Lithium isotope behavior in Hawaiian regoliths: Soil-atmosphere-biosphere exchanges
Lithium isotope behavior in Hawaiian regoliths: Soil-atmosphere-biosphere exchanges
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DOI：
10.1016/j.gca.2020.07.012
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发表时间：
2020-09-15
影响因子：
5
通讯作者：
Chadwick, Oliver A.
中科院分区：
文献类型：
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作者：
Li, Wenshuai;Liu, Xiao-Ming;Chadwick, Oliver A.
在使用锂同位素作为风化示踪剂之前，了解陆地土壤-大气-生物圈交换对锂地球化学行为的影响至关重要。我们研究了夏威夷科哈拉山波罗鲁熔岩形成的潮湿和干旱风化层剖面的锂地球化学特征。浅层风化层（0-1 m 深）保留了 Li（tau(Li, Nb) > 0），并在生物活性、近地表土壤层（潮湿，tau(Li, Nb) = 10.9；干旱，tau(Li, Nb) = 2.8）中显示出 Li 积累峰值，具有重 Li 同位素组成（潮湿，千分之 4.7-9.9；干旱，相对于玄武岩信号（千分之 2.2），千分之 4.0-13.9。深层风化层（>1 m）显示δ Li-7（平均千分之 3.3）与底层母玄武岩的成分（千分之 2.2）相当。浅层风化层中的锂丰度和同位素组成与深层风化层中的锂丰度和同位素组成的解耦意味着风化层对垂直位置锂化学的控制不同。浅层风化层中的锂地球化学很大程度上受到以下因素的影响：（i）大气沉积，（ii）植物循环，以及（iii）次生矿物形成。除了风化蚀变外，我们的数据表明，灰尘的添加主要影响潮湿风化层中的δLi-7，海洋气溶胶主要影响干旱风化层中的δLi-7。在潮湿的风化层中，向下渗流迁移允许更深、更高级的风化，而在干旱的风化层中，偶尔的润湿事件和随后的快速干燥可能占主导地位。因此，生物循环以及最近哈维火山喷发造成的亚洲尘埃和火山灰的沉积是导致土壤中锂向上富集以及浅层风化层中土壤三角洲Li-7成分增加的原因。这在植物覆盖密集、降雨量大的潮湿场地尤为重要。相比之下，湿润和干旱地区的深层风化层主要受孔隙流体渗滤和堆积的影响，从而继承了来自孔隙流体的重锂同位素信号。我们的研究强调化学风化过程中陆地锂循环的气候调节和长期被忽视的生物控制。 (C) 2020 Elsevier Ltd. 保留所有权利。
Understanding the influence of terrestrial soil-atmosphere-biosphere exchanges on Li geochemical behaviors is vital before using Li isotopes as a weathering tracer. We investigated Li geochemistry of the humid and arid regolith profiles formed on the Pololu lavas, the Kohala Mountain, Hawaii. The shallow regolith (0-1 m depth) retains Li (tau(Li, Nb) > 0) and displays peak Li accumulation in biologically-active, near-surface soil layers (humid, tau(Li, Nb) = 10.9; arid, tau(Li, Nb) = 2.8), with heavy Li isotopic compositions (humid, 4.7-9.9 parts per thousand; arid, 4.0-13.9 parts per thousand) with respect to the basalt signal (2.2 parts per thousand). The deep regolith (>1 m) demonstrates delta Li-7 (ave. 3.3 parts per thousand) comparable to the composition of the underlying parent basalt (2.2 parts per thousand). Decoupling of Li abundance and isotopic composition in the shallow regolith from those of the deep regolith implies different regolith controls on Li chemistry at vertical locations. The Li geochemistry in the shallow regolith has been substantially influenced by: (i) atmospheric deposition, (ii) plant cycling, and (iii) secondary mineral formation. In addition to weathering alteration, our data show that dust addition mainly influences delta Li-7 in the humid regolith, and marine aerosol largely affects delta Li-7 in the arid regolith. In the humid regolith, downward seepage migration allows for deeper and more advanced weathering, whereas occasional wetting events followed by rapid drying likely dominate in the arid regolith. Thus, biological cycling and the deposition of Asian dust and volcanic ash from the more recent Hawi eruptions are responsible for upward Li enrichment in soils and increases of soil delta Li-7 composition in the shallow regolith. This is particularly important in the humid site with dense plant coverage and heavy rainfall. By contrast, the deep regolith of the humid and arid sites is mostly affected by pore fluid percolation and accumulation, thus inheriting heavy Li isotope signals from pore fluids. Our study emphasizes climate-regulated and long-neglected biological controls on terrestrial Li cycling during chemical weathering. (C) 2020 Elsevier Ltd. All rights reserved.