Title: Reconciling redox proxy contradiction with active Fe-Mn shuttle in the Cryogenian Nanhua Basin of South China
Author: Meng Cheng*, Zihu Zhang, Jun Hu*, Haiyang Wang, Mengchun Cao, Chao Li
Journal: Palaeogeography, Palaeoclimatology, Palaeoecology
Date of Publication: Jun 1 2025
Volume: 667
DOI: 10.1016/j.palaeo.2025.112899
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 Corg/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 delta 34S 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 delta 34S 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 delta 34S 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 Fe-Mn shuttle. The operation of Fe-Mn 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 Corg/P and positive Ce/ Ce* in the Mn-carbonates may indicate a stronger Fe-Mn 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 Fe-Mn shuttle.
Key Words: Datangpo Formation; Mn carbonate; Seawater inflow; Primary productivity; Redox conditons, Chemocline
