Ediacaran chemostratigraphy and paleoceanography at a shallow marine setting in northwestern Hunan Province, South China

Abstract

The geochemical and lithological features of the Ediacaran strata at Yangjiaping in northwestern Hunan Province, South China, record information about the depositional environments, redox conditions, and carbon cycling in a shallow-water setting in this region. The chemostratigraphic profile of the inorganic carbon isotope reveals three negative anomaly intervals in the Ediacaran section that comprises the thick (∼360 m) Doushantuo and the thin (∼110 m) Dengying Formations. Two of the three intervals, at the base of the Doushantuo Formation and the PC/C boundary, correspond to the global trend. Another anomaly interval in the uppermost 120 m of the Doushantuo Formation appears large-amplitude fluctuations of the inorganic carbon isotope, as well of the oxygen isotope. This interval can be correlated to one of the anomaly intervals of the Three George section, either at the upper, or at the middle Doushantuo Formation. The latter correlation scheme implies association with the Gaskiers glaciation, and demands a substantial hiatus at the Doushantuo/Dengying boundary. Decoupling between organic and inorganic carbon isotopes was recognized in two intervals of the lower Doushantuo Formation and the upper Doushantuo to the lower Dengying Formations. The lower and upper intervals were likely attribute to unusual carbon cycling, reflecting from development and diminishing of the dissolved organic carbon (DOC) reservoir, respectively. Shallow-water sedimentary facies and high organic carbon isotopic values are characteristic of the Yangjiaping section, especially in the oolitic facies of the middle Doushantuo Formation and in the supratidal facies of the lower Dengying Formation. In these intervals, Yangjiaping was in an isolated lagoonal setting where the carbon cycle was independent of that of the open ocean in the South China region.