Patnaik R.,Panjab University |
Gupta A.K.,Wadia Institute of Himalayan Geology |
Naidu P.D.,National Institute of Oceanography of India |
Yadav R.R.,Birbal Sahni Institute of Paleobotany |
And 2 more authors.
Proceedings of the Indian National Science Academy | Year: 2012
Here, we present a review of the work done in India during 2007-2011 on various proxy records of monsoon variability preserved in the marine (Central Indian Basin, western, northern and eastern Arabian Sea and the Bay of Bengal) and terrestrial (Northeast, Gujarat, Himalayan Lakes, Siwaliks, Thar Desert, Ganga Valley, etc.) realms, in order to understand how Indian monsoon has evolved through the Tertiary and Quaternary periods. Though, there are clear indications of heavy rainfall occurrence throughout the Palaeogene and early Neogene, seasonality in rainfall pattern becomes apparent only in the Late Miocene record. Northern Hemisphere Glaciation played a major role in the evolution of Pliocene monsoon, whereas, glacial-interglacial cycles influenced the Pleistocene monsoonal variability. The Holocene, which is characterized by millennial-scale climatic fluctuations, started with a strong monsoonal phase, often known as "Holocene Climatic Optimum", lasted till the mid Holocene. This strong phase was followed by weak phases causing drier conditions mostly around -2,500-1,500, 1000, 650-450 yrs BP and the little Ice Age (AD. 1450-1850). © Printed in India.
Veena M.P.,Anna University |
Achyuthan H.,Anna University |
Eastoe C.,University of Arizona |
Farooqui A.,Birbal Sahni Institute of Paleobotany
Anthropocene | Year: 2015
Humans have continuously changed the landscape and vegetation cover of the earth, including deserts and lake margins. Vellayani Lake, Kerala, is being severely affected by human activity in the catchment basin. Population increase, leading to increased demand for agricultural land and water, is driving a positive feedback loop resulting in the accelerating siltation of the lake and lowering of water levels. A 145. cm sediment core represents deposition since 3000. yrs BP, but at least half of the sediment has been deposited since 1650 AD, possibly since 1950 AD. The sediment stratigraphy is disturbed, most likely by slumping since 1970, of an unstable sediment accumulation near the lake margin. The disturbance precludes detailed interpretation of paleoenvironmental proxies, textural variation, chemical weathering index, pollen and phytolith assemblages. The proxies provide evidence of drier climate up to 3000. yrs BP, and wetter conditions at about 1210. yrs BP. Low frequency of phytoliths morph types indicates reduction in vegetation cover and a significant increase in grassland since 2300. yrs BP due to climate warming, weakened southwest monsoon, deforestation reclamation of the lake margins for intensive agricultural practices. Occurrence of diatoms and sponge spicules indicate shallowing of the lake from 2300. yrs BP onwards. Occurrence of charcoal pieces post 3000. yrs BP represents human impact. Integration of all the results show that the Vellayani Lake has contracted because of a weakened Indian southwest monsoon since 3000. yrs BP, intense human impact in the form of deforestation, irrigation and agricultural practices. © 2015 Elsevier Ltd.
Tiwari R.K.,CSIR - Central Electrochemical Research Institute |
Yadav R.R.,Birbal Sahni Institute of Paleobotany |
Kaladhar Rao K.P.C.,CSIR - Central Electrochemical Research Institute
Geophysical Monograph Series | Year: 2012
The ability to distinguish different natural frequency modes from a complex noisy temperature record is essential for a better understanding of the climate response to internal/external forcing. Here we investigate the empirical orthogonal function and spectra of a newly reconstructed tree ring temperature variability record decoded from the western Himalayas for a period spanning 1227 A.D. to 2000 A.D., allowing frequency resolution of interdecadal and interannual oscillatory modes. The spectral analysis of first principal component (PC1) with ∼61.46% variance reveals the dominance of significant solar cycles notably peaking around 81, 32, 22, and 8-14 years. Although longer solar cycles are dominant and statistically significant at more than 95% confidence level, the average 11 year solar cycle peaking at a period ranging from 8 to 14 years is less significant (not >90%) and might indicate chaotic phenomena. Similar spectral analysis of PC2 (variance 26%) and PC3 (variance 13.05%) reveals interannual oscillations peaking at a period range of 2-8 years, which are probably related to the global aspect of the El Niño-Southern Oscillation phenomena. Our present analysis in the light of the recent ocean-atmospheric model results suggests that even small variation in solar output in conjunction with the atmospheric-ocean system and other related feedback processes could cause the observed abrupt temperature variability at the time of "criticality" through the triggering mechanism. © 2012. American Geophysical Union. All Rights Reserved.