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 (δ³⁴S_sulf), δ¹³C of carbonates (δ¹³C_carb), and bulk-rock elements. We find that highly positive (+23.60 ‰ to +29.90 ‰) and negative (as low as −12 ‰) δ³⁴S_sulf V-CDT are associated with samples exhibiting low and high detrital inputs, respectively. In contrast, total sulfur and δ¹³C_carb V-PDB show no relationship with terrigenous materials. Enrichment factors of molybdenum (Mo_EF) and uranium (U_EF), biogenic contents of nickel (Ni_bio) and copper (Cu_bio), 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.