Planktonic foraminiferal analyses were made on samples from the Pliocene to Pleistocene sediments of Holes 1039B and 1043A. The total number of planktonic foraminifers in each sample, the ratio of warm-water planktonic foraminifers to the assemblage, the ratio of Globigerinoides sacculifer, the ratio of surface-water dwellers (Globigerinoides group), and the ratio of intermediate-water dwellers (Neogloboquadrina group and Globorotalia menardii) have been estimated.
In Cores 170-1039B-18X through 1H, planktonic foraminifers in each sample are barren or rare in Zone N19 and Zone N21. In younger horizons, however, they are continuously and commonly found in each sample (Fig. F5). The ratio of warm-water planktonic foraminifers to the total assemblage temporarily decreases in Sample 170-1039B-18X-2, 46-49 cm, which is assignable to Zone N19 (Fig. F6). The horizon is estimated at older than 4.2 Ma, prior to the LO of G. nepenthes. It is suggested that the sea-surface temperature temporarily and abruptly drops in this horizon. In Zone N21, there is a high percentage of warm-water dwellers. These are recognized in fluctuations in Zone N22 and gradually decrease from Zone N19 to Zone N23.
G. sacculifer is a high salinity-tolerant, warm surface-water species. In the Caribbean Sea, the species became enriched at about 4.6 Ma, coincident with the closing of the Central American Seaway (Keller et al., 1989; Farrell et al., 1995). In Hole 1039B, the ratio of G. sacculifer shows ~10% percent through Zones N19-N23 (Fig. F6). No salinity change is therefore recognized on the East Pacific side in the duration. The maximum divergence of both surface- and intermediate-water dwellers of Hole 1039B begins in the lower part of Zone N22 and continues up to the present (Fig. F7).
In Cores 170-1043A-30X through 18X, planktonic foraminifers are barren or rare in Zone N19 and in the middle part of Zone N21 (Fig. F8). The ratio of warm-water planktonic foraminifers to the total assemblage abruptly decreases in Sample 170-1043A-27X-CC of Zone N19. In the horizon, there is a high percentage of cold-water planktonic foraminifers and the upwelling dweller Globigerinita glutinata is commonly found. The horizon is estimated at ~4.2-3.95 Ma, based on the coiling direction of Pulleniatina and the LO of G. nepenthes. The occurrence of G. sacculifer abruptly decreases in the lower part of Zone N18 and continues to Zone N19, temporarily increasing in the lower part of Zone N21 (Fig. F9). It appears that the surface seawater conditions changed in Zone N19. The maximum divergence of both surface- and intermediate-water dwellers in Hole 1043A is recognized in two horizons. Abundances of surface-water dwellers are recognized in Zone N17 and the lower to middle part of Zone N18, and the abundances of intermediate-water dwellers are recognized in the duration from the top part of Zone N21 to Zone N23 (Fig. F10).
Holes 1039B and 1043A can be correlated with each other based on planktonic foraminiferal biochronostratigraphy (Fig. F11). In Holes 1039B and 1043A, planktonic foraminifers are barren or rare in Zones N19 and N21 and are commonly found in most samples of Zones N22 through N23. Planktonic foraminiferal components of sediments consist of rich warm-water specimens. The ratio of warm-water planktonic foraminifers to the total assemblage temporarily decreases in four horizons (i.e., in Zone N19, the top part of Zone N21, the lower part of Zone N22, and the top part of Zone N22, respectively) in both holes (Fig. F11). These ratios indicate that the surface seawater temperature probably changed in those horizons. The surface seawater temperature drops in Zone N19 are dated at 4.2-3.95 Ma in Hole 1039B and 5-4.2 Ma in Hole 1043A. Those cooling events of Zone N19 are considered to be related to the closing of the Central American Seaway.
During the Pliocene to Pleistocene, the emergence of the Panama Isthmus is estimated to have occurred, separating the equatorial Pacific and Atlantic Oceans and the Caribbean Sea. This event strongly influenced peripheral regions, seawater temperature, ocean circulation, surface-water salinity, and faunal changes of planktonic foraminifers and radiolarians (Saito, 1976; Keigwin, 1982a, 1982b; Keller et al., 1989; Moore et al., 1993; Coates et al., 1992; Ibaraki, 1992, 1997).
The ratio of warm-water dwellers to the total assemblage gradually drops from the Pliocene to Pleistocene. G. sacculifer, a high salinity-tolerant species, has low occurrences in Zone N19. No salinity change is recognized on the East Pacific side.
Maximum divergence of both surface- and intermediate-water dwellers in Holes 1039B and 1043A begins at the top of Zone N21. This event considered to be related to the final closure of the Central American Seaway at that time.
The planktonic foraminiferal fauna changes ~4.6 Ma in the Caribbean Sea and changes in the planktonic foraminiferal assemblage in Zone N19 off Costa Rica might be related to the closing of the Central American Seaway.