Lower Cretaceous Volcanism Impact on the Palaeoenvironmental Evolution of the Urgonian Carbonate Platform, France

Climatic conditions were characterised by a warmer and more stable global climate during the early Cretaceous period. These conditions conducted to the large extension of carbonate platforms, particularly in subtropical to equatorial latitudes. However, the development of this carbonate platform was punctuated by several crises that affected the atmospheric and marine global carbon cycle. One of these occurred during the late Early Aptian period, associated with a significant positive excursion in carbon isotopes. This event, known as the Oceanic Anoxic Event OAE1a, is recognised at global scale and is characterised by the global demise of carbonate platforms and the accumulation of black shale in deeper environments. Prior to the definitive drowning of the Urgonian carbonate platform during the OAE1a, some precursor episodes occurred during the Barremian-Aptian transition, during which marls strongly enriched in orbitolinids were deposited. The orbitolinids are generally associated with mesotrophic conditions and high nutrient inputs. This episode, known as the “Lower Orbitolina Beds”, suggests a significant climatic change, characterised by wetter and warmer conditions. This led to an increase in detritism and a change in carbonate production. The intensified volcanic activity associated with the onset of the Ontong Java large igneous province may have triggered this increase in warm and humid conditions. To confirm the relationship between volcanism and the deposition of the Lower Orbitolina Beds, mercury measurements were conducted in several sections of the Urgonian carbonate platform in the Vercors area. Mercury is commonly used as an indicator of volcanic activity. To confirm this volcanic signature, measurements were taken of tellurium, as this element is more abundant than mercury in volcanic gas emissions. Furthermore, this marker is less susceptible to the influence of organic matter than Hg. The Ecouges section, along with the Gorges du Nan and the Gorges du Frou sections, show significant increases in Mercury and Tellurium levels in the marly intervals and paleosoils associated with the Lower Orbitolina Beds. These indications of volcanism during the Barremian-Aptian interval confirm the connection with the onset of the Ontong Java Plateau and the eutrophication of the Urgonian Carbonate Platform as a precursor to Oceanic Anoxic Event 1a in shallow marine environments.