Carbon isotope gradient of the Ediacaran cap carbonate in the Shennongjia area and its implications for ocean stratification and palaeogeography

Abstract

The geochemistry and paleogeography of the Doushantuo succession in the Shennongjia area have been insufficiently studied. Here, we report on the carbon and oxygen isotope compositions of Ediacaran Doushantuo cap carbonates from four sections (Longxi, Muyu, Yazikou and Songluo) in the Shennongjia area. A large C-isotopic gradient (~5‰) between the Longxi and Songluo sections, here identified for the first time, is inferred to have been dynamically maintained by photosynthesis in surface waters and anaerobic oxidation of dissolved organic carbon (DOC) in deep waters. Spatial variation in C-isotope chemostratigraphy among the four sections was related to Marinoan post-glacial sea-level elevation changes. At Longxi, a positive δ ¹³C_carb shift below the horizon containing tepee-like structure resulted from intense photosynthesis during early regression. A negative δ ¹³C_carb shift within the tepee-horizon was triggered by sulfate reduction and freshwater mixing with ¹³C-depleted dissolved inorganic carbon (DIC) during late regression. A positive δ ¹³C_carb excursion in the uppermost part of the cap carbonate was related to enhance primary productivity and organic matter burial during early transgression. At Muyu, the carbon isotopic variation tendency, which is similar to that at Longxi, may have been mildly influenced by the surface water environment. At Songluo, the positive δ ¹³C_carb excursion, up to -4‰ in the lower part of the cap carbonate, was probably associated with methanogenesis in deep waters during late transgression to early regression and subsequently disappeared due to decomposition of methane hydrate during late regression. At Yazikou, the consistently stable δ ¹³C_carb values around -4‰ indicate that the cap carbonate may have deposited at intermediate water depths. As evidenced by diagnostic sedimentary characteristics of the study sections, the palaeogeographic framework of the Shennongjia area exhibited deepening from SE to NW during the Early Ediacaran Period in the aftermath of Snowball Earth.

Keywords

Ediacaran / Doushantuo cap carbonate / carbon isotope / stratification / palaeogeography / Shennongjia

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