[B-CG06] 地球史解読：冥王代から現代まで

[BCG06-P06] 太古代-古原生代における海底堆積物の初期続成作用と鉄同位体比変動の数理モデリング

★招待講演

*雀地 遼平¹、田近 英一¹、渡辺 泰士^1,2 (1.東京大学、2.気象庁気象研究所)

キーワード：鉄同位体、続成作用モデル、縞状鉄鉱層、黄鉄鉱

Iron (Fe) isotopic compositions of Archean-Paleoproterozoic siderite (FeCO₃) and magnetite (Fe₃O₄) in iron formations (IFs) and of sedimentary pyrite (FeS₂) have the potential to constrain the oceanic redox environment and marine biosphere on early Earth. This is due to Fe isotopic fractionation that occurs during the formation of iron minerals through biotic and/or abiotic pathways, which are recorded in the marine sediments when they formed and experienced early diagenesis. However, the interpretation of Fe isotope ratios in IFs and pyrites is controversial and influenced by various factors, such as Fe sources, mode of primary precipitation, subsequent mineral transformations, and Fe isotopic fractionation during pyrite formation. Therefore, a quantitative framework that considers the processes controlling the preservation patterns of iron isotopic compositions in sediments would be required for comprehending the past environmental conditions and biological activity during the Archean and the Proterozoic. Here we developed a reaction-transport model of marine sediments to solve the steady-state concentrations at various sediment depth for organic matter (OM), oxidants (O₂, NO₃^-, MnO₂, Fe(OH)₃, Fe₃O₄, SO₄^2-, ΣDIC), reductants (Mn²⁺, Fe²⁺, Ca²⁺, ΣNH₄, ΣH₂S, S⁰, CH₄, H₂, FeS, FeS₂, FeCO₃), and acid-base speciation (carbonate, ammonia, sulfide, and phosphate species), as well as pH in pore water in sediment. Owing to the nonlinear and extensively coupled nature of the partial differential equations in our models, we utilized the mathematical package DVODE (Brown et al., 1989), drawing upon methods outlined by Boudreau (1996). Our model can simulate the preservation patterns of iron isotopic compositions in each Fe mineral during early diagenesis. This enables investigation of the effects of various factors, such as different seawater sulfate and oxygen concentrations, and Fe and organic matter fluxes, on the iron geochemical and isotopic behaviors in Archean-Paleoproterozoic marine sediment. Using this model, we will present the results of an extensive and systematic parameter studies on IFs and sedimentary pyrites formed in Archean and Paleoproterozoic marine environments.