WEATHERING LOSSES AND PHASE PARTITIONING OF ALUMINUM IN THE CRITICAL ZONE USING GALLIUM/ALUMINUM RATIOS
To investigate the extent of Ga/Al fractionation during weathering and phase partitioning, we measured Ga and Al in the top 10 m of soil at three sites: Southern Sierra CZO in California, Calhoun CZO in South Carolina, and Boulder Creek CZO in Colorado. The lithology of all sites were granitoids. Contemporary climatic conditions (mean annual precipitation and temperature) strongly varied among sites. Total Ga concentrations in soils ranged from 4 – 63 μg g-1 while total Al concentrations ranged from 19 – 167 mg g-1. Total Ga/Al ratios in soils ranged between 0.05 – 0.12 mmol mol-1. Surface horizons generally had lower Ga/Al ratios than deeper in the profile, suggesting preferential loss of Ga in the soil surface. A sequential extraction (ammonium acetate for exchangeable phase, hydrogen peroxide for organic matter bound phase, citrate-bicarbonate-dithionite for secondary oxide phase, and HF-HNO3 for residual phase) was conducted to determine partitioning of Ga and Al across various operationally-defined phases of sorption. Additional laboratory and field studies on Ga biogeochemistry are needed to further develop the application of the Ga/Al ratio for quantifying Al fluxes in the Critical Zone.