Bacterial iron oxide reduction in a terrigenous sediment-impacted tropical shallow marine carbonate system, north Jamaica

Discovery Bay, a carbonate-dominated embayment in north Jamaica, has been subject to inputs for 40 years of iron-rich bauxite sediment associated with the local mining and transport of processed bauxite. As such, this site is an ideal natural laboratory to study the records and impacts of iron oxide inputs upon geochemical, diagenetic, and microbial processes in tropical carbonate sediments.Total Fe contents in sites in the bay not receiving bauxite inputs are negligible and porewater Ca²⁺, SO₄²⁻ and Cl⁻ indicate that bacterial sulphate reduction is an important process. In contrast, surface sediments receiving bauxite inputs contain significant total Fe, from 44 μmol/g in shallow (5 m water depth) sites to 110 μmol/g in deeper (20 m water depth) sites. Up-core increases in total Fe record increased temporal inputs into the bay. Within these Fe-rich sediments porewater data shows the presence of Fe^II released by bacterial Fe^III reduction. There is no direct evidence for significant bacterial sulphate reduction in these sediments. Iron oxides within all bauxite-impacted sediments display a high potential reducibility, from 40% to 80% of the total Fe present as dithionite-extractable Fe^III. Experimental analysis of the potential susceptibility to, and rates of, bacterial Fe^III reduction, utilising Discovery Bay sediment and Shewanella putrefaciens CN32 (a known Fe^III-reducer) has confirmed the high bacterial reducibility of iron oxides within the sediment. Up to 75% of initial dithionite-extractable Fe^III in the sediments was reduced over 15 days.The presence of iron oxides within the Discovery Bay shallow marine carbonate systems has markedly altered the chemical diagenetic processes taking place, with a shift from apparent dominance of bacterial sulphate reduction at non-impacted (Fe-poor) sites, to highly significant bacterial Fe^III reduction in Fe-rich bauxite-impacted sediments. Given the perceived global increases in terrigenoclastic sediment inputs into tropical carbonate systems as a result of land-use and climate changes, coupled with the documented role that iron oxide reduction plays in nutrient and contaminant cycling in sediment systems, more research into the perturbation of early diagenesis by iron oxide inputs is required.     

To what extent do improved irrigation technologies extend aquifer life?

World financial support for Yemen has concentrated on improved irrigation technology as the primary means to reduce the rate of overexploitation of Yemen’s aquifers. An analysis of the impact of such investments on cumulative water abstraction, water savings and the life of an aquifer is presented. It shows that these investments have limited potential to extend aquifer life. The returns are highest in areas with significant remaining resources, and are not attractive in the most severely stressed areas. The analysis also shows that improved irrigation technologies increase the profitability of pumping for the farmer, exacerbating problems of over-abstraction. Finally, it shows that water savings depend on the hydrogeological situation. The policy implications of these findings are discussed in relation to Yemen, an exceptionally water-short country where most of the aquifers in groundwater-irrigated areas are severely over-drafted.     

Radius and limb topography of Mercury obtained from images acquired during the MESSENGER flybys

Analysis of images obtained by the MESSENGER spacecraft during its three flybys of Mercury yields a new estimate for the planet's mean radius of 2439.25±0.69 km, in agreement with results from Mariner 10 and Earth-based observations, as well as with MESSENGER altimeter and occultation data. The mean equatorial radius and polar radius are identical to within error, suggesting that rotational oblateness is negligible when compared with other sources of topography. This result is consistent with the small gravitational oblateness of the planet. Minor differences in radius obtained at different locations reflect regional variations in topography. Residual topography along three limb profiles has a dynamic range of 7.4 km and a root-mean-square roughness of 0.8 km over hemispherical scales. Following MESSENGER's entry into orbit about Mercury in March 2011, we expect considerable additional improvements to our knowledge of Mercury's size and shape.     

Chemical element concentrations in black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) and green ash (<Emphasis Type="Italic">Fraxinus pennsylvanica</Emphasis> Marsh.) leaves at the reclaimed Green Valley coal Mine,Indiana, USA

The Green Valley Mine in Indiana was abandoned in 1963 after 15 years of coal mining. Reclamation began in 1994 and ended in 1999 with the dense planting of twelve different tree species. By 2006, only green ash (Fraxinus pennsylvanica Marsh.) and black locust (Robinia pseudoacacia L.) had survived in any significant numbers. Given the acid mine drainage found at the site, we sought to determine whether element toxicity originating from mine waste was responsible for the extirpation of the trees. Leaf samples taken from 125 mine trees and 29 control trees were acquired and assayed for 41 element concentrations using inductively coupled plasma mass spectrometry (ICPMS). Except for sodium, there was no compelling evidence to indicate that toxic elemental levels were responsible for the widespread tree deaths. In addition to sodium toxicity, we suspect that micronutrient deficiency was a primary factor explaining the extirpation of trees at the mine.     

Counterurbanisation and the characteristics of persons migrating to West Cornwall

Studies at the macro-scale have indicated that many remote rural areas are for the first time in over a century experiencing population growth while metropolitan areas are losing population. In this paper theories of counterurbanisation directed towards explaining the redistribution of population at a national or regional scale are criticised and a more localised research framework is proposed. Such a framework is constructed for the examination of migration into West Cornwall. Results from a postal questionnaire survey of seven study areas in West Cornwall are used to illustrate differences between three population subgroups, namely, return migrants, non-return migrants and non-migrants. Clear differences emerge not only between the population subgroups but between the study areas as well. In conclusion, it is suggested that this middle-range approach should be adopted more widely in order to gain greater insight into the counterurbanisation process and to increase the prospect of relating macro-processes to local trends and human experience.     

Intertidal substrate modification as a result of mangrove planting: Impacts of introduced mangrove species on sediment microfacies characteristics

A programme of mangrove planting has been undertaken around the island of Rodrigues (SW Indian Ocean) since the mid-1980’s involving the introduction of the species Rhizophora mucronata. We examined three coastal embayment sites (Baie Diamant, Anse Goeland and Anse Pansia) in which planting has been undertaken over different time periods within the past 20 years. Planting has met with variable success in the different sites, probably due to variations in fluvial and groundwater influence. At two sites (Baie Diamant – first planted in 1990, and Anse Pansia – first planted in 1995) ecological data indicates that the mangroves are becoming well-established, and sedimentary evidence suggests that relatively rapid modification of intertidal substrates has occurred. This is evident in the form of significant increases in sediment organic-matter content (especially fibrous organic-matter) and an increase in the accumulation of sediment fines inside the mangroves. A strong correlation exists between the magnitude and depth of substrate modification and mangrove forest density, especially root and sapling density. At the third site, Anse Goeland (first planted in 2001), mangrove establishment has not been successful, many of the seedlings have died and no secondary colonisation has occurred. Sediment substrates show no deviation from background levels in terms of organic content or weight % fines content, and we find no evidence for mangrove planting influencing sediment substrates. Despite evidence for the development of a distinctive mangrove facies at Baie Diamant and Anse Pansia there is, however, no evidence as yet for a marked change in substrate geochemistry such as would be demonstrated by evidence of active bioclast dissolution – a common process in many natural (mature) mangrove substrates. We infer this to be a function of the present relative immaturity of the still developing mangrove substrates, but may also be a function of the apparent paucity of burrowing crabs which play an important role in nutrient cycling and sediment geochemistry. Thus whilst the mangroves in some of the study sites are reaching a stage where they are producing distinctive sedimentary facies, the systems appear to be in a state of progressive sedimentary and diagenetic modification as the floral and infaunal components of the mangroves continue to develop.