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Using relative-abundance changes of the radiolarian Cycladophora davisiana calibrated using O18/O16 values and C14 ages, we have established a high-resolution biostratigraphy for the Antarctic and subantarctic ocean covering the past 150,000 yr. Extremes of this species' relative abundance have been estimated to have occurred approximately 18,000, 35,000, 60,000, 85,000, 105,000, and 120,000 yr ago. Crosscorrelation between O18/O16 values and the most recent C. davisiana maximum shows that this maximum is isochronous over a broad area of the Antarctic and subantarctic sea floor.

Using diverse stratigraphic criteria, we selected 34 cores from the Atlantic and western Indian Ocean sectors of the Antarctic and subantarctic ocean containing a strong C. davisiana relative-abundance maximum near the core top, believing that this maximum occurred approximately 18,000 yr ago in the Antarctic and subantarctic. Using techniques including radiolarian-based paleoecological equations, Radiolaria per gram of sediment, and percentage of calicum carbonate, we show that 18,000 yr ago the Antarctic Polar Front was displaced north of its present position by as much as 7° of latitude. The subtropical convergence was little changed from its present position ; consequently, the width of subantarctic waters was reduced.

Using C. davisiana as a stratigraphie indicator, we estimate the sedimentation rate in one Antarctic core of the upper diatom-ooze layer (11.5 cm/1,000 yr) and the immediately underlying silty clay (2.7 cm/1,000 yr). The slower sedimentation rate of the silty clay strongly suggests that diatom production was inhibited during its deposition relative to production during deposition of the younger diatom ooze. We interpret this to indicate that summer ice cover extended nearly to lat 55°S 18,000 yr ago, whereas today it melts back to the Antarctic continent.

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