[B-CG07] 地球史解読：冥王代から現代まで

[BCG07-05] Boron isotope fluctuation during a snowball event: what caused the different duration of Neoproterozoic glaciations?

★Invited Papers

*門屋 辰太郎¹ (1.海洋研究開発機構)

キーワード：全球凍結イベント、新原生代、ボロン同位体比

The Earth underwent a global glaciation, also known as a Snowball Earth event, at least three times. The second and third snowball events occurred in a relatively short period during the Neoproterozoic while the second event lasted longer by a factor of four to fifteen than the third event. The difference in duration should have resulted from differences between the events, such as CO₂ degassing rate, CO₂ consumption rate, and/or planetary albedo. Nonetheless, it has still not been clear what caused the different durations. This study focuses on Boron isotopes (¹⁰B and ¹¹B) associated with the Neoproterozoic Snowball events. Some previous studies measured Boron isotopes and showed a negative excursion in the Boron isotope ratio (¹¹B/¹⁰B) after the third event while the ratio was almost constant after the second event. Boron isotopes are often utilized as a pH indicator. The Neoproterozoic Boron data were considered to indicate that ocean pH was lower after the third (shorter) snowball event than the second (longer) event. In other words, the data seems to imply that the atmospheric CO₂ level was higher after the third event than after the second event although this interpretation seems inconsistent with the difference in duration. This study simulates oceanic boron and shows oceanic boron concentration decreases owing to a lack of continental weathering during a snowball event. The decrease in the boron concentration also decreases the Boron isotope ratio (¹¹B/¹⁰B) because the residence time of ¹¹B is shorter than that of ¹⁰B. The effect of the concentration on the isotope ratio overwhelms that of the pH. Accordingly, the boron isotope ratio associated with a snowball event would indicate the variation of Boron concentration rather than pH. In addition, the decrease in boron concentration and, therefore ¹¹B/¹⁰B can be suppressed if continental weathering proceeds during a snowball event. Considering the Neoproterozoic Boron data, the negative excursion in the boron isotope ratio could indicate the decrease in the oceanic boron concentration owing to the lack of continental weathering during the third event. On the other hand, the constant value of the Boron isotope ratio could indicate that the oceanic boron concentration was also constant because the continental weathering proceeded during the second event. The difference in continental weathering, i.e., the uptake of atmospheric CO₂, is also consistent with the difference in duration. Though the reason causing different continental weathering is still unclear, the duration and boron isotope data of the Neoproterozoic snowball Earth events would indicate the variety of interaction between continent and ocean under global glaciations.