Mercury anomaly, Deccan volcanism, and the end-Cretaceous mass extinction

The contribution of the Deccan Traps (west-central India) volcanism in the CretaceousPaleogene (KPg) crisis is still a matter of debate. Recent U-Pb dating of zircons interbedded within the Deccan lava flows indicate that the main eruptive phase (>1.1 × 106 km3 of basalts) initiated ~250 k.y. before and ended ~500 k.y. after the KPg boundary. However, the global geochemical effects of Deccan volcanism in the marine sedimentary record are still poorly resolved. Here we investigate the mercury (Hg) content of the Bidart (France) section, where an interval of low magnetic susceptibility (MS) located just below the KPg boundary was hypothesized to result from paleoenvironmental perturbations linked to the paroxysmal Deccan phase 2. Results show Hg concentrations >2 orders of magnitude higher from ~80 cm below to ~50 cm above the KPg boundary (maximum 46.6 ppb) and coincident with the low MS interval. Increase in Hg contents shows no correlation with clay or total organic carbon contents, suggesting that the Hg anomalies resulted from higher input of atmospheric Hg species into the marine realm, rather than organic matter scavenging and/or increased runoff. The Hg anomalies correlate with high shell fragmentation and dissolution effects in planktic foraminifera, suggesting correlative changes in marine biodiversity. This discovery represents an unprecedented piece of evidence of the nature and importance of the Deccan-related environmental changes at the onset of the KPg mass extinction. INTRODUCTION Large igneous province (LIP) volcanism is a possible cause in four of the five major mass extinctions in Earth history. However, this link remains controversial because for most LIPs there is no direct evidence of the mass extinction within the volcanic sequences. The Cretaceous-Paleogene (KPg) boundary is the most famous example where there is a direct correlation between the KPg mass extinction and both a meteorite impact and massive Deccan Traps (west-central India) volcanism within a very short geological time interval (Chenet et al., 2007; Renne et al., 2015; Schoene et al., 2015). The mass extinction in planktic foraminifera was documented between Deccan lava flows in the Krishna-Godavari Basin >1000 km from the Deccan Traps; these lava flows correspond to magnetochron C29r (Keller et al., 2011). On a global basis, quantification of the paleoclimatic and paleoenvironmental effects of volcanism and their impacts on and contributions to the KPg mass extinction remain a challenge. One potential method to investigate these global effects is to identify key exceptional sedimentary sequences that span the KPg mass extinction interval. With such sections changes in key proxy elements such as mercury (Hg), which have been identified as possible markers for LIPs (e.g., Grasby et al., 2015), can be assessed and discussed in the context of both the meteorite impact and volcanism. Here we investigate the Hg composition of the Bidart (France) section (Fig. 1), considered one of the most complete KPg sections worldwide (Bonté et al., 1984; Font et al., 2014; Galbrun and Gardin, 2004), and where an interval of low magnetic susceptibility (MS) resulting from partial dissolution of detrital magnetite and disappearance of biogenic magnetite (magnetofossil) was hypothesized to be driven by paleoenvironmental perturbations induced by Deccan volcanism (Abrajevitch et al., 2015; Font et al., 2014). This peculiar interval of low MS, also observed in Gubbio (Italy) (Ellwood et al., *E-mail: [email protected] GEOLOGY, February 2016; v. 44; no. 2; p. 171–174 | Data Repository item 2016053 | doi:10.1130/G37451.1 | Published online 7 January 2016 © 2016 eological Society of A erica. For permission to copy, contact [email protected]. 120°E 60°E 0° 60°W 120°W 30°N