Radiolarian assemblage distributions and paleoceanography of the eastern equatorial pacific ocean during the last 127 000 years

Abstract

Mathematical treatment ( Q-mode factor analysis) of radiolarian species abundance data in equatorial Pacific core tops yields four oceanographically meaningful assemblages. The distribution of these assemblages in surface sediments can be related to the major surface and subsurface characteristics of the equatorial circulation system: western tropical mixed layer flow; coastal upwelling and the oxygen minimum layer; eastern equatorial divergence, upwelling and the Equatorial Undercurrent; and the subtropical water masses of both hemispheres. Changes in the distribution of radiolarian assemblages during the glacial-interglacial cycle spanning the last 127 000 yr are observed in two equatorial Pacific piston cores. These changes reflect fluctuations in the boundaries of equatorial water masses in response to variations in the position and strength of the tradewind system and tend to precede major glacial-interglacial transitions by several thousand years. A history of atmospheric circulation in the equatorial Pacific has been developed for the last 127 000 yr. Fluctuations in tradewind intensity, north-south shifts in the latitudinal position of the tradewind maxima, and perhaps a change in the zonality of the tradewinds during the last 127 000 yr have been inferred from downcore quartz abundance and radiolarian assemblage distribution patterns. Oxygen isotopic fluctuations during the last 127 000 yr indicate that continental ice volume was much greater during glacial stage 2 (29 000 B.P. to 11 000 B.P.) than during glacial stage 4 (73 000 B.P. to 61 000 B.P.). Large changes in atmospheric and oceanic circulation patterns also are indicated by the distribution of radiolarian assemblages during the two glacial periods. During stage 2, oceanic circulation was more intense, the southeast tradewind maximum was closer to the Equator and wind stress was more meridional than during stage 4.