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Climate of the Past An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/cp-2020-99
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-99
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Sep 2020

10 Sep 2020

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This preprint is currently under review for the journal CP.

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)

Thomas Munier1,2, Jean-François Deconinck1, Pierre Pellenard1, Stephen P. Hesselbo3, James B. Riding4, Clemens V. Ullmann3, Cédric Bougeault1, Mathilde Mercuzot5, Anne-Lise Santoni1, Emilia Huret6, and Philippe Landrein6 Thomas Munier et al.
  • 1Biogéosciences, UMR 6282, uB/CNRS, Université Bourgogne Franche-Comté, 6 Boulevard Gabriel, 21000 Dijon, France
  • 2ISTeP, UMR 7193, SU/CNRS, Sorbonne Université, 4 Place Jussieu, 75005 Paris, France
  • 3Camborne School of Mines and the Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9FE, UK
  • 4British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
  • 5Géosciences Rennes, UMR 6118, UR/CNRS, Université Rennes 1, Campus de Beaulieu, CS 74205 35042 Rennes cedex, France
  • 6Agence Nationale pour la gestion des déchets radioactifs, Centre de Meuse/Haute-Marne, RD 960, 55290 Bure, France

Abstract. High resolution 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 minerals are interpreted to be derived from the erosion of the Caledonian and Variscan massifs, including their basement and pedogenic cover. In the Cardigan Bay Basin, the variations in the proportions of smectite and kaolinite are inversely related to each other over the entire upper Sinemurian succession. As in the Pliensbachian, the stratigraphical distribution reveals an alternation of kaolinite-rich intervals reflecting strong hydrolysing conditions, and smectite-rich intervals indicating a semi-arid climate. Kaolinite is particularly abundant in the upper part of the obtusum Zone and in the oxynotum Zone, suggesting more intense hydrolysing conditions likely coeval with warm conditions responsible for an acceleration of the hydrological cycle. This interval is also marked by a negative excursion of δ13Ccarb and δ18Ocarb, which may confirm a warmer palaeoclimate, although these excursions may be exaggerated or overprinted by the carbonate diagenesis. In the north of the Paris Basin, the stratigraphical succession is much thinner compared to the Cardigan Bay Basin site, and the oxynotum Zone is either absent or highly condensed. The clay assemblages are dominantly composed of illite and kaolinite without significant stratigraphical trends, but a smectite-rich interval identified in the obtusum Zone is interpreted as a consequence of the emersion of the London-Brabant Massif following a lowering of sea-level. A long-term decrease of δ13Corg from the late oxynotum/early raricostatum zones is also recorded in the two sites and may precede or partly include the negative carbon-isotope excursion of the Sinemurian/Pliensbachian Boundary Event, recognised in most basins worldwide, and interpreted as a late pulse of the Central Atlantic Magmatic Province volcanism.

Thomas Munier et al.

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Thomas Munier et al.

Thomas Munier et al.

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Short summary
Clay minerals are witnesses of alteration conditions in continental environments. Lacking high-resolution data on clay minerals, this work highlights wet and semi-arid cycles at mid-latitude in the upper Sinemurian. The higher proportion of kaolinite in the upper part of the obtusum Zone and in the oxynotum zone indicates an increase in hydrolysis conditions in a warmer period confirmed by carbon and oxygen isotopes.
Clay minerals are witnesses of alteration conditions in continental environments. Lacking...
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