Figure F4 shows the number of D. brouweri and C. pelagicus specimens counted plotted against the Pliocene timescale and biozones. The underlying assumption in the following discussion is that fluctuations in the abundance of D. brouweri relative to C. pelagicus are primarily caused by changing surface water temperatures. A larger number of D. brouweri indicates relatively warmer waters and vice versa.
Surface water temperatures were warm during most of the early Pliocene at Site 1115 (Fig. F4). A minor cool interval occurred at ~4.5 Ma. Temperatures began to decline markedly only in the early mid-Pliocene. A pronounced incursion of cool water occurred at this site between ~3.3 and 3.1 Ma. Warm surface water returned during late Zones NN116a and NN16b. Temperatures began to fall again in late Zone NN16b (at ~2.7 Ma), a trend that continued into early Zone NN18 (until at least 2.5 Ma). The overall decline in temperature, which began at 2.7 Ma, was interrupted briefly by an influx of warmer water at ~2.3 Ma. Surface waters continued to cool after that time, with D. brouweri, the warm-water proxy used here, becoming extinct at the end of Zone NN18 (1.95 Ma).