Hardground, Gap and Thin Black Shale: Spatial Heterogeneity of Arrested Carbonate Sedimentation During the Jenkyns Event (T-OAE) in a Tethyan Pelagic Basin (Gerecse Mts, Hungary)

Müller, Tamás and Price, Gregory D. and Mattioli, Emanuela and Leskó, Máté Zs. and Kristály, Ferenc and Pálfy, József (2021) Hardground, Gap and Thin Black Shale: Spatial Heterogeneity of Arrested Carbonate Sedimentation During the Jenkyns Event (T-OAE) in a Tethyan Pelagic Basin (Gerecse Mts, Hungary). GEOLOGICAL SOCIETY SPECIAL PUBLICATIONS. pp. 1-38. ISSN 0305-8719 (print); 2041-4927 (online)

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Abstract

The Jenkyns Event or Toarcian Oceanic Anoxic Event (T-OAE) was an episode of severe environmental perturbations reflected in carbon isotope and other geochemical anomalies. Although well studied in the epicontinental basins in NW Europe, its effects are less understood in open marine environments. Here we present new geochemical (carbon isotope, CaCO3, [Mn]) and nannofossil biostratigraphic data from the Tölgyhát and Kisgerecse sections in the Gerecse Hills (Hungary). These sections record pelagic carbonate sedimentation near the margin of the Tethys Ocean. A negative carbon isotope excursion of ~6‰ is observed in the Tölgyhát section, in a condensed clay and black shale layer where the CaCO3 content drops in association with the Jenkyns Event. At Kisgerecse, bio- and chemostratigraphic data suggest a gap in the lower Toarcian. The presence of an uppermost Pliensbachian hardground, absence of the lowermost Toarcian Tenuicostatum ammonite zone, and the condensed record of the Jenkyns Event at Tölgyhát, together with a condensed Tenuicostatum Zone and the missing negative carbon isotope anomaly at Kisgerecse implies arrested carbonate sedimentation. A calcification crisis and sea-level rise together led to a decrease in carbonate production and terrigenous input, suggesting that volcanogenic CO2-driven global warming may have been their common cause. In the Early Jurassic, a series of severe environmental perturbations such as global warming (Suan et al., 2010), a second-order mass extinction (Caruthers et al., 2014), ocean anoxia (Pearce et al., 2008), and calcification crisis (Trecalli et al., 2012) happened in the early Toarcian (~183 Ma), collectively referred to as Toarcian Oceanic Anoxic Event (T-OAE) (Jenkyns, 1988; Jenkyns, 2010) or recently coined as the Jenkyns Event (Müller et al., 2017; Reolid et al., 2020). This time interval was characterised by enhanced marine primary production leading to the appearance of organic-rich sediments in marine and lacustrine settings under anoxic-euxinic conditions (Jenkyns, 1988; Xu et al., 2017). However, some other authors point to a high primary production occurring before the event and to a phytoplankton blackout concomitant with it (Bucefalo Palliani et al., 2002; Mattioli et al., 2009). The Jenkyns Event is coincident with a characteristic negative carbon isotope excursion (CIE) with a magnitude of ~5‰ (Hesselbo et al., 2000; Hermoso et al., 2009; Suan et al., 2015). This negative CIE is present in different substrates, e.g. marine biogenic carbonate, micrite, and in marine and terrestrial organic matter, implying a major perturbation of the carbon cycle affecting the exogenic carbon reservoirs (Suan et al., 2008a; Hermoso et al., 2009; Suan et al., 2010; Bodin et al., 2010; Hesselbo and Pieńkowski, 2011; Müller et al., 2020b). Furthermore, this carbon isotope anomaly can be also traced at multiple localities around the world suggesting a global extent of this event (Gröcke et al., 2011; Caruthers et al., 2011; Suan et al., 2011; Izumi et al., 2012; Al-Suwaidi et al., 2016). A second-order mass extinction is also associated with the Jenkyns Event severely affecting the biosphere (Caswell et al., 2009; Danise et al., 2013; Caruthers et al., 2014), with a remarkable temporal coincidence with the emplacements of the Karoo-Ferrar large igneous province (LIP) (Pálfy & Smith, 2000). The large amount of 12C that triggered the negative CIE has been hypothesized to be originated from volcanic degassing of CO2 from the Karoo-Ferrar LIP and/or thermogenic methane release from coal sediments in the Karoo Basin due to sill emplacements (McElwain et al., 2005; Svensen et al., 2007). Alternatively, methane-hydrate dissociation from marine sediments (Hesselbo et al., 2000; Kemp et al., 2005), methane release from terrestrial wetlands and/or from permafrost (Them et al., 2017; Ruebsam et al. 2019; Krencker et al. 2019), and upwelling of 12C-rich bottom water and stratification of water-column (Küspert, 1982; Schouten et al., 2000) have been suggested as well. The negative CIE and CO2 release are also coincident with global warming and a rapid temperature rise by ~7°C (Bailey et al., 2003; Suan et al., 2010). The globally elevated temperature resulted in the acceleration of the hydrological cycle, increased continental runoff and nutrient input into the ocean, further increasing primary productivity (Cohen et al., 2004; Jenkyns, 2010; Percival et al., 2016; Kemp et al., 2020). This high primary productivity led to oxygen depletion of the seawater resulting in the development of anoxic or euxinic water masses, occasionally even spreading to the photic zone (van de Schootbrugge et al., 2005; Pearce et al., 2008; Ruebsam et al., 2018). Thallium isotope studies revealed that seawater oxygen levels started to drop gradually already at the Pliensbachian/Toarcian boundary and reached euxinic conditions during the Jenkyns Event (Them et al., 2018). This is coherent with enhanced primary productivity occurring from the base of the Toarcian, followed by enhanced thermohaline stratification promoting anoxia in the core of the event (Bucefalo Palliani et al., 2002; Mattioli et al., 2009). A biocalcification crisis simultaneous with the Jenkyns Event severely affected both carbonate platforms and pelagic carbonate factories (Mattioli et al., 2009; Trecalli et al., 2012; Ettinger et al., 2021; Krencker et al., 2020; Müller et al., 2020b). Brachiopod δ11B-pH data imply that seawater pH started to drop, in multiple steps, already after the Pliensbachian/Toarcian boundary, reaching the minimum (~7.2; a total drop of 0.4–0.5 in pH values) immediately before and during the Jenkyns Event, that suggests a significant global lowering of seawater pH. A remarkable, rapid rise in pH at the onset of the Jenkyns Event indicates that organic carbon production and burial increased the withdrawal of atmospheric CO2 simultaneously with the start of the negative CIE. Carbonate system modeling revealed that the pH decrease was also accompanied by a substantial drop in seawater carbonate saturation (Ω<1) (Müller et al., 2020a). In order to better assess the global extent of this chain of environmental perturbations related to the Jenkyns Event oceanic records would be necessary but little oceanic crust of Toarcian age is preserved. Therefore, pelagic successions with a depositional setting close to the open ocean have a key importance (Suan et al., 2018). So far, only a handful of such sections have been studied from the sedimentary record of the Panthalassan margin in Japan and Canada (Gröcke et al., 2011; Caruthers et al., 2011) and from the NW Tethyan margin from European sections (Jenkyns et al., 1991; Vető et al., 1997; Kafousia et al., 2011; Neumeister et al., 2015; Polgári et al., 2016; Arabas et al., 2017; Suan et al., 2018; Müller et al., 2017; 2020b). To augment this dataset and to improve our understanding of the local and regional differences in the processes and expression of the Jenkyns Event, here we present new high-resolution carbon isotope, CaCO3 and [Mn] geochemical data, nannofossil biostratigraphy and mineralogical information from two sections, Tölgyhát and Kisgerecse from the Gerecse Hills in north-central Hungary. These sections are biostratigraphically well-constrained (Géczy, 1984; 1985; Kovács, 2012) and exhibit lithological variations that reflect the early Toarcian environmental and biotic changes. Paleogeographically, they represent condensed, pelagic records of the early Toarcian in an ocean-facing setting with proximity to the open Tethys Ocean (Fig. 1). Our study aims to characterize the changes in pelagic carbonate sedimentation and their relation to possible ocean acidification and calcification crisis that, compared to the extent and effects of anoxia, are lesser known components of the Jenkyns Event.

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