[U-05_30AM2] 生命-水-鉱物-大気相互作用

[U05-08] ケイ酸塩風化の見かけの活性化エネルギーに対する大気組成の影響

*菅崎 良貴¹、村上 隆¹ (1.東京大学大学院理学系研究科地球惑星科学専攻)

キーワード：ケイ酸塩風化, 二酸化炭素, フィードバック, 先カンブリア時代

Silicate weathering is a major sink of atmospheric CO₂. Because CO₂ is an important greenhouse gas, silicate weathering regulates not only the partial pressure of atmospheric CO₂ (Pco₂) but also the surface temperature (T). The apparent activation energy of silicate weathering represents the temperature dependence of silicate weathering and thus interrelates the intensity of silicate weathering, Pco₂ and surface temperature. It has been reported that solution composition can affect the apparent activation energy of dissolution/precipitation of silicates (e.g., Casey and Sposito, 1992; Lasaga, 1995; Cama et al., 1999). However, the relationship between the solution composition and the apparent activation energy of silicate reaction is not yet fully understood. To investigate the apparent activation energy of silicate weathering in a natural weathering system, we formulated the apparent activation energy of silicate weathering in three different scales, namely, (i) dissolution/precipitation of each mineral, (ii) elemental loss as the net reactions of the minerals and (iii) weathering flux from a weathering profile, based on the rate expressions in the three scales. It was found that, due to the effects of solution composition on the apparent activation energy, the temperature dependence of atmospheric CO₂ (ΔH^*co₂) affects the apparent activation energy of silicate weathering. Based on the formulated apparent activation energy, we estimated the apparent activation energy of silicate-weathering flux as a function of ΔH^*co₂. Then, the compensation law between the pre-exponential factor and the apparent activation energy of silicate-weathering flux was introduced from the literature, leading to the establishment of the relationship between silicate-weathering flux (Fco₂), T and ΔH^*co₂. Based on the Fco₂-T-ΔH^*co₂ relationship and the greenhouse effects of atmospheric CO₂ in the literature, we calculated the ratio of change in Fco₂ to that in Pco₂ as an indicator of silicate-weathering feedback in the Precambrian. The calculation revealed that when Pco₂ > 〜10^-0.5 atm, the feedback is negative and independent of Pco₂ and surface temperature. On the other hand, when Pco₂ < 〜10^-0.5 atm, the feedback is independent of Pco₂ but dependent on surface temperature; at low (< 〜30 ℃) and high (> 〜30 ℃) temperatures, the feedback is negative and positive, respectively. Due to the positive feedback, the conditions of Pco₂ < 〜10^-0.5 atm and T > 〜30 ℃ are unstable, and immediately change, with a slight change in Pco₂, to either the conditions of Pco₂ > 〜10^-0.5 atm or those of Pco₂ < 〜10^-0.5 atm and T < 〜30 ℃. When Pco₂ < 〜10^-0.5 atm and < 〜30 ℃, the feedback is not only negative, but also becomes more negative as temperature decreases, suggesting that global glaciations are harder to bring about than previously thought.