ACTIVE METHANOGENESIS DURING THE MELTING OF MARINOAN SNOWBALL EARTH: EVIDENCE FROM RARE EARTH ELEMENT

Abundant pyrite aggregates are discovered from the top 0.5 to 10 m interval of the Nantuo Formation (654-635 Ma) in the Yangtze Platform, South China. The sulfur isotope data suggest that the Nantuo pyrite aggregates were precipitated within sediment porewater with H₂S diffused from sulfidic seawater. Thus, the formation of Nantuo pyrite aggregates throughout the Yangtze Platform may indicate the development of oceanic euxinia during the melting of Marinoan snowball Earth. In order to further constrain the marine biogeochemical cycle during deglaciation, here we report the rare earth element (REE) compositions of the Nantuo pyrite. The Nantuo pyrites have distinctive REE patterns (normalized to the post-Archean Australian average shale (PAAS)) depending on the depositional environment. The pyrite from the basinal sections shows HREE-deplete pattern, while those from the slope environments enrich in MREE. HREE depletion might be attributed to active microbial activities that absorb HREE in organic matter, while MREE-enrichment may be related to the redox cycle of Fe-oxide that preferentially shuttles MREE. Besides, there’s a negative correlation between La/Yb (i.e. LREE/HREE) and Ni concentration, suggesting the methanogens might be an important source of organic matter during pyrite formation, since Ni is an essential element for methanogens. We suggest that methanogens preferentially scavenges HREE and consume seawater Ni. Remineralization of organic matter derived from methanogens releases Ni and HREE in porewater, lowering the LREE/HREE ratio and elevating Ni concentration. Thus, the HREE-depleted pattern of the basinal samples may be attributed to the active microbial activity in sediment porewater without significant remineralization of the biomasses, and the absence of MREE-enrichment may indicate inefficient redox cycle of Fe-oxides. In contrast, oxidation of bacterial biomasses (e.g., methanogens) as well as active redox cycle of Fe-oxides in the slope environments tends to alleviate the HREE-depletion pattern (LREE/HREE≈1) and enrich in MREE (MREE/LREE > 1) and Ni. Therefore, the REE data of the Nantuo pyrite confirm the active methanogenesis during the melting of Marinoan snowball Earth.