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)

Abstract. Clay mineral and stable isotope (C, O) data are reported from the upper Sinemurian (Lower Jurassic) of Cardigan Bay Basin (Llanbedr–Mochras Farm borehole, northwestern Wales) Paris (Montcornet northern France) to highlight prevailing environmental climatic conditions. In both basins, located at similar palaeolatitudes 30–35? N, clay assemblages comprise chlorite, illite, illite–smectite mixed layers (R1 I-S), smectite, kaolinite in various proportions. Because influence burial diagenesis authigenesis is negligible boreholes, minerals interpreted be derived erosion Caledonian Variscan massifs, including their basement pedogenic cover. Basin, variations proportions smectite inversely related each other through entire Sinemurian. As succeeding Pliensbachian, stratigraphic distribution reveals an alternation kaolinite-rich intervals reflecting strong hydrolysing conditions smectite-rich indicating a semi-arid climate. Kaolinite particularly abundant part obtusum zone oxynotum zone, suggesting more intense likely coeval with warm responsible for acceleration hydrological cycle. north succession less continuous compared site, as raricostatum either absent or highly condensed. The dominantly composed illite without significant trends, but interval identified consequence emersion London–Brabant Massif following lowering sea level. Following slight negative carbon excursion obtusum–oxynotum transition, long-term decrease ?13Corg late oxynotum–early zones recorded two sites may precede partly include Sinemurian–Pliensbachian Boundary Event, which recognised most basins worldwide signify pulse Central Atlantic Magmatic Province volcanism.