Nitrogen geochemistry and abnormal mercury enrichment of shales from the lowermost Cambrian Niutitang Formation in South China: Implications for the marine redox conditions and hydrothermal activity

Abstract

The Cambrian is an important period for life evolution during the geological history, and the basal Cambrian black shales with widespread occurrence worldwide record important paleoceanographic information during this critical period. The set of shales with stable distribution and thickness of about 200 m is also extensively developed in South China, which provides an important material for studying the marine redox environment, major geological events and biotic evolution. Here we conduct studies on the nitrogen cycle and mercury's abnormal enrichment based on mercury (Hg) concentrations and nitrogen (N) isotope compositions of the shales from the lower Cambrian Niutitang Formation in Tongren area of Guizhou Province, South China. The results show that both Hg and TOC exhibited abnormal enrichments at the bottom of the Niutitang Formation. Mo and U enrichment factors indicated a gradually oxidized redox evolution during the deposition of the Niutitang Formation, with two fluctuated redox cycles from reducing to oxic, and then from dysoxic to oxic conditions in the water column. And the abnormal enrichment of Hg during the early Cambrian may be related to the volcanic and hydrothermal activities. Nitrogen isotope (δ15N) has obvious negative shift at the bottom of the Niutitang Formation, which indicated that nitrogen fixation was enhanced under the anoxic condition in the water column, the organic carbon pool gradually reduced, and phosphorus (P) and other nutrient elements were available, leading to the enhancement of nitrogen fixation and an increase of bioavailable nitrogen in the water column. Nitrogen isotopes recorded the effects of early Cambrian biological evolution and marine redox environment on nutrient element circulation, with contemporaneous volcanic or hydrothermal fluid activities during this period.