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141.
Hadi Hajibeygi Jan Dirk Jansen Olwijn Leeuwenburgh Denis Voskov 《Computational Geosciences》2017,21(5-6):833-833
142.
Hilde Eggermont Oliver Heiri James Russell Mathias Vuille Leen Audenaert Dirk Verschuren 《Journal of Paleolimnology》2010,43(3):413-435
Fossil assemblages of chironomid larvae (non-biting midges) preserved in lake sediments are well-established paleothermometers
in north-temperate and boreal regions, but their potential for temperature reconstruction in tropical regions has never before
been assessed. In this study, we surveyed sub-fossil chironomid assemblages in the surface sediments of 65 lakes and permanent
pools in southwestern Uganda (including the Rwenzori Mountains) and central and southern Kenya (including Mount Kenya) to
document the modern distribution of African chironomid communities along the regional temperature gradient covered by lakes
situated between 489 and 4,575 m above sea level (a.s.l). We then combined these faunal data with linked Surface-Water Temperature
(SWTemp: range 2.1–28.1°C) and Mean Annual Air Temperature (MATemp: range 1.1–24.9°C) data to develop inference models for
quantitative paleotemperature reconstruction. Here we compare and discuss the performance of models based on different numerical
techniques [weighted-averaging (WA), weighted-averaging partial-least-squares (WA-PLS) and a weighted modern analogue technique
(WMAT)], and on subsets of lakes with varying gradient lengths of temperature and other environmental variables. All inference
models calibrated against MATemp have a high coefficient of determination (
r\textjack2 r_{\text{jack}}^{2} = 0.81–0.97), low maximum bias (0.84–2.59°C), and low root-mean-squared error of prediction (RMSEP = 0.61–1.50°C). The statistical
power of SWTemp models is generally weaker (
r\textjack2 r_{\text{jack}}^{2} = 0.77–0.95; maximum bias 1.55–3.73°C; RMSEP = 1.39–1.98°C), likely because the surface-water temperature data are spot measurements
failing to catch significant daily and seasonal variation. Models based on calibration over the full temperature gradient
suffer slightly from the limited number of study sites at intermediate elevation (2,000–3,000 m), and from the presence of
morphologically indistinguishable but ecologically distinct taxa. Calibration confined to high-elevation sites (>3,000 m)
has poorer error statistics, but is less susceptible to biogeographical and taxonomic complexities. Our results compare favourably
with chironomid-based temperature inferences in temperate regions, indicating that chironomid-based temperature reconstruction
in tropical Africa can be achieved. 相似文献
143.
Charline Giguet-Covex Fabien Arnaud Jérôme Poulenard Dirk Enters Jean-Louis Reyss Laurent Millet Jérome Lazzaroto Olivier Vidal 《Journal of Paleolimnology》2010,43(1):171-190
Sedimentological, geochemical and particle-size analyses were used to reconstruct the evolution of both trophic state and
hypolimnetic anoxia in Lake Bourget (French Alps) during the last century. Radionuclide dating (210Pb, 137Cs and 241Am) confirmed the annual rhythm of laminations in the upper sediment profile. In Lake Bourget, biochemical varves are triplets
composed of a diatom layer (spring lamina), a bio-precipitated calcite-rich layer (spring/summer lamina), and a layer rich
in organic matter and detrital particles (winter lamina). The onset of eutrophication and the first appearance of an anoxic
facies occurred simultaneously and were dated by laminae counting to AD 1943±1 year. Persistent anoxic conditions began in
AD 1960. Eutrophication is characterised by drastic increases in the flux of biogenic silica (mostly diatoms), lacustrine
organic matter, and larger calcite crystals (15–30 μm). The increase of organic matter also represents a marker of the onset
of anoxic conditions in the hypolimnion. Our results show that eutrophication was the main factor controlling anoxia in the
hypolimnion. This eutrophication was caused mostly by the inflow of untreated sewage effluents, and to a lesser extent, by
input of fertilizer-derived phosphorus during floods of the Rhone River and run-off from the lake catchment. The Rhone River,
however, can also be a source of re-oxygenation via underflows that originate during flood events. Oxygenation of the hypolimnion
is also controlled by low winter temperatures, which enable turnover of the lake. Thus, global warming, associated with a
forecasted reduction in precipitation, might reduce the efficiency of hypolimnetic re-oxygenation in Lake Bourget. 相似文献
144.
A simple box model of the circulation into and inside the ocean cavern beneath an ice shelf is used to estimate the melt rates
of Antarctic glaciers and ice shelves. The model uses simplified cavern geometries and includes a coarse parameterization
of the overturning circulation and vertical mixing. The melting/freezing physics at the ice shelf/ocean interface are those
usually implemented in high-resolution circulation models of ice shelf caverns. The model is driven by the thermohaline inflow
conditions and coupling to the heat and freshwater exchanges at the sea surface in front of the cavern. We tune the model
for Pine Island Glacier and then apply it to six other major caverns. The dependence of the melting rate on thermohaline conditions
at the ice shelf front is investigated for this set of caverns, including sensitivity studies, alternative parameterizations,
and warming scenarios. An analytical relation between the melting rate and the inflow temperature is derived for a particular
model version, showing a quadratic dependence of basal melting on small values of the temperature of the inflow, which changes
to a linear dependence for larger values. The model predicts melting at all ice shelf bases in agreement with observations,
ranging from below a meter per year for Ronne Ice Shelf to about 25 m/year for the Pine Island Glacier. In a warming scenario
with a one-degree increase of the inflow temperature, the latter glacier responds with a 1.4-fold increase of the melting
rate. Other caverns respond by more than a tenfold increase, as, e.g., Ronne Ice Shelf. The model is suitable for use as a
simple fast module izn coarse large-scale ocean models. 相似文献
145.
Justus E. E. van Beusekom Dirk Mengedoht Christina B. Augustin Mario Schilling Maarten Boersma 《International Journal of Earth Sciences》2009,98(2):251-260
From March 2002 to until April 2003 we investigated the seasonal nutrient and phytoplankton dynamics in the central Bornholm
Basin (Baltic Sea) within the framework of the German GLOBEC Project. We choose a nested approach consisting of vertical fluorescence
profiles, phytoplankton counts and nutrient analyses. The Fluoroprobe (MultiProbe, BBE Moldaenke) is capable of distinguishing
four algal groups (Cryptophyceae, Cyanophyceae, Chlorophyceae, Bacillariophyceae + Dinophyceae). Winter nutrient concentrations
were about 5 μM NO3 and 0.5 μM PO4 in the central Basin. The spring phytoplankton bloom was dominated by the diatom Skeletonema sp. and reached a maximum of about 270 μg C/l before the onset of the seasonal stratification. Protozooplankton was dominated
by the Mesodinium rubrum (a phototrophic ciliate = Myrionecta rubra) and reached a maximum biomass of about 200–300 μg C/l about 2 weeks after the demise of the diatom spring bloom. During summer,
the water column was stratified and a subsurface maximum developed near the thermocline consisting of Bacillariophyceae, Cryptophycea
and other phototrophic flagellates. Phytoplankton and protozooplankton biomass was generally low. Nutrient concentrations
point towards a nitrogen limitation during this period. The stratification period ended during September and surface nutrient
concentrations increased again. Protozooplankton reached a second maximum during September. With the Fluoroprobe small scale
structures in the plankton community could be detected like a subsurface Cryptophyceae maximum near the thermocline that however,
could not be confirmed by cell counts. The chlorophyll a estimate of the Fluoroprobe was in good agreement with the phytoplankton
biomass estimated from counts. We conclude that only by combining modern sensing technology with microscopy, the small-scale
dynamics and taxonomic spectrum of the plankton can be fully captured. 相似文献
146.
Michael Engel Simon Mischel Sabrina Quanz Sven Frei Dirk Radny Rike Voelpel Axel Schmidt 《水文研究》2024,38(5):e15118
Groundwater-surface water interactions (GSI) connect rivers and streams with riparian areas and the adjacent aquifer. Although these interactions exert a substantial control of quantity and quality of both groundwater and surface water, knowledge on GSI along rivers at the regional scale, particularly for inland waterways, is still limited. We investigated GSI along the river Moselle, an important federal inland waterway in Germany, by using radon and tritium to identify gaining (water flux from the aquifer to the surface water) and losing (water flux from the surface water to the aquifer) stream conditions, respectively. Gaining stream conditions were identified by continuously measuring radon along the river during boat surveys with a high spatial resolution (every 2 km) during intermediate (October 2020) and near low flow conditions (August/September 2021). The tritium concentrations in surface water and groundwater and the resulting tritium inventories were used to characterize losing stream conditions Monthly tritium inventories from 2017 to 2022 revealed a mean loss for the whole period of 20.3 % and a mean gain of 21.8%. Both were probably triggered by a combination of losing stream conditions and flood-induced mass transfer of water from the aquifer back into the river as well as discharge fluctuations. At the investigated site Lehmen there were direct indications of an influence of surface water due to elevated tritium concentrations in the groundwater (up to 13.3 Bq L−1). Using radon mass balance modelling, good agreements of simulated versus measured radon data with respect to two groundwater end-member scenarios were obtained during intermediate flow (Spearman's ρ: 0.97 and 0.99; MAE: 10.1 and 3.4 Bq L−1) and near low flow (Spearman's ρ: 0.97 and 0.99; MAE: 11 and 6.5 Bq L−1). Considerable groundwater inflow was limited to the meander of Detzem, where cumulated groundwater inflow of about 19 m3 s−1 (9.5% of total discharge) and 4.2 m3 s−1 (3.8% of total discharge) was simulated during intermediate and near low flow, respectively. However, the groundwater inflow was relatively low compared to alpine streams, for example. The study will help to better identify and quantify GSI at the regional scale and provide methodological guidance for future studies focusing on inland waterways. 相似文献
147.
A useful method for increasing the signal/noise ratio of refracted waves is Common-Midpoint (CMP)-refraction seismics. With this technique the shallow underground can be described in detail using all information (amplitude, frequency, phase characteristics) of the wavetrain following the first break (first-break phase). Thus, the layering can be determined and faults, weak zones, and clefts can be identified. This paper deals with the optimization of CMP-refraction seismics used in combination with the Generalized Reciprocal Method (GRM). Theoretical studies show a close relationship of both methods to the kinematics of wave propagation. Velocities and optimum offsets determined by the GRM can be used directly in the partial Radon transformation in CMP-refraction seismics. The integration of refracted waves leads to an increase in the signal/noise ratio but simultaneously the integration boundaries must be restricted to deal only with selective parts of the investigated refractor. The result of this process is an intercept-time section which can be converted directly to a depth section using standard refraction seismic techniques. Another possibility of depth conversion is the transformation of this intercept-time section to a `pseudo-zero-offset section', known from reflection seismics. Thus, zero-offset sections can be migrated using wave-equation techniques such as Kirchhoff migration. 相似文献
148.
Matthias Zillmer Dirk Gajewski & Boris M. Kashtan 《Geophysical Journal International》1998,132(1):159-166
In this article the interaction of plane waves with a weak-contrast interface between two weakly anisotropic half-spaces is investigated. The anisotropy dealt with is of a general type. The stress–displacement vectors of the plane waves are calculated by perturbation theory. By assuming that the jump in elastic parameters and density across the interface is small, one can derive a simple expression for the R qPqP coefficient. In cases in which the wave motion is restricted to a symmetry plane of an anisotropic medium, simple expressions for the R qSVqSV and R SHSH coefficients are also derived. 相似文献
149.
Dirk?Sachse Jens?Radke Reinhard?Gaupp Lorenz?Schwark Guido?Lüniger Gerd?GleixnerEmail author 《International Journal of Earth Sciences》2004,93(4):554-564
For the first time 18O and 13C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for 18O and 13C values of dolomitized carbonates and D values of individual n-alkanes. 18Ocarb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The 13Ccarb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since 13Ccarb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, 18Ocarb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high 18Ocarb and low 13Ccarb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C16 and n-C18 which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C27, n-C28, n-C29 and n-C30, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. Dwater values, reconstructed using the n-C16 alkane and 18O water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction. 相似文献
150.
Laboratory experiments with regular waves were used to investigate wave transmission by overtopping for a smooth, impermeable breakwater with a 1-in-4 slope.A resulting relationship between the transmission coefficient and a breakwater height above mean sea level normalized with a theoretical wave run-up height is reported in this paper. 相似文献