White(2002)による〔『Determining mineral weathering rates based on solid and solute weathering gradients and velocities: application to biotite weathering in saprolites』(69p)から〕


 Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.
 Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piemont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6 ×10-17 mol m-2 s-1for biotite. Faster weathering rates of 1.8 to 3.6 × 10-16 mol m-2 s-1 are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.

Keywords: Mineral weathering rates; Biotite weathering; Saprolites』

 風化勾配は、米国の北ジョージア州のPiemontおよびプエルトリコのLuquillo山脈のサプロライト質レゴリスにおける黒雲母の風化速度を計算するのに用いられた。PanolaでのMgとKに対する固体状態風化勾配は、 黒雲母について 3 〜 6 ×10-17 mol/m2/秒であった。 Rio Icacosレゴリスでは、1.8 〜 3.6 × 10-16 mol/m2/秒というもっと速い風化速度がMgとKの孔隙水勾配を基に計算された。相対的な速度は、プエルトリコの方がより温暖でより湿潤な熱帯気候であることと一致する。両方の天然からの速度は、報告されている黒雲母風化の実験による速度よりも3〜6桁小さい。』

1. Introduction
2. Methodology
 2.1. Calculation of solute concentration
 2.2. Calculation of solid-state concentrations
 2.3. Mass changes due to weathering
 2.4. Weathering gradients
 2.5. Weathering velocities
 2.6. Surface areas
 2.7. Parallel weathering rate equations
 2.8. Application to biotite weathering in saprolites
3. Results
 3.1. Stoichiometry of biotite weathering
 3.2. Solid-state weathering gradients
 3.3. Solute weathering gradients
 3.4. Biotite surface areas
 3.5. Determination of weathering velocities
 3.6. Calculation of weathering rates
4. Discussion
 4.1. Role of physical parameters
 4.2. Comparison of weathering gradients and velocities
 4.3. Comparison of biotite weathering rates
5. Summary