Carbonate platform demise and recovery at the Toarcian Oceanic Anoxic Event : high-resolution data from the Adriatic Carbonate Platform

Abstract

The sedimentological, stratigraphic, and geochemical signature of the Toarcian Oceanic Anoxic Event (OAE) on the Bahamian-style, Adriatic Carbonate Platform of Southern Europe is examined here. New stratigraphic sections correlated to previously published data show that a healthy and diverse metazoan carbonate factory characterized the flat-topped platform prior to the onset of the OAE. Carbon isotopes of marine organic matter and bulk carbonate, as well as trace element data (V, Ni, Mo, Mn, Hg) were used to identify the stratigraphic location of the OAE on the outer platform. A negative organic carbon isotope excursion of -2.2‰ coincident with enrichments in Mo and Fe was used to correlate to time equivalent strata in both the inner platform and adjacent basin. The onset of the OAE heralded a carbonate factory collapse on the outer platform that was driven by deoxygenation, eutrophication, and possibly ocean acidification. At the same time, a shallow intrashelf basin developed, while the adjacent deep-water basin accumulated black shales. The cessation of carbonate production throughout the OAE yielded a significant drowning unconformity on the outer platform as sea-level continued to rise, while restricted, organic-rich mudstones accumulated on the inner platform under a euxinic regime. Recovery of carbonate production is characterized mainly by calcitic fauna and ooids. By the Middle Jurassic, ooids generated on the outer margin spilled into the adjacent basin forming a thick deep-water oolitic fan, and they prograded to the inner platform, reestablishing a flat-topped carbonate platform.