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 Oce an were analyzed. Varieties of a single diatom species were found to be cha racteristic of two surface water masses bounded by the Kuroshio Current. Th alassionema nitzschioides var, incurvata, var. inflata and var, parva domin ate in warm saline waters in the Subtropical Gyre, while T. nitzschioides s ensu stricto is abundant in less-saline waters northwest of this current. T ime series data on percent abundances for the three T. nitzschioides variet ies in the T. nitzschioides complex are compared with path variations of th e current south of Japan. Inter-annual fluctuations in percent abundance ar e linked to changes in surface water masses over the sediment traps in asso ciation with migration of the Kuroshio paths. This finding makes the percen t abundance a useful tool for deciphering path migrations of the Kuroshio C urrent in the geologic past. Down-core analyses of two sediment cores colle cted beneath the modem Kuroshio Current were performed. A sharp increase in percent abundances from 12 to 9 kyr BP, with an abrupt change around II ky r BP, records the previously reported transition of the Kuroshio Current fr om 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 northwe stern Pacific in relation to changes in northward heat transport from the w estern equatorial Pacific Ocean by the western boundary current. (C) 1999 E lsevier Science B.V. All rights reserved.