Oceanography of the North Pacific during the last 18,000 years: Evidence from fossil diatoms

Abstract

Factor analysis of diatom sediment assemblages in the North Pacific defines five factors—Subtropical, Transitional, Subarctic, Production, and Okhotsk—which when mapped show distributions clearly related to those of water types and currents. The Subtropical, Transitional, and Subarctic factors exist in the corresponding oceanic Domains; the Production factor is confined to highly-productive cold-water environments of the Coastal Domain; the Okhotsk factor is probably an indicator of severe winter cooling and incomplete summer warming resulting in a subsurface temperature minimum. The factors can be treated as independent variables in a curvilinear regression analysis in which water temperature and productivity are dependent variables. Equations relating the factors to these parameters show a high degree of accuracy to estimate the parameters from a given fossil assemblage. Studies on a series of cores reveal a detailed picture of the North Pacific at 18,000 years B.P., during the peak of the last glacial event. Oceanographic effects included a strong cooling of surface and deeper waters in winter with incomplete summer warming, and heightened seasonal contrast, particularly along the Subarctic Front, which showed a seasonal latitudinal migration. Productivity was high throughout the northwestern Pacific, particularly along the Subarctic Front, probably due to increased shear between the gyres. After 18,000 years B.P., while temperature conditions remained glacial, productivity began to decrease. At about 14,000 years B.P. the steep gradients along the Front became more gentle, especially to the east, and the Transitional Domain was re-established. Winter temperatures warmed, and the subsurface layer of minimum temperature decreased as a result. By 8,000 years B.P. this transformation was complete, and the North Pacific assumed its present aspect.