Episodic wood loading in a mountainous neotropical watershed

The Upper Rio Chagres drains 414 km² of steep, mountainous terrain in central Panama. A tropical air mass thunderstorm on 10 July 2007 produced a flood across the basin that peaked at 720 m³ s^− 1 at a headwaters gage draining 17.5 km² and 1710 m³ s^− 1 at a downstream gage draining 414 km². The storm also triggered numerous landslides in the upper basin, which facilitated the formation of large logjams along portions of the channel where transport capacity of wood was reduced by a change in channel geometry such as a bend or channel expansion. During field work in February 2008, we characterized three jams with surface areas of 400–2450 m²; two of these jams resulted in storage of substantial (1100–8200 m³) sediment wedges upstream. We returned to these sites in March 2009 to document changes in the logjams and sediment storage. Drawing on observations made in the basin since 2002, and site visits during 2008 and 2009, we suggest that jams such as these last two years or less. We propose that wood dynamics in the Upper Chagres alternate between brief periods of moderate wood load in the form of large logjams and much longer periods of essentially no wood load, a situation that contrasts with the more consistent wood loads in catchments of similar size in temperate environments and with limited studies of more consistent wood load in tropical catchments with no landslides.     

LOST-Tree: a spatio-temporal structure for efficient sensor data loading in a sensor web browser

We present LOST-Tree, a new spatio-temporal structure to manage sensor data loading and caching in a sensor web browser. In the same way that the World Wide Web needs a web browser to load and display web pages, the World-Wide Sensor Web needs a sensor web browser to access distributed and heterogeneous sensor networks. However, most existing sensor web browsers are just mashups of sensor locations and base maps that do not consider the scalability issues regarding transmitting large amounts of sensor readings over the Internet. While caching is an effective solution for alleviating the latency and bandwidth problems, a method for efficiently loading sensor data¹ from sensor web servers is currently missing. Therefore, we present LOST-Tree as a sensor data loading component that also manages the client-side cache on a sensor web browser. By applying LOST-Tree, redundant transmissions are avoided, enabling efficient loading with cached sensor data. We demonstrate that LOST-Tree is lightweight and scalable, in terms of sensor data volume. We implemented LOST-Tree in the GeoCENS sensor web browser for evaluation with a real sensor web dataset.     

Implications of a non-adiabatic density gradient for the Earth's viscoelastic response to surface loading

The response of a viscoelastic Earth to the melting of the Late Pleistocene ice sheets has been the subject of a number of investigations employing PREM. In PREM, a non-adiabatic density gradient (NADG) exists in the upper mantle, and to understand the implications of this model it is thus important to examine the effects of this NADG on the Earth's response to surface loads. This paper is based on the assumption that the contribution to the depth dependence of the density that is not due to self-compression is due to compositional change. This contribution is referred to as 'non-adiabatic'. We evaluate the effects of a non-adiabatic density jump (NADJ) for the 670  km discontinuity and the NADG in the upper mantle by adopting a compressible earth model with both a compositional density gradient and a density jump. Numerical calculations based on these models indicate that the magnitude of the Earth's response associated with the NADG is much smaller than that associated with the NADJ at 670  km depth. It is also confirmed that the higher modes associated with the NADJ and the NADG are much more sensitive to the existence of an elastic lithosphere than the fundamental modes associated with the density jumps at the surface and core–mantle boundary.     

Oceanic loading on European laser-ranging sites

Summary. The expectation for high accuracies in laser ranging in the years to come and the increasing geodynamical implications in satellite geodesy require the investigation of the effect of time-dependent geodynamical phenomena on the laser-ranging sites. In this paper we evaluate the radial and horizontal displacements of the surface of the Earth and the gravity perturbation due to the loading effect of the M2 oceanic tide for 13 European laser-ranging sites. Most of them are situated in coastal regions.     

Sediment-chemistry response to land-use change and pollutant loading in a hypertrophic lake, southern Sweden

Responses to recent land-use changes and pollutant loading in the sediment of a hypertrophic lake in southern Sweden were studied by comparison of geochemical, pollen and magnetic records with historical land-use data. A chronology was constructed for the last two centuries by correlating changes in the pollen diagram to major events in the land-use history. Sediment accumulation was low (mean c. 0.2 g cm^-2 yr^-1) prior to 1800 AD, when less than 25% of the catchment was arable land. Reorganization of the agrarian system during the 19th century increased the annually tilled area by 300%, which accelerated soil erosion and substantially increased the accumulation of allochtonous matter in the lake. Since the turn of the century 90% of the catchment has been ploughed every year. The deposition of clastic matter in the lake has, however, decreased due to a gradual rerouting of the drainage system, which has reduced the effective catchment area by c. 85%.Authigenic vivianite (Fe3(PO4)2.8H2O) is a major P phase in the preindustrial non-sulphidic sediments, which suggests that the sediments at that time served as a fairly efficient sink for P. The arable expansion, increased manuring and, eventually, the introduction of artificial fertilizers during the 19th century led to a massive influx of nutrients, which elevated primary production in the lake. Subsequent development of bottom water anoxia around 1900, in combination with an additional pollutant burden of sulphate within the lake basin, led to major alterations of the biogeochemical cycles. The most critical change in the post-1900 sediments involved the cycling of Fe and P. The linkage between the lacustrine P and Fe cycles can explain that FeS formation was paralleled by a release of P from the sedimentary pool. This supply of P to the lake basin must have supplemented the nutrient supply by modern agriculture and contributed to recent hypertrophication. The bacterial sulphate reduction also affected the generation of alkalinity which supported a significant calcite precipitation in the post-1900 sediments.S is enriched 10-fold in the post-1900 sediments compared to preindustrial values. Along with the rise in S, soot particles derived from fossil fuel combustion appear in the sediments for the first time. Therefore, Bussj¨osj¨on is thought to be a good example of how a well-buffered, highly productive lake may respond to the pollution by sulphur from acid rain.     

Conservation of mass in tidal loading computations

Summary. A re-examination of methods for including mass conservation in tidal loading shows that the spherical harmonic correction of Farrell is incorrect. The effect of unconserved mass for a nearly ocean-covered earth shows that the proper spherical harmonic expansion of the Newtonian Green function is the average of the internal and external expansions.     

Inadvertent alkalization of a Florida lake caused by increased ionic and nutrient loading to its watershed

We evaluate the effects of land-use change since c.1890 on Little Lake Jackson in south-central Florida, USA. The lake currently is alkaline despite the prevalence of acidic lakes in the region. Watershed soils are acidic and poorly drained, but are underlain by limestone bedrock. Limnetic pH inferences, based on weighted-averaging tolerance regression of sedimented diatoms, indicate that Little Lake Jackson became significantly alkalized during the 1900s. Two driving forces that appear to be responsible for water-quality change are increased ionic loading and increased nutrient loading. Golf courses and residential lawns in the watershed receive substantial applications of lime, fertilizer, and irrigation with alkaline waters from deep wells, some of which reaches the lake in channelized runoff. Stormwater runoff and septic leachate also contribute to nutrient and solute loading. Sedimentary total P accumulation increased 5-fold and total N accumulation increased 3-fold since c. 1890. δ¹⁵N values suggest agricultural and septic sources for N loading. Sedimented pigments, inferred limnetic chlorophyll a values, and δ¹³C values of organic matter indicate that increased primary productivity occurred. Surface and subsurface inflow is nutrient-rich but low in hardness. Increased cation deposition in sediments indicates that ionic input might have reduced the lake’s natural resistance to alkalization. Lake waters remain low in ionic content, which suggests that the addition of base from carbonate sources is not responsible for all of the observed alkalization. Acid neutralization might have been facilitated by phosphate loading that led to increased base generation through greater nitrate assimilation. Inadvertent alkalization might occur commonly in regions where poorly buffered lakes are subject to significant ionic and nutrient loading from agriculture, turfgrass, and septic sources in their watersheds.     

Tidal gravity and ocean tide loading in Europe

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The results are presented from tidal gravity measurements at five sites in Europe using LaCoste and Romberg ET gravimeters. Improvements that we have made to the accuracies of these gravimeters are discussed. It is shown that the 'standard' calibration of the International Center for Earth Tides, used for worldwide tidal gravity profiles, is 1.2 per cent too high. The M₂ and O₁ observations are compared with model calculations of the Earth's body tide and ocean tide loading and it is shown that there is a very significant improvement in the agreement between observations and models compared to that obtained with previous tidal gravity measurements. For O₁, where the ocean tide loading and attraction in central Europe is only 0.4 per cent of the body tide, our measurements verify that the Dehant-Wahr anelastic body tide model gravimetric factor is accurate to 0.2 per cent. It is also shown that the effects of lateral heterogeneities in Earth structure on tidal gravity are too small to explain the large anomalies in previously published tidal gravity amplitudes. The observations clearly show the importance of conserving tidal mass in the Schwiderski ocean tide model. For sites in central Europe, the M₂ and O₁ observations and the models are in agreement at the 0.1 μgal (10⁻⁹ m s⁻²) level and tidal corrections to this accuracy can now be made to absolute gravity measurements.     

Historical loading record of sulfur in an Adirondack Lake

A model for the distribution of sulfur in the sediments of a lake in the Adirondack Mountains, New York, is developed to gain insight into the timing and magnitude of anthropogenic increases in sulfur loading. Surficial sulfur concentrations are about 3500 µg g^–1, increase to 6000–7000 µg g^–1 at a depth of about 6 cm, and then decrease downcore to background levels of about 2500 µg g^–1. Sulfate concentrations are about 60–80 µM in the overlying water and decrease rapidly below the sediment-water interface. This reflects assimilatory reduction and/or dissimilatory reduction of sulfate and incorporation into the inorganic and/or organic solid-phase fractions of the sediment. A mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion of sulfate from the overlying water with subsequent fixation in the sediment. Several historical scenarios of atmospheric sulfate loading rates were examined as model boundary conditions. Model results are compared with observed sedimentary sulfur profiles. The observed sediment sulfur profiles are best described using model boundary conditions showing increases in lake water sulfate concentrations since about 1940. Assuming as much as a decade of retention of sulfate within the terrestrial portion of the ecosystem, this suggests that significant increases in sulfate deposition rates began sometime after 1930 in this relatively remote area.     

Historical trace metal loading to a large river recorded in the sediments of Lake St. Croix,USA

The global cycling of anthropogenic trace metals intensified during the twentieth century, impacting aquatic systems throughout the world. There are, however, few quantitative records showing the history of this contamination in large rivers. Here we present a well-dated sedimentary record of trace metal accumulation in Lake St. Croix, a natural riverine lake on the St. Croix River (Minnesota and Wisconsin, USA), revealing the history of heavy metal inputs to the river over the past 200 years. Concentrations of Hg, Pb, Ag, Cd, Cr and Zn and stable Pb isotopes were measured in eight ²¹⁰Pb-dated sediment cores collected from profundal depositional areas throughout the lake. Time trends of trace metal concentrations and accumulation rates differed greatly between the upper lake (above Valley Creek) and the lower lake, reflecting contrasting sediment sources along the flow axis of the lake. For most of the study period (1800–2000 AD), sediment deposited throughout the lake derived almost exclusively from the suspended sediment load carried by the main-stem river into the lake. From 1910 through 1970, however, large inputs of eroded soils and stream channel sediments from side-valley tributaries resulted in greatly increased sediment and trace metal accumulation in the lower lake. Anthropogenic accumulation rates of Hg, Pb, Cd, Zn, and Ag in the upper lake correlate well with those from Square Lake, a small, relatively undisturbed nearby lake that has received trace metal inputs almost exclusively via atmospheric deposition. The similarity of these records suggests that atmospheric deposition was primarily responsible for trace metal accumulation trends in upper Lake St. Croix. Trace metal accumulation in the lower lake was also strongly influenced by atmospherically derived inputs, but metal contributions from native soils were important, as well, during the period of elevated sediment inputs from side-valley tributaries. Concentrations and accumulation rates of trace metals in both upper and lower lake sediments have decreased substantially since the 1970s due to decreased atmospheric inputs and sediment loadings, but accumulation rates remain well above pre-settlement values. Metal inputs to Lake St. Croix have been far lower than those to nearby Lake Pepin, located on the Mississippi River downstream of the Minneapolis-St. Paul metropolitan area, but there nevertheless remains a clear record of anthropogenic impact on the relatively pristine St. Croix River.     

The influence of a stratified rheology on the flexural response of the lithosphere to (un)loading by extensional faulting

We present a two-layered finite difference model for the flexural response of the lithosphere to extensional faulting. The model allows for three modes of flexure: (1) fully coupled, with the upper crust and mantle welded together by the lower crust; (2) fully decoupled, with the upper crust and mantle behaving as independent layers; and (3) partly decoupled, signifying that the response of the upper crust to small-wavelength loads is superimposed on the response of the entire lithosphere to long-wavelength loads. Which of these modes of flexure is to be expected depends on the rheology and especially the thermal state of the lithosphere. Coupled behaviour is related to a cold and strong lithosphere. The Baikal Rift Zone provides a typical example for this mode of flexure. A fully decoupled lithosphere is an exceptional case, related to anomalous high temperatures in the lower crust, and is observed in the Basin and Range province. The most common case is a partly decoupled lithosphere, with the degree of decoupling depending on the thickness and viscosity of the lower crust. This is inferred, for example, for the Bay of Biscay margin.     

State of stress in the oceanic lithosphere in response to loading

Summary. Flexure studies of the oceanic lithosphere constrained by bathymetry and gravity data suggest that the lithosphere behaves elastically over geological time-scales. For loads to be supported, however, large bending stresses (approaching 10 kb in some cases) are required at the top and bottom of the elastic plate.
These stress-differences can be significantly reduced by introducing more complex rheologies: we propose a model of layered lithosphere, consisting of a purely elastic upper layer, a transition zone with viscosity varying with depth and a perfectly plastic lower layer. The transition layer is grossly centred at the bottom of the elastic plate. Such a model results in a noticeable reduction of stress differences; reaching 60 per cent for flow laws representing creep mechanisms in olivine. When applied to a number of seamount loads, this model leads to maximum stress-differences which do not exceed 1–2kb.
The approach used in this study allows us to follow stress relaxation over time. Taking account of the thermal cooling of the lithosphere, we show that the elastic thickness of the lithosphere is stabilized after a given time, while the time required for stabilization is found to be of the order 5—6 per cent of the age of the lithosphere at the date of loading.     

Experimental study on DX pile performance in frozen soils under lateral loading

Experiments about working mechanism and mechanical characteristics of the DX model pile foundation under lateral dynamic and static loading were conducted by using a model system of the dynamic frozen ...