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The widespread adoption of the sediment fingerprinting approach to guide catchment management has been limited by the cost and the difficulty to prepare and process samples for geochemical and radionuclide analyses. Spectral properties have recently been shown to provide a rapid and cost-efficient alternative for this purpose. The current research objective was (i) to quantify the sediment source contributions in a 1.19-km2 rural catchment of Southern Brazil by using mid-infrared (MIR) spectroscopy, and (ii) to compare these results with those obtained with geochemical approach and near-infrared (NIR) and ultraviolet-visible (UV-VIS) spectroscopy methods. The sediment sources to discriminate were cropland surface (n = 20), unpaved roads (n = 10) and stream channel banks (n = 10). Twenty-nine suspended sediment samples were collected at the catchment outlet during nine significant flood events. The sources could be distinguished by MIR spectroscopy. Cropland and channel bank sources mainly differed in their clay mineral contents, but their similar organic matter content complicated the MIR-model predictions. Unpaved road contributions were discriminated from the other sources by their lower organic carbon content. When the results of the current research based on MIR spectroscopy are compared to those obtained using other sediment fingerprinting approaches, based on geochemistry and NIR and UV-VIS spectroscopy, an overestimation of channel banks contribution and an underestimation of cropland and unpaved road contributions is found. These results suggest that MIR spectroscopy can provide a useful tool that is non-destructive, rapid and cheap for tracing sediment sources in rural catchments and for guiding the implementation of soil and water conservation measures.

International audience; The 2.1‐billion‐year‐old (Ga) Francevillian series in Gabon hosts some of the oldest reported macroscopic fossils of various sizes and shapes, stimulating new debates on the origin, evolution and organization of early complex life. Here, we document ten representative types of exceptionally well‐preserved mat‐related structures, comprising “elephant‐skin” textures, putative macro‐tufted microbial mats, domal buildups, flat pyritized structures, discoidal microbial colonies, horizontal mat growth patterns, wrinkle structures, “kinneyia” structures, linear patterns and nodule‐like structures. A combination of petrographic analyses, scanning electron microscopy, Raman spectroscopy and organic elemental analyses of carbon‐rich laminae and microtexture, indicate a biological origin for these structures. The observed microtextures encompass oriented grains, floating silt‐sized quartz grains, concentrated heavy minerals, randomly oriented clays, wavy‐crinkly laminae and pyritized structures. Based on comparisons with modern analogues, as well as an average δ13C organic matter (Corg) composition of −32.94 ± 1.17‰ (1 standard deviation, SD) with an outlier of −41.26‰, we argue that the mat‐related structures contain relicts of multiple carbon pathways including heterotrophic recycling of photosynthetically derived Corg. Moreover, the relatively close association of the macroscopic fossil assemblages to the microbial mats may imply that microbial communities acted as potential benthic O2 oases linked to oxyphototrophic cyanobacterial mats and grazing grounds. In addition, the mat's presence likely improved the preservation of the oldest large colonial organisms, as they are known to strongly biostabilize sediments. Our findings highlight the oldest community assemblage of microscopic and macroscopic biota in the aftermath of the “Great Oxidation Event,” widening our understanding of biological organization during Earth's middle age.

This work assesses relationships between characteristic aggregate microstructures related to biological activity in soils under different long-term land use and the distribution and extractability of metal pollutants. We selected two neighbouring soils contaminated with comparable metal loads by past atmospheric deposition. Currently, these soils contain similar stocks, but different distributions of zinc (Zn) and lead (Pb) concentrations with depth. One century of continuous land use as permanent pasture (PP) and conventional arable (CA) land, has led to the development of two soils with different macro- and micro-morphological characteristics. We studied distributions of organic matter, characteristic micro-structures and earthworm-worked soil by optical microscopy in thin sections from A, B and C horizons. Concentrations and amounts of total and EDTA-extractable Zn and Pb were determined on bulk samples from soil horizons and on size-fractions obtained by physical fractionation in water. Large amounts of Zn and Pb were found in 2–20-µm fractions, ascribed to stable organo-mineral micro-aggregates influenced by root and microbial activity, present in both soils. Unimodal distribution patterns of Zn, Pb and organic C in size-fractions were found in horizons of the CA soil. In contrast, bimodal patterns were observed in the PP soil, because large amounts of Zn and Pb were also demonstrated in stable larger micro-aggregates (50–100-µm fractions). Such differing distribution patterns characterized all those horizons markedly influenced by earthworm activity. Larger earthworm activity coincided with larger metal EDTA-extractability, particularly of Pb. Hence, land use-related biological activity leads to specific soil microstructures affecting metal distribution and extractability, both in surface and subsurface horizons.