Planktonic foraminifera in the Western North Pacific during the past 150 000 years: Comparison of modern and fossil assemblages

Abstract

Planktonic foraminiferal abundance variations in 186 deep-sea piston core tops from the western North Pacific were used to create a calibration set to characterize the present-day Kuroshio gyre system. Q-mode factor analysis helped identify six faunal assemblages (93% variance) related to modern water masses: two tropical assemblages differing in relative dissolution, three assemblages representing subtropical, subpolar and transitional conditions, and a gyre-margin assemblage lying between the Kuroshio Current and the mainland of Asia. Multivariate regression produced transfer functions relating observed summer (August) and winter (February) sea-surface temperatures (SST) to the assemblages, with standard errors of estimate of 1.5°C and 2.5°C, respectively. Data from 24 piston cores was used to interpret variations of the Kuroshio gyre system during the past 150 000 yr. Stratigraphic control is based on oxygen isotopic measurements, the extinction of pink-pigmented Globigerinoides ruber at 120 000 B.P., coiling direction changes in Neogloboquadrina pachyderma (north of 30°N), and dissolution variations between resistant and fragile taxa. Major, dynamic changes in the circulation of the Kuroshio gyre occurred during the last two glacial maxima at 15–20 000 B.P. (isotopic stage 2) and at 140–150 000 B.P. (isotopic stage 6). These intervals are characterized by: (1) lowered sea levels which reduced flow into the Sea of Japan and Sea of Okh Okhotsk; (2) advance of the polar front from its modern position at about 42°N to 35–38°N; (3) southward penetration of the Oyashio Current along the Pacific coast of Japan to Tokyo; (4) 2–3°C lowered SST in tropical areas, little or no change in subtropical areas, and a 4–6°C decrease in transitional areas; and (5) an overall decrease in carbonate dissolution relative to interglacial stages. The maximum interglacial at 120 000 B.P. (isotopic substage 5e) shows a retreat of the polar front north of its modern position and an overall relative increase in interglacial carbonate dissolution that does not become severe until substage 5d.