Mercury and sulfur isotopic evidence for the linkages between the ca. 510 Ma Kalkarindji large igneous province and trilobite crisis

Abstract

The first major mass extinction of trilobites occurred at the transition from Cambrian Series 2 (CS2) to Miaolingian (M) and coincided with a large marine transgression and volcanic eruptions of a large igneous province (LIP). Understanding the causal links between these events is important in deciphering environmental changes and life evolution at that time. This paper presents S-Hg-C isotopic and Fe speciation data for calcareous shales from the CS2-M Yangliugang Formation in the Dongjin section, South China. In the lower part of this section (Interval I), calcareous shales have limited S isotopic differences between carbonate-associated sulfates (δ34SCAS) and pyrites (δ34SPy) with Δ34S values from –4.2‰ to +8.3‰; they also have high FeHR/FeT ratios from 0.5 to 0.66, that are indicative of a low SO42− content in anoxic seawater. Calcareous shales from Interval I display at least two Hg/TOC peaks (up to 207 ppb/wt%), coincident with volcanic eruptions associated with the ca. 510 Ma Kalkarindji LIP in Australia. In the middle part of the Dongjin section (Interval II), calcareous shales display negative excursions of Δ199Hg values, δ34SPy values and FeHR/FeT ratios, which were likely led by a large terrestrial input into a marginal basin from where Interval II deposited. Calcareous shales in the upper part of the section (Interval III) contain pyrite framboids with a mean diameter of <4 μm and high ratios of FeHR/FeT (>0.82) and FePy/FeHR (>0.78), indicating an euxinic depositional environment. They also record negative excursions in δ13C and δ202Hg (with low values down to –3.05‰ and –1.68‰, respectively), providing evidence for ocean upwelling. The negative shift of δ13C values in the Dongjin section was temporally comparable to the C isotope excursions that coincided with the Redlichiid-Olenellid Extinction Carbon Isotope Excursion (ROECE). We conclude that volcanic eruptions of the ca. 510 Ma Kalkarindji LIP enhanced the continental erosion rate, leading to a high terrestrial SO42− input and an accumulation of H2S in deep marginal basins at the end of CS2. The subsequent ocean upwelling (>506 Ma) brought anoxic/euxinic seawaters into the continental shelf, contributing to the mass extinction of Redlichiid and Olenellid.