Controls on redox-nutrient cycling in the Cretaceous greenhouse ocean: Insights from S isotope systematics


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Sabine.Kasten [ at ] awi.de

Abstract

Oceanic anoxic events (OAEs) were a frequent occurrence in the Cretaceous greenhouse ocean. Based on a variety of paleoredox indicators, euxinic water column conditions are commonly invoked for these OAEs. However, in a high resolution study of OAE3 deep sea sediments [1], revised paleoredox indicators suggest that euxinic conditions fluctuated with anoxic ferruginous conditions on orbital timescales. Building upon this, we here present new data for a continental shelf setting at Tarfaya, Morocco, that spans a period prior to, and during, the onset of OAE2. We again find strong evidence for orbital transitions from euxinic to ferruginous conditions. The presence of this distinct cyclicity during OAE2 and OAE3 in shallow and deep water settings, coupled with its occurrence on the anoxic shelf prior to the global onset of anoxia, suggests that these fluctuations were a fundamental feature of anoxia in the Cretaceous ocean. The observed redox cyclicity has major implications for the cycling of phosphorus, and hence the maintenance and longevity of OAEs. However, despite this significance, controls on the observed redox cyclicity are essentially unknown. Here, we utilize S isotope measurements (pyrite S and carbonate-associated S) from the deep sea and shelf settings to model oceanic sulphate concentrations across the redox transitions. Perhaps surprisingly, we find no evidence to suggest that ferruginous conditions arose due to extensive drawdown of seawater sulphate (as pyrite-S and organic-S) under euxinic conditions. Instead, S isotope systematics in the deep sea imply increased sulphate concentrations during ferruginous intervals. Based on these observations and other major element data, we infer that the redox cyclicity instead relates to orbitally-paced fluctuations in continental hydrology and weathering, linking the redox state of the global ocean to climate-driven processes on land. [1] März et al (2008) GCA, 72, 3703-3717.



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Not peer-reviewed
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Published
Eprint ID
34694
DOI 10.1180/minmag.2013.077.5.16

Cite as
Poulton, S. W. , Henkel, S. , März, C. , Urquhart, H. , Flögel, S. , Kasten, S. , Sinninghe Damsté, J. S. and Wagner, T. (2013): Controls on redox-nutrient cycling in the Cretaceous greenhouse ocean: Insights from S isotope systematics , Mineralogical Magazine - P: Goldschidt Abstracts 2013, 77 (5), p. 1991 . doi: 10.1180/minmag.2013.077.5.16


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