Earth system changes during the cooling greenhouse phase of the Late Cretaceous: Coniacian-Santonian OAE3 subevents and organic carbon-rich versus organic carbon-poor deposition

The Coniacian-Santonian (C-S) was a time of differentiation in marine sedimentation, characterized by organic carbon (OC)-rich black shales and carbonates interpreted as the last oceanic anoxic event, OAE3, versus OC-poor white/reddish limestones, chalk, and claystones known as Cretaceous Oceanic Red Beds (CORBs). Based on compiled geochemical and isotope proxy data of more than 95 study sites and sections, two high-resolution global carbon isotope curves for C-S carbonate and organic matter (OM) were reconstructed based on statistical analysis and discriminated three main levels of short amplitude (around 0.5‰), yet globally recognizable, carbon isotope excursions. These excursions, each some 0.4 to 0.7 Ma in duration, are characterized by regionally restricted benthic anoxia and sea-level highstands that best explain the OM accumulation during the OAE3 subevents defined herein as OAE3a (late mid-Coniacian, ca. 86.9 Ma, Kingsdown Event), OAE3b (late mid-Santonian, ca. 85.0 Ma, Horseshoe Bay Event), and OAE3c (late Santonian to Santonian-Campanian Boundary Event, ca. 83.5 Ma). For a better understanding of the C-S climate evolution on a regional to global scale, a global compilation of δ ¹⁸O from benthic and planktonic foraminifers and bulk carbonate was conducted and tested for pCO ₂ trends based on Δ ¹³C curves. Thus, the C-S palaeoclimate can be divided into (1) a steady state phase of warm greenhouse climate during the Coniacian, followed by (2) a hot greenhouse during the early Santonian that might be consistent with the activation of the Central Kerguelen large igneous province (LIP), and (3) a longer-term cooling of the warm greenhouse climate from the mid-Santonian onwards. The mechanism controlling OC-poor versus OC-rich deposition can be attributed mainly to palaeoceanographic conditions such as water column oxygenation and circulation pattern changes during the C-S. OM-rich deposition is largely restricted to the low-latitude Atlantic and adjacent epeiric and shelf seas. Areas of enhanced oceanic circulation systems with a westward-directed Tethyan current, and regional eddies of water mass flow had negative feedback on OM accumulation and preservation during the C-S, which resulted in well-developed water column oxygen content. This, in turn, oxidized OM and led to deposition of OM-poor facies and CORBs in large parts of the Late Cretaceous oceans.