Temperature maps of surface water in the North Atlantic for 18,000 B.P. have been reconstructed for the four seasons. Temperatures were estimated by transfer-function analysis of foraminiferal assemblages, and geometric patterns of surface waters were derived from water-mass-related assemblages of Coccolithophorida and Foraminifera. At 18,000 B.P. the Arctic Polar Front, which was characterized by a steep thermal gradient parallel and centered on lat 42°N, marked the fundamental dividing line for all climatic regimens between a northern dynamic zone and a southern area of relative stability. North of lat 42°N the glacial Atlantic was polar in character with wide areas of seasonal pack ice. The Norwegian-Greenland and Labrador Seas had year-round ice cover. The polar sea was dominated by a counterclockwise gyre. Subpolar and transitional surface water masses were squeezed into a narrow band between the polar front and the subtropical gyre, whose geometry differed only slightly from today. Increased upwelling occurred off the west coast of Africa, while the equatorial divergence-Benguela flow increased during Southern Hemisphere winter at 18,000 B.P. The greatest temperature differences between today and 18,000 B.P. are found in a latitudinal band from 42°N to 60°N, with differences in some areas exceeding 10°C. North and south of this maximum anomaly band, temperature differences are smaller. The upwelling region off Africa shows a 6°C anomaly. The subtropical gyre shows no statistically significant anomaly.

The relative abundances of 18 selected taxonomic groups of Radiolaria were determined for 145 trigger-weight core-top samples collected between long 80°E and 55°W and lat 35°S and 60°S. Seventy-two samples were considered to represent nonreworked recent sediments. A factor analysis of these 72 samples resolved Antarctic, subantarctic, and subtropical assemblages, with distributions closely corresponding to the main surface water masses in the area. Using the technique of Imbrie and Kipp, paleoecologic equations have been developed to estimate surface-water temperatures for August (winter) and February (summer). Estimated temperatures range from near 20°C in the northern parts of the area to 0°C at the location of the southernmost cores. The standard error of estimate of the equations is less than 1.5°C. The CaCO 3 content of the 145 surface-sediment samples shows that variations in the calcite compensation depth (CCD) for locations north of the Antarctic Polar Front depends mainly on varying dissolution. The CCD is about 4,800 m in the western basin of the Atlantic Ocean but is deeper than 5,200 m, at least locally, in the Indian Ocean. South of the Antarctic Polar Front the CCD is shallower than 3,700 m. Radiolaria are common in most samples, with the maximum number per gram of sediment being found in samples collected near or beneath the Antarctic Polar Front.

Using relative-abundance changes of the radiolarian Cycladophora davisiana calibrated using O 18 /O 16 values and C 14 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 O 18 /O 16 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.