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Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic

¹ Department of Earth and Environmental Sciences, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

View larger version (33K): [in this window] [in a new window]   Figure 1. Total organic carbon (TOC), 13Corg, 98/95Mo, [Mo], and Re/Mo data from the lower Toarcian sedimentary rocks exposed in Yorkshire, UK. Ammonite biostratigraphy, bed numbers, and 13Corg data are from Cohen et al. (2004) and Kemp et al. (2005) and references therein; present-day seawater Re/Mo ratio is from Crusius et al. (1996). Dark gray shading represents dark gray organic-rich mudrocks; pale gray shading represents medium-gray mudrocks; and brick pattern represents carbonate bands and nodules. The long-term 98/95Mo reproducibility of our in-house standard is 0.12 (2). The succession is divided into four intervals on the basis of their geochemical characteristics: -13.00 m to -0.73 m (Interval 1), -0.73 m to 5.80 m (Interval 2), 5.80 m to 20.85 m (Interval 3) and 20.85 m to 35.00 m (Interval 4). Interval 2 lasted 200 k.y. (Kemp et al., 2005; Kemp, 2006) and includes most of the period taken to represent the Toarcian oceanic anoxic event. Abbreviations: P.pa—Protogrammoceras paltum; D.—Dactylioceras; cl.—clevelandicum; ten.—tenuicostatum; sem.—semicelatum; Cl.—Cleviceras; H.—Harpoceras.

View larger version (39K): [in this window] [in a new window]   Figure 2. Details of geochemical changes in the upper Dactylioceras semicelatum and Cleviceras exaratum ammonite subzones, data sources as Figure 1. The four abrupt shifts in 13Corg are labeled A–D (Kemp et al., 2005; Kemp, 2006) and their stratigraphic positions are indicated by dashed gray lines. The four excursions in 98/95Mo are labeled I–IV and the intervals of decreasing 98/95Mo, representing the development of global marine anoxia, are highlighted in gray. Uncertainties on 98/95Mo values represent 2 standard error for each measurement.

View larger version (17K): [in this window] [in a new window]   Figure 3. Relationships between [Mo] and total organic carbon (TOC) (A), and between 98/95Mo and 1/[Mo] (B), used to resolve changes in the areal extent of marine anoxia during the Toarcian. Distinct grouping of samples reflects the four geochemically constrained stratigraphic intervals shown in Figure 1.