A positive C-isotope excursion induced by sea-level fall in the middle Capitanian of South China

Abstract

A new carbon isotope excursion was recovered from the Capitanian marine carbonates at the Rencunping (RCP) section of South China. Significantly, a pronounced excursion with elevated δ13Ccarb values over +5‰ was coeval with the conodont Jinogondolella prexuanhanensis Zone and resembles the Kamura event recorded in Tethys. The excursions in δ13Ccarb, constrained by conodont biostratigraphy, however, present inconsistent carbon cycle behaviors, especially between the separated basins in South China, and evidently reflect regional litho-facies controls. In addition, a transitional environment in association with a fall in sea level, was recovered from deposits of anoxic cherty carbonates subsequent to shallow-water carbonates around the positive excursion in δ13Ccarb. Accordingly, instead of this being a signal of global-scale climatic cooling, we suggest that the positive excursion in δ13Ccarb can be attributed to eutrophication effects regionally along a continental shelf. In this scenario, the increasing dissolved O2 level in the mixing zone that induced by the initial sea-level fall will efficiently impede denitrification and increase the bio-available N in N:P ratio to satisfy the demands of primary producers in surface waters. Subsequently, deposits of shallow-water carbonates comprising calcareous algae and massively-bedded lime muds accumulated widely around South China. These deposits appear to represent an unusual environment and ecosystem fertilized, perhaps, by the weathering products from the earliest stages of volcanism prior to the main Emeishan flood basalt eruptions at the Guadalupian-Lopingian boundary (GLB).