Abstract
Abstract. By studying the facies, geochemistry, and microfossil assemblages of the uppermost Pliensbachian and lower Toarcian of the Cueva del Agua section, I was able to appraise the impact of the Jenkyns Event in the eastern part of the South Iberian Palaeomargin (Western Tethys). Depleted oxygen conditions are envisaged for the Polymorphum–Serpentinum Zone boundary (lower Toarcian), represented by dark marls, precisely in a laminated interval. The decrease in the α diversity of foraminifera and ostracods, along with greater proportions of opportunists such as Lenticulina, Eoguttulina, and Cytherella just before the negative carbon isotopic excursion (CIE), would indicate a disturbance of the environmental conditions during the initial phase of the biotic crisis. The peak of the biotic crisis is characterized by an absence of trace fossils, increased organic matter content, an increased Mo∕Al ratio, and negative CIE and δ18O, as well as fewer specialist forms and more opportunists. This biotic crisis peak is related to oxygen-depleted conditions in the bottom waters and in the sediment pore water, while warming negatively affected microfauna – to the point of leaving a barren benthic horizon in the record. Recovery is evidenced by the occurrence of carbonate layers with hummocky cross-stratification and a decrease in organic matter content, the Mo∕Al ratio, and the trace fossil record. In terms of microfauna, the first phase of recovery shows highly abundant foraminifera, ostracods, and microgastropods, mainly opportunist forms. After the proliferation of opportunist forms, a second phase of recovery is marked by a progressive increase in α diversity.
Highlights
Research on environmental global events that affected past marine ecosystems offers us the chance to elaborate models for understanding ongoing worldwide changes
A perturbation of the carbon cycle indicated by a negative carbon isotopic excursion (CIE) affected marine environments (e.g. Jenkyns and Clayton, 1986; Sælen et al, 1996; Suan et al, 2010; Reolid, 2014a; Baghli et al, 2020) and land ecosystems
The main changes in geochemistry occur at the base of the Serpentinum Zone
Summary
Research on environmental global events that affected past marine ecosystems offers us the chance to elaborate models for understanding ongoing worldwide changes. The Toarcian oceanic anoxic event (T-OAE; Early Jurassic), known as the Jenkyns Event (Müller et al, 2017; Reolid et al, 2020a), is a very useful past analogue This global event was an abrupt palaeoenvironmental perturbation entailing marine transgression (Hallam, 1987; Pittet et al, 2014; Haq, 2018) coeval with a widespread deposition of black shales (Jenkyns, 1988; Bellanca et al, 1999; Röhl et al, 2001; McArthur, 2019), global warming (e.g. García Joral et al, 2011; Korte and Hesselbo, 2011; Suan et al, 2011; Danise et al, 2013; Them et al, 2017; Baghli et al, 2020; Piazza et al, 2020; Storm et al, 2020), and enhanced weathering Reolid et al (2020a) propose the use of the term T-OAE only when studying marine deposits holding evidence of oxygen-depleted conditions; the term Jenkyns Event (Müller et al, 2017) is more appropriate for the global changes that occurred during the early Toarcian (including the anoxic event)
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