Pett-Ridge,J.C., Derry,L.A. and Kurtz,A.C.(2009): Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico. Geochimica et Cosmochimica Acta, 73, 25-43.

『プエルトリコのルキジョ山脈の熱帯花崗岩質岩流域における風化過程と降塵量のトレーサーとしてのストロンチウム同位体』


Abstract
 Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major element alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100±700 mg cm-2 ka-1.Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified.』

1. Introduction
2. Site description
3. Saharan dust in the Caribbean
4. Methods
5. Results
 5.1. Water samples
 5.2. Soil, saprolite, and bedrock samples
6. Discussion
 6.1. Weathering processes
  6.1.1. Bedrock weathering
  6.1.2. Saprolite and soil weathering
  6.1.3. Influence of precipitation inputs on soil Sr
  6.1.4. Mineral separates
 6.2. Weathering fluxes and Sr mass balance
  6.2.1. Accounting for Sr components of weathering system
  6.2.2. Testing a previous model of weathering rates in the Icacos basin
  6.2.3. Watershed Sr mass balance
 6.3. Dust inputs
  6.3.1. Sr isotope-based calculation
  6.3.2. Nd isotope-based calculation of dust inputs
  6.3.3. Summary and implications of dust input
7. Conclusions
Acknowledgments
References


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