Carbon isotope evidence for sedimentary discontinuities in the lower Toarcian of the Lusitanian Basin (Portugal): Sea level change at the onset of the Oceanic Anoxic Event

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE), Early Jurassic, was an episode of rapid warming and deep perturbation of the carbon cycle, as suggested by the large carbon and oxygen isotope excursions recorded by various carbonate and organic materials of this age. Previous studies have shown that strata deposited immediately below the T-OAE are marked by widespread discontinuities, but their duration and synchronicity remain uncertain. In this study, we use the carbon isotope profiles of twelve sections from the Lusitanian Basin, Portugal, to correlate shallower and deeper settings and identify the durations and possible causes of these discontinuities. The obtained correlation suggests the existence of three major discontinuities. The first occurs close to the Pliensbachian–Toarcian boundary and locally corresponds to an interval of condensation due to rapid transgression. The second and most marked discontinuity occurs in the mid-upper part of the Polymorphum ammonite Zone. Depending on the location in the Lusitanian Basin, this discontinuity is interpreted to reflect either marine erosion (in deeper parts of the basin) or subaerial exposure (in the shallow parts of the basin), both induced by a high-amplitude sea-level fall. In intermediate parts of the basin, this discontinuity is interpreted as a transgressive ravinement surface due to subsequent sea-level rise. This latter transgression induced the formation of a third discontinuity of sedimentary condensation (uppermost Polymorphum Zone) that corresponds to the interval of highest rate of sea-level rise just before the onset of the T-OAE. The comparison of the carbon isotope record of the Lusitanian Basin with that of other European basins indicates that these discontinuities are present on a wide paleogeographical scale within the western Tethys, and correspond to marked changes in seawater temperatures and CO2 levels, implying a control by high-amplitude, likely glacio-eustatic sea-level changes.