Palaeoprecipitation record using O-isotope studies of the Himalayan Foreland Basin sediments, NW India

Abstract

The Himalayan foreland basin sediments were deposited during a crucial time period in the Himalayan orogeny. This period, in the Tertiary, has become major focus of research because of the large scale uplift of the Himalayan range which has greatly modified the climate of the Asian continent. The present South Asian monsoonal climate is believed to be a consequence of Himalayan uplift during this period. Also the Himalayan orogeny comprises a major regional tectonic event of the Earth. Hence, the study of these Himalayan foreland basin sediments are not only significant in revealing the palaeoclimatic history of the Asian continent but also paves a way for understanding of the effects of Himalayan orogeny on global climate. Here we report a palaeoprecipitation history of the Ramnagar sub-basin since Miocene times and its regional correlation to other sub-basins of the Himalayan mountain belt. An attempt has been made to use oxygen isotope ratios of pedogenic carbonates in Siwalik palaeosols as proxy palaeo-recorders to deduce the palaeoclimatic history of the Jammu region in the Ramnagar sub-basin for the first time.A total of 141 pedogenic carbonates have been analyzed from three Siwalik sections covering a lateral stretch of approximately 40-50Km along strike. The δ18O values vary from −12.31‰ to −5.89‰ and, when plotted against age, indicate that δ18O values, on average, become ~2‰ enriched from the oldest to the youngest analyzed sediments except for largely depleted δ18O values around 10Ma, 5Ma and 1.8Ma. This ~2‰ increase in calculated δ18O values could have been possible by an ~3°C increase in mean annual air temperature; conversely the largely negative δ18O values indicate periods of intense rainfall or stronger monsoonal conditions. An increase of 2.6‰ in δ18O values has been found from the 10Ma rainfall event to that at 1.8Ma which indicates a reduction of ~173mm in rainfall intensity and amount. Hence, precipitation reconstructions indicate a progressive increase in aridity from ~12Ma to Recent excluding a short term increases in rainfall or monsoon intensity at around 10Ma, 5Ma and 1.8Ma.This pattern of palaeoprecipitation change is similar to the changes documented in other sub-basins of the Himalayan mountain range. Further, monsoon intensification at around 10Ma, 5Ma and 1.8Ma is also observable in various other proxy records, thereby suggesting the role of significant regional tectonism in the evolution and intensification of the present South Asian monsoon climate. Our results indicate that from the Mid-Miocene to Recent, except for the periods of largely depleted δ18O values, the average δ18O value increased by ~2‰. The general increase in average δ18O values from Lower to Upper Siwalik sediments can be explained by changes in both continentality and latitude of the moisture source. It seems likely that this long term increase in δ18O values was due to the uplift of the Himalayas and the Tibetan Plateau in the north which acted as a barrier to the passage of airmasses from Central Asia in the north, thereupon with time restricting the source of airmasses to the south from the Indian Ocean.