Lithium isotope behavior in Hawaiian regoliths: Soil-atmosphere-biosphere exchanges

Abstract

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 (τLi,Nb > 0) and displays peak Li accumulation in biologically-active, near-surface soil layers (humid, τLi,Nb = 10.9; arid, τLi,Nb = 2.8), with heavy Li isotopic compositions (humid, 4.7–9.9‰; arid, 4.0–13.9‰) with respect to the basalt signal (2.2‰). The deep regolith (>1 m) demonstrates δ7Li (ave. 3.3‰) comparable to the composition of the underlying parent basalt (2.2‰). 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 δ7Li in the humid regolith, and marine aerosol largely affects δ7Li 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 δ7Li 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.