Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth’s surface environments

Abstract

Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H 4 SiO 4 solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth's surface environments. The results reveal that, in comparison to aqueous H 4 SiO 4 , heavy Si isotopes will be significantly enriched in secondary silicate minerals. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of 28 Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated, and the results support the previous statement that highly 28 Sienrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations. With the equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many of Earth's surface systems can be explained. For example, the change of bulk soil d 30 Si can be predicted as a concave pattern with respect to the weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesquioxides and poorly crystalline minerals reaches their maximum. When, under equilibrium conditions, the well-crystallized clays start to precipitate from the pore solutions, the bulk soil d 30 Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the d 30Si variations in the ground water profile. The equilibrium Si isotope fractionations among the quadracoordinated organosilicon complexes and the H 4 SiO 4 solution may also shed light on the Si isotope distributions in the Si-accumulating plants.

Response to Referee #1

“…The δ 30 Si Spicules average signature of the two siliceous sponge families from the compiled data presented in Fig. 2 (Hendry and Robinson, 2012, Wille et al, 2010, Hendry et al, 2010, with the equatorial Atlantic data (JC094)) show that the Hexactinellida class is significantly lighter than Demospongiae, with δ 30 Si Spicules =−2.66 ± 0.21 ‰ (CI (confi- dence interval) of the mean) and −1.91 ± 0.30 ‰ (CI of mean) respectively. However, it is important to take into consideration the environmental conditions of growth because δ 30 Si Spicules depends on the δ 30 Si and DSi concentration of seawater and the two groups live at different depth ranges and nutrient conditions.…”

“…Samples were acidified with 8 N HNO 3 and diluted with Milli-Q water to reach a pH of 2-3. This cleaning procedure followed the technique in Hendry et al (2010) and Hendry and Robinson (2012).…”

“…To eliminate the influence of these two parameters and resolve whether or not Demospongiae and Hexactinellida fractionate Si isotopes in different ways, a δ 30 Si residual has been calculated: δ 30 Si residual = 30 Si Observed − 30 Si Spiculesbestfit . Three best fits have been calculated assuming the hyperbolic relationship between DSi and δ 30 Si (Hendry and Robinson, 2012) to deconvolve the influence of fused skeleton. Equations (3), (4) and (5) (Fig.…”

“…Equations (3), (4) and (5) (Fig. 5a, c, e, respectively) with the compilation (a) of published data (Hendry and Robinson, 2012;Wille et al, 2010;Hendry et al, 2010), (b) of published and all JC094 data, and (c) of published and JC094 data without the fused spicules (F1 to F5). Only the samples with class identification are include in the three residual results.…”

Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites

“…In this study, we used the volume variable cluster model (He et al, ; Li & Liu, ; Liu, ) to calculate RPFRs for 26 Mg/ 24 Mg in langbeinite. The American Mineralogist Crystal Structure Database provided langbeinite lattice parameters (http://rruff.geo.arizona.edu/AMS/amcsd.php) for simulating crystal structure.…”