Continental weathering as a driver of Late Cretaceous cooling : new insights from clay mineralogy of Campanian sediments from the southern Tethyan margin to the Boreal realm

21 pagesInternational audienceNew clay mineralogical analyses have been performed on Campanian sediments from the Tethyan and Boreal realms along a palaeolatitudinal transect from 45° to 20°N (Danish Basin, North Sea, Paris Basin, Mons Basin, Aquitaine Basin, Umbria-Marche Basin and Tunisian Atlas). Significant terrigenous inputs are evidenced by increasing proportions of detrital clay minerals such as illite, kaolinite and chlorite at various levels in the mid- to upper Campanian, while smectitic minerals predominate and represented the background of the Late Cretaceous clay sedimentation. Our new results highlight a distinct latitudinal distribution of clay minerals, with the occurrence of kaolinite in southern sections and an almost total absence of this mineral in northern areas. This latitudinal trend points to an at least partial climatic control on clay mineral sedimentation, with a humid zone developed between 20° and 35°N. The association and co-evolution of illite, chlorite and kaolinite in most sections suggest a reworking of these minerals from basement rocks weathered by hydrolysis, which we link to the formation of relief around the Tethys due to compression associated with incipient Tethyan closure. Diachronism in the occurrence of detrital minerals between sections, with detrital input starting earlier during the Santonian in the south than in the north, highlights the northward progression of the deformation related to the anticlockwise rotation of Africa. Increasing continental weathering and erosion, evidenced by our clay mineralogical data through the Campanian, may have resulted in enhanced CO2 consumption by silicate weathering, thereby contributing to Late Cretaceous climatic cooling

In situ carbon and oxygen isotopes measurements in carbonates by fiber coupled laser diode-induced calcination: A step towards field isotopic characterization

Natural stable isotopes ratios (δ13Ccarb and δ18Ocarb) of carbonates archived in the geological record are routinely used to reconstruct local and global paleo temperatures and the secular evolution of the biogeochemical carbon cycle. The state-of-the-art technique, employed since the mid 20th century, to measure these isotopic ratios starts with field sampling followed by several steps of physical and chemical laboratory preparation including: (i) microdrilling and/or sawing and crushing, (ii) CO2 release by wet acid digestion, (iii) gas equilibration, purification and transfer, before (iv) gas phase IRMS measurements. While these steps are time and resource consuming, they provide accurate measurements of δ13Ccarb, δ18Ocarb and carbonate contents. This study presents a new protocol involving a compact and modernized laser calcination system that decreases drastically the analyses time by reducing the number of preparations steps together with offering the possibility of performing spatially resolved analysis at the mm scale. This new method is based on the use of a fiber coupled laser diode device emitting 30 W in the near infrared at 880 nm. The energy provided by the laser source induces the decomposition of calcium carbonate into lime and carbon dioxide. In this work, the CO2 was collected in sample tubes under a controlled atmosphere for offline analysis, however additional developments should permit online analysis in the near future. We analyzed 9 different types of carbonate minerals encompassing a range of isotopic compositions VPDB between +3.3 and − 18.2‰ and between −1.7 and − 14.6‰ for δ13Ccarb and δ18Ocarb, respectively. A comparison of isotopic results was performed for carbonate zones analyzed both by classic methods (micro-drilling followed by acid digestion) and laser calcination. This isotopic cross-calibration exercise shows a direct positive co-variation between both methods with a correlation coefficient of 0.99 and a regression slope of 1 within uncertainties for the δ13Ccarb values. The δ18Ocarb values also compared well with a correlation coefficient of 0.96, suggesting a constant gas-solid phase isotopic equilibrium between carbon dioxide and lime. The reproducibility of our laser calcination method performed on replicate analyses of dolomite, siderite and malachite shows a 1σ standard deviation of 0.31 and 0.77 for δ13Ccarb and δ18Ocarb, respectively. These reproducibilities are within the observed isotopic natural inhomogeneity of samples (up to 1.3 and 0.57‰ for the δ13Ccarb and δ18Ocarb, respectively) as assessed by microdrilling and acid digestion. Based on the suit of samples analyzed in this study, we demonstrate that (i) fiber coupled laser diode calcination enables accurate and reproducible C and O isotopic characterization of natural carbonates, (ii) physical effects during calcination do not introduce any isotopic fractionation for C and is accompanied by a constant isotopic offset for O over a range of isotopic compositions and mineral matrices. These findings pave the way for a new range of possibilities for carbonate δ13C and δ18O measurements directly in the field using rapid, portable, and easy to manipulate laser preparation devices paired with CRDS/IRIS optical-mass spectrometers