Regional and global proxies for varying ocean redox conditions at ∼1.57 Ga: A causal connection with volcanism-induced weathering

Abstract

We present rare earth element (REE) and Mo isotope data from the middle Gaoyuzhuang Formation in North China in order to contribute to the debate on the Mesoproterozoic marine redox state. Sequential leaching of the Gaoyuzhuang calcareous shales exhibits distinct enrichment patterns of trace metals between carbonate and authigenic phases, which are likely caused by element redistribution during organic matter degradation. Collectively, our results and literature data of the Gaoyuzhuang Formation show stratigraphically coupled variations in Ce anomalies and Mo stable isotope signatures. These geochemical shifts can be interpreted as a response to progressive oxygenation in regional and global settings, respectively. However, the oxygenation magnitudes expressed by Ce anomalies (Ce/Ce* = ∼0.8) and Mo isotopes (δ98Mo = 2.33‰) are markedly different. Rather than prevalent ocean oxygenation, we suggest that the modern-like seawater Mo isotope values represent accelerated input of reactive Fe from volcanism-induced weathering and subsequent burial of light Mo isotopes into the sediments. Furthermore, the documented transition coincides with the occurrence of macroscopic fossils, supporting a causal relationship between volcanism, environmental change, and biological evolution.

Keywords

• Sequential leaching
• Cerium anomalies
• Molybdenum isotopes
• Gaoyuzhuang formation
• North China