Alluvial fan aggradation during the MIS 6 sea-level lowstand in the lower reach of the Tenryu River, Japan

Abstract

Alluvial fans in coastal regions serve as valuable archives of past climate variability, but the potential insights that could be derived from their aggradational histories remain largely unexplored. In this study, the depositional history of the ~60-m-thick Iwatahara terrace deposits along the coastal Tenryu River, central Japan, was reconstructed based on post-IR IRSL dating of K-feldspar grains. Multi-grain measurements of both pIRIR_50/150 and pIRIR_50/225 signals, along with single-grain measurements of pIRIR_50/225, were conducted to estimate depositional ages. Fading corrections were applied using different models, and their validity was evaluated based on the corrected ages of lagoonal muds independently dated to MIS 7c. Among the various combinations of fading correction models and luminescence signals, the close agreement between the pIRIR_50/225 ages corrected using one of the models and the expected age range for MIS 7c led to the selection of this model–signal combination as a reasonable approach for constraining depositional age. The results revealed three distinct phases of fan aggradation during MIS 8–6: 255–245 ka, 220–210 ka, and a major episode at 180–160 ka. Notably, the 180–160 ka deposits overlie those attributed to the MIS 7 sea-level highstand (~215 ka), despite an overall sea-level fall after ~200 ka. This stratigraphic relationship may indicate that the effects of fluvial incision driven by sea-level fall were outpaced by a substantially increased sediment supply from upstream during this period. The significant aggradation at 180–160 ka may have been driven primarily by enhanced sediment supply, resulting from intensified East Asian summer monsoon precipitation and widespread slope instability linked to a lowered treeline and reduced vegetation cover during this glacial period. These findings underscore the importance of thick alluvial fan deposits as sensitive recorders of climatic fluctuations.