RECOVERY OF MARINE PRODUCTIVITY AFTER THE SNOWBALL EARTH: EVIDENCE FROM PYRITE IRON ISOTOPES

Diversification of eukaryotes immediately after the Marinoan snowball Earth glaciation (~650 – 635 Ma) implies the direct linkage between global glaciation and biological evolution. Previous studies indicate rapid recovery of marine primary productivity and the development of oceanic euxinia before cap carbonate precipitation. This scenario is supported by the massive precipitation of pyrite aggregates throughout the Yangtze Platform, South China. It is proposed that dissimilatory sulfate reduction (DSR) occurred in the water column, and diffusion of H2S from euxinic seawater into sediment porewater resulted in the syndepositional pyrite precipitation near the top of glacial deposits. However, diversification of eukaryotes requires the disappearance of sulfidic condition, which might be the consequence of pyrite precipitation scavenging seawater H2S. Thus, pyrite precipitation might be responsible for the change of seawater chemistry. Pyrite formation involves with both DSR and dissimilatory iron reduction (DIR), and removal of seawater euxinia requires excessive supply of ferrous Fe (Fe2+). Thus, abundant pyrite aggregate formation also implies active DIR in sediment porewater. In this study, we analyzed Fe isotope composition (δ56Fe) of pyrite aggregates from the Nantuo Formation. Pyrite from the slope section has the highest sulfur isotopes (δ34S) and δ56Fe of crustal composition, while the shelf samples have the lowest δ34S but the highest δ56Fe values. The Rayleigh distillation model indicate that pyrite precipitation in the basin and slope requires abnormally high content of reactive Fe, probably sourcing from the underlying deposits. Our study suggests a two-stage recovery of productivity in the aftermath of Marinoan glaciation. In the early stage, seawater was ferruginous due to insufficient organic matter input, while DIR in sediment porewater represents the major trajectory anaerobic organic matter degradation. In the second stage, DSR in seawater was fueled by enhanced organic matter input, driving seawater sulfidic. An excessive supply of Fe2+ from sediment resulted in the precipitation of pyrite aggregates, and seawater returned to the ferruginous condition.