Changes in foraminiferal assemblages and environmental conditions during the T-OAE (Early Jurassic) in the northern Lusitanian Basin, Portugal

Abstract

The foraminiferal assemblages and other microfossils from the Rabaçal area (Maria Pares section) in the Lusitanian Basin (Portugal) provide new insights regarding the Toarcian Oceanic Anoxic Event (T-OAE). Complementary analysis of the δ13C, total organic carbon (TOC) and geochemical redox and palaeoproductivity proxies reflect environmental changes that affected the microfossil assemblages. Detailed analyses of foraminiferal assemblages (their diversity, abundance, morphogroups and lifestyle) and geochemistry led us to discern phases of the biotic crisis and subsequent stages of recovery. Environmental instability, which began in the lattermost part of the Polymorphum Zone, is marked by a lesser abundance of foraminifera and ostracods, increased opportunist foraminifera, fluctuations in TOC values and the enrichment factor of phosphorous (PEF), as well as the onset of a negative carbon isotopic excursion (CIE). The peak phase of the biotic crisis corresponds with the lowest values for foraminiferal diversity and abundance (including a barren sample for benthic microfossils). The abundance and diversity of echinoderms, brachiopods, dinoflagellate cysts and calcareous nannofossils dropped sharply during the peak phase. This biotic crisis also entails extinction of the Order Metacopina and a local disappearance of echinoderms and brachiopods. Anoxic conditions at sea bottom did not develop, given the almost continuous record of foraminifera and trace fossils, yet oxygen-depleted conditions occurred. The lower part of the Levisoni Zone contains thin nodular limestones corresponding to a tempestitic–turbiditic facies. In this context, the re-oxygenation of bottom waters was favoured, as indicated by the presence of Thalassinoides and the low values of TOC and redox-sensitive elements (Mo, Ni and U). The survival phase in the wake of the biotic crisis is characterized by greater diversity and abundance of foraminifera and echinoderms (holothuroids and ophiuroids). The lowest δ13C values are recorded during this phase, with a dominance of opportunist (Eoguttulina) and facultative opportunist (Paralingulina tenera) foraminifera. Finally, recovery after the biotic crisis is evidenced by high values for specialists and potentially deep infaunal foraminifera, indicating oxic conditions in the sediment.