Three-dimensional modelling of a Holocene tufa system in the Lathkill Valley, north Derbyshire, using ground-penetrating radar

Unlithified and partly lithified carbonate sequences are ideally suited to the application of ground‐penetrating radar (GPR), augmented by percussion augering and shallow seismic techniques, all tied to present‐day topography using global positioning system (GPS) methods. This methodology provides the first clear information on the distribution and geometry of lithofacies within buried tufa complexes. The approach has been applied to a thick succession of Holocene tufas filling a gorge site along a 3·5‐km length of the River Lathkill, north Derbyshire. Earlier studies have demonstrated the presence of up to 16 m of tufas and sapropels associated with two transverse tufa dams (barrages). These strata have been accumulating throughout the Holocene, although tufa developments at present are of minor extent. Internal tufa morphologies are recorded by GPR as ‘bright’, laterally continuous reflections for lithified, concretionary and lithoclast‐rich horizons. The ‘brightest’ reflectors occur within well‐cemented barrages and delineate core areas and prograding buttress zones. In contrast, unlithified lime muds and sapropels produce low‐contrast reflections. Lithostratigraphic control and depth calibration of the GPR profiles was provided by percussion augering at selected sites. Six distinct lithofacies and four secondary barrages are identified in the study. Constructional and destructional events can be identified and correlated within the GPR profile network, and the internal growth morphologies of the barrages are apparent. GPR profiles also clearly define the evolution of the facies geometries. Three phases of tufa development can be recognized within the GPR data and greatly extend our understanding of Holocene tufa‐forming processes in valley sites: (a) Early Holocene barrage build‐ups but with limited paludal deposition; (b) Middle Holocene ponding and sapropel accumulation under ‘warm’ conditions; and (c) Late Holocene barrage termination and valley levelling, probably coincidental with anthropogenic activity. This type of multidisciplinary approach should be considered as an essential prerequisite to all biostratigraphic and geochemical studies of Holocene freshwater carbonate sites.     

Airborne MSS for land cover classification II

A basic methodology for land cover classification using airborne multispectral scanner (MSS) imagery is outlined. This includes waveband selection and radiometric calibration; correction for scan angle and atmosphere; training and classification and accuracy assessment. Refinements to this basic methodology include per‐field sampling and the addition of low‐pass filtering, image texture, prior probabilities and two dates of imagery.

For a study area in upland England, eight land covers were classified with a mean accuracy of 52.6 percent using the basic methodology. This was increased to 79.0 percent by using a suitability refined methodology. Per‐field sampling accounted for the largest proportion of this increase.     

Urban groundwater quality in sub-Saharan Africa: current status and implications for water security and public health

Groundwater resources are important sources of drinking water in Africa, and they are hugely important in sustaining urban livelihoods and supporting a diverse range of commercial and agricultural activities. Groundwater has an important role in improving health in sub-Saharan Africa (SSA). An estimated 250 million people (40% of the total) live in urban centres across SSA. SSA has experienced a rapid expansion in urban populations since the 1950s, with increased population densities as well as expanding geographical coverage. Estimates suggest that the urban population in SSA will double between 2000 and 2030. The quality status of shallow urban groundwater resources is often very poor due to inadequate waste management and source protection, and poses a significant health risk to users, while deeper borehole sources often provide an important source of good quality drinking water. Given the growth in future demand from this finite resource, as well as potential changes in future climate in this region, a detailed understanding of both water quantity and quality is required to use this resource sustainably. This paper provides a comprehensive assessment of the water quality status, both microbial and chemical, of urban groundwater in SSA across a range of hydrogeological terrains and different groundwater point types. Lower storage basement terrains, which underlie a significant proportion of urban centres in SSA, are particularly vulnerable to contamination. The relationship between mean nitrate concentration and intrinsic aquifer pollution risk is assessed for urban centres across SSA. Current knowledge gaps are identified and future research needs highlighted.     

Eelgrass beds and bare substrata – sparid and mugilid composition in contrasting littoral estuarine habitats

Seagrass beds serve as nursery grounds for many fish species and often play an important role in the juvenile stages of economically and recreationally important fishes. The eelgrass Zostera capensis is the dominant submerged macrophyte in permanently open South African estuaries and occupies large intertidal and subtidal areas within the Knysna system. The primary objective of this study was to compare the occurrence of sparids and mugilids in eelgrass and nearby bare sediment areas using sampling with seine nets. The hypothesis that was tested is that mugilids are dominant in unvegetated areas of the Knysna Estuary littoral whereas sparids predominate within eelgrass beds located in the same zone. The results indicate that the family Mugilidae is better represented at unvegetated sites when compared to members of the family Sparidae, with the exception of Lithognathus lithognathus, but that the dominant three sparids and dominant two mugilids were most abundant in sparse eelgrass beds that included both bare and vegetated areas within this habitat type. The main fish species responsible for the separation of fish assemblages associated with unvegetated sites versus those associated with vegetated areas were Rhabdosargus holubi, Liza dumerili, Liza richardsonii, Lithognathus lithognathus, Sarpa salpa and Diplodus capensis, with R. holubi, S. salpa and D. capensis being mainly attracted to eelgrass habitats and L. dumerili, L. richardsonii and L. lithognathus having a stronger affinity for bare areas. This finding confirms a previous estuarine study in the nearby Swartvlei lake littoral, which found that sparids dominated areas where aquatic macrophytes were extensive and that mugilids became more prevalent at those same sites when the macrophytes underwent complete senescence. The length-frequency distributions of some sparid and mugilid species differed considerably between eelgrass and bare sediment areas, whereas those of other species showed little or no difference between these two habitat types.     

Sedimentology and palaeoenvironment of the southeast Sicilian Tertiary platform carbonates

A thick succession of shelf and shelf-slope carbonates is developed in southeastern Sicily. Sedimentological aspects of the Oligocene to Tortonian sequence, including distribution and palaeoenvironmental factors, are considered. A new formation and four new members are proposed in the upper part of the succession.The region comprises a western half, typically exhibiting the principal deeper-water planktonic facies including marls, and an eastern area displaying typically inner-shelf carbonates and an attenuated succession. Coralline algal limestones and calcarenites are more typical of this latter region but there are also some volcanic intercalations. An attempt is made to relate the Hyblean succession to that of the Maltese Islands which lie at the southern end of this Tertiary carbonate platform.     

Palaeoenvironmental records in Holocene Spanish tufas: a stable isotope approach in search of reliable climatic archives

Tufa deposits are potential terrestrial archives of palaeoenvironmental and palaeoclimatic information. This study assesses the potential of stable isotopic archives from two closely juxtaposed Holocene tufa sites in SE Spain. The Ruidera site contains deep‐water lacustrine micrites and tufas, whereas the nearby Alcaraz site represents a shallow barrage tufa. Understanding site characteristics is critical to interpreting the stable isotopic variations. These Holocene lacustrine micrites have isotopic compositions consistent with modern European lake shore microbial carbonates, where the isotopic chemistry is strongly influenced by hydrological and residence time effects. All the lacustrine micrite δ¹³C values were influenced by microenvironmental microbial effects to some degree. Because of these effects, stable isotope data from lacustrine microbial micrites and tufas will not normally yield precise information on the isotopic composition of palaeoprecipitation, temperature or vegetation composition of an area. In contrast, Holocene tufas that formed in shallow, fast‐flowing riverine settings may contain valuable palaeoclimatic archives. The tufa deposits must be largely autochthonous, as at Alcaraz, where in situ reed stem encrustations are present. Records of relative change in air temperature and changes in the source of airmasses are potentially resolvable in the δ¹⁸O data. These interpretations can be verified by other independent climatic data where chronology is constrained. Variations in riverine tufa δ¹³C values probably record changes in local vegetation and/or soil respiration. Covariation between δ¹⁸O and δ¹³C values may be intrinsically linked to climatic factors such as aridity. Tentative palaeoclimatic interpretations for the middle Holocene at Alcaraz based on the isotope data suggest warming (or increasing influence of Mediterranean‐sourced precipitation) between approximately 5000–3000 radiocarbon years BP, accompanied by increased aridity. These interpretations are consistent with the sparse independent palaeoclimatic data and climate modelling results for the Holocene of SE Spain. This study supports the growing evidence that well‐chosen tufa sites could yield valuable palaeoclimatic information.     

New insights into biological influence on the geochemistry of freshwater carbonate deposits

Ambient temperature freshwater carbonates (“tufas”) have outstanding potential as palaeoclimate archives. However, it has proven difficult to constrain the behaviour of the geochemical parameters on which important proxy systems rely. Current thinking on the development of trace element ratios in tufa rests upon an assumption of precipitation driven by degassing of CO₂, with the final precipitate being in equilibrium with ambient river water. This study provides evidence that although supersaturation is largely regulated by degassing, precipitation in sterile flowing systems does not occur throughout the air-water interface, but only at lines of intersection between this interface and a solid surface. Precipitation at the bottom, as is normal in river sites, is not found to occur. However, when benthic microbial communities (“biofilms”) are present, extensive precipitation does occur within them in preference and/or addition to that at the air-water interface. Precipitation is therefore not purely a consequence of degassing, and some microbiological influence is necessary to generate precipitates analogous to nature.We further investigate whether these biofilms alter the behaviour of trace elements within tufa systems, in addition to modulating the sites and mechanisms of precipitation, via a series of experiments. Biofilms are shown to accumulate large inventories of calcium and other dissolved ions (Ba, Sr, and Mg), and we demonstrate that this process is chemoselective in favour of ions with low charge densities. These observations have consequences for interpretation of trace element records from tufas based on an assumption of equilibrium with river water, and also for other carbonate deposits associated with biofilm activity (travertines, marine stromatolites, lacustrine and soil carbonates, speleothems). Conversely, our study provides new insight into the reasons for discrepancy of tufa Mg/Ca ratios from expectation, and thus provides encouragement for the potential of Mg/Ca palaeothermometry in freshwater carbonate systems.     

河北承德黑山铁矿床热液成矿特征及流体包裹体研究

黑山大型钒钛磁铁矿矿床产于大庙斜长岩杂岩体中,是承德地区最重要的"大庙式"岩浆型铁矿床。笔者在矿区野外地质观察过程中发现,穿插于斜长岩中的铁磷矿脉、磁铁矿硫化物矿脉有热液成矿作用的显示,表明黑山铁矿床成因除传统认为的岩浆期结晶、熔离、矿浆贯入成矿作用外,还有热液期的成矿作用发生。本文对热液成矿期铁磷矿石中磷灰石和矿化蚀变石英中的流体包裹体进行了显微测温和激光拉曼光谱分析。结果表明,磷灰石中原生包裹体可以分为气液两相包裹体、含子矿物包裹体、含液态CO2三相包裹体、单液相、单气相包裹体5类,均一温度主要集中于180～420℃,盐度主要集中于6.2%～38.9%NaCleq,流体包裹体气相成分主要为CO2、N2和CH4,液相成分主要为H2O,固相成分主要为方解石、石盐、白云石及铁氧化物子矿物。石英中流体包裹体类型和成分与磷灰石中的类似,但固相成分未发现石盐和不透明金属子矿物,均一温度变化于149～422℃,盐度变化于5.7%～22.9%NaCleq。成矿流体为CaCl2-NaCl-H2O-CO2体系,均一温度和盐度呈现正相关连续渐变的特征。铁磷矿石的磷灰石中原生包裹体为流体包裹体,盐度高,子矿物种类复杂,组成中富含CO2和CH4等,这些特征显示成矿流体以岩浆热液为主;成矿机理可能与大气降水对岩浆热液的稀释有关。