Environmental isotopes (18O, 2H, and 87Sr/86Sr) as a tool in groundwater investigations in the Keta Basin, Ghana     

Niels O. Jørgensen  Bruce K. Banoeng-Yakubo 《Hydrogeology Journal》2001,9(2):190-201

Analyses of environmental isotopes (¹⁸O, ²H, and ⁸⁷Sr/⁸⁶Sr) are applied to groundwater studies with emphasis on saline groundwater in aquifers in the Keta Basin, Ghana. The ⁸⁷Sr/⁸⁶Sr ratios of groundwater and surface water of the Keta Basin primarily reflect the geology and the mineralogical composition of the formations in the catchments and recharge areas. The isotopic compositions of ¹⁸O and ²H of deep groundwater have small variations and plot close to the global meteoric water line. Shallow groundwater and surface water have considerably larger variations in isotopic compositions, which reflect evaporation and preservation of seasonal fluctuations. A significant excess of chloride in shallow groundwater in comparison to the calculated evaporation loss is the result of a combination of evaporation and marine sources. Groundwaters from deep wells and dug wells in near-coastal aquifers are characterized by relatively high chloride contents, and the significance of marine influence is evidenced by well-defined mixing lines for strontium isotopes, and hydrogen and oxygen stable isotopes, with isotopic compositions of seawater as one end member. The results derived from environmental isotopes in this study demonstrate that a multi-isotope approach is a useful tool to identify the origin and sources of saline groundwater. Electronic Publication  相似文献   

High δ18O and possible pre-eruptional Rb-Sr isochrons in cordierite-bearing Neogene volcanics from SE Spain     

Niels Crosley Munksgaard 《Contributions to Mineralogy and Petrology》1984,87(4):351-358

O- and Sr-isotope data are reported for cordieritebearing dacites and andesites forming part of the Neogene volcanic province of SE Spain. The almandine-bearing biotite-cordierite-labradorite dacite from the Cerro del Hoyazo with its numerous inclusions of metamorphic and igneous rocks has been studied in some detail. A syngenetic derivation of the Hoyazo dacite magma and part of its inclusions (interpreted as restite) by means of anatexis of (semi-)pelitic rocks has previously been proposed. ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios, thought to be closely representative of the original magmas, vary from +12.2 to +15.6 and 0.7095 to 0.7171, respectively. The metamorphic rock inclusions have ¹⁸O values (+13.0-+16.2) comparable to the range for the volcanics. These results support an anatectic origin for the entire suite of cordierite-bearing volcanics. The inclusions of basic igneous rocks in the Hoyazo dacite have, in comparison with the dacite, lower ¹⁸O values (+11.1-+13.1) and equal to lower ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7112), confirming an independent origin.The six analysed samples of the Hoyazo dacite show a strong linear correlation of ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr which, if interpreted as an isochron, yields an age of 210±17 Ma. Similar linear arrays for samples from the Mazarrón and Mar Menor areas yield nearly concordant ages. Samples for the Vera area define a 535±22 Ma line. These linear correlations may be interpreted either as (1) mixing lines, in which case the indicated ages have no geological meaning, or (2) true isochrons dating pre-eruptional events of the parent (meta-)sediments.  相似文献   

Geochemical Responses to a Major Transgression in Giant African Lakes     

Michael R. Talbot  Niels Bo Jensen  Tine Lærdal  Maria Letizia Filippi 《Journal of Paleolimnology》2006,35(3):467-489

Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content (TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the change in HI. Over the same time interval in all three lakes, δ¹³C_OM shows a falling trend of 5–8‰ . Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high δ¹⁵N values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline waterbody, while high δ¹⁵N later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency to stable chemical stratification as the water column deepened. In the case of δ¹³C, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic carbon (DIC) as runoff and soil–moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading. In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes. IDEAL Contribution No. 152.  相似文献   

Investigation of hydrochemical characteristics of groundwater from the Cretaceous-Eocene limestone aquifer in southern Ghana and southern Togo using hierarchical cluster analysis   总被引：4，自引：1，他引：4  

Tina Helstrup  Niels Oluf Jørgensen  Bruce Banoeng-Yakubo 《Hydrogeology Journal》2007,15(5):977-989

The most relevant controls on the water quality within the Cretaceous-Eocene limestone aquifer of the Keta Basin, Ghana, and the coastal sedimentary basin of Togo were assessed using Q-mode hierarchical cluster analysis (HCA) and mass-balance modelling. The pattern recognition technique of HCA was employed for partitioning hydrochemical data from a total of 65 surface and borehole samples from the study area into water groups. A spatial plot of the water groups consisting of samples from the limestone aquifer shows that the vast majority of samples belonging to the same group are located in close proximity to one another, suggesting the same processes and/or flow paths in the limestone aquifer system. Geochemical reaction models of selected water groups were constructed using PHREEQC-2. The hydrochemical compositions of the water groups and the mass-balance calculations indicate that the dominant processes and reactions responsible for the hydrochemical evolution in the system are: (1) carbonate equilibria, (2) silicate weathering reactions, (3) limited mixing with saline water, and (4) ion exchange. The combined use of HCA and mass-balance modelling has shown to be a useful approach in interpreting groundwater hydrochemistry in an area where large uncertainties exist in the understanding of the groundwater flow system.  相似文献   

Communicating uncertainty in the IPCC’s greenhouse gas emissions scenarios     

Niels J. Schenk  Sander M. Lensink 《Climatic change》2007,82(3-4):293-308

The issue of climate change required the development of the Special Report on Emission Scenarios (SRES) by the IPCC. The complexity of the subject and the unique science-policy relation resulted in confusion and discussions appeared in popular media like The Economist. This paper reviews scenario literature and SRES, identifies the most vulnerable elements in the communication of SRES. In the communication of GHG emission scenarios through SRES, the weaknesses that have been identified by the authors of this paper are the normative character of climate change assessment, the plausibility of the scenarios, and the risk of simplification of complex messages. The causes of these communicative issues have been identified as the intrinsic difficulties of interdisciplinary science, the uniqueness of the science-policy relation, and the need for a high degree of transparency. This paper suggests improving future communication of complex messages from scientists to their audience by means of clear reasoning when communicating with non-scientists, explicitly normative emission scenarios, and increased stakeholder participation in scenario development. Based on a presentation at the “IIASA-YSSP uncertainty seminars,” 22 July 2004, and the discussions thereafter.  相似文献   

Emergence of Civilization,Changes in Fluvio‐Deltaic Style,and Nutrient Redistribution Forced by Holocene Sea‐Level Rise     

Benjamin T. Pennington  Judith Bunbury  Niels Hovius 《Geoarchaeology》2016,31(3):194-210

During the mid‐Holocene, the first large‐scale civilizations emerged in lower alluvial systems after a marked decrease in sea‐level rise at 7–6 kyr. We show that as the landscapes of deltas and lower alluvial plains adjusted to this decrease in the rate of relative sea‐level rise, the abundance and location of resources available for human exploitation changed as did the network of waterways. This dynamic environmental evolution contributed to archaeological changes in the three fluvio‐deltaic settings considered herein: Egypt, Mesopotamia, and the Huang He in China. Specifically, an increase in the scale and intensity of agricultural practice, and the focussing of power toward a single city can be interpreted as responses to these environmental changes. Other archaeological observations, and the cultural trajectories leading to the formation of the Primary States also need to be considered in light of these evolving landscapes.  相似文献   

Weighting and smoothing of stomach content data as input for MSVPA with particular reference to the Barents Sea     

Bulgakova  Tatiana; Vasilyev  Dmitri; Daan  Niels 《ICES Journal of Marine Science》2001,58(6):1208-1218

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Long-term trends in ten non-target North Sea fish species   总被引：1，自引：1，他引：1  

Heessen  Henk J. L.; Daan  Niels 《ICES Journal of Marine Science》1996,53(6):1063-1078

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Changes in the North Sea fish community: evidence of indirect effects of fishing?   总被引：7，自引：4，他引：7  

Daan  Niels; Gislason  Henrik; G. Pope  John; C. Rice  Jake 《ICES Journal of Marine Science》2005,62(2):177-188

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Electromagnetic Subsurface Imaging. A Case for an Adaptive Born Approximation     

Niels B. Christensen 《Surveys in Geophysics》1997,18(5):477-510

The process of interpretation of electromagnetic data has many facets of which fast approximate interpretation techniques is an intriguing one. A new variant of the Born approximation – the Adaptive Born approximation – is presented and exemplified through 1D and 2D imaging techniques for transient electromagnetic data. The Adaptive Born approximation is generally applicable in approximate inversion schemes for inductive electromagnetic data as a one-pass imaging algorithm. Though it is as simple to use as the ordinary Born approximation, it offers a more accurate inverse mapping.In the first part of this paper an attempt will be made to give an overview of fundamental concepts in electromagnetic subsurface imaging relevant for approximate inverse mappings and to outline major trends in present day modeling and inversion of electromagnetic data. This is of course an impossible task – certainly for this author – and much important work will not be mentioned in the limited space of the following. My apologies to the people who are not mentioned and whose research is not given credit here though it should have been. Naturally, my choice of references reflects the schools and circles I have been subjected to, but I hope that the list of references to developments in electromagnetic methods will point to papers of importance and thereby to other references for the interested reader.  相似文献