Marie-Catherine Sforna

MapIT!: a simple and user-friendly MATLAB script to elaborate elemental distribution images from LA-ICP-MS data

The use of LA-ICP-MS imaging to study trace element distribution in solid samples has spread widely in the last few years throughout different disciplinary fields. This technique generates a large quantity of data that must be corrected for analytical and matrix effects and might not be easy to handle for non-experts. We present here a MATLAB® script, MapIT!, that allows fast treatment and correction of LA-ICP-MS data with minimum preparation of the raw data and the LA imaging in a short time (less than ∼30 minutes). The script is not specific for a single type of material, nor the spectrometer used for acquisition. It offers different types of calibration protocols (internal and external or only external) depending on the needs. We also present three application examples (a speleothem, a fossil tooth and a magmatic plagioclase sample) to show the potentiality of MapIT!. ESI accompanies this paper and elucidates the various steps of the data processing.

Patterns of metal distribution in hypersaline microbialites during early diagenesis: Implications for the fossil record

The use of metals as biosignatures in the fossil stromatolite record requires understanding of the processes controlling the initial metal(loid) incorporation and diagenetic preservation in living microbialites. Here, we report the distribution of metals and the organic fraction within the lithifying microbialite of the hypersaline Big Pond Lake (Bahamas). Using synchrotron-based X-ray microfluorescence, confocal, and biphoton microscopies at different scales (cm-μm) in combination with traditional geochemical analyses, we show that the initial cation sorption at the surface of an active microbialite is governed by passive binding to the organic matrix, resulting in a homogeneous metal distribution. During early diagenesis, the metabolic activity in deeper microbialite layers slows down and the distribution of the metals becomes progressively heterogeneous, resulting from remobilization and concentration as metal(loid)-enriched sulfides, which are aligned with the lamination of the microbialite. In addition, we were able to identify globules containing significant Mn, Cu, Zn, and As enrichments potentially produced through microbial activity. The similarity of the metal(loid) distributions observed in the Big Pond microbialite to those observed in the Archean stromatolites of Tumbiana provides the foundation for a conceptual model of the evolution of the metal distribution through initial growth, early diagenesis, and fossilization of a microbialite, with a potential application to the fossil record.

Sulfur isotope's signal of nanopyrites enclosed in 2.7 Ga stromatolitic organic remains reveal microbial sulfate reduction

Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7-Gyr-old Tumbiana Formation (Western Australia). The analyzed nanopyrites show a large range of δ34 S values of about 84‰ (from -33.7‰ to +50.4‰). The recognition that a large δ34 S range of 80‰ is found in individual carbonaceous-rich layers support the interpretation that the nanopyrites were formed in microbial mats through MSR by a Rayleigh distillation process during early diagenesis. An active microbial cycling of sulfur during formation of the stromatolite may have facilitated the mixing of different sulfur pools (atmospheric and hydrothermal) and explain the weak mass independent signature (MIF-S) recorded in the Tumbiana Formation. These results confirm that MSR participated actively to the biogeochemical cycling of sulfur during the Neoarchean and support previous models suggesting anaerobic oxidation of methane using sulfate in the Tumbiana environment.