An assessment of recent trophic changes in Windermere South Basin (England) based on diatom remains and fossil pigments

Cyanobacterial carotenoids and diatom remains have been analyzed in recent sediments from the Windermere South Basin (WSB) to study the trophic evolution experienced by the lake. Dates in the top 30 cm were specifically established through radionuclide (²¹⁰Pb and¹³⁷Cs) analyses. Diatom stratigraphy shows dominance of the centric diatomsCyclotella comensis andC. radiosa and several benthic taxa in the early postglacial. This indicates oligotrophy in the WSB during that period. This assemblage was replaced by another dominated by the diatomAsterionella formosa in the 1870's, as has been established from the²¹⁰Pb dating. From that date onwards, the lake underwent a progression towards eutrophy, indicated by the progressive increase inAulacoseira subarctica (c. 1930's),Fragilaria crotonensis (c. 1943), and more recently, of the centricsStephanodiscus parvus (c. 1971) andCyclotella meneghiniana (1988).Carotenoid stratigraphy reveals the differences between different sections of the core. Oscillaxanthin and myxoxanthophyll had very low records in the early and medium parts of the core, but increased fromc. 1950's, showing peaks atc. 1967, 1979 and 1987. Some of these peaks indicated a differential abundance ofOscillatoria, and are matched to those observed directly during the ongoing monitoring of the phytoplankton of the lake.The coincidence between the historic appearance of diatoms associated with nutrient-rich waters and the enhanced carotenoid occurrence suggest a common response to phosphorus enrichment, and that the progressive change towards eutrophy has been accentuated during the last twenty-five years.     

Geochemistry and petrogenesis of the marginal zone of the Mount Lowe Intrusion,central San Gabriel Mountains,California

The marginal zone of the Mount Lowe Intrusion (MLI) is composed of a mineralogically and geochemically distinctive suite of quartz poor, alkali-enriched monzodiorites and quartz monzodiorites. Low initial ⁸⁷Sr/⁸⁶Sr, Rb, and Rb/Sr coupled with high Ab/Or, K/Rb, Ba and Sr suggest that feldspathic continental crust did not contribute significantly to the magma. The inferred original magma composition is most consistent with experimental data for about 20% batch modal melting of eclogite or quartz eclogite of altered oceanic tholeiite composition. The high absolute Ba and Sr abundances require in addition an enriched component, perhaps derived from dehydration processes within a subducted slab. At the level of exposure, the range of observed compositions resulted from fractionation of hornblende, plagioclase, sphene, apatite and magnetite. Fractionation resulted in formation of a peraluminous, Mn — enriched residual liquid which crystallized garnet. The grandite — enriched composition of the garnet is inferred to be due to high pressure (ca. 6.7 kb), high crystallization, relative to typical plutonic garnets. The composition of the marginal zone of MLI has many chemical similarities to early Mesozoic plutonism in the Cordilleran continental margin magmatic arc of western North America.     

Desertification processes and impacts in irrigated regions

Desertification in irrigated regions is caused by man's activities but may be more rapid in periods of drought or abnormally high rainfall. It is caused by sedimentation of canals, use of poor quality water, or waterlogging or salinization because of rising groundwater tables caused by excessive applications of irrigation water. It can be avoided by good land management in catchments and irrigation areas but social, economic, and political factors cause difficulties. Direct costs due to loss of production from affected lands are less than indirect costs to national economies.     

The influence of urbanization of sinkhole development in central Pennsylvania

The karsted limestone valleys of central Pennsylvania contain two populations of sinkholes. Solution sinkholes occur in the Champlainian limestone units along the margins of the valleys. Solution sinkholes are permanent parts of the landscape and, although a nuisance to construction, do not present other problems. The second population is the suffosional or soil-piping sinkholes These occur on all carbonate rock units including the Beekmantown and Gatesburg dolomites that comprise the two principal carbonate aquifers in the valley. Suffosional sinkholes are the principal land-use hazard. Suffosional sinkholes are transient phonomena. They occur naturally but are exacerbated by runoff modifications that accompany urbanization Suffosional sinkholes are typically 1.5–2.5 m in diameter depending on soil thickness and soil type. The vertical transport of soil to form the void space and soil arch that are the precursors to sinkhole collapse is through solutionally widened fractures and cross-joints and less often through large vertical openings in the bedrock. The limited solution development on the dolomite bedrock combined with soil thickness, seldom greater than 2 m, limits the size of the sinkholes. All aspects of suffosional sinkhole development are shallow processes: transport, piping, void and arch formation, and subsequent collapse take place usually less than 10 m below the land surface Factors exacerbating sinkhole development include pavement, street, and roof runoff which accelerates soil transport Such seemingly minor activities as replacing high grass and brush with mowed grass is observed to accelerate sinkhole development. Dewatering of the aquifer is not a major factor in this region     

The effect of a shallow low viscosity zone on the apparent compensation of mid-plate swells

A simple model for mid-plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region. Finite element calculations have been used to determine the effects of the viscosity contrast, the layer thickness and the Rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell. As the viscosity decreases in the low viscosity zone, the effective local Rayleigh number for the top boundary layer of the convecting cell increases. Also, because the lower viscosity facilitates greater velocities in the low viscosity zone, the low viscosity layer produces proportionally greater horizontal flow near the conducting lid, causing the base of the conducting lid to appear like a free boundary. The change in the local Rayleigh number and in the effective boundary condition both cause the top boundary layer to thin. Through a Green's function analysis, we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth, whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations. By increasing the viscosity contrast, the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small. Therefore, shallow depths of compensation cannot be used to argue against dynamic support of mid-plate swells. Furthermore, we compared the distribution of the effective compensating densities, which is used to obtain the geoid, to that of Pratt compensation, which is often used to calculate the depth of compensation from geoid and topography data for mid-plate swells. For all of our calculations including those with no low viscosity layer, the effective gravitational mass distribution is more complex than assumed in simple Pratt models, so that the Pratt models are not an appropriate gauge of the compensation mechanism.     

The origin of Samoa: new evidence from Sr, Nd, and Pb isotopes

We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have²⁰⁶Pb/²⁰⁴Pb of 18.9–19.4,⁸⁷Sr/⁸⁶Sr of 0.7045–0.7055 and⁸⁷Sr/⁸⁶Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower²⁰⁶Pb/²⁰⁴Pb and¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between²⁰⁷Pb/²⁰⁴Pb and²⁰⁶Pb/²⁰⁴Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high-²⁰⁷Pb/²⁰⁴Pb,⁸⁷Sr/⁸⁶Sr, low-²⁰⁶Pb/²⁰⁴Pb and¹⁴³Nd/¹⁴⁴Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.     

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Estuaries and Coasts -     

Strontium Isotopic Signatures of the Streams and Lakes of Taylor Valley, Southern Victoria Land, Antarctica: Chemical Weathering in a Polar Climate

We have collected and analyzed a series of water samples from three closed-basin lakes (Lakes Bonney, Fryxell, and Hoare) in Taylor Valley, Antarctica, and the streams that flow into them. In all three lakes, the hypolimnetic waters have different ⁸⁷Sr/⁸⁶Sr ratios than the surface waters, with the deep water of Lakes Fryxell and Hoare being less radiogenic than the surface waters. The opposite occurs in Lake Bonney. The Lake Fryxell isotopic ratios are lower than modern-day ocean water and most of the whole-rock ratios of the surrounding geologic materials. A conceivable source of Sr to the system could be either the Cenozoic volcanic rocks that make up a small portion of the till deposited in the valley during the Last Glacial Maximum or from marble derived from the local basement rocks. The more radiogenic ratios from Lake Bonney originate from ancient salt deposits that flow into the lake from Taylor Glacier and the weathering of minerals with more radiogenic Sr isotopic ratios within the tills. The Sr isotopic data from the streams and lakes of Taylor Valley strongly support the notion documented by previous investigators that chemical weathering has been, and is currently, a major process in determining the overall aquatic chemistry of these lakes in this polar desert environment.     

Paleolimnology of a freshwater estuary to inform Area of Concern nutrient delisting efforts

The St. Louis River Estuary (SLRE), a freshwater estuary bordering Duluth, Minnesota, Superior, Wisconsin, and the most western point of Lake Superior (46.74°, ? 92.13°), has a long history of human development since Euro-American settlement ~ 200 years ago. Due to degradation from logging, hydrologic modification, industrial practices, and untreated sewage, the SLRE was designated an Area of Concern in 1987. Action has been taken to restore water quality including the installation of the Western Lake Superior Sanitary District in 1978 to help remove beneficial use impairments. A better understanding of historical impacts and remediation is necessary to help document progress and knowledge gaps related to water quality, so a paleolimnological study of the SLRE was initiated. Various paleolimnological indicators (pigments, diatom communities, and diatom-inferred phosphorus) were analyzed from six cores taken throughout the SLRE and another from western Lake Superior. Reductions in eutrophic diatom taxa such as Cyclotella meneghiniana and Stephanodiscus after 1970 in certain cores suggest an improvement in water quality over the last 40 years. However, in cores taken from estuarine bay environments, persistence of eutrophic taxa such as Cyclostephanos dubius and Stephanodiscus binderanus indicate ongoing nutrient problems. Sedimentary pigments also indicate cyanobacteria increases in bays over the last two decades. Diatom model-inferred phosphorus and contemporary monitoring data suggest some of the problems associated with excess nutrient loads have been remediated, but modern conditions (internal phosphorus loading, changing climate) may be contributing to ongoing water quality impairments in some locations. The integrated record of biological, chemical, and physical indicators preserved in the sediments will aid state and federal agencies in determining where to target their resources.