Heterogeneous oxygenation states in the Atlantic and Tethys oceans during Oceanic Anoxic Event 2

Abstract The Cenomanian–Turonian boundary (ca. 93.5 Ma) is marked by an episode of profound environmental change, including a major perturbation of the carbon cycle and an Oceanic Anoxic Event (OAE-2). Here, we present molybdenum (Mo) isotope variations within the OAE-2 interval for four sections from the western Tethys (Furlo and La Contessa) and the North-Atlantic (ODP site 1276 and DSDP site 367). The main target of this study is to investigate the extent of reducing conditions (truly global in extent or restricted to poorly-ventilated restricted deep basins), with particular reference to the relationship between the change in the oxygenation state of the ocean and the link to global perturbations of the carbon cycle recorded in carbon isotopes. All four sections show fluctuations in the redox sensitive trace metal (RSTE) distribution, suggesting rapid variations in local redox conditions, ranging from anoxic to euxinic. The RSTE enrichment factors (EFs) also suggest different depositional conditions and paleoceanographic processes in the western Tethys versus the North Atlantic. Whereas the North Atlantic sites show evidence of weak watermass restriction associated with the action of a particulate shuttle within the water column, the EFs of the Tethyan sections are characteristic of unrestricted marine systems. Mo isotopes show surprisingly negative values through the Tethyan sections. At the onset of OAE-2, an increasing trend in δ Mo 98 is observed, with values ranging from −0.6 to 0.6‰. During the second half of OAE-2, the δ Mo 98 curve shows a progressive shift towards more negative values. In the North Atlantic, δ Mo 98 signatures from ODP site 1276 show a similar behaviour as observed in the western Tethys. At DSDP site 367, Mo isotopes are generally heavier during OAE-2, fluctuating around an average value of 1.1‰. This is consistent with fully euxinic conditions and the black shales deposited may have recorded the seawater signature during OAE-2. The Mo isotope signature is lighter than the modern ocean value (2.3‰) and suggests a higher proportion of bottom water euxinia (∼10%). Although this estimate is greater than the euxinic fraction in the modern ocean, our results challenge the suggestion of a globally euxinic deep waters during OAE-2. The variations in Mo / TOC ratios within the North Atlantic and Tethys oceans are in good agreement with a general drawdown of Mo within the OAE interval, suggesting a general increase in anoxic conditions during OAE-2. Our results highlight the role of local parameters in the development of euxinia, even during period of widespread anoxia such as OAEs.