White A.F.

d30Si systematics in a granitic saprolite, Puerto Rico

Ziegler, K., OA Chadwick, AF White, and MA Brzezinski. 2005. (DSi)-si-30 systematics in a granitic saprolite, puerto rico. Geology 33 (10) (OCT): 817-20.

Abstract: 
Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (d30Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have d30Si values ;2.0‰ more negative than their parent mineral and the d30Si signature of the bulk solid is determined by the ratio of primary to secondary minerals. Mineral-specific weathering reactions predominate at different depths, driving changes in differing d30Sipore water values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates d30Sipore water signatures more positive than granite by up to 1.2‰; these reactions are the main contributor of Si to stream water and determine its d30Si value. Throughout the saprolite, biotite weathering releases Si to pore waters but kaolinite overgrowth formation modulates its contribution to pore-water Si. The influence of biotite on d30Sipore water is greatest near the bedrock where biotite-derived Si mixes with bulk pore water prior to kaolinite formation. Higher in the saprolite, biotite grains have become more isolated by kaolinite overgrowth, which consumes biotite-derived Si that would otherwise influence d30Sipore water. Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, d30Sipore water values are more negative than granite by up to 1.3‰ near the top of the saprolite.

Phosphorus and iron cycling in deep saprolite, Luquillo Mountains, Puerto Rico

Buss, Heather L., Ryan Mathur, Arthur F. White, and Susan L. Brantley. 2010. Phosphorus and iron cycling in deep saprolite, luquillo mountains, puerto rico. Chemical Geology 269 (1-2) (JAN 15): 52-61.

Abstract: 
Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut. Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ56Fe=0 to −0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ56Fe=−0.26 to −0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8×10−14 mol m−2 s−1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.

Iron and phosphorus cycling in deep saprolite, Luquillo Mountains, Puerto Rico

Buss H.L., Mathur R., White A.F., and Brantley S.L. 2010. Iron and phosphorus cycling in deep saprolite, Luquillo Mountains, Puerto Rico. Chem. Geol., 269, 52-61.

Abstract: 
Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut. Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ56Fe = 0 to − 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ56Fe = − 0.26 to − 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10− 14 mol m− 2 s− 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite. Keywords: Phosphorus; Iron isotopes; Saprolite; Apatite weathering rate; Fe(II)-oxidizing bacteria
Syndicate content