Nanofossil Evidence of the Early Toarcian Cretaceous Oceanic Anoxic Event

The Early Toarcian Oceanic Anoxic Event in the External Subbetic (Southiberian Palaeomargin, Westernmost Tethys): Geochemistry, nannofossils and ichnology

Authors:

Reilid et al

Abstract:

The analysis of macroinvertebrates, trace fossils, calcareous nannofossils and geochemistry from Pliensbachian to Lower Toarcian deposits of the Subbetic (Southern Spain) allows us to interpret the incidence of the Toarcian Oceanic Anoxic Event (T-OAE) in the boundary between the Tethys and the Hispanic Corridor. Trace fossils from the marl-limestone rhythmite (Upper Pliensbachian to Lower Toarcian) indicate an oxygenated bottom. A dark marly interval occurs above a fossil-rich condensed surface with a negative excursion of δ18O, interpreted as evidence of the Early Toarcian transgression and a climate warming. The carbon cycle perturbation is recorded by decreasing CaCO3 content and the negative excursion of δ13C at the base of the Serpentinum Zone, NJT 6 nannofossil Zone. The barren interval for nektonic and benthic organisms at the beginning of the Serpentinum Zone confirms the oxygen restricted conditions in the sea-floor. Environmental conditions were unfavourable for the development of calcareous nannoplankton, as attested by the drop in absolute abundances. The deep photic zone seemed to be particularly hostile to phytoplankton, as shown by the decrease in relative abundance of the deep-dweller Mitrolithus jansae, related to shoaling of the oxygen minimum zone in the water column. Increasing values of redox sensitive elements and total organic carbon, as well as the record of pyrite framboids < 5 μm size, confirm the anoxic conditions (probably euxinic conditions in any moment) at the base of the dark marly interval (NJT 6). The return to oxic or dysoxic conditions is confirmed by the record of fossil-poor marls with scarce macroinvertebrates and microfossils, as well as the recovery of trace fossils mainly (low-oxygen tolerant Chondrites tracemakers). Increasing abundance of calcareous nannofossils and deep-dweller M. jansae indicate ameliorated environmental conditions in the water column. Geochemistry supports the recovery of carbonate production (increasing CaCO3) and oxygen availability (decrease of TOC and redox sensitive elements).