Endomycorrhizae in Miocene paleosols: Implications in biotite weathering and accumulation of dolomite in plant roots (SW Madrid Basin, Spain)

This work provides evidence of arbuscular mycorrhizae (or endomycorrizhae) and their role in mineral weathering and soil carbonate production from Miocene paleosols. Formation of the paleosols-bearing mycorrhizae took place in marginal lacustrine and distal-fan deposits in the Madrid Basin. Endomycorrizhae fossils are preserved as carbonate in biotite-filled rhizoliths. Fossilized mycorrhizae are morphologically identical to those of living counterparts and consist of two linked parts. The extraradicular segment surrounding the root is represented by a calcitic mycelium comprising spores and two types of hyphae that are seen to attach and to corrode the mineral grains. The intraradicular part is made of a network of branched filaments, hyphal coils, arbuscules and likely vesicles that are mixed with the cortical root cells of the plants fossilized as dolomicrosparite aggregates. The preferential formation of the dolomite aggregates on biotite grains in rhizoliths is interpreted to reflect a focused interest of the plants on this mineral and indicates the mineral may have been used as a source of plant nutrients. The close association of the carbonate with the silicate further suggests that the precipitation of dolomite in the root cells and the weathering of the biotite resulted from interrelated processes. Main mechanisms to produce mineral dissolution include organic acids and chelate secretions by the plant roots and fungi which would remove nutrients. These results confirm that carbonate paleosols provide an excellent reservoir of information on ecological interactions and biogeochemical cycling, and expand the range of biogenic processes and their resultant products involved in the formation of carbonate soils. This approach has broad applications given the abundance of carbonate paleosols and the endomycorrhiza record dating back 400 million years

Participación microbiana en la formación de magnesita dentro de un ambiente lacustre evaporítico: Mioceno de la Cuenca de Madrid

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEBanco de Santanderpu

Magnesite formation by microbial activity: Evidence from a Miocene hypersaline lake

This paper provides an ancient analogue for biologically mediated magnesite in lacustrine hypersaline environments. Thin beds of massive to crudely laminated magnesitic marls occur interbedded with mudstone and evaporite facies deposited in a saline lake-mudflat sedimentary system during the Lower Miocene in the Madrid Basin, Central Spain. Exposure of this succession in a recently excavated tunnel and in collected cores offered a good opportunity to study magnesite precipitates that have been preserved in unaltered conditions by primary evaporitic minerals. Specifically, magnesite arranged as host matrix between and enclosed by displacive halite and glauberite crystals occurs closely associated with abundant microfossils embedded in sheets of organic matter, interpreted as the remains of extracellular polymeric substances (EPS). Even, many magnesite clots are seen to have resulted from the agglutination of fossilized bacterial bodies forming a biofilm. The close and pervasive association of microorganisms and magnesite is used as evidence that microbes played a fundamental role in the precipitation of this mineral. Additional traces of microorganisms and microbial activity observed in magnesite beds include the isotopic ratios in the carbonates, with the δ13C(PDB) value averaging −6.2‰, the presence of carbonaceous film-like structures and the association with patchly-distributed pyrite and celestite minerals. On the basis of the combined sedimentological, mineralogical, chemical and morphological signatures of the magnesitic deposits, it is postulated that carbonate crystals precipitated in microbial mats. Magnesite crystals together with celestite, local barite, and/or pyrite have been also observed to replace silicate and sulphate minerals. It is suggested that magnesite precipitation was biochemically coupled with the early dissolution of the associated sedimentary minerals. Microbes may have used the associated minerals as source of energy and/or of essential elements

The role of microbial mats in the movement of stones on playa lake surfaces

This paper sheds light into the debated mechanisms that move rocks across low gradient surfaces during storm episodes. Microbial mats are recognised to play a crucial role in sediment destabilisation and the subsequent motion of rocks in a playa lake from central Spain. Widespread countless scars are present on the playa lake sediment surface, each terminating at a stone, and/or a mound of overfolded fragments of microbial mats. All available objects, including cobble-sized stones, on the surface were transported for several metres leaving behind furrow-like tracks. The starting point of the trackswas found underwater on a surface veneered by microbial mats poorly attached to the substrate. The flotation and transportation of the microbial upper layer by wind-generatedwater currents are suggested to be a critical element promoting the destabilisation and subsequent transportation of the attached sediment, including the rocks. The sudden exposure of bubble-separated sediment is also envisaged as a source of buoyancy required to lift out the rocks at the starting point of the tracks and to reduce the friction on the base of the rock. At the end of the tracks the rocks and the sedimentmounds often overlap,which reveals that rockswere embedded and transported by sediment rafts across the playa lake surface. The combination of the specific sedimentary and weather conditions involved in the transportation of rocks occurs periodically in mudflats. The implications are that this is not a local phenomenon, but rather, is a geographically widespread sedimentary process that may have occurred repeatedly throughout geological time. The effects of storm episodes can be inferred in ancient mudflat deposits by the assemblage of variablysized stones and microbial-mat related structures

Silicate bioweathering and biomineralization in lacustrine microbialites: ancient analogues from the Miocene Duero Basin, Spain

The Miocene dolomite-chert microbialites studied here offer a complete record of the geochemical cycles of silicate weathering and the subsequent formation of secondary products. The microbialites were formed in lacustrine systems during the Miocene of the Duero Basin, central Spain. Mineralogical, chemical and petrographic results provide evidence of the mediation of microbes in early weathering and by-product formation processes. Irrespective of the composition, the surfaces of the grains were subject to microbial attachment and concomitant weathering. Palaeo-weathering textures range from surface etching and pitting to extensive physical disaggregation of the minerals. Extreme silicate weathering led to the complete destruction of the silicate grains, whose prior existence is inferred from pseudomorphs exhibiting colonial textures like those recognized in the embedding matrix. Detailed petrographic and microanalytical examinations of theweathering effects in K-feldspars show that various secondary products with diverse crystallinity and chemical composition can coexist in the interior of a mineral. The coexistence of by-products is indicative of different microenvironmental conditions, likely created by microbial reactions. Thus, the presence of varied secondary products can be used as a criterion of biogenicity. Intensive alteration of P-bearing feldspars suggests that mineral weathering may have been driven by the nutrient requirements of the microbial consortium involved in the precipitation of dolomite. The rock record provides useful information on mineral weathering mediated by microbes

Influencia del complejo de embalses Mequinenza-Ribarroja-Flix (Río Ebro) en la morfología del cauce situado aguas abajo

Cuando se construye un embalse, el cauce que se sitúa aguas abajo de la presa suele sufrir alteraciones de diversa índole fundamentalmente porque se modifican las características de su caudal líquido y sólido. En este trabajo se analiza el tipo de alteraciones que se han dado en el río Ebro a lo largo de un tramo de aproximadamente 40 km de longitud que se localiza aguas abajo del complejo de embalses de Mequinenza-Ribarroja-Flix. Las principales alteraciones reconocidas son: ausencia de sedimentación, degradación de barras en los puntos más próximos a la presa, revegetación de barras y orillas, y pérdida de sedimentos finos con el consiguiente acorazamiento del lecho y de las barras. Dos factores han propiciado tales modificaciones, que han conducido prácticamente a la estabilidad del cauce, son la presencia de abundantes gravas y una elevada tasa de asentamiento vegetal en la sección. Asimismo se analiza la evolución morfosedimentaria del río desde 1927 hasta la fecha de construcción de estos embalses, años sesenta, y se determina que dicha evolución estaba controlada por episodios de crecidas catastróficas. En el período de estudio se registraron dos eventos de esta naturaleza. En este trabajo se describen también tanto las consecuencias derivadas de los mismos, como el papel tan destacado que jugó en su desarrollo un afluente del Ebro, el río Ciurana. Finalmente, los resultados obtenidos referentes a la dinámica fluvial, anterior y posterior a la construcción de los embalses, se aplican a la evolución sedimentaria del Delta del Ebro, formado a poca distancia del tramo estudiado

Mg-rich carbonates mediated by a bacterium isolated from an extreme alkaline lake in Central Spain

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasFALSE(MINECO-FEDERpu

Mediation of endoevaporitic microbial communities in early replacement of gypsum by dolomite: a case study from Miocene lake deposits of the Madrid basin, Spain

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 ð18O values ranging from 22.99‰ and 23.79‰ and ð13C values ranging from 24.67‰ and 27.35‰, whilst ð13C values determined in the dolomite replacive of gypsum shows a small range of variation between 25.70‰ and 26.96‰. By contrast, ð18O 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 diluyed 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

Lower Miocene gypsum palaeokarst in the Madrid Basin (central Spain): dissolution diagenesis, morphological relics and karst end-products

The Miocene sedimentary record of the Madrid Basin displays several examples of palaeokarstic surfaces sculpted within evaporite formations. One of these palaeokarstic surfaces represents the boundary between two main lithostratigraphic units, the Miocene Lower and Intermediate units of the Madrid Basin. The palaeokarst formed in lacustrine gypsum deposits of Aragonian age and corresponds to a surface palaeokarst (epikarst), further buried by terrigenous deposits of the overlying unit. Karst features are recognized up to 5.5 m beneath the gypsum surface. Exokarst and endokarst zones are distinguished by the spatial distribution of solution features, i.e. karren, dolines, pits, conduits and caves, and collapse breccias, sedimentary fills and alteration of the original gypsum across the karst profiles. The development of the gypsum palaeokarst began after drying out of a saline lake basin, as supported by recognition of root tubes, later converted to cylindrical and funnel-shaped pits, at the top of the karstic profiles. The existence of a shallow water table along with low hydraulic gradients was the main factor controlling the karst evolution, and explains the limited depth reached by both exokarst and endokarst features. Synsedimentary fill of the karst system by roughly laminated to massive clay mudstone with subordinate carbonate and clastic gypsum reflects a punctuated sedimentation regime probably related to episodic heavy rainfalls typical of arid to semi-arid climates. Duration of karstification is of the order of several thousands of years, which is consistent with previous statements that gypsum karstification can develop rapidly over geologically short time periods

Microbially induced palygorskite-sepiolite authigenesis in modern hypersaline lakes (Central Spain)

This study gives insight into the mechanisms of formation of palygorskite-sepiolite minerals that have remained elusive for many years. The occurrence of palygorskite-sepiolite and minor smectite in association with a variety of authigenic sulphates and Ca-Mg carbonate mineral deposits was identified in silicon-poor, saline to hypersaline ephemeral lakes in Central Spain. By a combination of sedimentological, mineralogical and petrological techniques (XRD, SEM, FEGSEM and TEM) it is shown that very small, poorly ordered palygorskite-sepiolite crystals resulted from the aggregation of nanocrystals within the matrix of the microbial mats (EPS) as it dehydrates. The Mg-rich clays nucleated on Mg-enriched EPS that also contain variable amounts of Si, Mg, Al and Fe, mostly derived from the degradation of siliciclastic minerals. The crystals have a broad compositional and textural range, from poorly ordered palygorskite laths to pure sepiolite bundles, suggesting that they grow through successive stages. Changes in the chemical composition of the fibers occur even at microscale, which is explained by the variability of the biogeochemical conditions prevailing in the microenvironments where the crystals grow. The overall results provide evidence for a biomediated-crystallization of palygorskite-sepiolite in microbial sediments