Studying imprints of climatic and environmental change in a regional aquifer system in an arid part of India using noble gases and other environmental tracers
Zusammenfassung der Projektergebnisse
The Cambay Basin in the Indian state of Gujarat is a sedimentary basin situated in a semi-arid, monsoonal climate, providing multiple opportunities for hydrologic and palaeoclimatic research. Major fault zones imply geologic imprints, while the whole sedimentary aquifer sysrem comprises water with ages from recent up to 50.000 years, thus including the last glacial period. Previous work of the Indian partners of this project has established the age distribution and the presence of climatic signals in the groundwater of the Cambay Basin. The major aim of the present multi-tracer study was a comprehensive and quantitative investigation of the palaeoclimare record in this aquifer system. 14C and other tracers such as 3H, He isotopes, 222Rn and SF6 were used for dating, while stable water isotopes and in particular dissolved noble gases enabled the reconstruction of both palaeotemperature and palaeohumidity. The noble gas thermomerer is a unique tool to obtain reliable absolute palaeoremperatures, while the excess air component observed in noble gas statures of groundwater has been related to changes in precipitation, in particular in semi-arid regions. This study provides the first noble gas record from South Asia and to our knowledge the first quantitative information on continental palaeotemperature for the past 50.000 years in this region. The ^14C dating performed in this study generally agrees with the previous results and is qualitatively supported by He-Rn ages. However, a more comprehensive approach for the evaluation of 14C data was undertaken, including the first application of a new model for dead-carbon corrections in groundwater dating. He concentrations in general increase with age but also show specific imprints of geological features. In deep wells of the sedimentary basin and near major fault zones of the region, mantle He was determined to contribute up to 10% of the total rerrigenic He. In the crystalline basement, high concentrations of 222R and radiogenic 4He were found, and SF6 concentrations exceeded modern atmospheric equilibrium by several orders of magnitude. These findings provide insights in the geological processes in this tectonic basin as well as into mechanisms of natural SF6 production. Excess air and stable water isotope data enabled a consistent reconstruction of palaeohumidity, which in this region is synonymous to monsoon strength, a climatic parameter of major importance. Both signals revealed a dry glacial period followed by a humid Holocene, which was intermpted by a dry phase in the late Holocene. Monsoon strength obtained from excess air supports older local palaeoclimate studies in an independent approach novel for the region, confirming the notion that the monsoon intensity is governed by insolation. Noble gas temperatures show a cooler last glacial period compared to the warm Holocene, as expected. However, the quantification of the warming between the last glacial maximum and the Holocene is complicated by temperature variations within the Holocene. Whereas humid periods of the Holocene show noble gas temperatures close to modern airtemperatures, dry periods appear somewhat warmer. This difference is attributed to an offset between soil temperature (as measured by the noble gas thermometer) and air temperatures in dry climates. Comparing the dry phases of the last glacial period and the Holocene, we obtain a glacial - interglacial warming of (3.5 + 0.5) °C. This warming is lower than most results derived from previous noble gas studies, in particular one from the relatively nearby Oman, but it is in good agreement with other palaeotemperature records from tropical regions.
Projektbezogene Publikationen (Auswahl)
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2009. Klimarekonstruktion mit Edelgasen aus Grundwasser in einer indischen Trockenregion. DPG- Frühjahrstagung Hamburg, E-Verhandl. DPG, UP4
Wieser, M., T. Schneider, R. D. Deshpande, S. K. Gupta, W. Aeschbach-Hertig
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2009. Multitracer study of groundwater in a semiarid region in Northwest India. 19th Goldschmidt conference, Davos, 2009. Geochim. Cosmochim. Acta. 73 (suppl.): A1440.
Wieser, M., T. Schneider, W. Aeschbach-Hertig, R. D. Deshpande, S. K. Gupta
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(2010), Imprints of climatic and environmental change in a regional aquifer system in an arid part of India using noble gases and other environmental tracers. Dissertation, Heidelberg University, 204 pp.
Wieser, M.
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2010. Groundwater age and paleoclimate information derived from environmental tracers in a regional aquifer system in semiarid Northwest India. XXXVIII IAH Congress, Groundwater Quality Sustainability, Abstract Book, p. 533-535, University of Silesia Press, Krakow, 2010
Wieser, M., R. D. Deshpande, T. Schneider, W. Aeschbach-Hertig, S. K. Gupta
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2011. A temperature and monsoon record derived from environmental tracers in groundwater of Northwest India, International Symposium on Isotopes in Hydrology, Marine Ecosystems, and Climate Change Studies (book of abstracts), IAEA, Monaco, 2011
Wieser, M., W. Aeschbach-Hertig, T. Schneider, R. D. Deshpande, S. K. Gupta
