Multiproxy evidence for terrestrial and aquatic ecosystem responses during the 8.2 ka cold event as recorded at Højby Sø, Denmark     

Mikkel Ulfeldt Hede  Peter Rasmussen  Nanna Noe-Nygaard  Annemarie L. Clarke  Rolf D. Vinebrooke  Jesper Olsen 《Quaternary Research》2010,73(3):485-496

A sediment succession from Højby Sø, a lake in eastern Denmark, covering the time period 9400–7400 cal yr BP was studied using high-resolution geochemistry, magnetic susceptibility, pollen, macrofossil, diatom, and algal pigment analysis to investigate responses of the terrestrial and aquatic ecosystems to the 8.2 ka cold event. A reduced pollen production by thermophilous deciduous tree taxa in the period c. 8250–8000 cal yr BP reveal that the forest ecosystem was affected by low temperatures during the summer and winter/early-spring seasons. This finding is consistent with the timing of the 8.2 ka cold event as registered in the Greenland ice cores. At Højby Sø, the climate anomaly appears to have started 200–250 yr earlier than the 8.2 ka cold event as the lake proxy data provide strong evidence for a precipitation-induced distinct increase in catchment soil erosion beginning around 8500 cal yr BP. Alteration of the terrestrial environment then resulted in a major aquatic ecosystem change with nutrient enrichment of the lake and enhanced productivity, which lasted until c. 7900 cal yr BP.  相似文献   

Identifying and Removing Barometric Pressure Effects in Confined and Unconfined Aquifers   总被引：5，自引：0，他引：5  

Todd C. Rasmussen  Leslie A. Crawford 《Ground water》1997,35(3):502-511

  相似文献   

Monotoring shallow groundwater quality in agricultural watersheds in Denmark     

P. Rasmussen 《Environmental Geology》1996,27(4):309-319

Monitoring of shallow groundwater quality is part of the Danish Agricultural Watershed Monitoring Programme. The monitoring program provides fast and field-related information on impacts of changes in agricultural practices on a watershed scale. The monitoring concept described is carried out in six well-defined hydrological basins and includes new types of groundwater nests and soil water stations. Results from the first three years are presented through examples from the clayey till watershed Lillebæk and the sandy watershed Barslund Bæk. By averaging concentrations of solutes in groundwater sampled at the same depth on a monthly basis, it is possible to evaluate trends in the overall impact of climate and different agricultural practices in a watershed on the groundwater quality. Characteristic seasonal variations in nitrate concentrations related to water level fluctuations was observed. The monitoring has demonstrated significantly higher nitrate concentrations in groundwater sampled downgradient of fields receiving manure than in groundwater sampled downgradient of fertilized fields. Significant vertical nitrate reduction downgradient of manured fields was observed in both the sandy and the clayey watershed though above the redoxcline. In the clayey till watershed fast vertical transport and insignificant nitrate reduction was observed downgradient of fertilized fields probably due to fracture and macro-pore flow.  相似文献   

Dynamic interpretation of grain size relations in silt     

Jesper Bartholdy 《Geo-Marine Letters》1985,5(1):67-70

In the salt marsh deposits of the Danish Wadden Sea, a separation has successfully been made of two log-normally distributed grain size populations, which make up the major part of these fine-grained sediments. The coarse population consists almost exclusively of silt and is interpreted as material either deposited as resistant flocs or as single particles. The fine population consists of approximately 50% silt and 50% clay and is interpreted as particles settled as easily destroyable flocs. These interpretations are confirned by the fact that the relative abundance of the two populations is demonstrated to be related to the degree of exposure of salt marsh areas.  相似文献   

ON THE SENSITIVITY OF PRECIPITATION FORECASTS TO THE MOIST PHYSICS AND THE HORIZONTAL RESOLUTION OF NUMERICAL MODEL^*          下载免费PDF全文

Yu Xiaoding  Huang Xiang-Yu  Leif Laursen  Erik Rasmussen 《Acta Meteorologica Sinica》1997,11(4):432-445

The impacts of the enhanced model's moist physics and horizontal resolution upon the QPFs(quantitative precipitation forecasts) are investigated by applying the HIRLAM(high resolution limited area model) to the summer heavy-rain cases in China.The performance of the control run,for which a 0.5°×0.5°grid spacing and a traditional "grid-box supersaturation removal+Kuo type convective paramerization" are used as the moist physics,is compared with that of the sensitivity runs with an enhanced model's moist physics(Sundqvist scheme) and an increased horizontal resolution(0.25°×0.25°),respectively.The results show:(1) The enhanced moist physics scheme(Sundqvist scheme),by introducing the cloud water content as an additional prognostic variable and taking into account briefly of the microphysics involved in the cloud-rain conversion,does bring improvements in the model's QPFs.Although the deteriorated QPFs also occur occasionally,the improvements are found in the majority of the cases,indicating the great potential for the improvement of QPFs by enhancing the model's moist physics.(2) By increasing the model's horizontal resolution from 0.5°×0.5°,which is already quite high compared with that of the conventional atmospheric soundings,to 0.25°×0.25°without the simultaneous enhancement in model physics and objective analysis,the improvements in QPFs are very limited.With higher resolution,although slight amelioration in locating the rainfall centers and in resolving some finer structures of precipitation pattern are made,the number of the mis-predicted fine structures in rainfall field increases with the enhanced model resolution as well.  相似文献   

In situ U-Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes     

Birger Rasmussen  Ian R. Fletcher 《Geochimica et cosmochimica acta》2007,71(3):670-690

In situ U-Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250 Ma, whereas diagenetic xenotime provides a new minimum age of 3190 ± 10 Ma for deposition of the basal Soanesville Group, previously constrained between ∼3235 Ma and ∼2955 Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65 Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U-Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Specifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton. The 2.88 Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lineament, whereas the most common monazite age population (at 2.16 Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65 Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are readily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecognized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth.  相似文献   

Comparing spatial grain-size trends inferred from textural parameters using percentile statistical parameters and those based on the log-hyperbolic method   总被引：1，自引：0，他引：1  

Jesper Bartholdy  Christian Christiansen 《Sedimentary Geology》2007,202(3):436-452

The Folk&Ward (F&W) and the log-hyperbolic methods are applied to a small - and easy to overlook - number of typical sand sized grain-size distributions from the Danish Wadden Sea. The sand originates from the same source, and the pattern of change in the grain-size distributions is, therefore, exclusively linked to dynamic sorting. In general, the F&W parameters reflect the observed grain-size trends far better than the corresponding log-hyperbolic parameters. The log-hyperbolic “typical log grain size”, ν, is sensitive to changes in skewness and cannot replace mean grain size, Mz, in grain-size trend analysis. The four log-hyperbolic parameters describing dispersion δ, τ^− 1, κ^− 1 and ζ are not able to give an unambiguous picture of spreading trends. The F&W sorting parameter, Sd, can therefore not be replaced by any of the log-hyperbolic parameters in grain-size trend analysis. In skewed grain-size distributions, there seems to be a general defect in the log-hyperbolic approximation of the best represented tail. This makes χ less sensitive to indicate the correct sign of slightly skewed grain-size distributions than Sk. However, when examining trends, the relative change between χ and Sk by and large seems to be the same. The log-hyperbolic peakedness parameter, ξ, is preferable to the F&W peakedness (kurtosis) parameter, Kg, because the latter is hypersensitive to small, insignificant variations in a grain-size distribution. By contrast, the concept of peakedness as defined by ξ, seems to relate more directly to sedimentary environmental conditions. In conclusion, the log-hyperbolic method has both advantages and disadvantages when compared with the F&W method. It is thus suggested that a sensible combination of the two methods could be advantageous to sediment trend analysis.  相似文献   

Mesozoic–Cenozoic exhumation events in the eastern North Sea Basin: a multi-disciplinary study based on palaeothermal, palaeoburial, stratigraphic and seismic data     

Peter Japsen  Paul F. Green†  Lars Henrik Nielsen  Erik S. Rasmussen  Torben Bidstrup 《Basin Research》2007,19(4):451-490

Four Mesozoic–Cenozoic palaeothermal episodes related to deeper burial and subsequent exhumation and one reflecting climate change during the Eocene have been identified in a study of new apatite fission‐track analysis (AFTA^®) and vitrinite reflectance data in eight Danish wells. The study combined thermal‐history reconstruction with exhumation studies based on palaeoburial data (sonic velocities) and stratigraphic and seismic data. Mid‐Jurassic exhumation (ca. 175 Ma) was caused by regional doming of the North Sea area, broadly contemporaneous with deep exhumation in Scandinavia. A palaeogeothermal gradient of 45 °C km^?1 at that time may be related to a mantle plume rising before rifting in the North Sea. Mid‐Cretaceous exhumation affecting the Sorgenfrei–Tornquist Zone is probably related to late Albian tectonic movements (ca. 100 Ma). The Sole Pit axis in the southern North Sea experienced similar inversion and this suggests a plate‐scale response along crustal weakness zones across NW Europe. Mid‐Cenozoic exhumation affected the eastern North Sea Basin and the onset of this event correlates with a latest Oligocene unconformity (ca. 24 Ma), which indicates a major Scandinavian uplift phase. The deeper burial that caused the late Oligocene thermal event recognized in the AFTA data reflect progradation of lower Oligocene wedges derived from the uplifting Scandinavian landmass. The onset of Scandinavian uplift is represented by an earliest Oligocene unconformity (ca. 33 Ma). Late Neogene exhumation affected the eastern (and western) North Sea Basin including Scandinavia. The sedimentation pattern in the central North Sea Basin shows that this phase began in the early Pliocene (ca. 4 Ma), in good agreement with the AFTA data. These three phases of Cenozoic uplift of Scandinavia also affected the NE Atlantic margin, whereas an intra‐Miocene unconformity (ca. 15 Ma) on the NE Atlantic margin reflects tectonic movements of only minor amplitude in that area. The study demonstrates that only by considering episodic exhumation as an inherent aspect of the sedimentary record can the tectonic evolution be accurately reconstructed.  相似文献   

A new approach to interpreting Rock-Eval S₂ and TOC data for kerogen quality assessment     

Birger Dahl  Jrgen Bojesen-Koefoed  Anders Holm  Holger Justwan  Egil Rasmussen  Erik Thomsen 《Organic Geochemistry》2004,35(11-12):1461

A simple method for application in source potential mapping is used to assess the original oil and gas potentials in source rock horizons based upon Rock-Eval potential (S₂) and total organic carbon (TOC) values. The method assumes that kerogens consist of mixtures of end-members with assigned hydrogen index values. Based on suggested algorithms, the average amounts of oil-prone, gas-prone and inert organic material over source rock intervals are determined in TOC units. The method uses regression lines from plots of remaining hydrocarbon potentials (S₂) versus total organic carbon (TOC), and “quick-look” transparent overlays are used to read the appropriate kerogen mixture.Mineral matrix effects during pyrolysis, when strong, can cause erroneous results. This effect which occurs for oil-prone kerogens and adsorptive minerals can cause problems particularly for lean samples (S₂ = 0–3 mg HC/g rock) whilst the errors for richer samples are less.The method is applied on three sections of Upper Jurassic organic-rich rocks from the Danish North Sea sector, which are at different maturity stages. One of these sections is dominated by gas-prone material, one is dominated by oil-prone material and the third section contains a mixture of oil- and gas-prone material.The method has been compared with other methods that split kerogens in oil and gas generating potential and has given reasonable results.Experience using the method and a presented example suggest that sedimentological, system tract information may be derived from S₂ to TOC cross-plots. A constructed modelling example suggests that the end-member concept used in this approach may be used in forward type source rock prediction models when combined with sedimentological models. The resulting S₂–TOC plots can be used in order to check the forward modelling results against observed values.  相似文献   

Gold potential of the Mpanda Mineral Field, SW Tanzania: evaluation based on geological, lead isotopic and aeromagnetic data     

Henrik Stendal  Robert Frei  Sospeter Muhongo  Thorkild M. Rasmussen  Saidi Mnali  Faustin Petro  E. Brian Temu 《Journal of African Earth Sciences》2004,38(5):437-447

Gold and base metals of the Mpanda Mineral Field (MMF) is the focus of this paper. Gold veins and gold-bearing base metal occurrences are structurally controlled by conjugate sets of NW–SE and E–W trending faults and/or shear zones that crosscut high-grade metamorphic rocks and post-kinematic intrusions. It was anticipated that Palaeoproterozic country rocks could have been potential host rocks for the gold mineralisation in this area. This argumentation was based on Pb model ages of various deposits from the MMF. Recent fieldwork and Pb isotopic results presented herein indicate that epigenetic gold and base metal vein-type mineralisation in the MMF is post-Palaeoproterozoic.Our Pb isotope study concentrates on constraining the sources of metals in gold-bearing quartz reefs and base metal occurrences. Pb isotopes of whole rocks and minerals indicate that mineralisation was emplaced during the Neoproterozoic, contemporaneous with the intrusion of alkaline granites and carbonatite complexes (e.g., Sangu–Ikola carbonatite complex) at 720 Ma. The source of Pb in the mineral occurrences is compatible with that characteristic of the Palaeoproterozoic host rocks. Aeromagnetic data suggest that the gold-bearing, NNW–SSE trending area continues to the north beyond Mpanda town. Pb isotope results and aeromagnetic data have significant implications for future exploration programs within the region, in that the search should potentially focus on the defined geophysical borders and trendlines, and on Neoproterozoic, rather than Palaeoproterozoic vein systems.  相似文献