Weathering, dust, and biocycling effects on soil silicon isotope ratios

Abstract

Silicon isotope ratios (δ 30Si) of bulk mineral materials in soil integrate effects from both silicon sources and processing. Here we report δ 30Si values from a climate gradient of Hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy. The results demonstrate informative relationships between the mass fraction of soil Si depletion and δ 30Si. In upper (<1 m deep) soil horizons along the climate gradient, Si depletion correlates with decreases of residual δ 30Si values in low rainfall soils and increases in high rainfall soils. Strong positive correlation between soil δ 30Si and dust-derived quartz and mica content show that both trends are largely controlled by the abundance of these weathering-resistant minerals. The data also lend support to the idea that fractionation of Si isotopes in secondary phases is controlled by partitioning of silicon between dissolved and precipitated products during the initial weathering of primary basalt. Secondary mineral δ 30Si values from lower (>1 m deep) soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved Si in basalt-watershed rivers and driven by preferential 28Si removal from the dissolved phase during precipitation. In contrast, after correcting for the influence of dust, secondary mineral Si depletion and δ 30Si values in shallow (<1 m deep) soil horizons showed evidence of biocycling induced Si redistribution and substantially lower δ 30Si values than predicted. Low δ 30Si values in shallow soil horizons compared to predictions can be attributed to repeated fractionation as secondary minerals undergo additional cycles of dissolution and precipitation. Primary mineral weathering, secondary mineral weathering, dust accumulation, and biocycling are major processes in terrestrial Si cycling and these results demonstrate that each can be traced by δ 30Si values interpreted in conjunction with mineralogy and measures of Si depletion.