Signatures of global climatic events and forcing factors for the last two millennia from the active mudflats of Rohisa, southern Saurashtra, Gujarat, western India

摘要

The limited extent of instrumental records going back in time and control of various climatic variables on Indian Summer Monsoon (ISM) has reinvigorated the unprecedented effort in documenting climate changes for the last two millennia. Although extensive studies on late Holocene ISM reconstruction are available, comprehensive understanding on the influence of natural forcing factors on ISM and their plausible signatures is not arrived at till date. In view of this, the present study attempts to address the ISM variations during the last two millennia with special emphasis on natural forcing factors (solar and volcanic) and climate variables (ocean-atmospheric processes). The Saurashtra peninsula of Gujarat in western India receives majority of rainfall during ISM and hence it provides an ideal test-bed to study the ISM variability. A multiproxy approach has been adopted on a sediment core retrieved from the active mudflat of Southern Saurashtra which was chronologically supported by Pb-210, Cs-137 and C-14. The present study demonstrates vacillating climate with strengthened ISM during Roman Warm Period and Medieval Warm Period (2000 - 950 cal yr BP) as a result of increased solar irradiance interrupted by reduced ISM during Dark Ages of Cold Period (similar to 1500 cal yr BP). The plausible occurrence of volcanic eruption before the onset of Little Ice Age (500- 200 cal yr BP) caused the southward migration of Intertropical Convergence Zone (ITCZ) leading to enhanced western disturbances in the study area thereby resulting in cool and wet climate in the region. The study also emphasizes the increased El Nino events with gradual decline in the ISM since Little Ice Age. Further, the study underscores a climate warming during the last two centuries that corroborates well with the instrumental records. Thus, the present study has implication towards understanding the significant role of volcanic activity and solar variability in controlling the millennia scale climate oscillations with additional feedback mechanisms.