The Bonarelli level (Gubbio, Italy) reveals a potential correlation between the occurrence of oceanic anoxic events and large-scale magmatic activity

Abstract

Global and local paleo-environmental changes across Earth’s history can be tracked by investigating the sedimentary record. Large Igneous Provinces (LIPs), whose activity spans from 250 to 50 Ma, are known to be coeval to the Phanerozoic largest mass extinctions and the Oceanic Anoxic Events (OAEs), the latter identified in the marine sedimentary rocks through key geochemical proxies such as high organic matter content (up to 30%) and positive excursions in the δ13C values. The Bonarelli level, a ~0.9-m thick layer made up of organic-rich shales, cropping out at Valle della Contessa section in Gubbio (Italy) within the pelagic limestones of the Scaglia Bianca Formation, is the stratigraphic marker of OAE2 (Cenomanian-Turonian, ~93 Ma). This event was likely triggered by submarine volcanism of the High Arctic and Caribbean LIPs, although clear evidence is missing at present, that might be given by the study of geochemical markers like mercury (Hg) claimed as a signature within sedimentary layers for large volcanic eruptions.We present the results of X-ray diffraction, petrographic and geochemical (Total Organic Carbon, Hg concentration, trace elements, Hg, S and Sr isotopes) analyses on eleven rock samples collected from the Bonarelli level along with eleven rocks samples from the Scaglia Bianca Formation, to establish a potential link between OAEs and LIP volcanism.LIP-related basalts are known to have 1 to 4 µg/kg of Hg (Yin et al. 2022). Interestingly, our results show a sharp Hg anomaly up to ~1600 µg/kg measured in the black shales, pointing out episodes of large Hg emissions accompanied by volatile degassing. Such anomaly correlates positively with the concentration of chalcophile elements such as Cu, Ni and Fe and with the amount of sulfate (barite and jarosite) + sulfide (pyrite), likely the main Hg-bearing minerals. A major effect of organic matter accumulation on the Hg contents was excluded because of the high Hg/TOC ratios. Hg isotopic data suggest a (deep) mantle-derived magmatic contribution that is not accompanied by a similar mantle signature of Sr and S isotopes. In contrast, we observed a continental input for Sr and S signature controlled by diagenetic processes in marine environment (e.g., pyrite deposition in anoxic seawater), atmospheric oxidation and seawater mixing. We interpret these findings as evidence of large-scale magmatism, which triggered greenhouse state increasing continental weathering.  Yin, R., et al. (2022). Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle. Nature communications, 13(1), 948.