Ecosystem recovery after the Early Jurassic T-OAE in the Châabet El Attaris section of the Tunisian Atlas

Abstract

The flooding of the Pliensbachian shelf of the North-South Axis of Central Tunisia during the early Toarcian favoured the development of organic-rich facies in the most subsiding areas, such as that represented by the Châabet El Attaris section. The lower Toarcian strata in this section is represented by organic matter-rich sediments (dark laminated marls with low carbonate content) and the negative carbon isotope excursion (CIE) of the Jenkyns Event. The relatively high TOC values, increase of redox sensitive elements, and the absence of benthic organisms confirm the adverse conditions for life at the sea floor (suboxic to anoxic conditions) during the polymorphum Biozone and lower part of the levisoni Biozone. This indicates that the Jenkyns Event is expressed in this setting as the so-called Toarcian Oceanic Anoxic Event (T-OAE). The diversity and abundance of the calcareous nannoplankton was very low during the early Toarcian.After the T-OAE, the decrease in TOC and the low values of the geochemical redox proxies indicate a re‑oxygenation of the bottom waters, parallel with an increase of the carbonate content of the facies. After the negative CIE, the record of benthic macroinvertebrates (such as brachiopods, bivalves, ophiuroids and echinoids) and trace fossils (Zoophycos, Thalassinoides, Planolites, Chondrites, and locally Trichichnus) confirms the re‑oxygenation of the sea floor with increased diversity and abundance from the upper levisoni Biozone and bifrons Biozone. Benthic foraminifera colonization of the sea floor is evidenced by assemblages dominated by infaunal taxa, specially opportunist forms, such as Lenticulina, and progressive increase of diversity and abundance that confirms oxic pore-waters from the levisoni-bifrons biozone boundary. Planktic organisms attest to a progressive recovery of environmental conditions in the photic zone with increasing diversity and abundance of calcareous nannoplankton and radiolarians from the uppermost part of the levisoni Biozone and the bifrons Biozone. A first pulse after the negative CIE is represented by placoliths of the genus Lotharingius (shallow dweller) interpreted as a renewed input of nutrients to surface waters. The subsequent dominance of muroliths, especially of Crepidolithus crassus (a deep dweller), suggests a successive restoration of the deep-photic zone communities. These observations point towards enhanced oxygen availability in deep waters and pore waters due to improved basin ventilation.Taphonomic features of foraminifera pointing to dissolution in the bifrons Biozone are related to a lowered pH below the redox boundary within the sediment. In the same way, the presence of ferruginous moulds of microfossils (gastropods, bivalves, brachiopods, foraminifera, sponge spicules and radiolarians) with framboidal texture, actually preserved as goethite but originally pyrite, confirm the presence of reducing conditions within the sediment below the redox boundary. The record of Trichichnus confirms the reducing conditions in the deepest infaunal environments during the bifrons Biozone and is consistent with the development of pyrite moulds of microfossils early in the diagenesis.