NICKEL ISOTOPE EVIDENCE INDICATING ACTIVITY METHANOGENESIS DURING THE MELTING OF MARINOAN SNOWBALL EARTH

In South China, the Nantuo Formation that is dated between 654 and 635 Ma represents the Marinoan glacial deposits. Abundant pyrite aggregates are discovered in the top of the Nantuo Formation throughout the Yangtze Platform. The pyrite aggregate is composed of densely packed euhedral pyrite crystals, and has high sulfur isotopic compositions (δ³⁴S=13‰ ~ 24‰ in basin, 15‰ ~ +37‰ in slope). Thus, the pyrite aggregates may be precipitated within sediment porewater with H₂S diffused from sulfidic water column. The widespread pyrite aggregates may suggest rapid recovery of primary productivity during deglaciation. It can be further postulated that the deglaciation might be associated with emission of CH₄ from the ocean. In order to test whether there was active methanogenesis during the deglacial process, we measured the Ni isotopic composition of the Nantuo pyrite aggregates from four sections in South China. Ni is the key element in the enzyme for methanogenesis, and methanogens preferentially utilize light Ni (δ⁶⁰Ni_Methanogens - δ⁶⁰Ni_{culture medium} = -0.8‰), leaving heavy Ni in solution. Methanogenesis would generate organic matter with low δ⁶⁰Ni. If sulfate reducing bacteria (SRB) utilized organic matter derived from methanogens, pyrite would have low δ⁶⁰Ni values. δ⁶⁰Ni of pyrite samples from 3 basinal and 1 slope sections shows a wide range of variation (-0.7‰ ~ 1.5‰), and there is a negative correlation between Ni concentration in pyrite and δ⁶⁰Ni. The Ni isotope data can be interpreted as a binary mixing of two sources: seawater that has low Ni concentration (modern seawater: 300 ~ 600 ppt) and high δ⁶⁰Ni (modern seawater: δ⁶⁰Ni=1.4‰~1.8‰), and remineralization of methanogen-derived organic matter with lower δ⁶⁰Ni. It is noticed that pyrites from the slope sections have lower δ⁶⁰Ni than those from the basinal sections, suggesting more active methanogenesis probably due to higher organic matter input in the slope of Yangtze Platform. Furthermore, the wide range of variation in δ⁶⁰Ni (-0.4‰ ~+1.5‰) may indicate active methanogenesis that scavenged seawater Ni after the melting of snowball Earth. Finally, our study provides the direct geochemical evidence in supporting the development of oceanic euxinia and rapid recovery of primary productivity during the melting Marinoan snowball Earth.