Pliensbachian environmental perturbations and their potential link with volcanic activity: Swiss and British geochemical records
Introduction
The Early Jurassic interval witnessed important palaeoceanographic, climate and environmental changes, which were triggered by major tectonic processes leading to palaeogeographic reorganisation, volcanic activity, and associated increases in atmospheric CO2 (McHone, 1996; Marzoli et al., 1999; Jourdan et al., 2008). The supercontinent Pangaea started to break up during the latest Triassic leading to the opening of the Central Atlantic Ocean. As a consequence, numerous epicontinental basins were formed through “Europe”, and seaways opened connecting “Europe” to Panthalassa and the Arctic through the Hispanic Corridor (latest Sinemurian-early Pliensbachian) (Aberhan, 2001; Van de Schootbrugge et al., 2005; Korte and Hesselbo, 2011) and Viking Strait (Early Jurassic, less well dated) (Bjerrum et al., 2001; Surlyk, 2003; van de Schootbrugge et al., 2005). Furthermore, the Central Atlantic Magmatic Province (CAMP), which triggered the mass extinction event at the end of the Triassic, remained active during the Early Jurassic (Cohen et al., 1999, Cohen et al., 2004; Cohen and Coe, 2002; Olsen et al., 2003; Whiteside et al., 2007; Kuroda et al., 2010; Rühl et al., 2016). The Karoo-Ferrar Large Igneous Province, which impacted life and the environment during the early Toarcian may have also started earlier during the Pliensbachian (Pankhurst et al., 2000; Jourdan et al., 2008; Caruthers et al., 2014; Ivanov et al., 2017). In addition, carbon storage patterns (Suan et al., 2010; Silva et al., 2011; Caruthers et al., 2014), possibly glacio-eustatic dynamics (Suan et al., 2010; Krencker et al., 2019; Ruebsam et al., 2019), and orbital parameters (Martinez and Dera, 2015; Rühl et al., 2016) modulated the climate changes during this interval.
In this complex and changing world, the Toarcian Oceanic anoxic event (T-OAE ~183 myr), has been extensively studied (e.g., Jenkyns, 1988; Philippe and Thevenard, 1996; Wignall et al., 2005; Fantasia et al., 2018a, Fantasia et al., 2018b, Fantasia et al., 2018c; Suan et al., 2018 and references therein). However, an increasing number of studies highlight important climate and environmental changes prior to this event, which remain less well understood, including the Sinemurian-Pliensbachian boundary event, the upper Pliensbachian positive carbon isotopic event (CIE), the end-Pliensbachian spinatum negative CIE, and the Pliensbachian-Toarcian boundary event (e.g., Hesselbo et al., 2002; Suan et al., 2008, Suan et al., 2010; Porter et al., 2013; Bodin et al., 2016; Rühl et al., 2016; Bougeault et al., 2017; Peti et al., 2017; Deconinck et al., 2019; Baghli et al., 2020; Mercuzot et al., 2020; Schöllhorn et al., 2020).
We therefore studied the 8.7 Ma Pliensbachian interval (Rühl et al., 2016) prior to the early Toarcian within the lower part of the hemipelagic succession of the Breggia Gorge (Ticino, southern Switzerland, Fig. 1), with the aim to better assess the environmental change during this period and its potential causes (Schöllhorn, 2019). We selected the Breggia section because it constitutes a well-dated hemipelagic/pelagic setting close to the open Tethys Ocean (Fig. 1). We analysed thin sections and performed bulk-rock analyses to study the depositional context. We furthermore studied the carbon-isotope composition to recognise perturbations in the carbon cycle and the oxygen-isotope record, clay mineralogical analyses and phosphorus content, to study the eventual climate and environmental impact.
As volcanic emissions, coal combustion and intrusion-related metamorphism of organic-rich are the largest natural sources of mercury (Hg) in the biosphere (Pyle and Mather, 2003; Pirrone et al., 2010; Sanei et al., 2012; Grasby et al., 2015) Hg measurements have frequently been used to trace phases of intensified volcanism (Sanei et al., 2012; Grasby et al., 2015; Percival et al., 2015; Font et al., 2016; Thibodeau et al., 2016; Fantasia et al., 2018c; Sabatino et al., 2018). We decided therefore in addition to the aforementioned analyses to carry out the first Hg records spanning through the entire Pliensbachian in order to evaluate the occurrence of enrichments and determine their relation to late CAMP and early Karoo-Ferrar volcanic activity. We compared the results obtained on the Breggia section with a record that we measured along the Dorset coast (southern England).
Section snippets
Breggia section
The Breggia section is located in southern Switzerland (Canton Ticino), 16 km to the south of Lugano (45°86′44.78″N, 9°02′11.40″E; Fig. 1). The sediments of this section were deposited in the Lombardian Basin and more particularly in the Monte Generoso Subbasin (Winterer and Bosellini, 1981) along the southern Tethyan passive margin.
The Breggia section has attracted the attention of many geologists because of its continuity (Sinemurian-Cenomanian) (e.g., Stockar, 2003), of deposits
Material and methods
285 samples were collected at a 75-cm average spacing for carbon and oxygen-isotope measurements in the Breggia section. In addition, between 93 and 104 samples were analysed for bulk-rock and clay mineralogy, mercury and phosphorus content. Calcareous nannofossils were studied in 31 samples in selected intervals of the Breggia section in order to add precision on biostratigraphy, and 26 thin sections were prepared. Mercury analyses were performed on 108 samples of the Dorset section. Organic
Breggia section
The geochemical results are described for the Pliensbachian samples. The Toarcian sediments have already been described and interpreted by Fantasia et al., 2018a, Fantasia et al., 2018b.
Biostratigraphy of the Breggia section
The Breggia section was precisely dated by ammonites (Wiedenmayer, 1980) and for the uppermost Pliensbachian and Toarcian by nannofossils (Fantasia et al., 2018a). However, the Sinemurian-Pliensbachian boundary is not easily detectable and therefore some additional nannofossil analyses were performed. Nannofossils are scarce in general but one sample at −22.5 m contains Similiscutum spp. indicating an early Pliensbachian age (Mattioli and Erba, 1999). Thus the sediments corresponding to the
Conclusions
A set of geochemical analyses was performed on Pliensbachian hemipelagic sediments in the Breggia Gorge (southern Switzerland) close to the open Tethys (Schöllhorn, 2019). Negative carbon isotope excursions are recorded near the Sinemurian/Pliensbachian boundary, in the davoei, margaritatus, and spinatum Zs., and possibly near the Pliensbachian-Toarcian boundary. These events coincide with environmental conditions characterised by higher temperatures and increased runoff bringing in more
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered: as potential competing interests: this paper is part of a PhD thesis deposited in the Library of the University of Lausanne.
Acknowledgements
We would like to acknowledge the logistical and financial support of the University of Lausanne which financed the PhD thesis at the origin of this paper (Schöllhorn, 2019). We thank both Tiffany Monnier and Jean-Claude Lavanchy for their assistance in the laboratories, Alicia Fantasia and Yoann Chevalier for their help in the field. We thank the Cantonal Natural Museum of Lugano, Rudolf Stockar and Marco Antognini for the sampling permission in the Breggia gorges. Furthermore, we would like to
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