Authors:Fernando Gómez, Lucía Gómez-Peral, Anelize Bahniuk Pages: 97 - 99 Abstract: Sedimentary rocks are our window to understand the history and evolution of the surface of our planet. The sedimentary record, although biased by variable preservation conditions and post depositional processes, potentially preserve signals of the interaction between physical, chemical and biological processes that takes place at the interface between the Geosphere, the Hydrosphere/ Atmosphere and the Lithosphere. Geochemistry, integrated with Sedimentology, has become a standard approach to unravel these interactions and to provide some insights for the understanding of how sedimentary geochemistry is preserved within different sedimentary environments. Microbial life is ubiquitous at the Earth’s surface (Whitman et al., 1998), and has been present throughout our Planet’s history. It has evolved to exploit the energy provided by gradients in geochemical composition between rocks, organic materials and surface fluids, using this energy for maintenance and growth (Hoeler, 2007). Due to their abundance, chemical reactivity and metabolic activity (Konhauser, 2007), microbes play a central role in biogeochemical cycles (C, O, N, S, Fe, etc) at both, micro and macroscopic scales (Schlesinger and Bernhardt, 2020). The best example of these processes are the current chemical composition of our O2-rich atmosphere or the chemical redox gradients that exist at the sediment-water interface in most sedimentary environments around the world, where early diagenesis driven by organic matter degradation takes place (Aller, 2014). In addition, microbial mats and biofilms influence the precipitation/dissolution of minerals such as carbonates (explaining the the abundance of microbialites in the ancient rock record, Riding, 2011) as well as the rheological and mechanical behaviour of detrital sediments, where a classic example are known as MISS (Microbially induced sedimentary structures, Noffke et al., 2010). For these reasons, whenever we talk about sedimentary geochemistry we are actually talking about sedimentary biogeochemistry and geomicrobiology. This is so since microbial activity has been shaping the surface chemistry and composition of our planet, as previously recognized by the pioneering work of Vladimir Vernadsky in 1926 and later by Lourens Baas-Becking in 1934 (Knoll et al., 2012). Vernadsky and Baas-Becking early ideas have grown and become the lens through which we scientifically observe our world, making it clear that the Biosphere adds another layer of diversity and complexity in order to understand our planet as a system. PubDate: 2022-01-04 Issue No:Vol. 28, No. 2 (2022)
Authors:Maria Julia Arrouy, Lucía Gómez-Peral, Victoria Penzo, Camila Ferreyra, Daniel G. Poiré Pages: 101 - 120 Abstract: The well-preserved limestone succession, Loma Negra Formation (~40 m), in the Tandilia System was deposited in a shallow carbonate ramp under low energy conditions. The evolution in the depositional settings of the unit was indicated as deepening upwards varying from shallow-middle to outer ramp environment. The limestone fabric is assumed as the product of biologically controlled precipitation of micrite, where the terrigenous supply was limited. From detailed meso- and microscopic descriptions it is possible to recognize microbially induced sedimentary structures ‘MISS’ represented by typical microtextures related to microbial activity that appear represented throughout the entire formation. In addition, micro-stromatolites are observed in the unit associated with the microbial mats showing micro-columnar conical to domical morphologies. In the basal and middle Loma Negra Formation, hemispherical structures are recognized in the bed-tops and interpreted as bubbles-like and gas escape features associated with the microbial mat interaction. Their morphology is compared with oxygen bubbles produced by modern experimental modelling with photosynthetic cyanobacteria microbial mats. Moreover, hemispherical structures are associated with increasing gas pressure lifting grains and the organic components to the surface. This paper provides evidence to understand the possible causal relationship between microbial activity and seawater oxygenation. The high oxygen production revealed by geochemistry proxies and here proposed as probably associated with photosynthetic microbial activity might be a plausible explanation for the record of the documented Middle Ediacaran Oceanic Oxygenation Event in the Loma Negra Formation. PubDate: 2022-01-04 Issue No:Vol. 28, No. 2 (2022)
Authors:Lucía Maisano, I. Emma Quijada, Vanesa L. Perillo , Diana G. Cuadrado, L. Ariel Raniolo, Eduardo A. Gómez Pages: 121 - 132 Abstract: The present study traces a carbonate lamination in microbial mats in a siliciclastic coastal environment. The aim of this paper is to determine how physicochemical and biological processes influence the preservation of sedimentary structures such as ripple marks, covering it by carbonate precipitation. Such precipitation is a combination of the activity of microorganisms, and physicochemical factors. Biological factors, such as photosynthesis, composition and activity of the microbial community, and presence of exopolymeric substances (EPS), play a significant role in coastal environments, while high-energy hydrodynamic events supply seawater, providing calcium and carbonate ions, and trigger events like sediment transport, deposition, and erosion. The combination of these hydrodynamic events with the microbial activity creates physical sedimentary structures, such as ripple marks, which can be biostabilized. The study was conducted in Paso Seco (40°38’40´´S; 62°12´22´´W), a modern coastal flat within an elongated semi-closed basin colonized by microbial mats and categorized as a supratidal zone. Water samples were taken from different ponds within the flat with different degrees of evaporation, and in a tidal creek for hydrochemical analyses. Seawater level fluctuations were measured in the tidal flat over a 10-month period in 2018 using a HOBO water level logger. Ripple field formation over the tidal flat was documented after the occurrence of a strong storm and was monitored throughout three subsequent field trips, and sedimentary samples were taken in a field trip. Petrographic studies show a laterally continuous dense micritic calcite layer, 100–200 µm in thickness, covering the ripple structure, improving the preservation of these bedforms. The presence of microorganisms in the sedimentary environment enables the stabilization of ripple marks because their motility and abundant EPS secretion bind the sediment grains and generate an organic layer that protects them from erosion. On the other hand, the presence of microbial mats contributes to the early preservation of the ripples because they create the adequate conditions for carbonate precipitation. Our observations of modern sedimentary structures and the in situ study of their evolution and early lithification may provide an indicator of microbial colonization and stabilization of ripples in the paleoenvironmental reconstruction. PubDate: 2022-01-04 Issue No:Vol. 28, No. 2 (2022)
Authors:Inès Eymard, M. del Pilar Alvarez, Andrés Bilmes, Crisogono Vasconcelos, Camille Thomas, Daniel Ariztegui Pages: 133 - 151 Abstract: In recent years resulting investigations in living microbialites have provided significant data that have been critical to disentangle the role of the various biotic and abiotic processes contributing to their development. Despite these efforts separating the impact and magnitude of these processes remain a difficult task. At present the Maquinchao Basin in northeastern Patagonia, Argentina, contains both fossil and living microbialites. Thus, the region provides a unique opportunity to investigate the impact of intrinsic and extrinsic parameters in carbonate precipitation. Early investigations (Austral summer 2011) in living microbialites concluded that organomineralization was related to both photosynthetic activity in the more surficial layer (green), and sulfate-reduction in the lower part (beige). Field investigations in the same area four years later showed that the pounds previously containing abundant active mats had dried out, and in general revealed the absence of globular structured clusters of minerals in the microbial mats. Here we present microscale investigations using optical microscopy and SEM along with the 16SrRNA gene sequence diversity, and the physico-chemical parameters of the hosting waters. They were carried out in successive seasonal samplings in November 2015, April-May 2016, August 2016, February 2017, and March 2018. All microbialite samples show regular occurrences of sulfate reducing bacteria (SRB) along with filaments of unknown origin. Carbonates are observed associated with erect filaments in shallow and active running water locations whereas the mineral phase is located below organic matter film in comparatively deeper and calmer water areas. Additionally, seasonal changes in the physico-chemical properties of the hosting waters indicate that extrinsic parameters, especially evaporation, might play a more substantial role in the precipitation of these carbonates than previously proposed. The environmental differences between 2011 and 2015 in meteorological conditions, regional volcanic activity and associated deposits in the basin are analyzed. We concluded that they are likely responsible of the decrease of the mineralization processes, and particularly those associated with photosynthetic activity. These results call for caution when interpreting the degree of biological impact on the formation of microbialites in the geological record. Local extrinsic factors might have a changeable impact over time switching mineral precipitation from biotic to abiotic and vice-versa, which can be undistinguishable in fossilized microbialites. PubDate: 2022-01-04 Issue No:Vol. 28, No. 2 (2022)
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