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

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, northwestern 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 through the entire upper Sinemurian. As in the succeeding Pliensbachian, the upper Sinemurian stratigraphic 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. In the north of the Paris Basin, the succession is less continuous compared to the Cardigan Bay Basin site, as the oxynotum zone and the upper raricostatum zone are either absent or highly condensed. The clay assemblages are dominantly composed of illite and kaolinite without significant stratigraphic 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. Following a slight negative carbon isotope excursion at the obtusum–oxynotum zone transition, a long-term decrease in δ13Corg from the late oxynotum–early raricostatum zones is recorded in the two sites and may precede or partly include the negative carbon isotope excursion of the Sinemurian–Pliensbachian Boundary Event, which is recognised in most basins worldwide and interpreted to signify a late pulse of the Central Atlantic Magmatic Province volcanism.

Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) in the Mochras Borehole, Cardigan Bay Basin, NW Wales

“…Therefore, I-S R1 in Mochras is interpreted to be derived from chemical weathering of illite (Deconinck et al, 2019). The coeval increase of these primary clay minerals, I-S R1 and kaolinite, indicate that during this period physical erosion dominated over soil chemical weathering (Deconinck et al, 2019;Munier et al, 2021). This is similar to what is observed for the latest Pliensbachian in Mochras (Deconinck et al, 2019).…”

“…The predominantly detrital character of these clay minerals has been confirmed by TEM scanning of Pliensbachian smectite minerals, which revealed the fleecy morphology and lack of overgrowth (Deconinck et al, 2019). Therefore, the alternations of smectite and kaolinite are interpreted to reflect palaeoclimatic signatures of a changing hydrological cycle, with a year-round wet climate evidenced by high K/I ratios, and a more monsoon-like climate with seasonal rainfall with high S/I (Deconinck et al, 2019;Hollaar et al, 2021;Munier et al, 2021) (See Fig. 3).…”

“…Hence, smectite forms predominantly in soil profiles under a warm and seasonally dry climate (Chamley, 1989;Raucsik & Varga, 2008), and kaolinite in a year-round humid climate (Chamley, 1989;Ruffell et al, 2002). Similarly, alternating intervals of kaolinite and smectite dominance were observed for the Late Sinemurian (Munier et al, 2021) and the Pliensbachian of Mochras (Deconinck et al, 2019). The predominantly detrital character of these clay minerals has been confirmed by TEM scanning of Pliensbachian smectite minerals, which revealed the fleecy morphology and lack of overgrowth (Deconinck et al, 2019).…”

The optimum fire window: applying the fire-productivity hypothesis to Jurassic climate states

“…(Storm et al, 2020), shows the broad negative carbon-isotope trend around the SPB and the positive carbon-isotope excursion (CIE) in the Late Pliensbachian. (b) The smectite/kaolinite (Sm/K) ratio reflects changes in the hydrological cycle; data from Deconinck et al (2019) and Munier et al (2021). Peaks in smectite indicate greater climatic aridity (Deconinck et al, 2019;Munier et al, 2021).…”

On the occurrence of rare nannoliths (calcareous nannofossils) in the Early Jurassic and their implications for the end‐Triassic mass extinction

“…2013; Munier et al . 2021). During the Early Jurassic the sediments of the Mochras core were located between palaeolatitudes of 35° and 40°N (Torsvik et al .…”