Carbon-isotope stratigraphy of Early Cretaceous (Urgonian) shoal-water deposits: Diachronous changes in carbonate-platform production in the north-western Tethys

Abstract

Carbonate platforms are highly sensitive ecological systems that typically show rapid and characteristic response modes to environmental and climatic changes acting both on a regional scale and global scale. A widely accepted hypothesis proposes that the Late Barremian stepwise establishment of the Urgonian carbonate platform on the Northern Tethyan shelf was related to a gradual change from predominantly humid towards more arid greenhouse conditions. This climate change resulted in the reorganisation of the carbonate platform ecosystem from heterozoan towards photozoan-dominated assemblages and a decrease of organic-matter burial in Tethyan and Boreal basins. In order to decipher the palaeoenvironmental and climatic boundary conditions of these major changes in neritic and pelagic settings, a precise chronostratigraphy of Urgonian carbonate platform evolution is needed. Here, we provide an integrated stratigraphic framework of Lower Barremian to Lower Aptian Urgonian carbonate platform sections (Cluses, Forclaz) located at the northern rim of the Tethys (Subalpine Chains, ESE France), with special focus on sedimentological analyses and high-resolution carbon-isotope stratigraphy. A characteristic Barremian–Aptian carbon-isotope pattern permits precise platform-to-basin correlation with cyclostratigraphic and ammonite-dated pelagic and hemipelagic Tethyan sections in the Vocontian Trough (Angles/Combe-Lambert/Glaise) and Umbria Marche Basin (Gorgo a Cerbara). Similar to Helvetic shoal-water settings, the carbonate platform in the Subalpine Chains experienced a gradual transition from heterozoan- to photozoan-dominated ecosystems. This biogenic pattern points to a gradual change of the carbonate platform after the so-called “Early Barremian crisis” towards an oligotrophic rimmed platform system (Urgonian Limestone Formation). According to the chronostratigraphy of Urgonian carbonate platform evolution in the Subalpine Chains established here, the installation of a photozoan-dominated rudist-rich platform ecosystem, referred to as Urgonian limestones sensu stricto, occurred at the Early–Late Barremian transition. The onset of changes in the carbonate production mode (“heterozoan” versus “mixed heterozoan–photozoan” versus “photozoan”) clearly predates the onset of similar lithological changes in the Helvetic realm by about 1.0 to 1.2Myr. The established chronostratigraphic framework points to a link between the change towards photozoan-dominated limestones and fading palaeoenvironmental perturbations related to the mid-Barremian event, which is associated with major faunal turnovers and the onset of cyclic black-shale deposition in the central Tethyan realm. Judging from palaeoenvironmental proxies including kaolinite, phosphorus and black marl-limestone ratios, oscillating predominantly humid and relatively arid greenhouse conditions modulated the observed pattern in Urgonian carbonate platform production mode and the varying frequencies of black-shale deposits in the Tethyan and Boreal realms. Transient blooms of the orbitolinid foraminifera Palorbitolina lenticularis at the onset of a marked Upper Barremian positive carbon-isotope shift are interpreted to reflect increasing nutrient influx, most probably related to accelerated hydrological cycling and/or sea-level rise. These strata do not represent an over-regional correlatable lithostratigraphic unit, but likely an expression of progressive Northern Tethyan environmental and oceanographic change possibly related to Ontong Java large igneous province volcanism in the prelude of oceanic anoxic event 1a (OAE).