The Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE-2) in North–Central Tunisia: Insights from the thin-shelled bivalves “filament” bioevents and global climate change

Abstract

The Oceanic Anoxic Event 2 (OAE-2) occurred during the Cenomanian–Turonian (C/T) boundary interval and represents a major disturbance that affected the ocean-atmosphere system and the global carbon cycle. Biostratigraphy, sedimentology and geochemical proxies present key elements to better understand the effects of OAE-2 on the climate and marine ecosystems. Two sections covering the Cenomanian–Turonian transition at the Serj-Bargou area of North–Central Tunisia have been studied for bio-sedimentary events characterization and isotope geochemistry response to quantify changes in paleo-systems during the OAE-2. Combined biostratigraphic and isotope data of the studied sections allowed to identify four bioevents: (1) extinction of Rotalipora in the uppermost Cenomanian Pre-Bahloul beddings during the build-up of the Cenomanian–Turonian Boundary Event (CTBE) δ 13 C positive shift, (2) Heterohelix and globular planktonic foraminifera bloom during the deposition of the Bahloul black shale, (3) filament event during δ 13 C return to normal values, and (4) first occurrence of Helvetoglobotruncana helvetica during the transition to the marlstones of the Annaba Formation. This study focuses on the characterization of filament accumulations in relation to paleo-system and climate changes. A petrographic study shows thin-shelled bivalves filaments bloom during the uppermost Cenomanian Pre-Bahloul beddings and atop Unit 3 (U3) of the Cenomanian–Turonian Bahloul Formation. Shell beddings have been interpreted as transgressive or highstand lag deposits associated with storms, or as primary biogenic concentrations. In this study, the several accumulations of “filaments” recorded within the uppermost Cenomanian Pre-Bahloul beddings are associated with the last occurrence of Rotalipora cushmani biozone and its sequence stratigraphic significance recognized as a relatively deeper-water equivalent of a transgressive systems tract. Detailed sedimentological studies have allowed us to identify the occurrence of a tempestite bedding and laminae of well-sorted tiny debris of bivalves within the uppermost Cenomanian Pre-Bahloul beddings. These observations provide evidence that bottom water conditions were periodically affected by weaker currents than those responsible for a thick tempestite bedding, indicating the action of currents on the bottom, such as small erosional surfaces or small ripples. The described bedding with an exceptional buildup of thin-shelled bivalves could be interpreted as an announcement of the main transgressive event with the occurrence of bottom currents. The abundance of filaments atop the Cenomanian–Turonian interval coincides with the return to a normal δ 13 C (2.64‰). However, the obtained δ 18 O curve suggests that the Bahloul deposition occurred under a warmer climate than during the Pre-Bahloul and Annaba Formations deposition. Climatic change to warmer conditions is the main factor behind the high-concentration of filaments atop the Cenomanian–Turonian Bahloul Formation. Filament events associated with isotopic signatures, mass extinction, introduction of new pelagic species and tempestite are globally correlated. In particular, the filament event indicates a sea-level rise in response to eustatic and climatic changes. • The exceptional buildup of filaments recorded within the Pre-Bahloul bed is interpreted as an announcement of the main transgressive event. • δ 18 O curve suggests that the Bahloul deposition occurred under a warmer climate. • Climatic change to warmer conditions is the main factor behind the high concentration of filaments atop the CTBE. • At a global scale, the filament event illustrates a sea-level rise in relation to eustatic and climatic changes.