Earth system changes during the cooling greenhouse phase of the Late Cretaceous: Coniacian-Santonian OAE3 subevents and fundamental variations in organic carbon deposition.

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Date: June 2022
From: Earth-Science Reviews(Vol. 229)
Publisher: Elsevier Science Publishers
Document Type: Report
Length: 493 words

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Keywords OAE3; Black shales; CORB; Climate change; Palaeoceonography; Isotopes geochemistry Highlights * Widespread anoxia was regionally restricted to the equatorial Atlantic and adjacent epicontinental seas. * Two high-resolution global [delta].sup.13C.sub.carb and [delta].sup.13C.sub.org stacked curves for the C-S are presented. * We distinguished three short amplitude, yet globally recognizable, CIE defined as OAE3a, OAE3b, and OAE3c. * Global high-resolution [delta].sup.18O curves showed three phases of climate state for the C-S. * Palaeoceanographic and circulation patterns played a role in OC-poor versus OC-rich deposition. Abstract 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 0.50/00), 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 [delta].sup.18O from benthic and planktonic foraminifers and bulk carbonate was conducted and tested for pCO.sub.2 trends based on [DELTA].sup.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. Author Affiliation: (a) Geology Department, Faculty of Science, Minia University, 61519 Minia, Egypt (b) Department of Geology, Faculty of Earth Sciences, Geography and Astronomy, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria * Corresponding author. Article History: Received 19 August 2021; Revised 5 April 2022; Accepted 6 April 2022 Byline: Ahmed Mansour [ahmedmans48@mu.edu.eg] (a,*), Michael Wagreich [michael.wagreich@univie.ac.at] (b)

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Gale Document Number: GALE|A703357067