saprolite

Germanium–silicon fractionation in a tropical, granitic weathering environment

Lugolobi, Festo, Andrew C. Kurtz, and Louis A. Derry. 2010. Germanium-silicon fractionation in a tropical, granitic weathering environment. Geochimica Et Cosmochimica Acta 74 (4) (FEB 15): 1294-308.

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 2 lmol/ mol, with individual primary mineral phases ranging between 0.5 and 7 lmol/mol. The ratios in the bulk saprolite are higher (3 lmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 lmol/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 (1.1–1.6 lmol/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 (0.27–0.47 lmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite–bedrock interface. Massbalance 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 (1.2 lmol/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.

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

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher, R.C., Buss, H.L., Brantley, S.L., 2006.Aspheroidal weathering
model coupling porewater chemistry to soil thicknesses during
steady-state denudation. Earth Planet. Sci. Lett. 244, 444–457.

Abstract: 
Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher, R.C., Buss, H.L., Brantley, S.L., 2006.Aspheroidal weathering
model coupling porewater chemistry to soil thicknesses during
steady-state denudation. Earth Planet. Sci. Lett. 244, 444–457.

Abstract: 
Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.
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