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Abstract

Little is known about relative importance of climate as one of the external controls on deep-water depositional systems compared with eustasy and tectonics, due to sparse evidence. This study provides evidence of climatic control on turbidite deposition for the last 70 kyr along the Toyama Deep-Sea Channel (TDSC) in the central Japan Sea. The study is based on stratigraphic variations of coarse fraction, and frequency and thickness of surge-type and hyperpycnal-flow turbidites in three well-correlated cores collected along the TDSC. Four depositional phases have been identified in terms of temporal variations of turbidites; (1) from 72 to 22 ka, turbidite deposition fluctuated, with two peaks at 72–52 ka and 29–22 ka. The peaks are attributed to increase in sediment input to the TDSC induced by a large flux of ablation meltwater from glaciers and paraglacial processes during glaciation and deglaciation in the sediment source area. As a result, both hyperpycnites and surge-type turbidites increased along the TDSC. During 52–29 ka, turbidites deposition decreased, with a majority of surge-type turbidites, probably because fluctuating sea level induced collapses of delta slopes. (2) during 22–18 ka, turbidite deposition was reduced by decreased sediment input due to a cold and dry climate in the sediment source area. Consequent deposits were rich in finegrained hyperpycnites. (3) during 18–7 ka, turbidite deposition was most intensified due to increased sediment input from the sediment sources. Coarse debris trapped in the mountains during the previous phase began to be removed into coastal lowlands because of intensified rainfall and destabilized mountain slopes. Millennial-scale turbidite fluctuation has also been recorded in response to rapid climatic changes around the Younger Dryas period. Surge-type turbidites increased as a result of increased flow volume and efficiency. (4) During 7 ka-present, turbidite deposition was reduced as the sediment sources were gradually depleted in transportable debris. Resultant deposits are mostly fine-grained hyperpycnites in the distal reaches. The present study also sheds lights on difference of characteristics between surge-type and hyperpycnal-flow deposits. Hyperpycnites are generally finer grained than surge-type turbidites and contain abundant organic carbon derived from terrestrial plants. Hyperpycnal flows are considered to be slower and long-lived flows with estimated flow velocity of ~ 2 m/s and flow duration of the order of several days or more. Fines-rich hyperpycnal flows are high-efficiency flows that can transport sediments for long distances, whereas low-efficiency surge-type flows require large flow volumes to travel long distances. The difference in characteristics between hyperpycnal flows and surge-type flows would have resulted in different spatial and temporal distributions of deposits observed in the present cores. The study thus provides implications for assessment of past climate changes and sequence stratigraphic models, and also for prediction of hydrocarbon reservoir and source potential.

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