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  • 1
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    Sulfur and oxygen isotope fractionation during sulfate reduction coupled to anaerobic oxidation of methane is dependent on methane concentration (2014)
    Elsevier
    In:  Earth and Planetary Science Letters, 399 . pp. 61-73.
    Details
    Publication Date: 2017-03-06
    Description: Highlights • Naturally enriched AOM biomass was studied in high-pressure continuous incubation. • We report the first S- and O-isotope fractionation values by sulfate reduction coupled to AOM from culture studies. • There is a tight link between methane concentration and S- and O-isotope fractionation. • S- and O-isotope fractionation values indicate reversibility of energy limited microbial processes. • The wide range of environmental S- and O-isotope signatures can be explained. Abstract Isotope signatures of sulfur compounds are key tools for studying sulfur cycling in the modern environment and throughout earth's history. However, for meaningful interpretations, the isotope effects of the processes involved must be known. Sulfate reduction coupled to the anaerobic oxidation of methane (AOM-SR) plays a pivotal role in sedimentary sulfur cycling and is the main process responsible for the consumption of methane in marine sediments − thereby efficiently limiting the escape of this potent greenhouse gas from the seabed to the overlying water column and atmosphere. In contrast to classical dissimilatory sulfate reduction (DSR), where sulfur and oxygen isotope effects have been measured in culture studies and a wide range of isotope effects has been observed, the sulfur and oxygen isotope effects by AOM-SR are unknown. This gap in knowledge severely hampers the interpretation of sulfur cycling in methane-bearing sediments, especially because, unlike DSR which is carried out by a single organism, AOM-SR is presumably catalyzed by consortia of archaea and bacteria that both contribute to the reduction of sulfate to sulfide. We studied sulfur and oxygen isotope effects by AOM-SR at various aqueous methane concentrations from 1.4±0.6 mM1.4±0.6 mM up to 58.8±10.5 mM58.8±10.5 mM in continuous incubation at steady state. Changes in the concentration of methane induced strong changes in sulfur isotope enrichment (View the MathML sourceεS34) and oxygen isotope exchange between water and sulfate relative to sulfate reduction (θOθO), as well as sulfate reduction rates (SRR). Smallest View the MathML sourceεS34 (21.9±1.9‰21.9±1.9‰) and θOθO (0.5±0.20.5±0.2) as well as highest SRR were observed for the highest methane concentration, whereas highest View the MathML sourceεS34 (67.3±26.1‰67.3±26.1‰) and θOθO (2.5±1.52.5±1.5) and lowest SRR were reached at low methane concentration. Our results show that View the MathML sourceεS34, θOθO and SRR during AOM-SR are very sensitive to methane concentration and thus also correlate with energy yield. In sulfate–methane transition zones, AOM-SR is likely to induce very large sulfur isotope fractionation between sulfate and sulfide (i.e. 〉60‰〉60‰) and will drive the oxygen isotope composition of sulfate towards the sulfate–water oxygen isotope equilibrium value. Sulfur isotope fractionation by AOM-SR at gas seeps, where methane fluxes are high, will be much smaller (i.e. 20 to 40‰).
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.epsl.2014.04.047
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    The ikaite to calcite transformation: Implications for palaeoclimate studies (2022)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures. Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation at different marine sedimentary sites, including bacterial sulphate reduction and anaerobic oxidation of methane. The colours of the ikaites, from light brown to dark brown, indicate a high organic matter content, favouring high rates of bacterial sulphate reduction as the main driver of ikaite precipitation. Highest Mg/Ca ratios are found in the most unstable ikaites, indicating that Mg acts to destabilise ikaite structure.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    The role of trapped fluids during the development and deformation of a carbonate/shale intra-wedge tectonic mélange (Mt. Massico, Southern Apennines, Italy) (2020)
    Elsevier
    Details
    Publication Date: 2023-10-26
    Description: Numerous studies exist on exhumed tectonic mélanges along subduction channels whereas, in accretionary wedge interiors, deformation mechanisms and related fluid circulation in tectonic mélanges are still underexplored. We combine structural and microstructural observations with geochemical (stable and clumped isotopes and isotope composition of noble gases in fluid inclusions of calcite veins) and U-Pb geochronological data to define deformation mechanisms and syn-tectonic fluid circulation within the Mt. Massico intra-wedge tectonic mélange, located in the inner part of the central-southern Apennines accretionary wedge, Italy. This mélange developed by shear deformation at the base of a clastic succession. Deformation was characterized by disruption of the primary bedding, mixing, and deformation of relicts of competent olistoliths and strata within a weak matrix of deformed clayey and marly interbeds. Recurrent cycles of mutually overprinting fracturing/veining and pressure-solution processes generated a block-in-matrix texture. The geochemical signatures of syntectonic calcite veins suggest calcite precipitation in a closed system from warm (108°-147 °C) paleofluids, with δ18O vlaues between þ9‰ and 14‰, such as trapped pore waters after extensive 18O exchange with the local limestone host rock and/or derived by clay dehydration processes at T 〉 120 °C. The 3He/4He ratios in fluid inclusions are lower than 0.1 Ra, indicating that He was exclusively sourced from the crust. We conclude that: (1) intraformational rheological contrasts, inherited trapped fluids, and low-permeability barriers such as marlyshaly matrix, can promote the generation of intra-wedge tectonic mélanges and the development of transient fluid overpressure; (2) clay-rich tectonic mélanges, developed along intra-wedge décollement layers, may generate low-permeability barriers hindering the fluid redistribution within accretionary wedges.
    Description: Published
    Description: 104086
    Description: 1T. Struttura della Terra
    Description: 2T. Deformazione crostale attiva
    Description: JCR Journal
    Keywords: Tectonic mélange ; Fluid-rock interaction ; Stable and clumped isotopes ; Noble gases ; Fold and thrust belt ; 04.04. Geology
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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