erosion

Twelve testable hypotheses on the Geobiology of weathering

Brantley S.L., Megonigal J.P., Scatena F.N. et al 2010. Twelve testable hypotheses on the Geobiology of weathering. Geobiology. DOI: 10.1111/j.1472-4669.2010.00264.x

Abstract: 
Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth’s surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term.(4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes.(5) Biology shapes the topography of the Critical Zone.(6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws.(7) Rising global temperatures will increase carbon losses from the Critical Zone.(8) Rising atmospheric PCO2 will increase rates and extents of mineral weathering in soils.(9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering.(10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales.(12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

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.

REDUCTION OF BEDROCK BLOCKS AS CORESTONES IN THE WEATHERING PROFILE: OBSERVATIONS AND MODEL

Fletcher RC, Brantley SL. 2010. Reduction of bedrock blocks as corestones in the weathering profile: observations
and model. Am. J. Sci. 310:131–64

Abstract: 
the Espiritu Santo and Mameyes rivers within the Luquillo Experimental Forest (Puerto Rico) are interpreted as corestones, reduced from initial joint-bounded bedrock blocks by subsurface weathering. Maximum corestone size, expressed as the geometric mean of the three dimensions, S 3 abc, shows a smooth envelope when plotted against elevation. We postulate that, for each catchment, they represent in situ corestones within a stratified weathering profile, many tens of meters in thickness, that has been subsequently exhumed by younger erosion. We formulate a simplified one-dimensional model for reduction in corestone size within a steady-state weathering profile that incorporates: (i) vertical fluid transport of the reactant and the soluble products of chemical weathering; (ii) linear kinetics of corestone reduction; and, subsequently, (iii) erosion. The rate of advance of a steady-state weathering profile is a statement of the mass balance between entering reactants and weathering components, here idealized as H and albite. The mathematical relations, tie the laboratory-determined rate constant for dissolution of albite (k) to a generalized kinetic constant for the rate of decrease (K) in corestone diameter to the advance rate of the weathering profile (V ). The last parentheses contain an effective roughness at the scale of the weathering profile, where S0 is the maximum size of initial bedrock blocks, inferred to be set by initial bedrock fracture spacing, and 3L* is the profile thickness. The laboratory scale roughness value, , is the ratio of the surface area accessed by BET analysis to that of the corestone grain scale. In the model, erosion is not coupled with weathering, although the presence of corestones of finite size, SE>0, exiting at the erosional surface may be postulated to affect the erosional flux. The thickness of the corestone weathering profile derived for the model for the distance between bedrock and a corestone-free saprolite cap is approximately This expression is the product of the effective pH buffering-adjusted input reactant flux per unit area times a stoichiometeric factor linking this to net albite dissolution, divided by the rate of corestone size reduction at the input concentration of protons. Further, the profile thickness scales with the input “particle” size, S0. The model fit, which yields the ratio is consistent with a rate constant for albite dissolution that lies between laboratorymeasured and field-estimated values. Sensitivity to the reaction order of albite dissolution with respect to H, N, is small, except near the base of the profile. This model yields insights into the relationship between fracture spacing and the evolution of particle size and chemistry in weathering profiles.

Luquillo Mountains Puerto Rico A water energy and biogeochemical budgets program site

Larsen MC, Stallard RF. Luquillo Mountains, Puerto Rico–a water,
energy, and biogeochemical budgets program site. US geological
survey fact sheet. Washington, D.C.7 U.S. Geological
Survey; 2000. p. 163– 99.

Abstract: 
The Puerto Rico research site consists of the 113 square-kilometer Luquillo Experimental Forest (LEF), administered by the U.S. Forest Service, and the nearby Río Grande de Loíza drainage basin, an urbanized and agriculturally- developed watershed. This combined region serves as a terrestrial laboratory for the study of issues related to the global loss of tropical forest, and the associated changes in land-use practices. Findings from the WEBB research help scientists understand how vegetation, landscape, and people interact to affect the quantity and quality of water and the erosion of the landscape. The results of this work can be applied not only to Puerto Rico, but also to many other regions, where deforestation and rapid land-use change are issues.

Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced 10Be in river-borne quartz

Brown, E.T., Stallard, R.F., Larsen, M.C., Bourles, D.L., Raisbeck,
G.M., Yiou, F., 1998b. Determination fo predevelopment denudation
rates of an agricultural watershed (Cayaguas River, Puerto
Rico) using in-situ-produced Be in river-borne quartz. Earthand
Planetary Science Letters 160 (3-4), 723}728.

Abstract: 
Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic 10Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using 10Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.

The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico

LÓPEZ, TANIA DEL MAR; AIDE, T. MITCHELL; SCATENA F. N. 1998. The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico. Caribbean Journal of Science, Vol. 34, No. 3-4, 298-307, 1998.

Abstract: 
The Revised Universal Soil Loss Equation (RUSLE) was used in conjunction with a Geographic Information System to determine the influence of land use and other environmental factors on soil erosion in the Guadiana watershed in Puerto Rico. Mean annual erosion, suspended sediment discharge, and the rainfall-erosion factor of the RUSLE increased with annual rainfall. Median soil erosion rates varied among the seven land uses: bare soil (534 Mg ha-1 yr-1), open canopy forest (26 Mg ha-1 yr-1), agriculture (22 Mgha-1 yr -1), pasture (17 Mg ha -1 yr -1), less dense urban (15 Mg ha-1 yr -1), closed canopy forest (7 Mg ha -1 yr -1), and dense urban (1 Mg ha-1 yr -1). The differences between open canopy forest, agriculture, pasture, and less dense urban were not significantly different but median values for open canopy forests were slightly greater because they occurred on steep slopes. The five-year average sediment delivery ratio for the basin was 0.17, which is comparable to delivery ratios estimated for watersheds of similar size. Simulations of different land use configurations indicate that reforestation of 5% of the watershed with the highest erosion rates would decrease basin wide erosion by 20%. If the entire watershed was reforested, soil erosion would be reduced by 37%.
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