Reduced channel morphological response to urbanization in a flood-dominated humid tropical environment

Phillips C.B., Scatena F.N. Reduced channel morphological response to urbanization in a flood-dominated humid tropical environment. Earth Surface Processes and Landforms. 2012. DOI: 10.1002/esp.3345

Urbanization through the addition of impervious cover can alter catchment hydrology, often resulting in increased peak flows during floods. This phenomenon and the resulting impact on stream channel morphology is well documented in temperate climatic regions, but not well documented in the humid tropics where urbanization is rapidly occurring. This study investigates the long-term effects of urbanization on channel morphology in the humid sub-tropical region of Puerto Rico, an area characterized by frequent high-magnitude flows, and steep coarse-grained rivers. Grain size, low-flow channel roughness, and the hydraulic geometry of streams across a land-use gradient that ranges from pristine forest to high density urbanized catchments are compared. In areas that have been urbanized for several decades changes in channel features were measurable, but were smaller than those reported for comparable temperate streams. Decades of development has resulted in increased fine sediment and anthropogenic debris in urbanized catchments. Materials of anthropogenic origin comprise an average of 6% of the bed material in streams with catchments with 15% or greater impervious cover. At-a-station hydraulic geometry shows that velocity makes up a larger component of discharge for rural channels, while depth contributes a larger component of discharge in urban catchments. The average bank-full cross-sectional area of urbanized reaches was 1.5 times larger than comparable forested reaches, and less than the world average increase of 2.5. On average, stream width at bank-full height did not change with urbanization while the world average increase is 1.5 times. Overall, this study indicates that the morphologic changes that occur in response to urban runoff are less in channels that are already subject to frequent large magnitude storms. Furthermore, this study suggests that developing regions in the humid tropics shouldn’t rely on temperate analogues to determine the magnitude of impact of urbanization on stream morphology. Copyright © 2012 John Wiley & Sons, Ltd.

Spatial and temporal patterns of water quality indicators in reef systems of southwestern Puerto Rico

Otero E.E. Spatial and temporal patterns of water quality indicators in reef systems of southwestern Puerto Rico. Carribbean journal of science, Vol 45, Is 2-3, 2009, pp. 168-180

Records of cosmogenic radionuclides 10Be, 26Al and 36Cl in corals: First studies on coral erosion rates and potential of dating very old corals

Lal, D., CD Gallup, BLK Somayajulu, L. Vacher, MW Caffee, AJT Jull, RC Finkel, RC Speed, and A. Winters. 2005. Records of cosmogenic radionuclides be-10, al-26 and cl-36 in corals: First studies on coral erosion rates and potential of dating very old corals. Geochimica Et Cosmochimica Acta 69 (24) (DEC 15): 5717-28.

We present results of measurements of cosmogenic 10Be, 26Al and 36Cl, and the indigenous (intrinsic) concentrations of the stable elements Be, Al and Cl in 120–200 kyr old corals from Barbados and Puerto Rico. The concentration levels of these radionuclides in the corals lie in the range 104 to 108 atoms/g. A comparison of the measured nuclide concentrations with those expected to be produced in the corals by nuclear interactions of energetic cosmic radiation shows that (i) the radionuclides 26Al and 36Cl are derived from in situ cosmic ray interactions in the corals after their formation, but that (ii) the radionuclide 10Be owes its provenance in the coralline lattice primarily due to incorporation of dissolved beryllium in seawater in the lattice structure of the corals.

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.

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.

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.

A record of recent change in terrestrial sedimentation in a coral-reef environment, La Parguera, Puerto Rico: A response to coastal development?

Ryan, K. E., J. P. Walsh, D. R. Corbett, and A. Winter. 2008. A record of recent change in terrestrial sedimentation in a coral-reef environment, la parguera, puerto rico: A response to coastal development? Marine Pollution Bulletin 56 (6) (JUN): 1177-83.

Increased sediment flux to the coastal ocean due to coastal development is considered a major threat to the viability of coral reefs. A change in the nature of sediment supply and storage has been identified in a variety of coastal settings, particularly in response to European colonization, but sedimentation around reefs has received less attention. This research examines the sedimentary record adjacent to a coastal village that has experienced considerable land-use change over the last few decades. Sediment cores were analyzed to characterize composition and sediment accumulation rates. Sedimentation rates decreased seaward across the shelf from 0.85 cm y1 in a nearshore bay to 0.19 cm y1 in a fore-reef setting. Data reflected a significant (up to 2) increase over the last 80 years in terrestrial sediment accumulating in the back-reef setting, suggesting greater terrestrial sediment flux to the area. Reef health has declined, and increased turbidity is believed to be an important impact, particularly when combined with additional stressors.

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.

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.

Review of Use of Isotopes in Studying the Natural History of Puerto Rico

Evaristo J. Review of Use of Isotopes in Studying the Natural History of Puerto Rico. University of Pennsylvania. 2012.

This review summarizes the earth and environmental science research papers in Puerto Rico that used isotopic techniques between 1965 and 2011. The range of applications in isotope-related research in Puerto Rico has grown steadily, led by the ubiquitous utility of stable isotope ratios in biogeochemical (δ13C, δ15N) and ecological (δ13C, δ15N, δD) research. Moreover, research in climatology has grown in recent years, spanning from the evaluation of the fidelity of isotope records (δ18O, δ13C) as an environmental proxy to the elucidation of multidecadal variability for paleoclimate reconstructions (δ18O and Sr/Ca). On the other hand, in addition to using isotope ratios, hydrological studies in Puerto Rico have also used trace element data to answer flow source (δD, δ18O, 87Sr/86Sr) and solute source (Ge/Si) questions, as well as in examining groundwater/surface flow relationships (222Rn). Finally, various isotope data have been used in trying to understand geomorphological (10Be, δ30Si) and geophysical (Pb, Nd, and Sr) phenomena. It is hoped that this review will be able to contribute to stimulating future interests in isotope-related research as applicable in the LCZO or Puerto Rico, in particular, and/or in comparable humid tropical settings, in general.

Causes and Effects of Noise in Landscape Dynamics

Jerolmack D.J. Causes and Effects of Noise in Landscape Dynamics. Eos, Vol. 92, No. 44, 1 November 2011.

Geomorphology, the study of landscape form and change, is at a crossroad. Quantification of patterns on Earth’s surface has revealed surprising and robust order. Transport equations rooted in physics that relate material flux of sediment to environmental drivers have been derived and are capable of simulating realistic-looking topography [Dietrich et al., 2003]. Yet despite this rapid progress and an explosion of interest in the field, scientists are unable to predict sediment transport rates in rivers to better than an order of magnitude—they can only qualitatively anticipate the response of landscapes to land use or climatic changes. In addition, some argue that the sedimentary archive of landscape evolution is dominated by randomness [Sadler and Strauss, 1990], calling into question researchers’ ability to reconstruct environmental change from the rock record. A major obstacle to progress is the lack of understanding of the nature and origins of “noise” in sediment transport. “Noise” here refers to the seemingly random or inexplicable fluctuations in transport rate that can occur without any external perturbation. Transport noise can obscure the relations between cause and effect in landscape evolution. Two key questions help to focus research efforts: (1) How do noisy dynamics arise in a steady environment? (2) How can robust, ubiquitous patterns persist in a noisy environment? Addressing these questions will require the adoption of theoretical and experimental approaches that are outside the realm of traditional geomorphology.

Exploration of the Geological Formations of the Luquillo Mountain Range of North Eastern Puerto Rico using X-ray Florescence

Nawal C.. Exploration of the Geological Formations of the Luquillo Mountain Range of North Eastern Puerto Rico using X-ray Florescence. Master's Capstone. University of Pennsylvania 2011.

X-ray Florescence (XRF) is a widely used non-destructive method that measures the elemental composition of materials. This technology was applied to investigate the rocks and sediments in the Luquillo Mountains / El Yunque region of Puerto Rico. Initial testing of wet and dry sediments revealed that the machine records higher elemental concentrations in dry compared to wet sediments as it seems that the water molecules interfere with the X-ray beam on wet samples. The XRF method on dried samples produced reliable results and allowed for the chemical separation of the five basic bedrock types found in the Luquillo Mountains. Of the volcanoclastic the Fajardo Formation can be distinguished from the others by its concentration of Barium (Ba) and Rubidium (Rb). The Unnamed formation was distinguished by Copper (Cu) and the Hato Puerto Formation was distinguished by Nickel (Ni) and Strontium (Sr). The Rio Blanco granodiorite is the youngest rock type of the region and was the only formation whose elemental chemistry was not distinguishable from the othersapparently because it was formed directly from the basic magma that also formed the Luquillo Mountains volcanic rocks. Recent studies have found high levels of Mercury (Hg) in Luquillo stream water. Knowing that the Luquillo region was heavily mined for Gold (Ag) and Silver (Au), the Hg used in historic mining is a possible source of the elevated Hg values. The XRF analysis indicated small quantities of Hg in some rocks but no Hg was found in the sediments and soils surrounding the historic mining sites. Therefore if Hg had been used in historic mining operations it is no longer apparent in the sediments and has presumably been removed by erosion of the site.
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