Mercury isotope evidence for marine photic zone euxinia across the end-Permian mass extinction

Abstract

The driving forces, kill and recovery mechanisms for the end-Permian mass extinction (EPME), the largest Phanerozoic biological crisis, are under debate. Sedimentary records of mercury enrichment and mercury isotopes have suggested the impact of volcanism on the EPME, yet the causes of mercury enrichment and isotope variations remain controversial. Here, we model mercury isotope variations across the EPME to quantitatively assess the effects of volcanism, terrestrial erosion and photic zone euxinia (PZE, toxic, sulfide-rich conditions). Our numerical model shows that while large-scale volcanism remains the main driver of widespread mercury enrichment, the negative shifts of Δ199Hg isotope signature across the EPME cannot be fully explained by volcanism or terrestrial erosion as proposed before, but require additional fractionation by marine mercury photoreduction under enhanced PZE conditions. Thus our model provides further evidence for widespread and prolonged PZE as a key kill mechanism for both the EPME and the impeded recovery afterward.

Deglacial volcanism and reoxygenation in the aftermath of the Sturtian Snowball Earth

“…The elevated Hg contents and Hg/TOC in the lower portion of interval II, enriched by up to 2.5× relative to interval I, are comparable to or slightly lower than the Hg enrichment during Phanerozoic mass extinction events that have been linked to large-scale volcanism (18,19,(35)(36)(37)(42)(43)(44)(45)(46). Therefore, the Hg enrichments in interval II provide evidence for the enhanced loading of Hg from volcanism (Fig.…”

“…As such, any potential influences of Hg anomalies due to enhanced organic burial can be minimized by normalizing Hg against total organic carbon (TOC) (18,19,32). In addition, Hg isotopes undergo both Hg-MDF and Hg-MIF (33,34), which can distinguish Hg sources and transformations during Hg biogeochemical cycles (20,(35)(36)(37). Hg-MDF, reported as δ 202 Hg, occurs in numerous biogeochemical processes (34).…”

Recurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran

“…In addition, the Hg concentration in crustal igneous and metamorphic rocks is typically <10ppb 30 , 31 , much lower than the >100ppb (and frequently >1000ppb) THg in almost all Doushantuo shales, which also argues against terrestrial Hg from continental weathering as a dominant source of Hg. Moreover, we simulated the shift in Δ 199 Hg caused by enhanced terrestrial weathering using a simplified Hg isotope box-model (see details of the model in Supplementary Text S4 ) 32 , and found that even under a scenario of 50× increase in terrestrial Hg input, the Δ 199 Hg of the ocean can only be decreased by 0.02‰ (Fig. 4 ), which is an order of magnitude lower than the extent of Δ 199 Hg excursions during the E intervals (−0.19‰ to −0.29‰).…”

“…Recurrent and widespread PZE in the Ediacaran ocean may have also inhibited the diversification and ecological expansion of early macroscopic eukaryotic life by limiting their habitable space via H 2 S poisoning. H 2 S toxification during PZE was a potent kill mechanism for some of the largest mass extinctions, such as the end-Permian 20 , 32 , 66 and end-Triassic mass extinctions 67 . Emerging geochemical data suggest a redox control on the rise and fall of the Ediacara biota 4 , 5 , 68 .…”

Subaerial volcanism broke mid-Proterozoic environmental stasis

“…These processes, in turn, result in positive Hg-MIF and negative δ 202 Hg compositions within atmospheric Hg(II) and sediments dominated by atmospheric Hg(II) deposition ( 42 ). Furthermore, photochemical reduction of aqueous Hg(II) to Hg(0) in the photic zone of euxinic waters, as observed through the end-Permain mass extinction ( 43 ), could give rise to negative excursions of Hg-MIF of odd-mass isotopes (Δ 199 Hg and Δ 201 Hg) within organic-rich sediments. The converse holds true for organic-poor sediments, characterized by predominantly positive Δ 199 Hg and Δ 201 Hg values due to a more substantial contribution from atmospheric Hg(II) deposition.…”