Sedimentologic Expression of the Cretaceous OAEs in a Tropical Epicontinental Sea

Abstract:

The acidification and deoxygention of modern oceans are major environmental concerns to the international community. The effects of ocean acidification and deoxigention in the biogeochemical cycles of modern tropical oceans are poorly constrained mainly due to the lack of empirical and quantitative data. The Cretaceous World witnessed several period of potential ocean acidification and deoxygenation, which resulted from the rapid additions of volcanic derived CO2 to the atmosphere. The effects of ocean acidification and deoxygenation on the Cretaceous biogeochemical cycles are evidenced mainly by major global C-isotope anomalies. These anomalies parallel the occurrence of organic rich black shales as well as major decreases in the deposition of shallow marine carbonates worldwide. Here we use detailed C- and Sr- chemostratigraphy as well as published bioestratigraphic information and volcanic zircon U-Pb ages to precisely constrain the geochemical and sedimentologic expression of the Cretaceous OAES along a tropical epicontinental sea, the La Luna Sea. Our multi-pronged approach allows identifying the occurrence of several of the Cretaceous Oceanic Anoxic Events (OAEs) in carbonate units paleogeographically located along the northern most part of the La Luna Sea, i.e. Weissert-OAE-(Palanz and Rosablanca Formations), Faraoni-(Rosablanca Formation), AOE1a-(Paja and Fomeque Formations, Cogollo Group), OAE1c-(Cogollo Group), OAE2-(Cogollo Group), OAE3-(La Luna Formation). These events are preserved in highly euxinic - organic rich “black shales” successions deposited along the deepest part of the seaway at the Middle Magdalena Valley and Cundinamarca Basin; Weiser-OAE-(Lutitas de Macanal Formation), OAE1a-(Paja Formation, Fomeque Formation), OAE1C-(San Gil Formation). Regional changes in depositional settings and sedimentary facies preserving the different Cretaceous OAEs were likely the result of the combined action of regional changes in paleogeography and tectonic regimes and changes in the physicochemical conditions of the Cretaceous seawater along the tropical La Luna epicontinental seaway.