Varieties of a single cosmopolitan diatom species associated with surface water masses in the North Pacific

A time series of 65 sediment trap samples, representing 48-month changes in diatom flora, and 62 sediment core-tops from the western North Pacific Ocean were analyzed. Varieties of a single diatom species were found to be characteristic of two surface water masses bounded by the Kuroshio Current. Thalassionema nitzschioides var. incurvata, var. inflata and var. parva dominate in warm saline waters in the Subtropical Gyre, while T. nitzschioides sensu stricto is abundant in less-saline waters northwest of this current. Time series data on percent abundances for the three T. nitzschioides varieties in the T. nitzschioides complex are compared with path variations of the current south of Japan. Inter-annual fluctuations in percent abundance are linked to changes in surface water masses over the sediment traps in association with migration of the Kuroshio paths. This finding makes the percent abundance a useful tool for deciphering path migrations of the Kuroshio Current in the geologic past. Down-core analyses of two sediment cores collected beneath the modern Kuroshio Current were performed. A sharp increase in percent abundances from 12 to 9 kyr BP, with an abrupt change around 11 kyr BP, records the previously reported transition of the Kuroshio Current from a predominantly offshore path to its modern path. Path migrations of the Kuroshio Current may be inferred from percent abundances of these diatoms, which comprise a new proxy for delineating the paleoclimate of the northwestern Pacific in relation to changes in northward heat transport from the western equatorial Pacific Ocean by the western boundary current.