The role of LIPs in Phanerozoic mass extinctions: An Hg perspective

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All of the Big Five mass extinctions (BFMEs) of the Phanerozoic are postulated to have been associated with eruptions of large igneous provinces (LIPs), although the role of LIP magmatism in some of these biocrises remains contentious. Mercury (Hg)-system proxies are now widely used to identify volcanic fluxes to stratigraphic successions. Here, we review Hg proxies and isotopic records for the BFMEs as well as seven non-BFME LIP events in the Phanerozoic to evaluate the extent to which Hg-system data support a link with LIP magmatism. LIP influence is assessed based on Hg/TOCEF of >3 (i.e., enrichment factors of Hg normalized to total organic carbon for the event interval relative to the pre- and/or post-event background) and on characteristic Δ199Hg shifts. The relationships of Hg-system anomalies to eruption volume, distance from the putative LIP source, depositional water depth, and average sedimentation rates were evaluated by multiple regression analysis (MRA). No LIP is universally registered by sedimentary Hg deposits: positive “hits” are recorded variably by 15-71% of the sections contemporaneous with individual major LIPs. Because Hg anomalies can have multiple origins, Hg-isotope data are crucial to recognition of the source of LIP-associated Hg anomalies. For the end-Permian and end-Triassic mass extinctions, average Hg/TOCEF values for all sections are respectively 4.7 and 4.2 (with 71% and 67% of sections yielding values >3) and Δ199Hg shifts are >±0.10‰, providing evidence of LIP influence. For seven non-BFME LIP events, average Hg/TOCEF ranges from 1.8 to 4.2, with 15% to 67% of the sections exhibiting anomalous Hg enrichment and isotopic shifts (−0.15‰ to +0.08‰). In contrast, the Late Ordovician and Late Devonian mass extinctions exhibit less common Hg enrichment (<30% of sections with Hg/TOCEF >3) and Δ199Hg shifts mostly <±0.04‰, indicating no fundamental change in Hg sources during these events and, thus, providing no support for an LIP trigger of these biocrises. Rather, the Hg anomalies associated with these events were likely due to localized Hg uptake related to depositional water depth and/or watermass redox conditions (i.e., euxinia). The end-Cretaceous mass extinction yields negative Δ199Hg shifts (−0.16‰ to −0.01‰) associated with local volcanism, suggesting limited influence by LIP magmatism. Among the variables examined by MRA, the LIP source (i.e., subaerial vs. submarine), distance from LIP source, and sedimentation rates show by far the strongest relationships to Hg anomalies. In summary, our results indicate that (1) Hg-concentration data must be supplemented by Hg-isotope data to accurately evaluate LIP influences in sedimentary systems, and (2) the extant Hg-system data support the dominant role of LIPs in the end-Permian and end-Triassic mass extinctions but not in the Late Ordovician and Late Devonian biocrises.