Historical development of the saltmarsh at Wells,Maine

Comparisons of maps and aerial photographs dating from the late 1700s to the present document the recent development of an 8 km² saltmarsh that is situated behind a barrier spit in southern Maine. Tidal channels that were relatively narrow in 1794 became wider by 1872. The reduction of marsh bordering tidal channels is interpreted as evidence that marsh accretion could not keep pace with rising sea-level. This suggests that the rate of sea-level rise had increased, although a change in discharge or sediment load caused by extensive settlement and land clearance may also have been involved. Meander patterns of the tidal streams changed considerably throughout the time period covered by the maps, demonstrating that the streams of this marsh are more dynamic than some others that have been widely reported in the literature. These differences in stream dynamics are probably related to the differences in sedimentological structure of the marshes. Between 1872 and 1956 the barrier spit eroded on its inside (shoreward) edge, probably in response to the construction of riprapping and houses along the spit, and the subsequent reduction of overwash and aeolian transport of sediment. Modification of the tidal inlet and adjacent marsh during the 1960s, including jetty construction, dredging, and filling of portions of the marsh surface, affected the marsh only locally. One tidal stream has been migrating rapidly apparently in response to compaction of peat by dredge spoils and consequent local disruption of the marsh hydrology. Except for this migration, erosion of the marsh edge occurred immediately after the inlet modifications; planimetric changes in the marsh and its streams have been minor since then.     

Ecological perspectives on rock surface weathering: Towards a conceptual model

A wide range of information is now available on the ecology of rock surface microorganisms (including bacteria, fungi and algae) and lichens and their role in weathering in particular locations. External environmental conditions as well as rock characteristics determine the type of rock surface community, and thus the style of biological weathering. A simple, preliminary conceptual model can be drawn, which relates biological weathering activity to an environmental stress gradient and which is applicable at a range of scales. Further research needs carrying out to test and extend this model in order to make some progress towards the important goal of assessing the overall importance of biological weathering to geomorphology.     

The privatisation of secondary schooling and the desegregation of central Johannesburg

African education controlled by the state's Department of Education and Training has collapsed. Insufficient spending, low standards and persistent student boycotts have made township schools undesirable places of learning. African students with any financial means now seek secondary education at multi-racial private schools in affluent white suburbs or at street-academies in the centre of Johannesburg. The establishment of schooling services for African pupils in the traditionally white central business district contributes to the wider process of desegregation of South Africa's major urban area, Johannesburg.     

Remeasurement of weathering rates,St. Paul's Cathedral,London

Remeasurements of weathering rates at six sites on the base of the balustrade of St. Paul's Cathedral gave substantially lower mean results (0.06 mm a^?1, 1980–1985) than for earlier results (0.14 mm a^?1, 1980–1981). Statistical analysis of the data showed that for five of the sites there were no significant differences between the measurement periods. The only significant spatial difference was for the southwest (exposed) site which was greater than the other sites for 1980–1981, but not at later times. This site also showed the only significant decrease over time. If this site is included in the statistical analysis, the whole set shows a significant decrease over time; omitting this site from the data removes any significant differences over time. The results for this one site for 1980–1981 thus have significant bias on the data body as a whole. Methodological problems, such as rock erosion by the probe and temperature effects, are thought to have minimal effect on the interpretation of the data. Calibrations showed that measurements were accurate to ±0.006 mm and that only results of a change ≥ 0.02 mm were reliably interpretable as different. Remeasurements of the Lead Plug Index (LPI) for the top of the balustrade in 1987 gave a mean rate for 1718–1985 of 0.081 mm a^?1. This is not significantly different from the 1718–1980 mean rate of 0.078 mm a^?1 but the data are not directly comparable, the 1987 sample being smaller than the 1980 set. For the 1987 data, the only statistically significant differences are for the southeast rate ≧ northeast and for a rise in rate in the northeast quadrant. The LPI data are comparable to most of the 1980–1985 MEM data. The conclusion is that most erosion rates have neither increased nor decreased significantly in the measurement time, except for a significant decrease at one (southwest) site from 1980–1981 to 1981–1982. Such a sustained level of erosion gives cause for concern in the context of stone conservation. Causative factors of air quality are discussed and while SO₂ levels have decreased, NO₂ and smoke levels show increases. An overall decrease in stone decay in association with the decrease in SO₂ levels is thus not in evidence.     

Factors Controlling Sulfide Geochemistry in Sub-tropical Estuarine and Bay Sediments

The primary factors that control the concentration of total reduced (inorganic) sulfide in coastal sediments are believed to be the availability of reactive iron, dissolved sulfate and metabolizable organic carbon. We selected nine sites in shallow (<3 m), close to sub-tropical, estuaries and bays along the central Texas coast that represented a range in sediment grain size (a proxy for reactive iron), salinity (a proxy for dissolved sulfate), and total organic carbon (a proxy for metabolizable organic carbon). Based on these parameters a prediction was made of which factor was likely to control total reduced sulfide at each site and what the relative total reduced sulfide concentration was likely to be. To test the prediction, the sediments were analyzed for total reduced sulfide, acid volatile sulfide, and citrate dithionate-extractable, HCl-extractable and total Fe in the solid phase. Using solid-state gold–mercury amalgam microelectrodes and voltammetry, we determined pore water depth profiles of Fe(II) and ΣH₂S and presence or absence of FeS_(aq). At five of the nine sites the calculated degree of sufildization of citrate dithionite-reactive-iron was close to or greater than 1 indicating that rapidly reactive iron was probably the limiting factor for iron sulfide mineral formation. At one site (salinity = 0.9) dissolved Fe(II) was high, ΣH₂S was undetectable and the total reduced sulfide concentration was low indicating sulfate limitation. At the last three sites a low degree of sulfidization and modest total reduced (inorganic) sulfide concentrations appeared to be the result of a limited supply of metabolizable organic carbon. Fe(II)–S(-II) clusters (FeS_(aq)) were undetectable in 10 out of 12 bay sediment profiles where ΣH₂S was close to or below detection limits, but was observed in all other porewater profiles. Acid volatile sulfide, but not total reduced sulfide, was well correlated with total organic carbon and ranged from being undetectable in some cores to representing a major portion of total reduced sulfide in other cores. Although predicted controls on total reduced sulfide were good for very low salinity water or sandy sediments, they were only right about half the time for the other sediments. The likely reasons for the wrong predictions are the poor correlation of total organic carbon with grain size and differing fractions of metabolizable organic carbon in different sedimentary environments. Differences in sediment accumulation rates may also play a role, but these are difficult to determine in this region where hurricanes often resuspend and move sediments. This study demonstrates the need to examine more complex and often difficult to determine parameters in anoxic “normal marine” sediments if we are to understand what controls the concentration and distribution of sulfides.     

Simulation of the dissolution of weathered versus unweathered limestone in carbonic acid solutions of varying strength

A simulation was undertaken within a climatic chamber to investigate limestone dissolution under varied carbonic acid (H₂CO₃) strengths as a possible analogue for future increases in atmospheric CO₂ arising from global warming. Twenty‐eight samples cut from a block of Bath (Box Hill) limestone from Somerville College, Oxford, which had been removed during restoration after 150 years in an urban environment, were weighed and placed in closed bottles of thin plastic containing varying concentrations of H₂CO₃. Half of the stone samples were derived from exposed surfaces of the stone block (weathered) while the others were obtained from the centre of the block on unexposed surfaces (unweathered). The purpose of this was to compare dissolution of previously weathered versus unweathered surfaces in strong (pH 4·73) versus weak (pH 6·43) solutions of H₂CO₃. A temperature of c. 19 °C was maintained within the chamber representing a plausible future temperature in Oxford for the year 2200 given current warming scenarios. The simulation lasted 25 days with a few stone samples being removed midway. Stone samples show reduced weight in all cases but one. There was greater dissolution of stone samples in a strong H₂CO₃ solution as conveyed by higher concentrations of total hardness and Ca²⁺ in the water samples as well as enhanced microscopic dissolution features identified using SEM. The simulation confirms that enhanced atmospheric CO₂ under global warming, given adequate moisture, will accelerate dissolution rates particularly of newly replaced limestone building stones. However, previously weathered surfaces, such as those on historical stone exposed for a century or more, appear to be less susceptible to the effects of such increased rainfall acidity. Conservation techniques which remove weathered surfaces, such as stone cleaning, may accelerate future decay of historical limestone structures by increasing their susceptibility to dissolution. Copyright © 2006 John Wiley & Sons, Ltd.     

Application of GIS Processing Techniques for Producing Mineral Prospectivity Maps—A Case Study: Mesothermal Au in the Swayze Greenstone Belt,Ontario, Canada

A Geographic Information System (GIS) is used to prepare and process digital geoscience data in a variety of ways for producing gold prospectivity maps of the Swayze greenstone belt, Ontario, Canada. Data used to produce these maps include geologic, geochemical, geophysical, and remotely sensed (Landsat). A number of modeling methods are used and are grouped into data-driven (weights of evidence, logistic regression) and knowledge-driven (index and Boolean overlay) methods. The weights of evidence (WofE) technique compares the spatial association of known gold prospects with various indicators (evidence maps) of gold mineralization, to derive a set of weights used to produce the final gold prospectivity map. Logistic regression derives statistical information from evidence maps over each known gold prospect and the coefficients derived from regression analysis are used to weight each evidence map. The gold prospectivity map produced from the index overlay process uses a weighting scheme that is derived from input by the geologist, whereas the Boolean method uses equally weighted binary evidence maps.The resultant gold prospectivity maps are somewhat different in this study as the data comprising the evidence maps were processed purposely differently for each modeling method. Several areas of high gold potential, some of which are coincident with known gold prospects, are evident on the gold prospectivity maps produced using all modeling methods. The majority of these occur in mafic rocks within high strain zones, which is typical of many Archean greenstone belts.     

Hailstorm Risk Assessment in Rural New South Wales

The risk of hail damage at a particular location depends on the frequency and severity (intensity) of hailfall. Three data sets were used to determine the relative risk of hail damage in the various weather forecasting districts of the state of New South Wales, Australia. Two of the data sets were observational data and the third was a set of crop insurance data. The crop insurance data was the least spatially-biased data available for rural areas. Combined data revealed that hailstorms were most frequent in the tablelands and most severe in the north of the state where there is summer-dominant rainfall.