Continental weathering and climatic changes inferred from clay mineralogy and paired carbon isotopes across the early to middle Toarcian in the Paris Basin

Abstract

Lower Toarcian strata (Lower Jurassic) have been extensively studied with a view to understanding the oceanographic, climatic and biological processes that drove the Earth's system into an Oceanic Anoxic Event (OAE). For this time period, the evolution of the European marine seaways is now relatively well constrained owing to multiple geochemical studies, but investigations regarding climatic trends in the continental realm remain sparse. In the present study, we test the clay mineralogy as a continental climate-sensitive proxy in the well-documented Sancerre core (southern Paris Basin). We compare variations in the kaolinite content with pCO 2 fluctuations (derived from paired carbon isotopes; Δ 13 C = δ 13 C carb δ 13 C org), taking advantage of the detailed chemostratigraphic, palaeoenvironmental and sequence stratigraphy frameworks established for this core material. The results indicate a substantial decrease in kaolinite abundance at the end of the Pliensbachian, which is compatible with a long-term diminution in continental weathering and an inferred temperature decrease. The early Toarcian, prior to the carbon cycle perturbation and deposition of black shale facies, remained relatively cold with minima in both the proportion of kaolinite and reconstructed pCO 2. The mineralogical and geochemical responses across the prominent negative carbon isotope excursion (CIE) itself are not univocal. The first of four steps that compose the negative limb of the CIE at Sancerre is associated with decreased kaolinite and pCO 2 , and increased carbonate oxygen isotope ratios. Taken together, these trends are compatible with a transient cooling phase immediately preceding the onset of black shale deposition. Conversely, the subsequent steps are marked by substantial enrichment in kaolinite that matches increased osmium isotope ratios measured in Yorkshire, providing compelling evidence for rapid increases in continental weathering and riverine runoff forced by intensification of greenhouse conditions during the CIE. Relaxation in the intensity of continental weathering, as suggested by resumed low kaolinite abundance is seen immediately after the cessation of CO 2 input (after the fourth step of the CIE). The interval spanning the upper portion of the early Toarcian and the middle Toarcian records a subsequent long-term increase in the proportion of kaolinite synchronous with significant clay enrichment of the sediment. Continued greenhouse conditions, even after the recovery from the carbon isotope perturbation and "regional" black shale deposition, are likely related to sustained CO 2 emission by Karoo-Ferrar volcanism through the considered interval.

Globally enhanced mercury deposition during the end-Pliensbachian extinction and Toarcian OAE: A link to the Karoo–Ferrar Large Igneous Province

“…Because Bornholm preserves a near-terrestrial environment, the ammonite biostratigraphy applied to most other Toarcian sections cannot be utilized. However, the size and shape of the CIE preserved in the Bornholm wood is similar to those from Lower Toarcian sediments at other locations (Hesselbo et al, 2007;Hesselbo and Pieńkowski, 2011). Available palynology also indicates identification of Lower Toarcian sediments at this locality (Koppelhus and Nielsen, 1994).…”

Fungal decomposition of terrestrial organic matter accelerated Early Jurassic climate warming

“…A single noticeable peak of a dinoflagellate bloom (Fig. 2, Mechowo borehole) reflecting an intermittently high runoff and nutrient supply (indicated also by increase in phosphorus content27 and slightly preceding the onset of T-OAE, can be correlated with the marine plankton bloom heralding the T-OAE in the Paris Basin2631. The aggregate content of land palynomorphs (spores and pollen grains) in kerogen does usually not exceed 10–20%.…”

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin, Llanbedr (Mochras Farm) borehole, Wales

“…This early Toarcian global event has been the subject of numerous studies in various sedimentary basins, which have highlighted the important role of global warming, attested by a dramatic increase in the temperature of ocean waters and enhanced hydrolysis on continents (e.g. Jenkyns, , ; Raucsik & Varga, ; Dera et al ., ; Harazim et al ., ; Hermoso & Pellenard, ; Korte et al ., ; Müller et al ., ; Them et al ., ; Izumi et al ., ). By contrast, Pliensbachian deposits have been less studied, while this stratigraphic interval also shows significant climatic fluctuations (Korte et al ., ; Gómez et al ., 2016a; Price et al ., ).…”