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.
