Fluctuated redox condition in Lower Triassic microbialite: evidence from magnetic mineral assamblage

Marine anoxia events, as indicated by geochemical proxies, have been widely studied as a possible mechanism driving the end-Permian mass extinction, which also inhibited the subsequent biotic recovery. However, redox changes in the aftermath of extinction and their further ecological effects are still controversial. Here, we report the characteristics of rock magnetic changes in several sections of the Permian-Triassic transition in South China and reinterpret the changes in redox conditions during this period. Our study shows that the magnetic mineral assemblages and magnetic characteristics considerably altered in the aftermath of the extinction, implying significant environmental changes. In microbialite sections developed in the shallow-water carbonate platform after the extinction, the variation of magnetic mineral assemblages records the fluctuated redox conditions under oxic and euxinic conditions, which explains the cleavage between continuous oxic or euxinic conditions suggested by paleontological and pyrite framboid evidence in earlier work, respectively. Due to the sensitivity of magnetite and hematite to the redox change, their preservation and the relative change between these two minerals have the potential to reflect the redox state. Hence, we propose a new magnetic proxy to reflect the redox conditions in the ancient ocean. This proxy correlates well with other geochemical redox proxies and the abundance changes of aerobic fossils, supporting its considerable reliability. In all, our study indicates that the microbialites were affected by frequent anoxia events while the photosynthetic oxygen production of cyanobacteria in the microbialites may be the key factor for buffer effect. Under the rapid increase in ocean temperature and an overall seawater oxygen depletion during the Permian-Triassic transition, microbialite and the microbes within it thus provide a more favorable habitat for the surviving metazoans.