Periodic Reporting for period 2 - MicroS (Micro-scale δ34S variation of sulfide species)
Período documentado: 2022-10-01 hasta 2023-09-30
Life exerts significant influence on the redox landscape of Earth; however, to what extent environmental factors, i.e. background oxygen levels and bio-essential nutrient availability, influenced the early diversification of eukaryotes remains debated. To solve this conundrum, accurate reconstructions of redox landscapes from critical junctions in Earth’s history are necessary. The often-compared metasedimentary rocks from the ca. 2.0 billion-year-old Onega Basin (Karelia, Russia) and Francevillian Basin (Gabon) have been central to reconstructing Earth’s early oxygenation and following environmental change. However, a growing body of conflicting interpretations of paleoredox conditions from ancient sedimentary rocks has complicated such efforts. One of the most debated paleoredox proxies is the sulfur (δ34S) isotope record. The MicroS project addresses the uncertainties under which (bio)geochemical sulfur cycling occurred during periods of significant change in Earth’s atmosphere-ocean evolution, the degree to which geochemical signals reflect local or global processes, and how these signatures are ultimately preserved in the rock record. To better understand the environmental information in the ancient Onega and Francevillian rocks, the project investigates the generation and transfer of geochemical signals under modern depositional conditions in structurally similar basins such as the Gulf of California (e.g. semi-restricted bathymetry, tectonic activity, hydrocarbon seepage).
Establishing a geological context via detailed characterization of the studied rock successions is fundamental to interpreting geochemical proxy data accurately. Therefore, a lithological and petrographic inspection of sample material from the Onega, Francevillian, and the Gulf of California localities is critical to assess the environmental meaning of the laboratory-based bulk extraction and high-resolution analytical techniques results. During the screening of samples, special attention was paid to textures (e.g. crystal size, shape, zoning, secondary overgrowths, veins) used to distinguish earlier and later pyrite generations and characterization of associations between co-existing sulfur species (e.g. organic sulfur, gypsum, barite). The sample’s bulk 34S values were determined using isotope ratio mass spectrometry (IRMS), and chemical speciation maps of sulfur were created using a combination of synchrotron µ-X-ray fluorescence (µ-XRF) imaging and X-ray absorption near edge structure (XANES) spectroscopy. The latter synchrotron-based technique’s results were used to target areas of interest for in situ δ34S measurements by secondary ion mass spectroscopy (SIMS). As an alternative to SIMS, the project also broadened the use of the more common laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) by developing a methodological protocol for micro-scale δ34S measurements.
Former interpretations of the δ34S excursions recorded in the Onega and Francevillian successions were assessed by combining the observation- and laboratory-based techniques results and comparison to modern Gulf of California sediments. Looking for signs of secondary alteration (e.g. veins, fractures, and secondary mineralization) and correlations between δ34S patterns and petrographic (e.g. crystal size/growth/zoning) data allowed to discriminate primary environmental signals from those of later processes (i.e. diagenesis, metamorphism, weathering). Placing those results into the specific geological context of the studied basins provided the premise to test the former interpretations of large-scale excursions inferred from bulk34S data and concomitant implications for changes in the ocean’s redox landscape inferred from the Onega and Francevillian Basins.
Contrary to earlier research, the outcomes of the MicroS project indicate that the most appropriate interpretation of the ancient Francevillian and Onega Basin’s bulk pyrite δ34S records is that they represent a combination of signals from diagenesis and late-stage processes in sulfate- and organic-rich strata. These post-depositional processes can effectively obscure the isotopic signal of the earliest pyrite generation, which reflects the conditions during deposition. To deconvolve (bio)geochemical signals from early deposition and later overprints to ensure robust interpretations of the pyrite record, the project advances the combined use of traditional geological (petrography, sedimentology) and novel high-resolution techniques in geoscience (i.e. wet-chemical extractions, SIMS, XANES, and LA-ICP-MS) to identify pyrite-generating processes.
While the primary focus is on pyrite, similar approaches can be applied to various mineral systems to clarify the timing of mineralizing events and identify geochemical proxy data that record primary depositional signals. Thus, the MicroS project serves to guide research aimed at enhancing understanding of pivotal events in Earth’s history by highlighting the need for critical reappraisal of conceptual models using bulk geochemical proxy data as primary depositional signals linked to global-scale (bio)geochemical processes.
The MicroS project’s findings have been disseminated through various channels, including open-access publication, conference proceedings, oral or poster presentations at seminars and conferences, and outreach activities.
Former interpretations of the δ34S excursions recorded in the Onega and Francevillian successions were assessed by combining the observation- and laboratory-based techniques results and comparison to modern Gulf of California sediments. Looking for signs of secondary alteration (e.g. veins, fractures, and secondary mineralization) and correlations between δ34S patterns and petrographic (e.g. crystal size/growth/zoning) data allowed to discriminate primary environmental signals from those of later processes (i.e. diagenesis, metamorphism, weathering). Placing those results into the specific geological context of the studied basins provided the premise to test the former interpretations of large-scale excursions inferred from bulk34S data and concomitant implications for changes in the ocean’s redox landscape inferred from the Onega and Francevillian Basins.
Contrary to earlier research, the outcomes of the MicroS project indicate that the most appropriate interpretation of the ancient Francevillian and Onega Basin’s bulk pyrite δ34S records is that they represent a combination of signals from diagenesis and late-stage processes in sulfate- and organic-rich strata. These post-depositional processes can effectively obscure the isotopic signal of the earliest pyrite generation, which reflects the conditions during deposition. To deconvolve (bio)geochemical signals from early deposition and later overprints to ensure robust interpretations of the pyrite record, the project advances the combined use of traditional geological (petrography, sedimentology) and novel high-resolution techniques in geoscience (i.e. wet-chemical extractions, SIMS, XANES, and LA-ICP-MS) to identify pyrite-generating processes.
While the primary focus is on pyrite, similar approaches can be applied to various mineral systems to clarify the timing of mineralizing events and identify geochemical proxy data that record primary depositional signals. Thus, the MicroS project serves to guide research aimed at enhancing understanding of pivotal events in Earth’s history by highlighting the need for critical reappraisal of conceptual models using bulk geochemical proxy data as primary depositional signals linked to global-scale (bio)geochemical processes.
The MicroS project’s findings have been disseminated through various channels, including open-access publication, conference proceedings, oral or poster presentations at seminars and conferences, and outreach activities.
The project provides new insights into the effects of secondary alteration, highlights the complexity of interpreting ancient sulfur isotope records, and assesses the appropriateness of using bulk δ34S records in paleoenvironmental reconstructions. The project’s results demonstrate that a thorough understanding of the geological context and mechanisms associated with sulfur cycling and mineral formation is necessary to interpret bulk δ34S records accurately. The MicroS project advances the coupled use of traditional geological and novel high-resolution techniques, such as SIMS, µ-XRF, and XANES in Early Earth studies. While neither XANES nor SIMS instruments are standard equipment in most laboratories, then the more common LA-ICP-MS offers a fast-screening method for high-resolution sulfur isotope measurements. Given that LA-ICP-MS instruments are used to sample and analyze the surfaces of solid materials in scientific and commercial laboratories, broadening its use to δ34S measurements provides new possibilities in geochemical investigations.