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We acknowledge the financial support from the UnivEarths Labex program of Sorbonne Paris Cite (ANR 11-IDEX-00005-02). CH and AL acknowledge support from the Max Planck Society. EES and RB were funded by the NASA Exobiology grant NNX16AI37G and the Virtual Planetary Laboratory of the NASA Astrobiology Institute and were technically assisted by Andy Schauer. Interpreting the organic carbon content (TOC) and stable carbon isotopic composition (δ13C) of organic matter in the sedimentary rock record depends on our capability to accurately measure them, while excluding sources of contamination. This however becomes increasingly problematic as we analyze samples with ever-lower organic carbon content. Accordingly, organic solvents are sometimes used to remove contaminating traces of modern organic matter from ancient rock samples. However, especially for very low TOC samples, traces of solvents or their impurities remaining in the sample may contribute a significant organic contamination that can impact the bulk measurements of both TOC and δ13C values. This study, including three independent investigations performed in different laboratories, is the first detailed examination of the effect of cleaning treatments on the reliability of TOC and δ13C values in a range of natural rock samples and synthetic materials with low TOC content from below detection limit to 3330 ppm. We investigated the four most common organic solvents used to remove modern organic matter: dichloromethane (DCM), n-hexane, methanol and ethanol, and evaluated the effect of grain size and mineralogy. We find that (i) cleaning treatments with methanol, n-hexane and dichloromethane contaminate rock samples when used directly on sample powder, regardless of the grain size; (ii) this pollution buffers the natural variability and homogenizes the δ13C values of samples around the isotopic composition of the solvent, i.e. between −27 and −29‰; (iii) the extent of contamination depends on the solvent used, DCM being the most contaminating (up to 6000 ppm) and ethanol the only solvent that does not seem to contaminate rock samples above our detection limit; (iv) sample mineralogy also exerts an influence on the extent of contamination, clay minerals being more prone to adsorb contaminants. We conclude that the response of carbon concentrations and the stable carbon isotopic composition of organic matter in geological samples to cleaning treatments is neither negligible nor systematic when investigating samples with low carbon content. Postprint

The Great Oxidation Event (GOE) has been defined as the time interval when sufficient atmospheric oxygen accumulated to prevent the generation and preservation of mass-independent fractionation of sulphur isotopes (MIF-S) in sedimentary rocks. Existing correlations suggest that the GOE was rapid and globally synchronous. Here we apply sulphur isotope analysis of diagenetic sulphides combined with U-Pb and Re-Os geochronology to document the sulphur cycle evolution in Western Australia spanning the GOE. Our data indicate that, from ~2.45 Gyr to beyond 2.31 Gyr, MIF-S was preserved in sulphides punctuated by several episodes of MIF-S disappearance. These results establish the MIF-S record as asynchronous between South Africa, North America and Australia, argue for regional-scale modulation of MIF-S memory effects due to oxidative weathering after the onset of the GOE, and suggest that the current paradigm of placing the GOE at 2.33–2.32 Ga based on the last occurrence of MIF-S in South Africa should be re-evaluated., The Great Oxidation Event (GOE) is considered to have occurred at 2.33–2.32 Ga based on the last occurrence of MIF-S in South Africa. Here, based on sulphur isotope analysis of samples from Western Australia, the authors show preservation of MIF-S beyond 2.31 Ga and call for a re-evaluation of GOE timing.

International audience; Charcoal fragments from the Neolithic settlements of Lake Chalain (Jura Mountains, France) 17 were characterized by dendro-anthracology (charcoal-pith distance, tree-ring width, 18 earlywood/latewood proportion) and ring-scale isotope geochemistry (13 C) to assess the 19 relevance of this combined approach for paleoclimate reconstructions. Two differing climatic 20 periods were investigated: (i) a climatic deterioration period characterized by cool and moist 21 conditions and (ii) a climatic improvement period characterized by slightly less precipitation 22 and warmer temperature. Latewood proportion in charcoal tree-rings was similar for the two 23 studied climatic periods. However, the charcoal tree-rings exhibited width and 13 C-content 24 significantly different between the two studied periods, in agreement with previously inferred 25 climatic difference. Monitoring ring-to-ring 13 C variation within each charcoal fragment 26 revealed no noticeable climatic trend, for none of the studied periods. However, calculation of 27 the difference in 13 C-content between earlywood and latewood of a given tree-ring suggested 28 2 that the cool and moist climatic period also corresponded to higher seasonal contrast than 29 the dryer climatic period. Although this exploratory study needs further confirmation, it opens 30 promising developments for paleoclimatic reconstructions based on the stable carbon 31 isotope composition of archaeological charcoals: the potential for recording subtle 32 paleoclimatic variations and seasonal contrasts.

International audience; One of the most serious issues with deciphering the evolution of organisms and their biogeochemical environments from the ancient rock record is the difficulty in obtaining well-preserved samples. Although not much can be done to avoid diagenetic and metamorphic alteration when they have occurred, alteration due to weathering can be avoided by working on drill core samples. This implies however that the amount of sample is limited, which may in turn restrain the number of possible chemical and isotopic analyses that can be performed. In order to save sample we show here that the chemical protocol used for the sulfur sulfide extraction (for later sulfur isotope analyses) is also suitable to decarbonate samples (for later organic carbon isotope analyses). In the case of carbonated rocks, both sulfur sulfide extraction and decarbonation require high amounts of sample so that coupling them may save a significant amount of sample and time. In addition it allows both organic carbon (TOC and δ 13 C) and sulfur isotope composition measurements to be performed on the exact same powder , which is essential when trying to understand couplings between S and C cycles in heterogeneous samples. We thus tested the efficiency of the acidic chromium solution, commonly used to extract sulfur from sulfide, for sample decarbonation on various Archean rocks. Our results show that no significant carbon isotope fraction-ation is caused by this new decarbonation protocol, even for the samples with low organic carbon content. The chromium solution seems to be perfectly adapted for the analysis of organic matter in the ancient rock record, at least when the rock samples have experienced low greenschist facies metamorphism. Further tests will be needed to verify if this protocol can also be used for less mature organic matter.