Million-year-scale alternation of warm-humid and semi-arid periods as a mid-latitude climate mode in the Early Jurassic (Late Sinemurian, Laurasian Seaway) 5 Thomas Munier1,2, Jean-François Deconinck1, Pierre Pellenard1, Stephen P. Hesselbo3, James B. Riding4, Clemens V. Ullmann3, Cédric Bougeault1, Mathilde Mercuzot5, Anne-Lise Santoni1, Émilia Huret6, Philippe Landrein6 10 1 Biogéosciences, UMR 6282, uB/CNRS, Université Bourgogne Franche-Comté, 6 Boulevard Gabriel, 21000 Dijon, France. 2 ISTeP, UMR 7193, SU/CNRS, Sorbonne Université, 4 Place Jussieu, 75005 Paris, France. 3 Camborne School of Mines and the Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9FE, UK. 4 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. 15 5 Géosciences Rennes, UMR 6118, UR/CNRS, Université Rennes 1, Campus de Beaulieu, CS 74205 35042 Rennes cedex, France. 6 Agence Nationale pour la gestion des déchets radioactifs, Centre de Meuse/Haute-Marne, RD 960, 55290 Bure, France. Correspondence to: Thomas Munier (
[email protected]) 20 Abstract. Clay mineral and stable isotope (C, O) data are reported from the upper Sinemurian (Lower Jurassic) of the Cardigan Bay Basin (Llanbedr [Mochras Farm] borehole, northwest Wales) and the Paris Basin (Montcornet borehole, northern France) to highlight the prevailing environmental and climatic conditions. In both basins, located at similar palaeolatitudes of 30– 35°N, the clay mineral assemblages comprise chlorite, illite, illite-smectite mixed-layers (R1 I-S), smectite, and kaolinite in various proportions. Because the influence of burial diagenesis and authigenesis is negligible in both boreholes, the clay 25 minerals are interpreted to be derived from the erosion of the Caledonian and Variscan massifs, including their basement and pedogenic cover.