Sedimentary organic matter and δ13CKerogen variation on the southern Iberian palaeomargin (Betic Cordillera, SE Spain) during the latest Pliensbachian–Early Toarcian.

The Early Toarcian Oceanic Anoxic Event (T-OAE) is characterised by major palaeoenvironmental and palaeoceanographical changes (ocean anoxia, global warming, ocean acidification, among other), and a severe perturbation of the global carbon cycle. Although widespread oceanic anoxia was a significant control on the occurrence of organic-rich facies during the T-OAE in several locations of the northern Tethyan margin, the effects of anoxia, oceanic productivity, and dilution are poorly understood in other worldwide locations, such as the southern Tethyan and Iberian margins. In this study, we report new geochemical [total organic carbon (TOC), total sulphur (TS), total carbonate content (CaCO 3), δ13C from kerogen isolates (δ13C Kerogen)], palynofacies and thermal maturation data from the Upper Pliensbachian–Lower Toarcian successions in La Cerradura and Fuente Vidriera, External Subbetic domain, Betic Cordillera (SE Spain). The obtained results indicate that these successions have low TOC contents (~ 0.3 wt.%), with TOC reaching up to 0.46 wt.% around the Polymorphum–Serpentinum ammonite zone boundary in La Cerradura. δ13C Kerogen negative carbon isotope excursions (CIE) are observed at the base of the Polymorphum Zone, the Pliensbachian–Toarcian Event (Pl-Toa), and base of the Serpentinum Zone (T-OAE). Overall, the low maturity kerogen assemblages are dominated by terrestrial OM and with a minor marine component. Small increments in terrestrial OM, non-opaque phytoclasts, and palynomorphs are observed just before and during the T-OAE in the studied area. During the Early Toarcian, in the External Subbetic domain, accumulation and preservation of OM was low and with a significant terrestrial contribution, similarly to other locations around the southern margins of Iberia and Tethys. The dominance of the opaque phytoclast subgroup, the occurrence of Classopollis , and the relatively more positive δ13C of terrestrial kerogen suggest that OM was mostly sourced from an area characterised by a semi-arid climate (i.e. with pronounced arid periods) during most of the Polymorphum Zone. The T-OAE negative CIE is accompanied by an increase in terrestrial OM, with a slight increase in the non-opaque phytoclast subgroup and terrestrial palynomorph subgroup. The increase of non-opaque phytoclast and terrestrial palynomorphs are in line with the posited enhancements of the hydrological cycle and export of terrestrial OM into marine environments during the T-OAE. This study suggests a link between 13C fractionation during photosynthesis in C3 plants and climate during the Early Toarcian. This link suggests coupling of the water and terrestrial carbon cycles before and during the T-OAE. • Early Toarcian sediment successions of the External Subbetic are mainly organic matter lean. • The studied palynofacies assemblages suggest a dominantly semi-arid climate. • During the T-OAE, an increase in land plant-derived organic matter is observed. • A link between 13C fractionation during photosynthesis in C3 plants and climate during the Early Toarcian is suggested here. [ABSTRACT FROM AUTHOR]