Fluvio–sedimentary response of the monsoon-fed Indian rivers to Late Pleistocene–Holocene changes in monsoon strength: reconstruction based on existing 14C dates

Abstract

The primary objective of the present study is to identify major phases of alluviation in the Indian region since the abrupt Deglacial intensification of the monsoon (∼15 cal ka BP) on the basis of analysis of 68 radiocarbon dates from two major hydro-geomorphic regions of India: the Central Ganga Basin (CGB) and the Deccan Peninsula (DP). The recognition of main phases of alluviation and incision has been achieved by evaluating the temporal distribution and clustering of the radiocarbon dates from alluvial sequences. The clusters were detected on the basis of the interpretation of the summed probability distribution plots derived by using OxCal version 4.0.1 and CALPAL (version May 2006) software packages. The summed probability plots reveal that periods of alluviation in the CGB, represented by three clusters (13.9–12.3, 11.9–11.2 and 9.8–9.0 cal ka BP) occur roughly before the onset of Early Holocene monsoon optimum phase. Two other clusters occur in the intervals 3.6–2.8 and 1.1–0.9 cal ka BP. The peak monsoon period generally lacks clusters of radiocarbon dates implying fluvial erosion and channel incision. This period also shows clustering of radiocarbon dates of the abandoned channels. In comparison, 14C dates from DP alluvial units form clusters at 16.4–14, 12.8–11.2, 10.8–8.9, 8.1–6.7 and 5.1–3.9 cal ka BP, indicating an association with the Deglacial–Early Holocene humid phase. Alluviation in the DP appears to have continued, more or less, uninterrupted till the middle of the Holocene epoch. The beginning and end of the discernible gap in the radiocarbon dates of CGB (9.0–3.6 cal ka) broadly corresponds with the two well-established short-term events of the Holocene, 8.2 and 4.2 ka cal BP. In comparison, the prominent gap of DP radiocarbon dates (3.9–2.1 ka cal BP) approximately begins with the 4.2 ka cal BP short-term event (onset of aridity) and ends with the 2.0 ka cal BP enhanced monsoon event. Notwithstanding the inter-regional differences in the fluvio–sedimentary response in the India region, the clusters of radiocarbon dates indicate that the century to millennium scale variations in fluvial activity in the Indian subcontinent were intimately linked to long-term fluctuations in the monsoon strength during the Late Quaternary.