| Records |
| Author |
Heaton, T.H.E.; Talma, A.S.; Vogel, J.C. |
| Title |
Origin and history of nitrate in confined groundwater in the western Kalahari |
Type |
Journal Article |
| Year |
1983 |
Publication |
Journal of Hydrology |
Abbreviated Journal |
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| Volume |
62 |
Issue |
1 |
Pages |
243-262 |
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| Abstract |
Data are presented for nitrate, dinitrogen and argon concentrations and 15N14N ratios in groundwater, with radiocarbon ages up to 40,000 yr. for three confined sandstone aquifers in the western Kalahari of South West Africa/Namibia. The nitrate is probably generated within the soil of the recharge areas, and its production rate during the period 3000-40,000 B.P. has remained between 0.5 and 1.6 meq NO−3l−1 of recharge water, with ° 15N between + 4 and + 8‰. Variations in the amount of nitrate and of “excess air” in groundwater recharge are found, and can only reflect changes in the environmental conditions during recharge. They must therefore be caused by the climatic changes that have taken place during the past 25,000 yr. |
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0022-1694 |
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| Call Number |
THL @ christoph.kuells @ Heaton1983243 |
Serial |
282 |
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| Author |
Belz, L.; Schüller, I.; Wehrmann, A.; Köster, J.; Wilkes, H. |
| Title |
The leaf wax biomarker record of a Namibian salt pan reveals enhanced summer rainfall during the Last Glacial-Interglacial Transition |
Type |
Journal Article |
| Year |
2020 |
Publication |
Palaeogeography, Palaeoclimatology, Palaeoecology |
Abbreviated Journal |
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| Volume |
543 |
Issue |
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Pages |
109561 |
| Keywords |
-Alkanes, -Alkanols, Late Quaternary, Organic geochemistry, Palaeohydrology, Southern Africa |
| Abstract |
Conventional continental geoarchives are rarely available in arid southern Africa. Therefore, palaeoclimate data in this area are still patchy and late Quaternary climate development is only poorly understood. In the western Kalahari, salt pans (playas, ephemeral lakes) are common and can feature quasi-continuous sedimentation. This study presents the first climate-related biomarker record using sediments from the Omongwa Pan, a Kalahari salt pan located in eastern Namibia. Our approach to reconstruct vegetation and hydrology focuses on biogeochemical bulk parameters and plant wax-derived lipid biomarkers (n-alkanes, n-alkanols, and fatty acids) and their compound-specific carbon and hydrogen isotopic compositions. The presented record reaches back to 27 ka. During the glacial, rather low δ2H values of n-alkanes and low sediment input exclude a strong influence of winter rainfall. n-Alkane and n-alkanol distributions and δ13C values of n-hentriacontane (n-C31) indicate a shift to a vegetation with a higher proportion of C4 plants at the end of the Last Glacial Maximum until the end of Heinrich Stadial I (ca. 18–14.8 ka), which we interpret to indicate an abrupt excursion to a short wetter period likely to be caused by a temporary southward shift of the Intertropical Convergence Zone. Shifts in δ2H values of n-C31 and plant wax parameters give evidence for changes to drier conditions during early Holocene. Comparison of this dataset with representative continental records from the region points to a major influence of summer rainfall at Omongwa Pan during the regarded time span and demonstrates the potential of southern African salt pans as archives for biomarker-based climate proxies. |
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0031-0182 |
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THL @ christoph.kuells @ belz_leaf_2020 |
Serial |
104 |
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| Author |
Lim, S.; Chase, B.M.; Chevalier, M.; Reimer, P.J. |
| Title |
50,000years of vegetation and climate change in the southern Namib Desert, Pella, South Africa |
Type |
Journal Article |
| Year |
2016 |
Publication |
Palaeogeography, Palaeoclimatology, Palaeoecology |
Abbreviated Journal |
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| Volume |
451 |
Issue |
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Pages |
197-209 |
| Keywords |
Climate reconstruction, Late Quaternary, Namib Desert, Pollen, Rock hyrax middens, South Africa |
| Abstract |
This paper presents the first continuous pollen record from the southern Namib Desert spanning the last 50,000years. Obtained from rock hyrax middens found near the town of Pella, South Africa, these data are used to reconstruct vegetation change and quantitative estimates of temperature and aridity. Results indicate that the last glacial period was characterised by increased water availability at the site relative to the Holocene. Changes in temperature and potential evapotranspiration appear to have played a significant role in determining the hydrologic balance. The record can be considered in two sections: 1) the last glacial period, when low temperatures favoured the development of more mesic Nama-Karoo vegetation at the site, with periods of increased humidity concurrent with increased coastal upwelling, both responding to lower global/regional temperatures; and 2) the Holocene, during which time high temperatures and potential evapotranspiration resulted in increased aridity and an expansion of the Desert Biome. During this latter period, increases in upwelling intensity created drier conditions at the site. Considered in the context of discussions of forcing mechanisms of regional climate change and environmental dynamics, the results from Pella stand in clear contrast with many inferences of terrestrial environmental change derived from regional marine records. Observations of a strong precessional signal and interpretations of increased humidity during phases of high local summer insolation in the marine records are not consistent with the data from Pella. Similarly, while high percentages of Restionaceae pollen has been observed in marine sediments during the last glacial period, they do not exceed 1% of the assemblage from Pella, indicating that no significant expansion of the Fynbos Biome has occurred during the last 50,000years. These findings pose interesting questions regarding the nature of environmental change in southwestern Africa, and the significance of the diverse records that have been obtained from the region. |
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0031-0182 |
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THL @ christoph.kuells @ lim_50000years_2016 |
Serial |
107 |
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Zhang, Y.; Liu, X.; Yuan, S.; Song, J.; Chen, W.; Dias, D. |
| Title |
A two-dimensional experimental study of active progressive failure of deeply buried Qanat tunnels in sandy ground |
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Journal Article |
| Year |
2023 |
Publication |
Soils and Foundations |
Abbreviated Journal |
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63 |
Issue |
3 |
Pages |
101323 |
| Keywords |
Qanat tunnel, Sand, Failure effect, Soil arching, Model test |
| Abstract |
As an ancient underground hydraulic engineering facility, the Qanat system has been used to draw groundwater from arid regions. A qanat is a horizontal tunnel with a slight incline that draws groundwater from a higher location and delivers it to lower agricultural land. During long-term water delivery, the qanat tunnel has experienced different degrees of aging and collapse, which may result in the significant ground settlement and even disasters. This paper developed a two-dimensional laboratory system to investigate the influence of progressive failure on the stability of deeply buried qanat tunnels. The developed system is fully instrumented with a particle image velocimetry (PIV) system and earth pressure and displacement monitoring. A special cylindrical membrane tube is designed and connected to an advanced pressure–volume controller to simulate the step-wise failure process of the tunnel. Three model tests were conducted on a dry sand considering the buried qanat tunnels at three different depths. Experimental results clearly show the progressive evolution of soil arching effect in the dry sand associated with the progressive failure of the tunnels. The failure of the Qanat ground starts from the vault and develops upwards, which is closely related to the evolution of stress contour at three consecutive stages. Ground surface settlement and volume loss corresponding to three burial depths were compared. A deeply buried qanat tunnel has a small effect on surface settlement. Earth pressure evolution on the 2D plane shows the load redistribution when the qanat collapses. The maximum arch and the initial point of the limit state correspond to a volume loss of 12.5 % and 50 %, respectively. For the collapse of the deep buried qanat tunnel, ground earth pressure evolution can be divided into a stress-increasing region, stress-decreasing region, and no redistribution region. Furthermore, a multi trap-door model considering soil expansion is proposed to describe the progressive failure behavior and its effects. |
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0038-0806 |
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no |
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THL @ christoph.kuells @ Zhang2023101323 |
Serial |
274 |
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| Author |
Li, X.; Shen, K.; Li, Q.; Deng, Y.; Zhu, P.; Wang, D. |
| Title |
Roll-over behavior in current-voltage curve introduced by an energy barrier at the front contact in thin film CdTe solar cell |
Type |
Journal Article |
| Year |
2018 |
Publication |
Solar Energy |
Abbreviated Journal |
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| Volume |
165 |
Issue |
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Pages |
27-34 |
| Keywords |
AlO HRT layer, Band alignment, CdTe solar cell, Roll-over behavior |
| Abstract |
Roll-over phenomenon in the current–voltage (J–V) curve is often observed in a CdTe thin film solar cell. The roll-over phenomenon, which is occurred near the open-circuit voltage in a light J–V curve, is due to Schottky energy barrier formed at the CdTe/metal interface in a CdTe solar cell back contact. In this study we report a J–V roll-over phenomenon which is induced by an energy barrier at the front contact of a CdTe solar cell. Two kinds of oxides, namely, Al2O3 and SnO2, were deposited as high-resistance transparent (HRT) layer between the window layer CdS and the fluorine doped tin oxide (FTO) front electrode in CdTe solar cells. These two oxides present much different electronic band alignment with FTO and CdS. SnO2 formed almost no energy barrier with CdS, this allowed smooth transport for photo-generated electrons from CdTe to CdS and FTO. However, Al2O3 formed a high energy barrier with CdS. The rather high energy barrier with a value of 3.43 eV at the CdS/Al2O3 interface induced a J–V roll-over phenomenon in a CdTe thin film solar cell, which dramatically led to a quick decrease for the cell device efficiency. The electron transport at the FTO/Al2O3/CdS interface is governed by tunneling effect. The results presented in this study demonstrate that the band structure at the front electrode plays an important role for the performance of a CdTe thin film solar cell. |
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0038-092x |
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no |
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THL @ christoph.kuells @ li_roll-over_2018 |
Serial |
187 |
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