Trace fossils and rhizoliths as a tool for sedimentological and palaeoenvironmental analysis of ancient continental evaporite successions.

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

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

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

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

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

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

Petrología y geoquímica isotópica de los depósitos de magnesita de la cuenca de Calatayud (provincia de Zaragoza). Interpretación paleoambiental

Los sedimentos miocenos de la Cuenca de Calatayud, en particular los correspondientes a las unidades Inferior e Intermedia, contienen nllmerosos niveles de magnesita, que aparecen comúnmente asociados con depósitos lacustres sulfatados (anhidrita, yeso, glauberita), así como con margas y arcillas. L¡t magnesita de la Cuenca de Calatayud es de tipocriptocristalino, presentándose en capas masivas de color blanquecino, de espesor variable (mm a dm), entre arcillas, margas, niveles de yeso laminado y yeso nodular. Se han distinguido cuatro asociaciones de facies (MLH, Mys, Myp, MIN) con presencia de magnesita. Estas asociaciones de facies se ordenan básicamente en dos tipos de secuencias sedimentarias principales que reflejan, en un caso, la evolución de subambientes lacustres de alta salinidad y, en otro, de salinidad moderada a alta. La textura de la magnesita, observada mediante M.E.B. en muestras con alta pureza en este mineral, se caracteriza por presentar mosaicos densos y homogéneos de cristales romboédricos euhedrales a subeuhedrales, con tamaño alrededor de 1 m.Los valores de isótopos estables (0180, 013C) de la magnesita (n=34) muestran una ciertadispersión, con valores de 0180 de -4,8 a +4,9 %0 PDB y valores negativos de 013C, de -9,3 a +0,4 %0 PDB, más bajos en los niveles de magnesita correspondientes a las asociaciones de facies MLH y Mys. Las características estratigráficas, texturales e isotópicas sugieren que la magnesita se formó en condiciones evaporíticas, posiblemente por precipitación primaria a partir de soluciones concentradas con mezcla de aguas meteóricas, ricas en CO2 biogénico, aunque los valores negativos de o\3C pueden interpretarse también como reflejo de la actividad bacteriana en la precipitación de la magnesita. El análisis de las asociaciones de facies en que aparece la magnesita y su evolución secuencial permiten concluir que los depósitos de magnesita de la Cuenca de Calatayud se formaron tanto en condiciones lacustres hipersalinas como de salinidad moderada a alta

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

Microbial weathering of Fe-rich phyllosilicates and formation of pyrite in the dolomite precipitating environment of a Miocene lacustrine system

Sedimentary sequences composed of selenite gypsum, mudstone, and dolomite microbialites cropping out in the eastern part of the Madrid Basin were deposited in a mudflat – saline-lake system during the Miocene. In some dolomite beds enriched in detrital grains, dolomite crystals occur intergrown with framboidal pyrite clusters that in many cases are arranged along the associated phyllosilicate cleavages, which accounts for the mineral disaggregation, and eventually for the mineral replacement. Highresolution FE-SEM investigations across the interface between the pyrite and the phyllosilicate show that a single phyllosilicate is replaced by iron sulphides in different microsites, with retention of some aspects of the parent structure. Pyrite formed in the sediment as texturally complex framboids, coexisting with amorphous to poorly crystalline iron-rich sulphide phases, and include carbonaceous filaments with biogenic isotopic signatures (δ 13 C = -22.95 ‰). Microprobe analytical data indicate the weathered phyllosilicates to be significantly depleted in Fe relative to Si and Al. The selective release of Fe has not involved the mineral transformation to secondary phyllosilicates, which suggests a preferential microbial colonization of the Fe-bearing minerals rather than a thermodynamically driven degradation of those minerals. Depleted δ 34 S values in pyrite further suggest that sulphate microbial reduction to sulphide was active within the sediment during the alteration of the silicates. This paper gives new insight into the microbial weathering of phyllosilicates and the subsequent formation of pyrite through a sequence of intermediate products enriched in iron. These processes were coeval with the microbially-mediated precipitation of dolomite, which further reinforces the role of the microbes in the formation of the sulphides