Robert Stallard

... 3. Once representative microspherule populations of the late Eocene layers have been compositionally analyzed, a late Eocene occurrence of microtektites, or related crysta'-bearing microspherules, can be ... Benchmark Papers in Geology, Dowden, Hutchinson and Ross. ...

... Jeffrey E. Richey Fisheries Research Institute, University of Washington, Seattle 98195 James T. Brock Department of Biology, Idaho State University, Pocatello 83201 Robert J. Naiman Woods Hole Océanographie Institution, Woods Hole, Massachusetts 02543 Robert С ...

Streams on uniformly rainforest-covered, but lithologically very diverse Barro Colorado Island in central Panamá show remarkable differences in their runoff response to rainfall. This lithological diversity is reflected in equally diverse soilscapes, and our objective was to test the hypothesis that contrasting runoff responses derive from soilscape features that control the generation of overland flow. We determined the soil saturated hydraulic conductivity (Ks) of two neighboring, but hydrologically contrasting catchments (Lutz Creek with a flashy and Conrad Trail with a delayed response to rainfall), and quantified the spatial and temporal frequency of overland flow occurrence. The median Ks values at a depth of 12.5 cm are large enough to rule out Hortonian overland flow, but a marked decrease in Ks in Lutz Creek catchment at 30 cm suggests the formation of a perched water table and the generation saturation overland flow; the decrease in Ks in the Conrad Trail catchment is more gradual, and a perched water table is expected to form only at depths below 50 cm. In Lutz Creek, overland flow was generated frequently in time and space and regardless of topographic position, including near the interfluve, with very low thresholds of storm magnitude, duration, intensity and antecedent wetness, whereas in Conrad Trail, overland flow was generated much less frequently and then only locally. We conclude that soilscape features and microtopography are important controls of overland flow generation in these catchments. Our results contribute to the growing evidence that overland flow and forests are not a priori a contradiction in terms.

We report SHRIMP UPb ages of 49 zircons from a sand sample from the lower Orinoco River, Venezuela, and Nd model ages of the fine sediment load from the main river and tributaries. The UPb ages reflect individual magmatic or metamorphic events, the SmNd model ages reflect average crustal-residence ages of the sediment sources. Together they allow delineation of the

Abstract Modal sand compositions in nine rivers draining the proximal part of the Amazonian retroarc foreland basin in Peru and Bolivia provide support for the Recycled Orogen provenance model of Dickinson and Suczek. Average compositions of 52 sand samples ...

The behavior and distribution of dissolved (< 0.4 μm) Al, Cu, Fe, Mn, Ni, Ti, Zn, Si, and organic carbon have been examined in the inshore and offshore estuarine waters of New Jersey during two seasons (September-October 1986 and June-July 1987). Water samples collected had average salinities of 22%. for inshore and 30%. for offshore. The removal or addition normalized to the inshore concentration of an element during transport offshore was calculated in this salinity range. Dissolved Si was removed by nearly 50%, and dissolved Fe by 15-30% in both seasons. Removal did not occur for other metals. Dissolved organic carbon was conservative. In offshore water, dissolved Fe, Al, and Ti seemed to be associated with the same colloidal particles. During estuarine mixing, most of the dissolved Fe, probably associated with colloidal oxyhydroxide, was removed; only the Fe that was associated with Al and Ti remained stable. As a result of removing oxyhydroxide, all the dissolved Fe, Al, and Ti in the high-salinity offshore water occurred in the same colloidal particles, possibly clays. This conclusion is further supported by chemical evidence in suspended sediments. The correlation between Al, Fe, and Ti in suspended particles generally indicates the same pattern as that for dissolved metals. These smaller metal-bearing particles (< 0.4 μm) might have been supplied through deflocculation of the larger ones (> 0.4 μm). These results imply that there is a need to further examine the influence of coastal water on the form of dissolved (< 0.4 μm) Fe in the near-shelf seawater. The dissolved Fe in the near-shore region is probably colloidal. This is also possible for some regions of the open ocean where dissolved Fe is supplied mainly by near-shore sources.

Deglaciation of the Laurentide Ice Sheet in North America was accompanied by sequestration of organic carbon in newly exposed soils. The greatest rate of land exposure occurred around 12,000 to 8,000 years ago, and the greatest increase in the rate of carbon sequestration by soils occurred from 8,000 to 4,000 years ago. Sequestration of carbon in deglaciated peat lands continues today, and a steady state has not been reached. The natural rate of carbon sequestration in soils, however, is small relative to the rate of anthropogenic carbon dioxide production.

This paper examines the linkages between the carbon cycle and sedimentary processes on land. Available data suggest that sedimentation on land can bury vast quantities of organic carbon, roughly 10 15 g C yr −1 . To evaluate the relative roles of various classes of ...

ABSTRACT CONTINENTAL weathering and mid-ocean-ridge hydrothermal circulation are the two main processes that release dissolved silicon to the ocean. It has been hypothesized1 that temporal variations in the relative strengths of these two processes might be estimated from the Ge/Si ratio of opal in marine sediments, because the chemistry of germanium is similar to that of silicon2–5. We analysed river water from the Orinoco basin and soils from Venezuela and Panama for germanium and silicon, to determine the relationship between river Ge/Si ratios, drainage basin lithology and chemical-weathering intensity. Our results, presented here, indicate that chemical-weathering intensity, defined as the fraction of original bedrock silicon dissolved during continental weathering, is related to the Ge/Si ratio in river water and is inversely related to river silicon concentrations. Changes in global climate, sea level or patterns of plate-tectonic interactions could change chemical-weathering intensity and complicate the interpretation of the marine opal Ge/Si record.

... surfaces show extensive laterization (Zormeveld, 1969), bauxitization (Aleva, 1979, 1984; Menendez and Sarmentero, 1984), and karst development on quartzite and granite bed-rock (Szczerban, 1973; Blancaneaux and Pouyl-lau, 1977; Grabert, 1978; Shaw, 1980; Chalcraft ...

... 2). Assuming that ero sion is a constant and continuous process, these steady-state concentrations may be used to quan tify average rates ... The fol lowing procedure [8,9] applies 1OBe (half-life = 1.5-106 yr) in riverborne quartz (a favorable min eral for studies of in situ-produced ...

In 1996, the Smithsonian Tropical Research Institute and the Republic of Panama's Environmental Authority, with support fromthe United States Agency for International Development, undertook a comprehensive program to monitor the ecosystem of the Panama Canal watershed. The goals were to establish baselineindicators for the integrity of forest communities and rivers. Based on satellite image classification and ground surveys, the2790 km2 watershed had 1570 km2 of forest in 1997, 1080 km2 of which was in national parks and nature monuments. Most of the 490 km2 of forest not currently in protected areas lies along the west bank of the Canal, and its managementstatus after the year 2000 turnover of the Canal from the U.S. to Panama remains uncertain. In forest plots designed to monitorforest diversity and change, a total of 963 woody plant specieswere identified and mapped. We estimate there are a total of 850–1000 woody species in forests of the Canal corridor. Forestsof the wetter upper reaches of the watershed are distinct in species composition from the Canal corridor, and have considerably higher diversity and many unknown species. Theseremote areas are extensively forested, poorly explored, and harbor an estimated 1400–2200 woody species. Vertebrate monitoring programs were also initiated, focusing on species threatened by hunting and forest fragmentation. Large mammals are heavily hunted in most forests of Canal corridor, and therewas clear evidence that mammal density is greatly reduced in hunted areas and that this affects seed predation and dispersal. The human population of the watershed was 113 000 in 1990, and grew by nearly 4% per year from 1980 to 1990. Much of this growth was in a small region of the watershed on the outskirts of Panama City, but even rural areas, including villages near and within national parks, grew by 2% per year. There is no sewage treatment in the watershed, and many towns have no trashcollection, thus streams near large towns are heavily polluted. Analyses of sediment loads in rivers throughout the watershed did not indicate that erosion has been increasing as a result ofdeforestation, rather, erosion seems to be driven largely by total rainfall and heavy rainfall events that cause landslides.Still, models suggest that large-scale deforestation would increase landslide frequency, and failure to detect increases inerosion could be due to the gradual deforestation rate and the short time period over which data are available. A study of runoff showed deforestation increased the amount of water fromrainfall that passed directly into streams. As a result, dry season flow was reduced in a deforested catchment relative to aforested one. Currently, the Panama Canal watershed has extensive forest areasand streams relatively unaffected by humans. But impacts of hunting and pollution near towns are clear, and the burgeoningpopulation will exacerbate these impacts in the next few decades.Changes in policies regarding forest protection and pollution control are necessary.

Several samples of quartz were etched hydrothermally at 300°C in etchams of controlled dissolved silica concentration in order to measure the critical concentration, Ccrit, above which dislocation etch pits would not nucleate on the quartz surface. Ccrit for 300°C was theoretically predicted to be 0.6 C0 and the measured Ccrit, was 0.75 ± 0.15 C0 ( C0 is the equilibrium concentration). Above this value, some dislocation etch pits formed, but the rate of formation significantly decreased. These results are the first experimental validation of etch pit formation theory under hydrothermal conditions. Dune sands showed a generally angular and pitted surface when etched in dilute solutions, while sands etched at C ~ Ccrit showed less angular pitting. Analysis of a soil profile developed in situ on the Parguaza granite, Venezuela, revealed a gradual change from angular, pitted grain surfaces at the top of the profile to rounded surfaces on grains sampled just above bedrock. Since quartz dissolution without surface pitting continues deep in the profile, the Si concentration must exceed Ccrit, at depth. These results indicate that for C > Ccrit, dissolution occurs at edges and kinks on the surface of quartz and very few pits form; in contrast, at C ≪ Ccrit, dislocation etch pits grow rapidly, contributing to the overall dissolution rate.