Thibault, Nicolas Rudolph4; Ruhl, Micha4; Ullmann, Clemens Vinzenz4; Korte, Christoph4; Kemp, David2; Hesselbo, Stephen3
1 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet2 Open University3 University of Exeter4 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet
The early Toarcian (~183 Ma ago) was characterized by massive carbon burial and a pronounced negative carbon-isotope excursion (CIE) in marine carbonate and marine and terrestrial organic matter. These features along with the high abundance of redox sensitive trace metals in that interval led to the recognition of a major oceanic anoxic event (OAE). More recently, an earlier companion of the early Toarcian CIE has been documented at the Pliensbachian/Toarcian (Pl/To) boundary in sections of NW Europe, but its expression in the sediment and possible causes are less constrained. One of the most studied areas for this interval is the Cleveland Basin, UK, which is well-studied for litho-, bio- and chemostratigraphy. Here, we present a new dataset of high-resolution element data produced by hand-held X-ray fluorescence analysis to test for the expression of redox-sensitive trace metals and detrital elements across the late Pliensbachian to middle Toarcian of the Cleveland Basin. The Pl/To boundary CIE is associated with low Total Organic Carbon (TOC<2%), a high degree of pyritization (S/Fe) and enrichments in Cu, Zn and Ni, which together suggest that water-mass restriction and anoxia occurred in this interval, probably with periodical re-oxygenation events that prevented massive deposition of organic matter. Trends from redox-sensitive elements such as Fe, V, Mo and U are in agreement with previous findings and scenarios of basin restriction across and after the early Toarcian OAE (McArthur et al., 2008). An interval of maximum enrichment in these elements immediately after the CIE is a feature very similar to the recent observations of Hermoso et al. (2013) in the Paris Basin. This suggests a similar tempo of regional anoxia and black shale deposition in NW Europe. These results also shed light on the behaviour of elements associated with organic matter and the sulphur cycle such as Ni, Cu, Zn and As. Cu is well-correlated to the TOC whereas As shows an enrichment in the interval of black shale deposition after the CIE and two distinct correlation lines with sulphur, one before and during the event, and one after the CIE. This suggests that more As could be incorporated in pyrite in the Cleveland Basin after the CIE, because the availability of sulphur was more limited after the global anoxia. Therefore, these data reinforce the interpretation of localized water-mass restriction and anoxia in NW Europe after the CIE. Detrital elements (Al, Si, Ti, Zr) are used as proxies for sea-level change and match well previous sequence stratigraphic interpretations in the Cleveland Basin. Black shale deposition and anoxia were favored by a major transgression and appear to be paced by 3rd to 4th order sea-level sequences. References: Hermoso, M., Minoletti, F., Pellenard, P., 2013. Black shale deposition during Toarcian super-greenhouse driven by sea level. Climate of the Past Discussions 9, 4365-4384 McArthur, J.M., Algeo, T.J., van de Schootbrugge, B., Li., Q., Howarth, R.J., 2008. Basinal restriction, black shales, Re-Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography 23, PA4217, doi: 10.1029/2008PA001607.