Anomalous mercury enrichment in Early Cambrian black shales of South China: Mercury isotopes indicate a seawater source

Abstract

Extremely elevated Hg levels up to the μgg−1 range were found in sulfide-rich black shale and phosphorite in the lowermost part of the Early Cambrian black shale sequences on the Yangtze Platform in South China. In this study, mercury isotopes were used to help resolve the origin of this anomalous Hg enrichment. Mass independent fractionation signatures of Hg isotopes with positive Δ199Hg values of 0.13 to 0.24‰ and positive Δ200Hg values of 0.05 to 0.10‰ were observed, indicating seawater is an important source of Hg. We hypothesize that upwelling of nutrient-rich waters from the open ocean resulted in high bioproductivity in the photic zone with concomitant scavenging of Hg from seawater. Advanced decay of biomass and remineralization under anoxic to euxinic conditions combined with very low clastic input led to enrichment of Hg and a broad spectrum of other redox-sensitive and biogenic metals up to 107 compared to modern seawater. Such spectacular hydrogenous metal enrichment has previously been observed in the oxic deep-sea environment only, i.e. manganese crusts/nodules.