Intensified continental chemical weathering and carbon-cycle perturbations linked to volcanism during the Triassic–Jurassic transition

Abstract

Direct evidence of intense chemical weathering induced by volcanism is rare in sedimentary successions. Here, we undertake a multiproxy analysis (including organic carbon isotopes, mercury (Hg) concentrations and isotopes, chemical index of alteration (CIA), and clay minerals) of two well-dated Triassic–Jurassic (T–J) boundary sections representing high- and low/middle-paleolatitude sites. Both sections show increasing CIA in association with Hg peaks near the T–J boundary. We interpret these results as reflecting volcanism-induced intensification of continental chemical weathering, which is also supported by negative mass-independent fractionation (MIF) of odd Hg isotopes. The interval of enhanced chemical weathering persisted for ~2 million years, which is consistent with carbon-cycle model results of the time needed to drawdown excess atmospheric CO2 following a carbon release event. Lastly, these data also demonstrate that high-latitude continental settings are more sensitive than low/middle-latitude sites to shifts in weathering intensity during climatic warming events.

Mercury evidence for combustion of organic-rich sediments during the end-Triassic crisis

“…Mercury has been used in T-J transitional research to track mantle-derived volcanic inputs to continental as well as shallowto intermediate-depth marine sections proximal to the Pangean supercontinent 11,14,[17][18][19]26,27 (Fig. 1).…”

“…At the epicontinental St. Audrie's Bay site 18 , background Hg fluxes were dominantly of terrestrial origin (Δ 199 Hg −0.4‰ to −0.2‰), but rising Δ 199 Hg within the extinction interval (−0.17‰ to −0.07‰) resulted from either dominantly atmospheric deposition (Δ 199 Hg −0.17‰ to −0.07‰) or a mixture of atmospheric and terrestrial sources that were weighted toward the former. For the two terrestrial settings (Haojiagou and Qilixia) 19 , the relatively higher Δ 199 Hg values near the extinction interval (−0.2‰ to −0.1‰, and −0.05‰ to +0.05‰ for Haojiagou and Qilixia, respectively) than that during the background interval (−0.4‰ to −0.2‰ for Haojiagou, and −0.2‰ to −0.05‰ for Qilixia) document mixing of terrestrial and atmosphere sources. The negative Δ 199 Hg values (ca.…”

“… Geo-map adapted from Ron Blakey https://deeptimemaps.com/ , (©2016 Colorado Plateau Geosystems Inc.) Red star shows the study site (Katsuyama section), located in the center of Panthalassa. Circles and squares represent other marine and continental sections, respectively, for which mercury data have been generated, including Hg concentrations and isotopes for Nevada 17 , 18 , St Audrie’s Bay 14 , 18 , Levanto 18 , Haojiagou, and Qilixia 19 ; Hg concentrations for Arroyo Malo, Astartekløft, Igounane, Kuhjoch, Partridge Island (from ref. 14 ); Stenlille-1/Stenlille-4, Rødby-1, and Norra Albert/Albert-1 (from ref.…”

“…1). Both marine and terrestrial sections show elevated Hg concentrations and Hg/TOC ratios during the T-J extinction interval, and these enrichments have been linked to volcanic sources based on near-zero Δ 199 Hg values [17][18][19] . However, recent studies have painted a more nuanced picture, revealing that terrestrial and nearshore sections received Hg from multiple sources, including seawater, terrestrial materials (e.g., vegetation, soil), and the atmosphere [18][19][20][21][22] .…”

“…Both marine and terrestrial sections show elevated Hg concentrations and Hg/TOC ratios during the T-J extinction interval, and these enrichments have been linked to volcanic sources based on near-zero Δ 199 Hg values [17][18][19] . However, recent studies have painted a more nuanced picture, revealing that terrestrial and nearshore sections received Hg from multiple sources, including seawater, terrestrial materials (e.g., vegetation, soil), and the atmosphere [18][19][20][21][22] . Although processes of removal of Hg from pelagic seawater are complex 23,24 , analysis of a pelagic open-ocean section far from continental influences is needed to gain a better understanding of atmospheric Hg fluxes during the T-J transition 15 .…”