Spatio-temporal variability in microfossil and geochemical records of Cenomanian-Turonian oceanic anoxic event-2: a review

Abstract

The Cenomanian-Turonian boundary oceanic anoxic event (OAE-2; Bonarelli event; ∼94 Ma) is associated with a species turnover in the marine calcareous microfossil record, widespread marine anoxia, black shale deposition and positive δ¹³C excursions. This study reviews 141 CTBE sites globally, aiming to understand micropaleontological, geochemical, and sedimentological expressions of OAE-2. There is a clear palaeogeographic and palaeobathymetric heterogeneity in the development of OAE-2 marine anoxia. A majority of the documented OAE-2 sites are from deep marine environments. The calcareous nannoplankton and benthic foraminifera record a diversity decline, while planktic foraminifera shows community level shifts and no major mass extinction. The variability of total organic carbon in OAE-2 sediments across sites (<1 to >10 wt. %) and the δ¹³C profiles (diachronous) have been attributed to different mechanisms of anoxia development. The increased primary “productivity model” gains support from productivity proxies (e.g., Ba, P, Cu, Ni), redox-sensitive elements (e.g., Mn, Mo, U, V, As), and eutrophic genera (e.g., benthic foraminifera Gabonita spp., calcareous nannofossils Biscutum spp. and Zeugrhabdotus erectus). The enhanced organic carbon “preservation model” in stratified (semi) restricted basins gains support from sites with lower enrichment of redox-sensitive elements and oligotrophic flora and fauna in the OAE-2 records. Geochronology of the Caribbean and the High Atlantic Large Igneous Province events (CLIP and HALIP) mark them as likely triggers of the OAE-2 related global perturbation of marine biogeochemistry. Volcanic triggers may have caused climate warming, altered hydrological cycles, enhanced continental weathering, shifts in ocean circulation, and nutrient flows. Addressing knowledge gaps, further research is urged, utilizing innovative proxies, and exploring underrepresented depositional systems to comprehensively understand OAE-2 onset and biotic crisis.