Carbon and oxygen isotopes analyses for the Sinian carbonates in the Helan Mountain, North China

Abstract

We conducted detailed studies of the Sinian carbonates from the Helan Mountain, Ningxia by both field investigation and laboratory analyses of carbon and oxygen isotopes. The carbon isotopic compositions of the Sinian Zhengmuguan Formation shows variations with regularity in the five studied sections, the δ 13C values all gradually decrease from bottom to top, with a total range from −4.51‰ to 0.11‰, and the biggest negative excursion reaches up to −6.88‰. In addition, abundant macro-body fossils were also found from the Tuerkeng Formation in this study, similar to those found from the Dengying Formation in the South China Block. This observation firmly constrained the age for the Zhengmuguan and Tuerkeng formations to the Sinian period. Here we interpreted that the Zhengmuguan Formation formed during a glacial period when the extremely cold climate substantially decreased the amount of burial of organic matter in the ocean and thus led to the general negative δ 13C excursion. The increasing δ 13C values in the Tuerkeng Formation dolomites imply the ending of the glacial period in the Zhengmuguan epoch, biological recovery and increase of organic matter burial in the ocean occurred during this period. Comparing our obtained carbon and oxygen isotopes of the Zhengmuguan and Tuerkeng formations with those of coeval carbonate successions from other places all over the world, the Zhengmuguang-aged glacial deposits might be deposited after the Gaskiers glaciation. This study provides important information for paleo-ocean, paleogeographic and biological evolution in the North China Block during the Ediacaran period and adds new carbon and oxygen isotopes data for the global Sinian carbon-oxygen isotopes dataset.