Kochi University, Japan

Using core-top sediment samples, we measured the modern distribution of Thalassionema species and their morphological types in the Pacific Ocean. Strong correlations between Thalassionema distributions in the sediments (T. pseudonitzschioides and six morphological variants of T. bacillare and T. nitzschioides) and the modern gyral circulation systems over sediments in the Pacific were found. Two morphological variants of T. nitzschioides s. 1. and T. pseudonitzschioides were common in sediments under the Subpolar Gyre and under the region northwest of the Kuroshio Front in the North Pacific. One morphological type of T. nitzschioides s. 1. and two morphological variants of T. bacillare were widely distributed in the sediments under the Subtropical Gyres of the North and South Pacific and under the Equatorial Current System. Three types of T. nitzschioides s. 1. were primarily restricted to the sediments south of the Subtropical Convergence in the South Pacific. The Kuroshio Front that forms between the Subtropical and Subpolar Gyres and the Subtropical Convergence in the South Pacific are distinct boundaries of the distributions of Thalassionema species in the Pacific Ocean.