『Abstract
Germanium-silicon (Ge/Si) ratios were determined on quartz diorite
bedrock, saprolite, soil, primary and secondary minerals, phytolith,
soil and saprolite pore waters, and spring water and stream waters
in an effort to understand Ge/Si fractionation during weathering
of quartz diorite in the Rio Icacos watershed, Puerto Rico. The
Ge/Si ratio of the bedrock is 2μmol/mol, with individual primary
mineral phases ranging between 0.5 and 7μmol/mol. The ratios in
the bulk saprolite are higher (～3μmol/mol) than values measured
in the bedrock. The major saprolite secondary mineral, kaolinite,
has Ge/Si ratios ranging between 4.8 and 6.1μmol/mol. The high
Ge/Si ratios in the saprolite are consistent with preferential
incorporation of Ge during the precipitation of kaolinite. Bulk
shallow soils have lower ratios (1.1-1.6μmol/mol) primarily due
to the residual accumulation of Ge-poor quartz.
Ge/Si ratios measured on saprolite and soil pore waters reflect
reactions that take place during mineral transformations at discrete
depths. Spring water and baseflow stream waters have the lowest
Ge/Si ratios (0.27-0.47μmol/mol), reflecting deep initial weathering
reactions resulting in the precipitation of Ge-enriched kaolinite
at the saprolite-bedrock interface. Mass-balance calculations
on saprolite require significant loss of Si and Al even within
1 m above the saprolite-bedrock interface. Higher pore water Ge/Si
ratios (～1.2μmol/mol) are consistent with partial dissolution
of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase
up through the saprolite and into the overlying soil, but never
reach the high values predicted by mass balance, perhaps reflecting
the influence of phytolith recycling in the shallow soil.』
1. Introduction
2. Study site
3. Methods
3.1. Sample collection and processing
3.1.1. Rock and mineral samples
3.1.2. Soil and saprolite samples
3.1.3. Water samples
3.1.4. Soil paste experiments
3.1.5. Plant phytolith extractions
3.2. Sample analyses
4. Results
4.1. Quartz diorite parent material
4.2. Saprolite and soil
4.3. Phytoliths
4.4. Water samples
4.4.1. Soil paste experiments
5. Discussion
5.1. Ge/Si fractionation by weathering
5.2. Mass-balance calculations
5.2.1. Bedrock-saprolite weathering transformation
5.2.2. Saprolite and soil weathering
5.3. Phytolith influence in Puerto Rico
5.4. Comparison of silica tracers: Ge/Si vs. δ30Si
6. Conclusions
Acknowledgments
References