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Lookup NU author(s): Dr Tatiana Goldberg, Professor Simon Poulton, Professor Thomas Wagner
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During the Cretaceous greenhouse, episodes of widespread ocean deoxygenation were associated with globally occurring events of black shale deposition. Possibly the most pronounced of these oceanic anoxic events (OAE’s) was the Cenomanian–Turonian OAE2 (∼94 Ma). However, although certain redox sensitive trace metals tend to be preferentially sequestered in sediments deposited under anoxic conditions, with Mo drawdown being specifically prone to euxinic settings, these elements are generally somewhat depleted in sediments deposited during OAE2. Tounderstand the driving factors responsible for this depleted trace metal drawdown, wehave studied a low latitude section from the proto-North Atlantic Ocean (Tarfaya S57), where existing biomarker and iron–sulphur data point to a dominantly euxinic water column, with periodic transitions to ferruginous (Fe-rich) water column conditions. Weutilise a variety of redox proxies (Fe-speciation, redox sensitive trace metals and Mo isotopes), which, incombination, allows us to evaluate the detailed nature of ocean redox conditions and hence controls on trace metal drawdown. The results suggest that seawater δ98Mo values may have ranged between ∼0.6 and 1.1 during OAE2, likely connected to changes in the local Mo reservoir as a consequence of low and probably heterogeneous concentrations of Mo in the ocean. The very low Mo/TOC ratios at Tarfaya and elsewhere in the proto-North Atlantic may support a model in which deep-water circulation was partially restricted within and between the North Atlantic and other ocean basins. Wepropose that the combination of a low and possibly heterogeneous δ98Mo of seawater Mo, together with low Mo/TOC ratios, points to a large decrease in the global oceanic Mo reservoir during OAE2, reflecting a major global scale increase in Mo drawdown under persistent euxinic conditions.
Author(s): Goldberg T, Poulton SW, Wagner T, Kolonic S, Rehkämper M
Publication type: Article
Publication status: Published
Journal: Earth and Planetary Science Letters
Year: 2016
Volume: 440
Pages: 81-91
Print publication date: 15/04/2016
Online publication date: 22/02/2016
Acceptance date: 02/02/2016
ISSN (print): 0012-821X
ISSN (electronic): 1385-013X
Publisher: Elsevier BV
URL: http://dx.doi.org/10.1016/j.epsl.2016.02.006
DOI: 10.1016/j.epsl.2016.02.006
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