The lithium and magnesium isotope signature of olivine dissolution in soil experiments
The lithium and magnesium isotope signature of olivine dissolution in soil experiments
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DOI：
10.1016/j.chemgeo.2020.120008
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发表时间：
2021-01
期刊：
影响因子：
3.9
通讯作者：
Philip A. E. Pogge von Strandmann;P. Renforth;A. Joshua West;M. Murphy;Tu-Han Luu;Gideon M. Henderson
中科院分区：
文献类型：
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作者：
Philip A. E. Pogge von Strandmann;P. Renforth;A. Joshua West;M. Murphy;Tu-Han Luu;Gideon M. Henderson
这项研究从两个土芯的风化实验中得到了土壤及其排水的锂镁同位素比值，一个土芯在表层添加橄榄石，另一个土芯作为对照。实验设计模拟了在土壤中添加橄榄石以用于二氧化碳封存和/或作物施肥，以及在火山沉积期间等自然表面添加活性矿物。更广泛地说，这项研究提供了一个更好地理解同位素分馏如何记录风化过程的机会。在实验开始时，排出两个岩芯的水的镁同位素组成与土壤可交换池相似。随着橄榄石溶解的进行，两个核中的成分向不同的方向演化。质量平衡计算表明，水δ26 mg值受碳酸盐和硅酸盐(后者仅在橄榄石岩心)的一致溶解以及一个同位素分馏的可交换库控制。对于Li来说，离开岩心底部的水最初具有相同的同位素组成，但随着橄榄石溶解的进展，水的同位素组成发生了变化。对于镁和锂来说，与可交换库的交换显著地延缓和分馏了向下的运输。这一观察结果对利用微量元素或同位素监测强化风化有意义，因为在可交换库向新的平衡演化期间，溶解速度和通量将被低估。
This study presents lithium and magnesium isotope ratios of soils and their drainage waters from a well-characterised weathering experiment with two soil cores, one with olivine added to the surface layer, and the other a control core. The experimental design mimics olivine addition to soils for CO2sequestration and/or crop fertilisation, as well as natural surface addition of reactive minerals such as during volcanic deposition. More generally, this study presents an opportunity to better understand how isotopic fractionation records weathering processes. At the start of the experiment, waters draining both cores have similar Mg isotope composition to the soil exchangeable pool. The composition in the two cores evolve in different directions as olivine dissolution progresses. Mass balance calculations show that the water δ26Mg value is controlled by congruent dissolution of carbonate and silicates (the latter in the olivine core only), plus an isotopically fractionated exchangeable pool. For Li, waters exiting the base of the cores initially have the same isotope composition, but then diverge as olivine dissolution progresses. For both Mg and Li, the transport down-core is significantly retarded and fractionated by exchange with the exchangeable pool. This observation has implications for the monitoring of enhanced weathering using trace elements or isotopes, because dissolution rates and fluxes will be underestimated during the time when the exchangeable pool evolves towards a new equilibrium.