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.

Fig. 1. Geobal paleogeography of the Late Triassic (~200 Ma).

Adapted from Ron Blakey, http:// https://deeptimemaps.com/, © 2016 Colorado Plateau Geosystems Inc. Yellow stars represent the study sites, including. The Haojiagou section (~60°N, high latitude) on the North China Craton, and the Qilixia section (~30–40°N, low/middle latitude) on the South China Craton. Circles and squares represent other marine and continental sections, respectively, for which mercury data have been generated, including Hg concentrations and isotopes for Nevada^, , St. Audrie’s Bay^, , and Levanto; and Hg concentrations for Arroyo Malo, Astartekløft, Igounane, Kuhjoch, Partridge Island; Stenlille-1/Stenlille-4, Rødby-1, Norra Albert/Albert-1, Csővár, Arroyo Alumbre, Lombardy Basin, Mt. Sparagio, and Haojiagou.

Fig. 2. Profiles of Haojiagou (Upper, HJG) and Qilixia (Lower, QLX) sections.

(a, g) Organic carbon isotope (δ¹³C_org, ‰); (b, h) Mercury concentrations (Hg, ppb); (c, i) Ratios of mercury to total organic carbon (Hg/TOC, ppb/wt.%); (d, j) Mass independence fractionation of odd-Hg isotope (Δ¹⁹⁹Hg, ‰); (e, k) Chemical index of alteration (CIA) and (f, l) clay minerals. The eccentricity cycle and ages are from Sha et al. and Li et al. for Haojiagou and Qilixia respectively. The red crosses in represent δ¹³C_org data from Sha et al.. Open and purple filled circles in c and i represent samples with TOC < 0.2 wt.% and ≥ 0.2 wt.%, respectively. ICW Intense chemical weathering interval; TJT Triassic–Jurassic transition; ME mercury-enriched interval. The red and green bar at the base of column d and j represent the volcanic (V) and terrestrial (T) compositions of Δ¹⁹⁹Hg respectively. The arrow for the ICW represents the uncompleted records in the Early Jurassic for QLX. The horizon bars of the Δ¹⁹⁹Hg profiles represent standard deviation (2σ) values. Abbreviations: Sy System, St (sub)stage, F formation, B bed, Z sporomorph assemblage zone, Ec. eccentricity cycle, M member, Jur. Jurassic, Het. Hettangian, ZZC Zhenzhuchong, ICIE Initial carbon isotope excursion, MCIE Main carbon isotope excursion, PCIE precursor carbon isotope excursion. Note: full geochemical data are in Supplementary Figs. 2–3. Source data are provided as a Source Data file.

Fig. 3. Crossplot of geochemical proxies.

Δ¹⁹⁹Hg versus Hg/TOC for Haojiagou (a) and Qilixia (b) sections, as well as Δ¹⁹⁹Hg versus Δ²⁰¹Hg (c) and CIA (d) for HJG and QLX sections. The gray, purple, and blue ellipses in (a) and (b) represent the background, volcanic, and terrestrial endmember sources, respectively. The volcanic endmember is based on the most positive values of MIF (Δ¹⁹⁹Hg = ~0.1‰) and most elevated Hg/TOC values (~600–800 ppb/wt.%) in the study sections, which likely reflect dominant volcanic influence. For the terrestrial endmember, we assumed values based on the maximum Hg/TOC (~300 ppb/wt.% and 1000–1200 ppb/wt.% for HJG and QLX, respectively) and Δ¹⁹⁹Hg values similar to the background (~−0.3‰ to −0.4‰ and −0.2‰ to 0‰ for HJG and QLX, respectively). The purple dashed arrows in a and b represent a two-component mixing model between the background (low Hg/TOC, negative Δ¹⁹⁹Hg) and the volcanic endmember (high Hg/TOC, positive Δ¹⁹⁹Hg). The blue dashed arrows in a and b represent increasing terrestrial Hg inputs (high Hg/TOC, negative Δ¹⁹⁹Hg) into the system. The range of Δ¹⁹⁹Hg values for volcanisms, soil, and continental plants and continents are from Yin et al.. The bars of the Δ¹⁹⁹Hg and Δ²⁰¹Hg distributions represent standard deviation (2σ) values. ME mercury-enriched interval. Source data are provided as a Source Data file.

Fig. 4. Ternary diagrams of A-CN-K for the Haojiagou.

(a) and Qilixia (b) sections. Purple and blue symbols represent samples from intensely chemically weathered (ICW) and background intervals, respectively. The green arrows represent weathering trends. A = Al₂O₃, CN = CaO* + Na₂O, K = K₂O; Chl chlorite, Gi gibbsite, Kao kaolinite, Kfs K-feldspar, Sm smectite. Other details as in Supplementary Figs. 2 and 3. Source data are provided as a Source Data file.

Fig. 5. Model results of LOSCAR for the T–J boundary carbon-cycle perturbation. The weatherability (k_silw) factors were setted to 1.0, 1.1, 1.2, and 1.3.

a Atmospheric pCO₂ level. b Silicate weathering flux. The gray shaded rectangle represents the interval of intense chemical weathering (ICW), yielding elevated silicate weathering fluxes. The horizontal dashed lines represent the background values of atmospheric pCO₂ (a) and silicate weathering flux (b) before the CAMP eruptions.