Reconciling redox proxy contradiction with active FeMn shuttle in the Cryogenian Nanhua Basin of South China

Abstract

The Cryogenian Period is one of Earth's most critical time intervals for biological innovation and manganese (Mn) accumulation. The formation of the Mn ores in the Cryogenian Nanhua Basin (South China) has been attributed to early diagenesis with transient oceanic oxygenation evidenced by low C_org/P and high Ce/Ce*, but reconstruction of the ocean redox with Fe speciation revealed persistently anoxic water conditions in the basin. To address this disagreement, we conducted new δ³⁴S measurements of pyrite and carbonate-associated sulfate (CAS), together with bulk major and trace element compositions in three drill cores of the Datangpo Formation collected from the Nanhua Basin. The δ³⁴S values of both CAS and pyrite were significantly higher than the estimated seawater sulfate in the contemporary open ocean, likely reflecting the hydrographic restriction of the Nanhua Basin and a small sulfate reservoir size in the open ocean. Additionally, the superheavy δ³⁴S of pyrite is higher than its paired CAS, which can be best explained by partial oxidation of H₂S during the inflow of open ocean seawater, or the emission of volatile organosulfur, both indicating the bottom water was not fully oxygenated. Partial oxidation of H₂S might be caused by the episodic inflow of open ocean seawater, which would result in fluctuations in the depth of the chemocline, enhancing the activity of the FeMn shuttle. The operation of FeMn shuttle might adsorb P and Ce in shallow oxic waters, transport them to the deeper anoxic waters, and release them back into the water column. Therefore, we propose that the low C_org/P and positive Ce/Ce* in the Mn‑carbonates may indicate a stronger FeMn shuttle effect rather than oxic water conditions. The contradictory results of different redox proxies in the Cryogenian Nanhua Basin thus can be reconciled with widespread anoxia in the open ocean, strong basin restriction, and episodic seawater inflow that caused enhanced FeMn shuttle.