Stallard R.F.

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

Brown, E. T., R. F. Stallard, M. C. Larsen, D. L. Bourlès, G. M. Raisbeck, and F. Yiou. 1998. Determination of predevelopment denudation rates of an agricultural watershed (cayaguas river, puerto rico) using in-situ-produced 10Be in river-borne quartz. Earth and Planetary Science Letters 160 (3-4): 723-8.

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 (Cayagua´s 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 Cayagua´s. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayagua´s Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.

Denudation rates determined from the accumulation of in situ-produced loBe in the Luquillo Experimental Forest, Puerto Rico

Brown, E. T., R. F. Stallard, M. C. Larsen, G. M. Raisbeck, and F. Yiou. 1995. Denudation rates determined from the accumulation of in situ-produced 10Be in the luquillo experimental forest, puerto rico. Earth and Planetary Science Letters 129 (1-4): 193-202.

Abstract: 
We present a simple method for estimation of long-term mean denudation rates using in situ-produced cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin. The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term average denudation of = 43 m Ma^-1, consistent with mass balance results.

Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed

Shanley, J. B., W. H. McDowell, and R. F. Stallard (2011), Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed, Water Resour. Res., 47, W07515, doi:10.1029/2010WR009788.

Abstract: 
The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains, northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates among the highest in the world. We pooled stream chemistry and total suspended sediment (TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008. During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in 1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4), and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall, stream composition shifts toward that of precipitation, diluting 90% or more in the largest storms, but maintains a biogeochemical watershed signal marked by elevated K and dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang” pattern, initially increasing with flow but then decreasing at the highest flows as it becomes depleted and/or vigorous overland flow minimizes contact with watershed surfaces. TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive power of large storms in a landslide‐prone landscape. The relations of TSS and most solute concentrations with stream discharge were stable through time, suggesting minimal long‐term effects from repeated hurricane disturbance. Nitrate concentration, however, increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first‐order decay pattern. The combined data sets provide insight about important hydrologic pathways, a long‐term perspective to assess response to hurricanes, and a framework to evaluate future climate change in tropical ecosystems.

Possible Environmental Factors Underlying Amphibian Decline in Eastern Puerto Rico: Analysis of U.S.Government Data Archives

STALLARD, R. F. 2001. Possible environmental factors
underlying amphibian decline in eastern Puerto Rico:
analysis of U.S. government data archives. Conservation
Biology 15:943–953.

Abstract: 
The past three decades have seen major declines in populations of several species of amphibians at high elevations in eastern Puerto Rico, a region unique in the humid tropics because of the degree of environmental monitoring that has taken place through the efforts of U.S. government agencies. I examined changes in environmental conditions by examining time-series data sets that extend back at least into the 1980s, a period when frog populations were declining. The data include forest cover; annual mean, minimum, and maximum daily temperature; annual rainfall; rain and stream chemistry; and atmospheric-dust transport. I examined satellite imagery and air-chemistry samples from a single National Aeronautics and Space Administration aircraftflight across the Caribbean showing patches of pollutants, described as thin sheets or lenses, in the lower troposphere. The main source of these pollutants appeared to be fires from land clearing and deforestation, primarily in Africa. Some pollutant concentrations were high and, in the case of ozone, approached health limits set for urban air. Urban pollution impinging on Puerto Rico, dust generation from Africa (potential soil pathogens), and tropical forest burning (gaseous pollutants) have all increased during the last three decades, overlapping the timing of amphibian declines in eastern Puerto Rico. None of the data sets pointed directly to changes so extreme that they might be considered a direct lethal cause of amphibian declines in Puerto Rico. More experimental research is required to link any of these environmentalfactors to this problem.

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.

Research plan for the investigation of water, energy, and biogeochemical budgets in the Luquillo mountains, Puerto Rico

Larsen, M.C., Collar, P.D., and Stallard, R.F., 1993, Research plan for the investigation of water, energy, and biogeochemical budgets in the Luquillo mountains, Puerto Rico: U.S. Geological Survey Open-file Report 92-150, 19 p.

Abstract: 
The Luquillo mountains of eastern Puerto Rico are the site of U. S. Geological Survey (USGS) research into biogeochemical and geomorphic processes that control the movement and transformation of water, energy, bedrock weathering products, and nutrients in the earth-surface environment. This study was begun in 1990 and is scheduled to last three years, with the possibility of being extended for further data collection. The study area for this research effort includes the 113 square kilometers Luquillo Experimental Forest (LEF) that is administered by the U. S. Forest Service. The LEF has been the site of ongoing research since 1988 as part of the National Science Foundation's Long Term Ecological Research program. In addition, comparative studies are being conducted in the Río Grande de Loíza basin (Loíza basin), an urban and agriculturally developed 600 square kilometers watershed located immediately to the west of the LEF. The principal elements of the study described in the report are as follows: Determination of biogeochemical budgets: water, energy, carbon, nutrient, ion, sediment, and gas budgets will be calculated in two LEF watersheds instrumented with meteorologic, soil, hydrologic, and ground-water monitoring equipment. A biweekly time series of samples is being collected. In addition, intensive sampling is undertaken during selected storms. Study of weathering, erosion, and mass-wasting processes in undeveloped watersheds of contrasting lithology: chemical-weathering, erosion, and mass-wasting processes in watersheds underlain by the two dominant rock types, volcaniclastic and quartz diorite, are being compared. The effects of mass wasting on biogeochemical cycling in each rock type will be evaluated through a compilation of physical, chemical, and mineralogic properties for a chronosequence of landslides. Water and sediment budgets will be used to develop a conceptual model of hillslope hydrology and landform evolution. Comparison of weathering and gas flux in developed and forested watersheds: paired basins were selected and gaged in the relatively undisturbed LEF and in the agriculturally developed Loíza basin. Budgets of all aqueous constituents will be compared and contrasted in the developed and forested basins of similar lithology. Gas-flux differences (carbon dioxide, nitrogen dioxide, methane) between developed and undeveloped areas will be evaluated using chamber techniques and the results related to land-use differences. Measurement of reservoir and agricultural pond gas fluxes: methane production is being measured in selected reservoirs and agricultural ponds in and near the Loíza basin and LEF. A regional methane budget will be calculated.

Denudation rates determined from the accumulation of in situ-produced 10Be in the luquillo experimental forest, Puerto Rico

Brown, E.T., Stallard, R.F., Larsen, M.C., Raisbeck, G.M., Yiou,
F., 1995b. Denudation rates determined from the accumulation of in
situ-produced Be in the Luquillo Experimental Forest, Puerto
Rico. Earthand Planetary Science Letters 129, 193}202.

Abstract: 
We present a simple method for estimation of long-term mean denudation rates using in situ-produced cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin. The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term average denudation of ≈ 43 m Ma−1, consistent with mass balance results.

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.
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