Opposing Effects of Rainfall and Aerosol Washout on Soil Silicon Isotope Ratios

Secondary mineral precipitation in soils fractionates silicon isotopes relative to soil solution. Increasing secondary mineral prevalence, in conjunction with overall loss of Si from soil, drives bulk soil d³⁰Si values lower with progressive soil development. Here we report the net effect of two opposing processes on bulk soil d³⁰Si: climate driven weathering and washout of mineral aerosols. The study utilizes a climosequence located on a 170 ka lava flow on Kohala Mountain in Hawaii. Rainfall ranges from 160 to 2500 mm yr^-1 and is a primary driver for weathering and soil development. Total mineral dust addition to soil also increases with rainfall, from approximately 25 to 200 kg m^-2. A climatic rainfall threshold of 1400 mm yr^-1, corresponding to the point at which annual soil water balance becomes positive, divides the soils into two categories with regards to d³⁰Si evolution. On the drier side of the threshold, net soil Si loss is associated with secondary mineral precipitation fractionating Si. Bulk soil d³⁰Si declines from -0.6 to -2.4‰ with Si loss from 35% to 80% depletion relative to lava. On the wetter side of the threshold, dissolution consumes both primary and secondary minerals, driving more extensive Si loss from 77% to 90% depletion relative to lava. These losses, combined with greater dust inputs with increased rainfall, increase the contribution of weathering resistant, dust-derived quartz and mica to the Si pool. The d³⁰Si of dust minerals is similar to lava (-0.5‰), causing soil d³⁰Si to increase from -2.5 to -0.8‰ with extreme weathering. These results highlight another role for dust in rejuvenating weathered soils. Further, while quartz and mica strongly constrain the evolution of bulk soil d³⁰Si, more easily weathered dust minerals (e.g. plagioclase) are likely to dominate the evolution of the d³⁰Si supplied from soils to rivers and ultimately the oceans.