The response of two Tethyan carbonate platforms to the early Toarcian (Jurassic) oceanic anoxic event: environmental change and differential subsidence

Chemostratigraphic analyses (87Sr/86Sr, d13Ccarb) of limestones from twoJurassic platform-carbonate sequences in Italy (Trento and Campania–Lucania Platforms) illustrate previously established trends found in pelagicsediments and skeletal carbonates from biostratigraphically well-calibratedsections elsewhere in Europe. Chemostratigraphic correlations between theplatform-carbonate successions and appropriate intervals from well-datedreference sections allow the application of high-resolution stratigraphy tothese shallow-water peritidal carbonates and, furthermore, elucidate the faciesresponse to the Early Toarcian Oceanic Anoxic Event (OAE). Lower Jurassic(Toarcian) levels of the western Trento Platform (Southern Alps, NorthernItaly) contain spiculitic cherts that appear where rising carbon-isotope valuescharacterize the onset of the OAE: a palaeoceanographic phenomenoninterpreted as driven by increased nutrient levels in near-surface waters.There is a facies change to more clay-rich facies at the level of the abruptnegative carbon-isotope excursion, also characteristic of the OAE, higher in thesection. The Campania–Lucania Platform (Southern Apennines, SouthernItaly) records a change to more clay-rich facies where carbon-isotope valuesbegin to rise at the beginning of the OAE but the negative excursion, higher inthe section, occurs within oolitic facies. Although, in both examples, the EarlyToarcian OAE can be recognized by a change to more clay-rich lithologies, thisfacies development is diachronous and in neither case did the platform drown.Although the Trento Platform, in the south-west sector studied here, wasadversely affected by the OAE, it did not drown definitively until LateAalenian time; the Campania–Lucania Platform persisted throughout theJurassic and Cretaceous. Differential subsidence rates, which can be calculatedusing comparative chemostratigraphy, are identified as a crucial factor in thedivergent behaviour of these two carbonate platforms: relatively fast in the caseof the Trento Platform; relatively slow in the case of the Campania–LucaniaPlatform. It is proposed that where water depths remained as shallow as a fewmetres during the OAE (Campania–Lucania Platform), dissolved oxygen levelsremained high, nutrient levels relatively low and conditions for carbonatesecretion and precipitation remained relatively favourable, whereas morepoorly ventilated and/or more nutrient-rich waters (Trento Platform) adverselyinfluenced platform growth where depths were in the tens of metres range. Thestage was thus set for drowning on the more rapidly subsiding western marginof the Trento Plateau and a pulse of oolite deposition post-dating the OAE wasinsufficient to revitalize the carbonate factory.