Selenium, arsenic and molybdenum variation and bio-radiation in the Ediacaran-Cambrian interval

Abstract

The Ediacaran-early Cambrian period is critical for the evolution of multicellular life. However, the distribution and enrichment patterns of biological trace elements and their correlations with the bio-changes are not yet fully understood. Here we present the variation patterns of trace elements in the Yangtze Gorges area, South China. Compared to the upper continental crust, selenium (Se), cadmium (Cd), arsenic (As), and molybdenum (Mo) are the most enriched and varied trace elements in the whole Ediacaran and early Cambrian Formations, the concentrations of which vary from 10−5> to 30.1, 0.02 to 163.1, 5×10−5> to 196.5, and 0.02 to 289.0mg/kg, respectively, with average values of 1.3, 1.0, 14.0, and 8.0mg/kg, average enrichment factors (EF) of 27.0, 10.7, 9.7, and 5.4, and EF variations of 601.2, 1663.6, 131.0, and 192.6, respectively. The significantly enriched (EF>10) strata sequences of Se and As mainly include the lower and upper Ediacaran Doushantuo member II (DST-II), DST-IV, bottom and upper Yanjiahe Formation (Precambrian-Cambrian), and Shuijingtuo Formation (early Cambrian). Mo displays similar significantly enriched strata with Se and As from DST-IV and above. Compared to the Cryogenian Nantuo Formation, Se, As, and Mo are enriched and concentrated in the whole Ediacaran and early Cambrian strata, especially at the early Ediacaran Doushantuo Formation, the diachronous Yanjiahe Formation (Precambrian-Cambrian) and the early Cambrian Shuijingtuo Formation, which are the horizons of the great bio-changes. Moreover, the significantly enriched strata of Se, As, and Mo are also consistent with intervals of great bio-changes.