Mercury evidence of intense submarine volcanism and hydrothermal activity during a mid-Tournaisian anoxic event in the Carnic Alps

Abstract

The mid-Tournaisian Event, or Lower Alum Shale Event (LASE), is linked to severe facies changes including pelagic carbonates and widespread black siliceous shales and radiolarites in many parts of the world. For the first time, high-resolution inorganic geochemistry and framboidal pyrite analyses were applied to decipher depositional condition changes during the mid-Tournaisian event in the Carnic Alps. The predominance of tiny pyrite framboids, high Corg/P, U/Th, and V/Cr ratios, generally higher enrichments of redox-sensitive metals such as Mo, U, V, and depletion in Mn suggest anoxic/euxinic conditions at the sea bottom. Moreover, we found very large anomalous Hg spikes (with maximum values reaching 3650 ppb) for the first time in the mid-Tournaisian deep-water marine succession of the Carnic Alps. Our results suggest intense volcanism during the LASE, which triggered the mid-Tournaisian anoxic event. The most likely candidate for the observed Hg anomalous contents is submarine arc volcanism and hydrothermal activity connected with the collision of the peri-Gondwanan terranes Paleo-Adria and probably Armorican Terrane Assemblage (ATA) and/or Noric terrane during the first phase of the Variscan orogeny.