Abstract
The Oceanic Anoxic Event 2, at the Cenomanian-Turonian boundary (∼93.9 Ma), was an episode of widespread burial of organic matter in marine sediments, underlined by a positive carbon-isotope (δ13C) excursion observed worldwide. Within this episode of O2-depleted conditions, a short interval of cooling, termed as the Plenus Cold Event, has been recorded in many sites and sections in the northern hemisphere (Tethyan domain, Western Interior Seaway, proto-North Atlantic Ocean). But, its record and its impact on the biogeochemical cycles of carbon and nitrogen in the southern part of Central Atlantic Ocean has not been explored yet. Here, we present a detailed geochemical study of the Deep Sea Drilling Project site 367 (Cape Verde) based on a compilation of previous and new data of carbon and nitrogen isotope signals as well as trace element concentrations. The aim of this study is to better constrain the evolution of oxygenation in the water column and the associated changes in nitrogen cycle before and during the Oceanic Anoxic Event 2 in order to understand the paleoceanographic and environmental consequences of the Plenus Cold Event at one of the deepest site of the Central Atlantic Ocean. Our new dataset improves the resolution of the δ13C curve for this site, and we propose a new chemo-stratigraphic frame of the carbon excursion allowing for a better identification of the short-term negative carbon isotope excursion associated to the Plenus Cold Event. The detailed evolution of redox-sensitive proxies (Mo, U, V, Fe, Cu, Ni enrichments and Corg/Ptotal) and isotopic signals (δ13Corg and δ15Ntotal) evidence that this deep site was impacted by this cooling event. While anoxic conditions prevailed in bottom waters before and during the onset of the Oceanic Anoxic Event 2 characterized by euxinic NH4+-rich water column, this cooling event was accompanied by reoxygenation of the water column, which had affected the behavior of the redox-sensitive elements and caused changes in nitrogen biogeochemical cycling.
Highlights
The Cenomanian-Turonian boundary (CTB, ∼93.9 Ma ago) is marked by a major environmental disturbance namely the Oceanic Anoxic Event 2 (OAE-2; Schlanger and Jenkyns, 1976)
The widespread deposition of these black shales is associated to a short-term (
In order to have the highest resolution for geochemical signals, the results obtained in this study (n 42) have been compiled with those of previous studies (n 17 from Kuypers et al, 2002, Kuypers et al, 2004 and from H-J., Brumsack, unpublished data, for selected major and trace elements (Fe, Mn, P, Ni, Cu, Co, and Zn) on the same 17 samples; n 29 from Forster et al, 2007 and n 14 from Westermann et al, 2014)
Summary
The Cenomanian-Turonian boundary (CTB, ∼93.9 Ma ago) is marked by a major environmental disturbance namely the Oceanic Anoxic Event 2 (OAE-2; Schlanger and Jenkyns, 1976). The positive CIE, resulting from preferential burial of 12C-rich organic carbon in marine sediments and observed in many marine DSDP-ODP-IODP sites and continental sections and cores, is used to define the extent of the biogeochemical OAE-2 (Gale et al, 1993; Kuypers et al, 2002) This increased sedimentary organic matter (OM) burial rate is usually argued to reflect increasing nutrient delivery at basin scales leading to increasing primary productivity subsequently transferred to the sedimentary record by deposition of organicrich deposits. As a direct consequence of vigorous and sustained primary productivity, a profound deoxygenation of the water column in many parts of the world ocean, but well expressed in the protoNorth Atlantic and in the Central Atlantic
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