Regional to global correlation of Cenomanian-early Turonian sea-level evolution and related dynamics: New perspectives

Abstract

Major sea-level cycles occurred in the Cenomanian-early Turonian greenhouse world and impacted depositional conditions and ecosystems across the paleo-shelf seas. These sea-level cycles have been interpreted from various paleogeographic settings around the globe, such as the Western Interior Seaway (North America), the Proto-North Atlantic, Western Europe, and eastern Tethys (SE India). However, their drivers remain poorly understood and may include glacio-, aquifer-, thermo-, and/or tectono-eustasy. Uncertainties also persist regarding the timing, synchronicity, and magnitude of Cenomanian-early Turonian eustatic cycles. By combining palynological data from northern Africa (Gindi Basin, Egypt) with data available in the literature, a detailed synthesis of Cenomanian palynostratigraphy and sea-level dynamics is presented. Age-diagnostic spores, pollen, and organic walled dinoflagellate cysts (dinocysts) are correlated to global marine biozonation schemes, which provide a comprehensive biostratigraphic framework for the Cenomanian-early Turonian. Additionally, palynological data enable the identification of an early late Cenomanian Dinopterygium bio-event marked by the highest abundances of dinocysts. This bio-event can be correlated to the Neolobites ammonite bio-event and the Jukes-Browne Carbon Isotope Event. The bio-events stratigraphically constrain with a major transgression, which occurred in the early late Cenomanian, slightly preceding Oceanic Anoxic Event 2 (OAE2). Another major transgression spans the late Cenomanian-early Turonian, referred to the Plenus transgression bio-event, and consistent with the onset of the OAE2. Regional to global correlations indicate that these transgressive events reflect eustatic sea-level rises that can be recognized throughout the Tethys, Proto-North Atlantic, Europe, Western Interior Seaway, and India. These transgressions occurred within <1.0 Myr with modest magnitudes of 10–60 m. Rates of sea-level change has commonly been attributed to glacio-eustasy, which is however difficult to reconcile with a probably ice-free Cenomanian-early Turonian greenhouse world. Both transgressions coincide with phases of rising temperatures, whereby warming was most pronounced during the early late Cenomanian transgression. However, we can only speculate whether rising temperatures indicate the demise of polar glaciations. Eustatic processes, including tectono-eustasy, and to some extent aquifer- and thermo-eustasy, likely played a role in the sea-level rise during the early late Cenomanian and early Turonian. Environmental changes associated with the early late Cenomanian transgression may have triggered the onset of OAE2 possibly exacerbated by sluggish ocean circulation in a warming greenhouse world where sea ice formation was limited.