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.     

Velocity structure of upper-mantle transition zones beneath central eurasia from seismic inversion using genetic algorithms

We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.
Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s⁻¹ over a depth range of 400–415 km, a gradient of 0.0433 s⁻¹, and from 9.25–9.60 km s⁻¹ over a depth range of 415–435 km, a gradient of 0.0175 s⁻¹. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s⁻¹ at 655 km depth to 10.70 km s⁻¹ at 660 km depth, a gradient of 0.055 s⁻¹.     

Dialogue, metaphors of dialogue and understandings of geography

Increasingly, human and physical geographers alike describe their research practices as involving dialogue of some kind. However, the widespread popularity of the term belies some very different understandings of its meaning and methodological implications. In this paper we reflect on these different conceptions of 'dialogical' geography as a way, first, of illustrating the broad range of understandings of research methodology now current in the discipline and, second, of identifying their implications for three long–standing controversies in the discipline: relativism and the truth of geographical knowledge, the associated dualisms between subject/object, nature/society, and realism/constructionism, and the potential for unity between human and physical geography. We argue that, while dialogue is a potentially fruitful way of understanding and practising geography, that the defence of it, like the attacks on it, is often misconceived.     

Thorium adsorption in the ocean: reversibility and distribution amongst particle sizes

Experiments have been carried out to determine whether the adsorption of Th by marine suspended particles is a reversible process. The results indicate reversibility on a time scale of hours. The distribution of Th between the ‘dissolved’ state (<0.22 μm particle size) and various particle size fractions has been compared with predictions based on a hyperbolic size distribution of marine particles assuming spherical geometry and surface adsorption. Thorium is present in small particles to a much smaller degree than such a model would predict, so that the use of 1 μm pore size filters to separate ‘dissolved’ and ‘particulate’ Th does not introduce serious errors. The implications of these results on the reversible exchange model of Bacon and Anderson (1982) and on the settling model of Tsunogai and Minagawa (1978) are discussed.     

Multi‐criteria evaluation and least‐cost path analysis for optimal haulage routing of dump trucks in large scale open‐pit mines

A new raster‐based GIS model that combines multi‐criteria evaluation and least‐cost path analysis was developed to determine the optimal haulage routes of dump trucks in large scale open‐pit mines. The model logic can consider multiple criteria simultaneously (i.e. speed, water body, ore body, curve, visibility, haul road maintenance) and can rate the adverse factor scores of truck movement using fuzzy membership functions. After establishing the weights of five factors by pairwise comparisons, the average adverse score grid can be generated by the weighted linear combination of factor and constraint scores. New software, called Dump Traveler, was implemented to improve the availability of the developed model. An application to the Roto South pit in the Pasir open‐pit coal mine, Indonesia, showed that the software can provide rational solutions to determine the optimal routes on truck haulage operations. Moreover, the layout of available haul roads can be evaluated to consider the trade‐off between road maintenance costs and the potential for traffic jams. Variations of weights for factors were found to be sensitive to the optimal haulage routes determined by least‐cost path analysis. The software provides both optimal routes on truck haulage operations and approximately estimated travel times along the routes, therefore it can support other truck dispatching software that mainly considers scheduling problems.     

Does land use matter in an arid Environment? A case study from the Hoanib River catchment, north-western Namibia

The effect of intensive grazing and browsing of domestic stock and wildlife on the number of species and abundance of vegetation was investigated in the Hoanib River catchment, north-western Namibia. The seasonal abundance of ground cover, bare earth, canopy cover, annual grass, perennial grass and annual forbs were measured in each of the focus-study areas. In three of the focus areas where the ranges of both domestic stock and wildlife were restricted either by fencing or water availability, impact on the vegetation was greatest. The final focus area was a more ‘open range’ system that allowed for the free movement of wildlife. Under these conditions the species abundance and availability of browsing and grazing was greater than the other focus areas during both the wet and dry seasons. However, very little difference in abundance and availability of vegetation was observed between focus areas in both seasons regardless of landuse. There is generally a low abundance of perennial grasses and browse species affording the ecosystem little resistance and resilience to disturbance caused by grazing and drought.     

Multimode surface waveform tomography of the Pacific Ocean: a closer look at the lithospheric cooling signature

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the   V_SV   structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in   V_SV   with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average   V_SV   trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.     

Regional and annual variability in common eider nesting ecology in Labrador

Nesting densities are often used to estimate breeding population size and with other measures of reproductive performance can be useful indicators of population status. These aspects of breeding biology often show considerable spatial and temporal variation. Between 2000 and 2003, we surveyed nesting common eiders ( Somateria mollissima ) on 172 islands in three archipelagos (Nain, Hopedale, Rigolet) on the Labrador coast. Rigolet was the largest archipelago (2834 km²) followed by Nain then Hopedale, and island density varied inversely with archipelago size. Overall means were: nest density 52.0 ± 141.9 (SD) nests/ha; nest initiation 12 June ± 12 days; clutch size 3.7 ± 1.2 eggs/nest; egg volume 98.8 ± 10.4 cm³; and clutch volume 392.3 ± 135.0 cm³. Rigolet had the highest average egg volumes and nest densities, the highest single island nest density of 1053 nests/ha, and the earliest average nest initiation date. We found significant differences in nest densities among archipelagos and across years; significant archipelago and year interactions were detected for nest initiation date and clutch size. Significant differences were found among individual islands for all response variables except egg volume. For egg volume, within-archipelago island differences were not significant, but between-archipelago differences were significant. Thus egg volume may be a useful diagnostic to identify population affiliation.