Organic Carbon Isotope Evidence For Late Ediacaran-early Cambrian Ocean Stratification In South China

The Ediacaran period is characterized by the extremely negative carbon isotope anomalies with magnitude from≥+5‰down to≤-12‰which have been documented almost all over the world. It is traditionally argued that the carbon isotope records can be used as a useful tool for global correlation of Ediacaran strata, particularly for those sections lacking age-diagnostic biostratigraphic markers and/or precise radiometric age determinations. However, published Ediacaranδ¹³C_carb curves from different continents show major inconsistencies not only in age but also in number and magnitude, this is partially because the origin of these anomalies remains largely controversial. South China is one of the most well-preserved location for the Ediacaran succession, it contains relatively conformable carbonate/shale sections along a platform-to-basin transect and can sever as an ideal place to solve a series key questions in this period. During the past two decades, numerous studies have been done on the Ediacaran carbon isotope chemostratigraphy in South China, however, most of the previous researches only emphasize the carbon isotope temporal variation but ignored the spatial variation, and put much energy on the Doushantuo period but overlook the late Ediacaran. Moreover, there is a generally lack of paired carbonate and organic carbon isotope research in South China. Here we present high-resolution carbon isotope records from the late Ediacaran to the early Cambrian in two different sedimentary environments. Our results show that: 1) theδ¹³C_org profile from the platform section overall displays an obvious decoupling relationship with that ofδ¹³C_carb; 2) two negativeδ¹³C_org excursions associated with two negativeδ¹³C_carb anomalies are recognized at the uppermost Doushantuo Formation and Precambrian/Cambrian(P(?)/C) boundary respectively in the platform section, during which theδ¹³C_org excursion at P(?)/C boundary was rarely reported; and 3) an up to 7‰δ¹³C_org gradient exists between shallow water platform and deep water basin during the Dengying period. These three characters were barely reported in the previous studies. If the decoupling ofδ¹³C_carb andδ¹³C_org did result from the remineralization of a large DOC reservoir just as suggested, then the DOC reservoir may not have been completely consumed at 551 Ma, it may last to at least early Cambrian, or even longer. We argue that the carbon isotope excursions of both carbonate and organic carbon at P/C boundary were caused by the sustained anoxic deep ocean formed by the ocean stratification. Theδ¹³C_org gradient in the Dengying period indicate perhaps strong chemoautotrophic recycling of organic carbon in anoxic/euxinic conditions below the chemocline because chemoautotrophic organisms including sulfur oxidizing bacteria use recycled carbon during carbon fixation and their biomass is up to 15‰depleted in ¹³C relative to primary photosynthate.