a microbial origin is established for the formation of dolomite associated with gypsum in Miocene evaporite lake deposits of the Madrid Basin, central Spain. In these deposits, dolomite is present as both intercalated carbonate beds, locally showing domal stromatolite structures between packages of selenite Christmas tree-like gypsum, and patches replacing macrocrystalline gypsum. Texture of the dolomite is characterized by crystal aggregates showing a variety of crystal sizes and morphologies, e.g., platelets, rhombs, micro-rods, and rings, whilst larger crystals are commonly spherical and/or wheat-grain shaped. Organic remains, in the form of filaments, shrubs, micro-fibrils, and strands, are also common and contain significant amounts of carbon. These textural features are also recognized in dolomite replacing gypsum, where Fe oxide and sulfide as well as celestite are ubiquitous mixed with the dolomite groundmass. The dolomite, whether primary or replacing gypsum, is poorly ordered and slightly Ca-rich, thus non stoichiometric. Stable-isotope compositions are characterized by negative values for both oxygen and carbon. Dolomite beds featuring domal stromatolites have d 18 O values ranging from 22.99% and 23.79% and d 13 C values ranging from 24.67% and 27.35%, whilst d 13 C values determined in the dolomite replacive of gypsum shows a small range of variation between 25.70% and 26.96%. By contrast, d 18 O values of replacive dolomite oscillate in a wider range (from 23.04% to 27.99%). Formation of the dolomite was associated mainly with microbial mats, having taken place in relatively diluted lake water. Further evaporative concentration resulted in precipitation of gypsum crystals sealing the mats and creating endoevaporitic microenvironments in which endolithic cyanobaterial activity produced extensive boring and corrosion of the gypsum crystals. Hiatuses in gypsum growth caused an intensification of the corrosion process and favored the precipitation of dolomite mediated by microbes, resulting in pervasive replacement of the sulfate.

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.

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Recognition of evaporite formations from continental Tertiary basins of Spain provides evidence that trace fossils (including rhizoliths) can be abundant in some saline lake systems and their study helps in palaeoenvironmental interpretation of ancient continental evaporite sequences. Six main types of trace fossils have been distinguished and include: (1) networks of small rhizoliths; (2) large rhizoliths; (3) tangle-patterned small burrows; (4) isolated large burrows; (5) L-shaped traces; and (6) vertebrate tracks. Rhizoliths were related to both marginal areas of hypersaline lakes and lakes of moderately high saline waters. In these settings, pedoturbation resulted from colonization by grasses and bushes of distinct lake subenvironments. The activity of burrowing invertebrate faunas was especially intense in lakes of moderately concentrated brines from which gypsum was the main evaporite mineral deposited. A specific gypsum lithofacies ('bioturbated gypsum deposits') forming thick, massive beds has a widespread occurrence in many of the basins. Tangle-patterned small burrows and minor isolated large burrows constitute the typical trace fossil types within the gypsum. The traces are interpreted as having been caused by burrowing insect larvae, probably chironomids, coleopterans and annelids. The behaviour of these organisms in recent lake environments yields information about the salinity range of lake waters from which gypsum precipitated. Concentration values averaging 100-150 g/l may be thus deduced though some organisms involved in the formation of the traces can tolerate higher salinities. The combined analysis of lithofacies and trace fossils from the lacustrine evaporite sequences contributes to the study of distinct saline lake subenvironments as well as changes in the sedimentary evolution of the lake systems. Consequently, trace fossils can provide valuable insight for palaeoenvironmental analysis of at least some evaporite formations that accumulated in continental settings.

This research work is centred on continental lacustrine gypsum deposits of Miocene age cropping out in the easternmost part of the Madrid Basin. These gypsum deposits, accumulated in a continental saline lake, are characterized by a spectacular, distinctive Christmas-tree morphology and a peculiar dolomite replacement. A combination of microscopic (petrography and scanning electron microscopy) and analytical techniques (fluid inclusion microthermometry, X-ray energy dispersive spectroscopy and X-ray diffractometry) was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum. The objectives were to characterize the continental brine from which the mineral precipitated, and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits. Data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid (lake water) of low to moderate total salinity (between 20 and 90 g/L NaCl). Secondary fluid inclusions represent interstitial lake brine in contact with gypsum, slightly enriched in total salt content as crystal formation proceeded. Textural, ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells. Our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes.