『Abstract
Silicon isotope ratios (δ30Si) of bulk mineral materials
in soil integrate effects from both silicon sources and processing.
Here we report δ30Si 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 δ30Si. In upper (<1 m deep) soil horizons
along the climate gradient, Si depletion correlates with decreases
of residual δ30Si values in low rainfall soils and
increases in high rainfall soils. Strong positive correlation
between soil δ30Si 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 δ30Si 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 δ30Si
values in shallow (<1 m deep) soil horizons showed evidence of
biocycling induced Si redistribution and substantially lower δ30Si
values than predicted. Low δ30Si 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 δ30Si values
interpreted in conjunction with mineralogy and measures of Si
depletion.』
1. Introduction
2. Methods
2.1. Soil sampling, chemical, and mineralogical analyses
2.2. Halloysite purification and phytolith extraction from plants
2.3. Isotopic analyses
3. Results
3.1. Climate gradient δ30Si values and mineralogy
3.2. Si depletion and mineralogy influence on δ30Si
3.3. Soil, water extracts, and phytoliths at 180 mm rainfall
4. Discussion
4.1. Dust and Si depletion effects along a soil age gradient
4.2. Climate gradient Si mass-balance
4.3. Primary and secondary mineral weathering effects
4.4. Biologic silicon cycling effects
4.5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References