Papers by Chris Vasconcelos
Fossilized Extracellular Polymeric Substances And Microfossils Preserved In Ancient Dolomite
Qatar Foundation Annual Research Conference Proceedings Volume 2014 Issue 1, 2014
Dolomite is an important constituent of many economically important gas and oil reservoir rocks. ... more Dolomite is an important constituent of many economically important gas and oil reservoir rocks. Studies conducted in modern environments combined with microbiological laboratory experiments have shown that microbes and their extracellular polymeric substances (EPS) play an important role for the formation of primary dolomite at Earth's surface conditions. These studies showed that, at low temperature, Mg is incorporated into the carbonate mineral exclusively in the presence of specific organic molecules. However, because the organic molecules involved in the mineralization process rarely survive metamorphism and are usually not preserved in the carbonate mineral, identifying microbial dolomite in ancient rocks represents a challenging task. It remains, as yet, unclear what percentage of sedimentary dolomite is a primary microbially mediated precipitate vs. a secondary replacement product that formed during diagenesis or at high temperatures during metamorphism. A useful approach for evaluating the microbial origin of ancient dolomite is that of searching for microfossils and other microstructures of biological origin associated with the dolomite crystals. Here, we present the results of scanning electron microscopy (SEM) investigations of various ancient sedimentary dolomites. Several investigated samples include abundant filamentous microstructures that we interpret to be mineralized EPS. In some cases, these filaments form a well-structured alveolar pattern, whose architecture appears too complex to be the result of an abiotic process. In order to reinforce our interpretation, we also investigated modern biofilms, which include microstructures of EPS that are morphologically identical to the mineralized and preserved EPS in ancient dolomites. In some rare cases, we also found microfossils (i.e. mineralized cells) in close spatial association with the dolomite crystals. These occurrences are remarkable, considering that it is commonly thought that only chert has the potential of preserving the soft tissues constituting microbial cells. Although we cannot prove conclusively that the fossilized cells and EPS mediated the formation of the adjacent dolomite crystals, our results indicate that dolomite formation took place in environments where microorganisms and biofilms were an important component of the depositional setting. The "microbial factor", which has been proven to be essential for the precipitation of dolomite at low temperature in modern environments, may, therefore, have also been active during the formation of these ancient carbonates.
Characterization of environmental conditions during microbial Mg-carbonate precipitation and early diagenetic dolomite crust formation: Brejo do Espinho, Rio de Janeiro, Brazil
Geological Society, London, Special Publications, 2015
For many years, sedimentary dolomite rocks have been considered to be primarily a replacement pro... more For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. Th...
Modern Dolomitic Stromatolites from Lagoa Vermelha, Brazil
ABSTRACT Classic examples of modern stromatolites, such as those found in Shark Bay (Western, Aus... more ABSTRACT Classic examples of modern stromatolites, such as those found in Shark Bay (Western, Australia) and Highborne Cay (Bahamas), furnish important insights to better interpret the environmental conditions of ancient biogeochemical process leading to calcification. Although these living examples may serve as modern analogues for ancient stromatolites, they do not precipitate primary dolomite, a carbonate mineral commonly found in Precambrian stromatolites and, thus, do not represent the full range of environmental conditions existing in early Earth history. Here, we report on the first recognized occurrence of microbiolite stromatolitic structures, associated with Ca-dolomite, growing in a hypersaline coastal lagoon, Lagoa Vermelha, near Rio de Janerio, Brazil. We examined the microbial community using microelectrode measurements, which indicate high rates of photosynthesis, aerobic respiration, sulfate reduction, sulfide oxidation and fermentation. The ratio between precipitation and dissolution inside the microbial mat has been estimated. The high pH recorded during daytime in the microbial mats reflects increased alkalinity associated with progressive increase of Mg in the mineral phase with depth below the surface, whereas the detection of zero-valent sulfur and polythionates produced by sulfide oxidizing bacteria indicates an additional process increasing alkalinity. Ca+2 microsensors studies and silver foil experiments, combined with the EDS/SEM examination of the biofilm, provide evidence also for the importance of EPS in the calcification processes. This geomicrobiology approach, linking studies of population, processes and products, permits us to estimate the role that specific microorganisms play during high Mg-calcite and Ca-dolomite precipitation. As dolomitic stromatolites were abundant in the Precambrian, the Lagoa Vermelha microbial community, which is adapted for survival under extreme salinity variations, may represent a modern analogue for a microcosm, which was active on the early Earth and may have evolved similarly in response to comparable extraterrestrial environments.
Sedimentology, 2014
The study of microbial carbonates has acquired new significance with the recognition that they re... more The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time. Additionally, microbialites have a demonstrated economic potential to serve as excellent hydrocarbon reservoirs. The Lower Cretaceous Codó Formation, located in the Parnaiba Basin of northeast Brazil, This article is protected by copyright. All rights reserved. comprises a unique stratigraphic sequence of up to 20 m thick, well-preserved carbonate microbialites. Deposited in a continental basin during the initial break up and separation of South America from Africa in the Early Cretaceous, this lacustrine carbonate sequence provides an excellent example to investigate the palaeoenvironmental conditions controlling microbialite facies development. Based on macroscopic and microscopic observations of outcrop and drill core samples, four microbialite facies (stromatolite, lamina, massive and spherulite) were defined and distinguished by textures and microbial fossil content. Changes in facies type are related to alternating palaeo-water depths, as reflected by 87 Sr/ 86 Sr cycles resulting from fluctuations in the sources of meteoric water. Clumped isotope measurements of stromatolitic fabrics yield precipitation palaeo-temperatures with an average value of 35°C. The δ 18 O values of bulk carbonate (-6.8 to-1.5‰ Vienna Pee Dee Belemnite) imply precipitation from water with calculated δ 18 O values between-1.6 and 1.8‰ Vienna Standard Mean Ocean Water, reflecting precipitation from variably modified meteoric waters. The δ 13 C values of bulk carbonate (-15.5 to-7.2‰ Vienna Pee Dee Belemnite) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter. Combined, the data indicate that the evolution of the Codó Formation occurred in a closed lacustrine palaeoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations. The results provide new insights into palaeoenvironmental settings, biogenicity and early diagenetic processes involved in the formation of ancient carbonate microbialites and, by extension, improve the knowledge of the reservoir geology of correlative units in deep waters offshore Brazil.
Scientific Reports, 2014
How to cite this article: Sánchez-Román, M. et al. Microbial mediated formation of Fe-carbonate m... more How to cite this article: Sánchez-Román, M. et al. Microbial mediated formation of Fe-carbonate minerals under extreme acidic conditions. Sci. Rep. 4, 4767;
Chemical Geology, 2011
We studied the formation of Mg-rich carbonate in culture experiments using different aerobic bact... more We studied the formation of Mg-rich carbonate in culture experiments using different aerobic bacterial strains and aqueous Mg/Ca ratios (2 to 11.5) at Earth surface conditions. These bacteria promoted the formation of microenvironments that facilitate the precipitation of mineral phases (dolomite, huntite, high Mg-calcite and hydromagnesite) that were undersaturated in the bulk solution or kinetically inhibited. Dolomite, huntite, high Mg-calcite, hydromagnesite and struvite precipitated in different proportions and at different times, depending on the composition of the medium. The Mg content of dolomite and calcite decreased with an increasing Ca concentration in the medium. The stable carbon isotope composition of the Mg-rich carbonate precipitates reflected the isotope composition of the organic compounds present in the media, suggesting that microbial metabolism strongly influenced the carbon isotope composition of biomediated carbonates. We observed that Ca-enriched carbonate precipitates have relatively low carbon isotope composition. These results provide insights into the mechanism(s) of carbonate formation in natural systems, and they are of fundamental importance for understanding modern environments in which carbonate minerals form as a window into the geologic past.
Journal of the Brazilian Chemical Society, 2013
O histórico de acumulação em sedimentos de produtos de combustão produzidos na área metropolitana... more O histórico de acumulação em sedimentos de produtos de combustão produzidos na área metropolitana ao redor da Baía de Guanabara foi avaliado através de black carbon (BC) e hidrocarbonetos policíclicos aromáticos (HPA) como indicadores. A concentração de BC variou de 0,23-0,51%, com uma tendência crescente de acúmulo de massa ao longo dos últimos 118 anos, mas a proporção BC/carbono orgânico caiu nos últimos 30 anos, possivelmete reflexo de eutrofização. Observaram-se as maiores concentrações de HPAs pirolíticos nos períodos entre 1925 e 1976, condizendo com o período subsequente de desmatamento, urbanização e industrialização que ocorreu na bacia da Guanabara. Estes HPAs foram melhor caracterizados pelo índice diagnóstico BFl/(BFl+BePy). Os dados obtidos permitiram caracterizar os principais eventos que influenciaram o padrão dos produtos de combustão: queima de biomassa e de derivados de petróleo, a crise econômica nos anos 1970 e 1990 e a entrada de etanol na matriz energética brasileira. The accumulation history of combustion products from the metropolitan area around Guanabara Bay was evaluated using black carbon and polycyclic aromatic hydrocarbons as indicators. BC concentration varied between 0.23 and 0.51%, with an increasing mass accumulation tendency over the last 118 years, whereas a decrease in the values of the ratio BC/organic carbon was observed in the upper sediment layers, probably reflecting the enhancement of bay's eutrophication process in the last 30 years. Higher concentrations of pyrolytic PAH were observed between 1925 and 1976, being consistent with the subsequent deforestation, urbanization, and industrialization that occurred in the Guanabara basin in the period. These PAHs were best characterized by the BFl/(BFl+BePy) diagnostic ratio. The data obtained allowed the characterization of the main events that influenced the combustion patterns in the region: burning of biomass and fossil fuel, economic crisis of 1970 and 1990 and the introduction of ethanol in the Brazilian energetic matrix.
Geochimica et Cosmochimica Acta, 2017
Carbonate clumped isotope thermometry is based on the temperature-dependent formation of 13 C 18 ... more Carbonate clumped isotope thermometry is based on the temperature-dependent formation of 13 C 18 O 16 O 2 2ion groups within the lattice of solid carbonate minerals. At low temperatures the bonds between rare, heavy 13 C and 18 O isotopes are thermodynamically favored, and thus at equilibrium they are present in higher than random abundances. Here we calibrate the use of this temperature proxy in a previously uncalibrated carbonate phase-dolomite [CaMg(CO 3) 2 ]over a temperature range that extends to conditions typical of shallow crustal environments, by determining the ∆ 47 values of CO 2 extracted from synthetic or natural (proto)dolomites grown at known temperatures from 25 to 350ºC and analyzed in two different laboratories using different procedures for sample analysis, purification and post-measurement data treatment. We found that the Δ 47-1/T 2 dependence for (proto)dolomite is linear between 25 and 350°C, independent of their Mg/Ca compositions or cation order (or the laboratory in which they were analyzed), and offset from, but parallel to, the theoretical predictions of the ∆ 63 dependence to temperature of the abundance of the 13 C 18 O 16 O 2 isotopologue inside the dolomite and calcite mineral lattices as expected from ab-initio calculations (Schauble et al., 2006). This suggests that neither the equilibrium constant for 13 C-18 O clumping in (proto)dolomite lattice, nor the experimental fractionation associated with acid digestion to produce CO 2 , depend on their formation mechanism, degree of cation order and/or stoichiometry (ie., Mg/Ca ratio) and/or δ 18 O and δ 13 C compositions (at least over the range we explored). Thus, we suggest the following single ∆ 47-1/T 2 linear regression for describing all dolomite minerals: ∆ 47CDES90 = 0.0428 (± 0.0033) * 10 6 /T 2 + 0.1174 (± 0.0248) (r 2 = 0.997), with T in kelvin and ∆ 47 in the Carbon Dioxide Equilibrium Scale (CDES) of Dennis et al. (2011) and referring to CO 2 extracted by phosphoric acid digestion at 90°C. The listed uncertainties on slope and intercept are 95% confidence intervals. The temperature sensitivity (slope) of this relation is lower than those based on low temperature acid digestion of carbonates, but comparable to most of those based on high temperature acid digestion (with however significantly better constraints on the slope and intercept values, notably due to the large range in temperature investigated and the large number of ∆ 47 measurements performed here, n = 67). We also use this dataset to empirically determine that the acid fractionation factor associated with phosphoric acid digestion of dolomite at 90°C (∆* dolomite90) is about + 0.176‰. This is comparable
Reconstructing Historical Changes in Combustion Patterns by Means of Black Carbon
The study of microbial carbonates has acquired new significance with the recognition that they re... more The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time.
For many years, sedimentary dolomite rocks have been considered to be primarily a replacement pro... more For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. The mud undergoes an early diagenetic transformation into a 100% dolomite crust on the margins of the lagoon. The biomineralization process, characterized by the variations of the physico-chemical conditions in this environment during the annual hydrological cycle, is integrated with isotopic analysis to define the early diagenetic processes responsible for the formation of both dolomitic mud and crust. The carbon isotope values indicate a contribution of respired organic carbon, which is greater for the crust (d 13 C ¼ 2 9.5‰ Vienna Pee Dee Belemnite (VPDB)) than mud (d 13 C ¼ 21.2‰ VPDB). The oxygen isotope values reflect a moderate degree of evaporation during mud formation (d 18 O ¼ 1.1‰ VPDB), whereas it is greatly enhanced during early diagenetic crust formation (d 18 O ¼ 4.2‰ VPDB). The clumped isotope formation temperature derived for the Brejo do Espinho mud is 34 8C, whereas it is 32 8C for the crust. These temperatures are consistent with the upper range of measured values during the dry season when the lagoon experiences the most hypersaline conditions.
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Papers by Chris Vasconcelos