The contemporary stable isotope hydrology of Lake Suigetsu and surrounding catchment (Japan) and its implications for sediment-derived palaeoclimate records

The Lake Suigetsu sediment cores exemplify a high-quality archive of palaeoclimatic change in East Asia during the past 150 ka. Robust interpretation of stable isotope-based proxy reconstructions from the Suigetsu cores can be aided by a greater understanding of the factors affecting the isotope composition of the lake and how it relates to that of precipitation. Here we use extended contemporary monitoring to establish the factors affecting the stable isotope composition (delta 18O, delta 2H and d-excess) of precipitation, river water and lake water in the catchment surrounding Lake Suigetsu, central Japan. We show that the composition of precipitation is influenced by the dual East Asian Monsoon system, producing minima in delta 18O and delta 2H and semi-annually varying d-excess values across the year. These signals are then transferred to the lake system, where they are combined with secondary local influences on lake water composition: homogenisation with existing catchment waters, a catchment transit lag, the interaction with saline water from the nearby Sea of Japan, and evaporative enrichment during summer. Our observations suggest that the palaeo-isotope composition of Lake Suigetsu was closely related to the behaviour of the East Asian Monsoon. We highlight lake stratification and proxy seasonality as critical components of signal interpretation.