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Articles) Was Extracted with a Tio Mass Spectrometer Clim. Past, 17, 1547–1566, 2021 https://doi.org/10.5194/cp-17-1547-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. 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, Émilia Huret6, and Philippe Landrein6 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 Correspondence: Thomas Munier ([email protected]) Received: 16 July 2020 – Discussion started: 10 September 2020 Revised: 19 May 2021 – Accepted: 28 May 2021 – Published: 21 July 2021 Abstract. Clay mineral and stable isotope (C, O) data are drolysing conditions likely coeval with warm conditions re- reported from the upper Sinemurian (Lower Jurassic) of sponsible for an acceleration of the hydrological cycle. In the Cardigan Bay Basin (Llanbedr–Mochras Farm borehole, the north of the Paris Basin, the succession is less continu- northwestern Wales) and the Paris Basin (Montcornet bore- ous compared to the Cardigan Bay Basin site, as the oxyno- hole, northern France) to highlight the prevailing environ- tum zone and the upper raricostatum zone are either absent mental and climatic conditions. In both basins, located at or highly condensed. The clay assemblages are dominantly similar palaeolatitudes of 30–35◦ N, the clay mineral assem- composed of illite and kaolinite without significant strati- blages comprise chlorite, illite, illite–smectite mixed lay- graphic trends, but a smectite-rich interval identified in the ers (R1 I-S), smectite, and kaolinite in various proportions. obtusum zone is interpreted as a consequence of the emer- Because the influence of burial diagenesis and authigene- sion of the London–Brabant Massif following a lowering of sis is negligible in both boreholes, the clay minerals are in- sea level. Following a slight negative carbon isotope excur- terpreted to be derived from the erosion of the Caledonian sion at the obtusum–oxynotum zone transition, a long-term 13 and Variscan massifs, including their basement and pedo- decrease in δ Corg from the late oxynotum–early raricosta- genic cover. In the Cardigan Bay Basin, the variations in tum zones is recorded in the two sites and may precede or the proportions of smectite and kaolinite are inversely re- partly include the negative carbon isotope excursion of the lated to each other through the entire upper Sinemurian. Sinemurian–Pliensbachian Boundary Event, which is recog- As in the succeeding Pliensbachian, the upper Sinemurian nised in most basins worldwide and interpreted to signify a stratigraphic distribution reveals an alternation of kaolinite- late pulse of the Central Atlantic Magmatic Province volcan- rich intervals reflecting strong hydrolysing conditions and ism. smectite-rich intervals indicating a semi-arid climate. Kaoli- nite is particularly abundant in the upper part of the obtusum zone and in the oxynotum zone, suggesting more intense hy- Published by Copernicus Publications on behalf of the European Geosciences Union. 1548 T. Munier et al.: Alternation of humid and arid periods in the upper Sinemurian 1 Introduction borehole (Storm et al., 2020), and in the southern Alps in Italy from shallow-water carbonate platforms to deep off- The Early Jurassic is characterised by major palaeogeo- shore environments (Masetti et al., 2017). This excursion co- graphical changes induced by the breakup of Pangaea. This incides with increasing proportions of two thermophilic paly- geodynamic evolution is accompanied by intense volcanic nomorph taxa, Classopollis classoides, a terrestrially derived activity corresponding to the Central Atlantic Magmatic pollen grain, and Liasidium variabile, a marine dinoflagellate Province (CAMP) beginning at the Triassic–Jurassic bound- cyst, suggesting that the obtusum–oxynotum zone transition ary ∼ 201:5 million years ago (Marzoli et al., 1999; McHone, was a warm and/or dry interval. Liasidium variabile, a reli- 2000; Davies et al., 2017), and it is likely responsible for able index for the upper Sinemurian in northwestern Europe the end-Triassic mass extinction (see e.g. Korte et al., 2019, (Brittain et al., 2010; van de Schootbrugge et al., 2019), may and references therein). The breakup of Pangaea led to the have invaded the Tethys Ocean from Panthalassa after the opening of the Hispanic and Viking corridors, connecting the opening of the Hispanic Corridor (van de Schootbrugge et Tethys Ocean to Panthalassa and the Arctic Ocean, respec- al., 2005). This species is particularly abundant in the oxyno- tively (Bjerrum et al., 2001; van de Schootbrugge et al., 2005; tum zone, and the name Liasidium Event is used to describe Damborenea et al., 2013; Porter et al., 2013). The disinte- the complex of environmental changes at this time (Hesselbo gration of Pangaea resulted in the formation of many sedi- et al., 2020). mentary basins, and palaeogeographical changes led to ex- Humidity is also a key parameter of climate, but it is still changes of water masses that triggered climate fluctuations poorly documented over this period. Palynological data are with colder intervals (Dera et al., 2011) over a prolonged focused on Classopollis pollen, which is very common in the greenhouse period (Chandler et al., 1992; Dera et al., 2009a, obtusum and oxynotum zones, whether in the Cardigan Bay 2015; Korte et al., 2015). (Wall, 1965; van de Schootbrugge et al., 2005), the Cleve- Reconstructions of seawater temperatures through the land (Riding et al., 2013), or the Lusitanian (Poças Ribiero et Early Jurassic are mostly deduced from δ18O measurements al., 2013) basins. of belemnite rostra and some other mollusc shells, no- The composition of clay mineral assemblages can be a re- tably oysters. For the late Sinemurian, oxygen isotope data liable climate indicator (Chamley, 1989; Ruffell et al., 2002; 18 18 (δ Ocarb and δ Obel) show increasing values over time Raucsik and Varga, 2008; Dera et al., 2009b) provided their (Dera et al., 2011), indicating cooler ocean temperatures as dominant detrital origin can be demonstrated. Clay mineral recorded, for example, in the Cleveland Basin (Hesselbo et assemblages may reflect the intensity of hydrolysis during al., 2000; Korte and Hesselbo, 2011) and the Asturian Basin pedogenic processes and runoff conditions on land masses, (Gómez et al., 2016). However, warmer conditions also seem and they thus specify humidity variations from the signal to have prevailed episodically, for example, during the oxyno- recorded in marine sedimentary series. In the upper Sine- tum zone (Riding et al., 2013; Hesselbo et al., 2020). murian, variations of clay mineral assemblages have been The carbon cycle also shows perturbations, with nega- studied on several outcrops and boreholes from the British 13 tive carbon isotope excursions recorded either by δ Ccarb Isles (Jeans, 2006; Kemp et al., 2005; Hesselbo et al., 2020) 13 or δ Corg. The best documented of these excursions is the and in the Montcornet borehole north of the Paris Basin Sinemurian–Pliensbachian Boundary Event (SPBE or S-P (Debrabant et al., 1992), but at low resolution, only for a Event), which is recognised in many areas, including, among short interval, or in successions affected by strong clay min- others the Cleveland Basin (Hesselbo et al., 2000; Jenkyns eral diagenesis. Here we attempt, through a study of detri- et al., 2002; Korte and Hesselbo, 2011), the Wessex Basin tal clay mineral assemblages and fluctuations in stable iso- in Dorset (Jenkyns and Weedon, 2013; Price et al., 2016; topes (C and O) of upper Sinemurian strata from the Llanbedr Schöllhorn et al., 2020a), the Cardigan Bay Basin in west- (Mochras Farm) and the Montcornet boreholes, to estimate ern Wales (van de Schootbrugge et al., 2005; Hesselbo et the intensity of chemical weathering and hydrolysis, as well al., 2013; Storm et al., 2020), the Lusitanian Basin (Duarte as the magnitude of carbon cycle changes. et al., 2014; Plancq et al., 2016), the Lombardian Basin and Trento platform (Franceschi et al., 2019), the Paris Basin (Peti et al., 2017; Bougeault et al., 2017), and the Central 2 Geological background High Atlas Basin of Morocco (Danisch et al., 2019; Mercu- zot et al., 2020). This negative excursion is recorded in car- During the Early Jurassic, the Paris and Cardigan Bay basins 13 13 bonate rocks (δ Ccarb), belemnite rostra (δ Cbel), and or- were located to the northwest of the Tethyan domain. This 13 ganic matter (δ Corg), including fossil wood. area corresponded to an archipelago of large islands inherited 13 Another δ Corg negative excursion was also first recog- from Caledonian and Variscan massifs (Thierry et al., 2000). nised in the upper part of the obtusum zone and through These continental masses, such as the London–Brabant Mas- the oxynotum zone in eastern England (Riding et al., 2013; sif, the Massif Central, the Armorican Massif, and the Welsh Hesselbo et al., 2020), and it is recorded in Dorset (south- High, were surrounded by an epicontinental sea (Fig. 1). An ern England; Jenkyns and Weedon, 2013), in the Mochras excellent sedimentary record of the Early Jurassic is pre- Clim. Past, 17, 1547–1566, 2021 https://doi.org/10.5194/cp-17-1547-2021 T. Munier et al.: Alternation of humid and arid periods in the upper Sinemurian 1549 Figure 1.
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