Anomalously high enrichment of mercury in early Cambrian black shales in South China

Abstract

The widespread lower Cambrian black shales reflect a significant anoxic event during a global transgression, recording synergistic changes in the marine environment and biological evolution. Extremely high Hg (714–6187 ppb, average is 3380 ppb) contents and Hg to total organic carbon ratios (Hg/TOC: 120–765 ppb/wt.%) were observed in the Danquan section of South China, suggesting that extensive volcanic activities may occur during the early Cambrian. The redox condition of the Danquan section is euxinic as evidenced by multiple redox proxies, which include U/Th, V/Cr, Corg/P and authigenic enrichment of redox sensitive elements (MoEF, UEF). The Co*Mn – Al crossplot revealed that black shale developed in different geographical settings (upwelling setting for the Interval I, and restricted condition for the Interval II). Besides, the sedimentary Hg is hosted in different phases in the two intervals: in organic matter for the Interval I, and in sulfides for the Interval II. Moreover, the anomalous enrichment of Hg in both intervals document the Hg controlled by geographical settings and host phases. Thus, volcanic activities and geographical settings both constrain the evolution of life, as well as redox conditions.