The origin of abnormally 13C-depleted organic carbon isotope signatures in the early Cambrian Yangtze Platform

Abstract

A significant negative organic carbon isotope (δ13Corg) excursion occurred in the lower member of the lower Cambrian Qiongzhusi organic-rich shales in the Sichuan Basin, which is also reported in other contemporaneous strata throughout the Yangtze Platform. In this study, 47 lower Cambrian drill core samples were collected to investigate the possible formation mechanism of this negative δ13Corg excursion using multiple geochemical approaches. Primary paleoproductivity, redox conditions and organic carbon isotopic compositions were determined for these core samples of the Qiongzhusi Formation in the Sichuan Basin. The results suggest that the degree of microbially mediated carbon cycling and its contribution to the total sedimentary organic matter has an important influence on organic carbon isotopic compositions. An extensive transgression during the deposition of the lower member of the Qiongzhusi Formation brought abundant nutrients to primary producers and thus stimulated blooms, which is evidenced by the high primary productivity in the lower member of the Qiongzhusi Formation. In addition, marine transgression also provided sulfide to the seawater. Enhanced primary productivity accelerated the consumption of bottom water oxygen and thus promoted the establishment of euxinic bottom water conditions. The euxinic conditions promoted an anaerobic carbon cycle mediated by sulfate-reducing bacteria and chemoautotrophic bacteria, which resulted in the dissolved inorganic carbon reservoir becoming more depleted in 13C. Once these 13C-depleted carbons are utilized by primary producers, the 13C-depleted carbons will participate in the carbon cycle. This series of processes ultimately resulted in lower δ13Corg values within the lower Cambrian Qiongzhusi organic-rich black shales. A shift to positive and rather stable δ13Corg values in the relatively organic-poor sediments indicates a smaller contribution of 13C-depleted bacterial biomass (e.g., sulfate-reducing bacteria and chemoautotrophic bacteria), which is evidenced by more oxygenated ocean bottom conditions in the study area.