Organic carbon isotope gradient and ocean stratification across the late Ediacaran-Early Cambrian Yangtze Platform

Abstract

Organic carbon isotope (δ 13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ 13Corg values of the late Ediacaran Dengying Formation range from −29‰ to −24‰. In the deep-water Longbizui section, δ 13Corg values from time-equivalent strata of the Dengying Formation are mostly between −35 and −32‰. These new data, in combination with δ 13Corg data reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δ 13Corg gradient. High δ 13Corg values (>−30‰) occur mostly in shallow-water carbonate rocks, whereas low δ 13Corg values (<−32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ 13Corg variations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ 13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ 13Corg values (>−30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ 13Corg data and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δ 13Corg values (<−32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ 13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.