17:15 〜 19:15
[BCG06-P02] 初期地球における堆積物初期続成作用と鉄同位体の挙動に関する地球化学的検討
キーワード：黄鉄鉱中の鉄同位体記録、初期続成作用モデル、太古代古原生代
The iron (Fe) isotopic composition (d56Fe) of sedimentary pyrite provides crucial insights into the redox evolution of Earth's early oceans. During the late Archean and early Paleoproterozoic (2.8-2.0 Ga), bulk pyrite d56Fe values disperse within a broad range (up to ~3 per-mill), including exceptionally low minimum values (~-3.5 per-mill), that has never been observed in Phanerozoic sediments. The minimum d56Fe values declined toward ~2.65 Ga, followed by an increase into the early Paleoproterozoic. These geochemical records would reflect dramatic changes in the marine Fe cycle. While previous studies have attributed these trends to the seawater d56Fe variations caused by oxidative precipitation of Fe(OH)3 in the seawater column, the role of early diagenetic processes, such as microbial iron reduction, in modifying seawater and pyrite d56Fe values remains unclear. Here, we develop a reaction-transport model of marine sediments to quantify Fe isotope dynamics during early diagenesis. The model is validated using modern sediments from the Santa Barbara Basin, an analog for the low-oxygen, iron-rich environments characteristic of early Earth.
Our simulations assuming Archean and early Paleoproterozoic conditions show that minimum pyrite d56Fe values primarily reflect seawater d56FeFe2+ under a broad range of the assumed values of seawater SO42- while exhibiting minor diagenetic shifts (~0 to -0.5 per-mill). We further show that variations in Fe(OH)3/organic matter (CH2O) flux ratios (mol/mol) can generate pyrite with d56Fe values up to ~+3 per-mill higher than seawater by diagenetic processes, largely encompassing the isotopic variations observed across different stratigraphic formations. Furthermore, under high Fe(OH)3/CH2O flux ratios (> ~3), the depositions of Fe-oxides and siderite dominated and the burial of isotopically heavy Fe enhanced. This effect intensified as the ratio declined down to ~3, which would have contributed to the lowest global seawater d56FeFe2+ values observed around 2.65 Ga. As this ratio further decreases, pyrite deposition increased, and heavy Fe removal was suppressed. This would have caused the gradual increase of the seawater d56FeFe2+ values in the early Paleoproterozoic. We further discuss that the global decline in Fe(OH)3/CH2O ratios would be caused by the increase of oxygenic photosynthesis, which would have reshaped the marine Fe cycle and produced the long-term trend of the pyrite d56Fe values. This study provides a quantitative framework for discussing the role of early diagenesis in Fe isotope fractionation on early Earth and its relationship with the Earth's oxygenation.
Our simulations assuming Archean and early Paleoproterozoic conditions show that minimum pyrite d56Fe values primarily reflect seawater d56FeFe2+ under a broad range of the assumed values of seawater SO42- while exhibiting minor diagenetic shifts (~0 to -0.5 per-mill). We further show that variations in Fe(OH)3/organic matter (CH2O) flux ratios (mol/mol) can generate pyrite with d56Fe values up to ~+3 per-mill higher than seawater by diagenetic processes, largely encompassing the isotopic variations observed across different stratigraphic formations. Furthermore, under high Fe(OH)3/CH2O flux ratios (> ~3), the depositions of Fe-oxides and siderite dominated and the burial of isotopically heavy Fe enhanced. This effect intensified as the ratio declined down to ~3, which would have contributed to the lowest global seawater d56FeFe2+ values observed around 2.65 Ga. As this ratio further decreases, pyrite deposition increased, and heavy Fe removal was suppressed. This would have caused the gradual increase of the seawater d56FeFe2+ values in the early Paleoproterozoic. We further discuss that the global decline in Fe(OH)3/CH2O ratios would be caused by the increase of oxygenic photosynthesis, which would have reshaped the marine Fe cycle and produced the long-term trend of the pyrite d56Fe values. This study provides a quantitative framework for discussing the role of early diagenesis in Fe isotope fractionation on early Earth and its relationship with the Earth's oxygenation.