Mercury anomalies and the timing of biotic recovery following the end-Triassic mass extinction

Abstract

The end-Triassic mass extinction overlapped with the eruption of the Central Atlantic Magmatic Province (CAMP), and release of CO2 and other volcanic volatiles has been implicated in the extinction. However, the timing of marine biotic recovery versus CAMP eruptions remains uncertain. Here we use Hg concentrations and isotopes as indicators of CAMP volcanism in continental shelf sediments, the primary archive of faunal data. In Triassic-Jurassic strata, Muller Canyon, Nevada, Hg levels rise in the extinction interval, peak before the appearance of the first Jurassic ammonite, remain above background in association with a depauperate fauna, and fall to pre-extinction levels during significant pelagic and benthic faunal recovery. Hg isotopes display no significant mass independent fractionation within the extinction and depauperate intervals, consistent with a volcanic origin for the Hg. The Hg and palaeontological evidence from the same archive indicate that significant biotic recovery did not begin until CAMP eruptions ceased.

Figure 1. Geography of the T-J interval and age model.

(a) Palaeogeographic map of the T-J interval showing the hypothesized extent of CAMP with the study site and other key T-J localities marked (with a red dot and black dots, respectively), modified from Greene et al. (b) Comparison of ammonoid biostratigraphy and U-Pb dates from interbedded volcanic ashes in Peru and U-Pb ages for CAMP magmatism largely from N. America and Morocco (top grey bar). The initiation of CAMP volcanism is coincident with the end-Triassic extinction (typically indicated by the last occurrence of C. crickmayi) and continues through the earliest Jurassic.

Figure 2. Mercury and organic carbon geochemistry of T-J interval, Muller Canyon, Nevada.

Plots show (a) δ¹³C_org, (b) Hg, (c) Hg/TOC and (d) Δ¹⁹⁹Hg for Muller Canyon, Nevada, along with lithology and key ammonites. Marker beds N3, N9 and N11 are after Guex et al. In panel b, error bars on Hg are 2 s.d. In panel c, error bars represent the combined error on Hg (2 s.d.) and TOC (1 s.d.) concentration measurements. In panel d, errors on individual Δ¹⁹⁹Hg measurements are as reported in Supplementary Table 1 and represent either 2 s.e.m. of sample replicates or 2 s.d. of the JT Baker standard, whichever is higher (see the Methods for further explanation). The vertical grey bar in panel d is centred on Δ¹⁹⁹Hg=0.00‰ and extends from −0.05‰ to +0.05‰. Samples for which the Δ¹⁹⁹Hg values and associated error bars fall within the shaded grey region can be considered to have Δ¹⁹⁹Hg values within experimental error of zero, and thus no measureable mass independent fractionation (MIF).

Figure 3. Summary of key features of the T-J interval, Muller Canyon, Nevada.

Panels compare (a) Hg chemostratigraphy (this study), (b) ammonite species diversity from Guex et al.; (c) benthic palaeoecology and microfacies modified from Ritterbush et al.; and (d) ecosystem state for Muller Canyon, Nevada, in association with lithology and key dates. Nevada ash date is from Schoene et al. and approximate ages of key ammonites are from Wotzlaw et al., and Schoene et al. (see Fig. 1b). These comparisons show that significant biotic recovery follows the Hg anomalies and provide evidence that biotic recovery began after the cessation of CAMP magmatism.