[M-IS14] 古気候・古海洋変動

[MIS14-P04] Evolution of the global osmium cycle in the history of the Earth

★Invited Papers

*渡辺 泰士¹、松本 廣直² (1.東京大学、2.筑波大学)

キーワード：オスミウム、全球凍結

The isotope signature of seawater osmium (Os) reflects the relative contribution of multiple processes such as weathering of rocks and sediments and hydrothermal activities, serving as an important proxy for constraining the past biogeochemical cycles. Previously, long-term continuous reconstructions of osmium cycles have been limited to Cenozoic, while recent accumulation of osmium isotopic data allowed the understanding of the long-term osmium evolution for the last 140 million years (Matsumoto et al., 2021; 2022; 2024; Percival et al., 2025). In this presentation, we first review the evolution of seawater Os isotopic evolutions and reveal the global osmium cycle from the Cretaceous to Cenozoic. Then we extend our analysis to the early Earth. We employ a biogeochemical C–Sr–Os–Mg cycle model (Li and Elderfield, 2013), which is driven by input data of carbon isotope signatures of carbonate and organic matters, reconstructions of Sr and Os isotope signatures, and estimation of the past seafloor spreading rate. Using this model, we constrain the global osmium cycles for the last 140 million years to understand the factors controlling the osmium isotope signatures during this period. We show that the weathering rate of basalts should have been two or three times higher during the Cretaceous than the present, while the sediment weathering of organic matters should vary between 0.3 and 1.4 times relative to the present to explain the marine osmium isotopic evolutions. We further show that the sediment weathering rate should become as low as 0.1 times relative to present to explain the low seawater ¹⁸⁷Os/¹⁸⁸Os, which is consistent with the estimate by the previous study (Li and Elderfield, 2013). This result infers that the importance of the basalt and sediment weatherings in driving the variations of marine Os isotope signatures in the Cretaceous and Cenozoic. We further employ a model of C–P–O₂–Fe–S–Os biogeochemical cycles developed based on the previous study (Watanabe et al., 2023) to discuss the evolution of the marine Os cycles in the Earth's history. During the Paleoproterozoic and Neoproterozoic, multiple snowball Earth events would have occurred. In the aftermath of the snowball Earth events, a pervasive hot environment that led the complete deglaciation increased the seawater ¹⁸⁷Os/¹⁸⁸Os transiently, which is consistent with the estimated initial seawater ¹⁸⁷Os/¹⁸⁸Os in the aftermath of the Paleoproterozoic snowball Earth event (Sekine et al., 2011) and the Neoproterozoic snowball Earth event (Rooney et al., 2020). On the basis of the simulated seawater ¹⁸⁷Os/¹⁸⁸Os, we will further discuss the uncertainty in the current understanding of the global osmium cycle.