Abstract
The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.
Highlights
The end-Triassic extinction (ETE) event is characterized by a strong negative excursion of δ 13Corg recorded in New York Canyon (Nevada, USA) and worldwide[7,8] correlated to a initial negative excursion of δ 13Cwood measured in fossil leaf and wood from East Greenland[1,12,15] and by a marine regression in the upper Rhaetian of Austria, England and Nevada (Fig. 1)
The ammonoid recovery during the early Jurassic is associated with progressive increase of δ 13Corg, decrease of δ 18O and a significant sea level rise illustrated by the deposition of silty sediments overlying the bivalve-rich bed in the Muller Canyon Member (New York Canyon, NV)
This warming period is associated with an abrupt change in plant diversity observed in Greenland[1,15] and with a second negative δ 13C recorded in the Hettangian Psiloceras planorbis beds that postdate the ETE
Summary
Petrological constraints on primary magmas indicate that the mantle is hotter and melts more extensively to produce LIP lavas or continental flood basalts (CFB) than for current oceanic islands basalts[47]. The S-rich melts/fluids released from the base of the cratonic lithosphere do not directly reach the surface, but enrich progressively shallower levels of the lithospheric mantle, levels which could be remobilized sequentially during the thermal erosion of the lithosphere (Fig. 4a,d); (II) If lithospheric erosion is sufficiently shallow, scavenging of sulfur-rich melts/fluids could either reach the surface and release significant amounts of sulfur to the atmosphere or be mixed with the first CFB magma pulses producing initial high SO2 flux to the atmosphere (Fig. 4b,e) The former process could be linked to the emission of small volumes of alkaline lavas and carbonatites associated with translithospheric fracturing as observed in various LIP areas[71]; (III) The last stage (Fig. 4c,f) corresponds to an advanced degree of thermal erosion of the lithosphere, sufficient to lead to significant melting and extrusion of the lavas observed in the Karoo-Ferrar area and in the CAMP.
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