Secular variations in carbon isotopic compositions of carbonates from Proterozoic successions in the Ming Tombs Section of the North China Platform

Abstract

Thick, Proterozoic, unmetamorphosed carbonate-dominated successions are widely distributed in the North China Platform. The Ming Tombs Section near Beijing is a well-studied stratigraphic section and has been selected for the study of secular variations in the carbon isotopic composition of marine carbonates. In ascending order, this section includes the Changzhougou, Chuanlinggou, Tuanshanzi, Dahongyu and Gaoyuzhuang formations in the Changcheng Group; the Yangzhuang, Wumishan, Hongshuizhung, and Tieling formations in the Jixian Group; and the Jing'eryu Formation in the Qingbaikou Group. Whereas the depositional ages of the Changzhougou, Chuanlinggou and Tuanshanzi formations of the Changcheng Group are older than 1600 Ma, the Gaoyuzhuang, Yangzhuang and Wumishan formations belong to the Mesoproterozoic Era with ages evidently greater than ∼1300–1200 Ma, and the Jing'eryu Formation of the Qingbaikou Group were deposits of the early Neoproterozoic Era. δ 13C values vary only within a narrow range between −1.6 and +1.5 permil in the early Mesoproterzoic Gaoyuzhuang, Yangzhuang and Wumishan formations. Secular variation in δ 13C of carbonates from the early Neoproterozoic Jing'eryu Formation is more significant, showing values from −0.5 to +3.3 permil. The secular variations in δ 13C values of carbonates from the Proterozoic successions in the Ming Tombs Section can be broadly correlated with those in the Jixian Section of the North China Platform. They compare well with the established global isotopic curve of the Mesoproterozoic and Neoproterozoic eras.