Episodic marine anoxia, primary production, and weathering controls on carbon‑sulfur cycling in the Early Cambrian Yangtze Block

Abstract

The Early Cambrian Period has received significant attention for decades owing to its geological records of unprecedented skeletal fossils, suggesting a major bio-event known as “the Cambrian Explosion.” Notwithstanding, related primary production and marine redox variation with respect to evolution of biota remain elusive. These factors are intimately linked to marine carbon‑sulfur cycling, promoting an extensive use of related isotopes (e.g., δ13C and δ34S) as paleoenvironmental proxies. Nevertheless, bulk-rock δ34S and δ13C may reflect multigenerational 34S/32S and 13C/12C isotopic fractionations. Herein, we analyzed drill cores from continuous depth profiles in the recently discovered Qingchuan-Ziyang-Yibin intrashelf basin for δ34S of sedimentary sulfate (δ34Ssulf), δ13C of carbonates (δ13Ccarb), and bulk-rock elements. We find that highly positive (+23.60 ‰ to +29.90 ‰) and negative (as low as −12 ‰) δ34Ssulf V-CDT are associated with samples exhibiting low and high detrital inputs, respectively. In contrast, total sulfur and δ13Ccarb V-PDB show no relationship with terrigenous materials. Enrichment factors of molybdenum (MoEF) and uranium (UEF), biogenic contents of nickel (Nibio) and copper (Cubio), as well as total organic carbon (TOC), are all widely distributed among the samples. Integration of these findings with existing data from shallow to deep marine environments in the Yangtze Block, supports the following conclusions: (1) Five episodes of widespread marine anoxia coincide with extensive primary production. (2) Inputs of terrigenous materials promote 32S-enriched sulfate, which adds to subsequent biochemical fractionations, resulting in generally complex pathways for δ34S of pyrite. (3) Terrigenous nutrients were critical to extensive marine primary production.