Reply to the comment on “Non-uniqueness and interpretation of the seawater 87Sr/86Sr curve” by John M. McArthur and Paul B. Wignall

Abstract

Waltham and Gröcke (2006) introduced a new methodology for analysis of the seawater strontium-isotope curve, which showed that changes to a source-flux (e.g., the continental weathering flux) or a source-isotope ratio (e.g., the hydrothermal flux isotope ratio) could be estimated simply by scaling and shifting the time-derivative of the isotopic curve. We used this methodology to analyse the Lower Jurassic and in particular the Toarcian oceanic anoxic event (T-OAE). The power of this approach is nicely illustrated by the fact that, as a consequence, the scale of the T-OAE strontium-isotope excursion is shown to be controlled almost entirely by the time-duration of this event. However, the comment by McArthur and Wignall (M&W) argues that the change in the Toarcian portion of the curve is solely controlled by sedimentation rate and that the weathering event as produced by Waltham and Gröcke (2006) is an artefact of this factor. No criticisms in the comment were associated with our mathematical model itself, and thus this shows our approach’s usefulness for understanding the strontium-isotope cycle on geological time scales. There is unequivocal evidence for an environmental perturbation during the T-OAE that resulted in global warming, an intensified hydrological cycle and increased weathering (see [Hesselbo et al., 2000], [Hesselbo et al., 2007], [Cohen et al., 2004], Kemp et al., 2005 D.B. Kemp, A.L. Coe, A.S. Cohen and L. Schwark, Astronomical pacing of methane release in the Early Jurassic period, Nature 437 (2005), pp. 396–399. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (60)[Kemp et al., 2005] and [McElwain et al., 2005]). Many independent lines of evidence support the Toarcian weathering event and suggest that the degree of condensation proposed by M&W is extreme with limited sedimentological evidence and constraint, especially considering the uncertainties of chronostratigraphic constraint for the Jurassic. The exact magnitudes of all the above require additional, concentrated research efforts. We eagerly await the outcome of such research endeavours and the information they will provide to us for understanding the global perturbation during the Early Jurassic, T-OAE.