Distinctive changes in the size of specimens of the Neogene genus Reticulofenestra have been observed by numerous authors. Young (1990) studied size distribution patterns in the middle Miocene to Pliocene Reticulofenestra specimens in the western Indian Ocean and the Red Sea, and was the first to define the small Reticulofenestra event (SRE), in which the specimens >5µm in size virtually disappear in late Miocene Zones NN9 (CN8) and NN11 (CN9). Rio et al. (1990) identified the "paracme of Reticulofenestra" based on the disappearance of large (>7 µm in diameter) specimens of Reticulofenestra from Zone CN8 to the middle of Zone CN9 in Indian Ocean sediments. Takayama (1993) divided various Reticulofenestra species into four groups based on their sizes and showed characteristic stratigraphic distribution patterns of them throughout the Neogene.
We measured the size and shape of Reticulofenestra coccoliths in Zones CN3 to CN12 from Holes 998A and 999A. Based on these data, we observed intervals where there are distinctive changes. We can define several new biostratigraphic events based on these changes as well as observe the size changes proposed by Takayama (1993). We recognized three major intervals within the Neogene sections studied, from top to bottom: Intervals I, II, and III.
This interval corresponds to Zone CN12 in Holes 998A and 999A. In this interval, medium (5-7 µm in diameter), small (3-5 µm in diameter) and very small (<3 µm in diameter) Reticulofenestra specimens are observed. Interval I corresponds to the upper part of Interval A1 of Takayama (1993). Four biostratigraphic units were recognized in this interval based on abrupt decreases in size and/or sudden disappearance of characteristic species of Reticulofenestra.
This interval is characterized by the dominant occurrence of very small specimens of Reticulofenestra (<3 µm in diameter) with few larger specimens (>6 µm in diameter; Fig. 5, Fig. 6).
The upper boundary of this interval is defined by the abrupt and temporal disappearance of larger specimens of Reticulofenestra (6-7 µm in diameter in Hole 998A and 5-7 µm in diameter in Hole 999A). These specimens are present characteristically in the upper part of this interval, whereas the lower part of the interval is characterized by small to medium Reticulofenestra specimens (Fig. 5, Fig. 6).
The upper boundary of this interval is defined by the disappearance of Reticulofenestra ampla. This horizon lies close to the LO of Discoaster tamalis (Sato et al., 1991; Kameo et al., 1995), the boundary between Subzones CN12c and CN12b. R. ampla, described by Sato et al. (1991) in the Indian and the North Atlantic Oceans, is an elliptical medium-sized species (>5 µm in diameter) and is similar to but smaller than R. pseudoumbilicus. The former species characteristically is in the uppermost part of Subzone CN12a and disappears near the LO of D. tamalis. Throughout this interval, small to very small specimens of Reticulofenestra are abundant (Fig. 5, Fig. 6).
In the lower part of Subzone CN12a, relatively large specimens (6-7 µm in diameter) of Reticulofenestra are observed. These larger specimens include circular forms of Reticulofenestra that suddenly disappear within Subzone CN12a in Holes 998A, 999A, and 1000A. This circular form is assigned to a variety of Reticulofenestra minutula. Small to very small specimens are also abundant in this interval (Fig. 5, Fig. 6).
This interval lies in Zone CN11 to the middle part of Zone CN8 (middle Miocene to lower Pliocene). We have defined three subintervals (Fig. 5, Fig. 6).
The upper boundary of this interval corresponds to the LO of R. pseudoumbilicus. In the lower part of the interval, medium-sized Reticulofenestra specimens (5-7 µm in diameter) are dominant, whereas the middle to upper part of this interval contains smaller sized Reticulofenestra specimens with some larger sized specimens. Interval IIa closely correlates to the lower part of Interval A1 of Takayama (1993; Fig. 5, Fig. 6).
The upper boundary of this interval is characterized by the temporary disappearance of larger specimens of Reticulofenestra (>10 µm in diameter in Hole 998A and 7 µm in diameter in Hole 999A) near the base of Subzone CN10b (Fig. 5, Fig. 6). In this interval, small to large Reticulofenestra specimens (>3 µm in diameter) are dominant, whereas very small Reticulofenestra specimens (<3 µm in diameter) are rare. This interval corresponds to the upper part of Interval A2 of Takayama (1993).
The boundary between Intervals IIb and IIc is defined by the re-entrance of common large (>7 µm in diameter) specimens of Reticulofenestra. This interval is dominated by small to very small specimens of Reticulofenestra (<5 µm in diameter), whereas Interval IIb contains abundant medium to large specimens (Fig. 5, Fig. 6). The upper boundary of this interval lies in Subzone CN9b in Hole 998A and Subzone CN9a in Hole 999A. This difference may be caused by uncertainties in the FO of Amaurolithus spp. in Hole 999A, the event that defines the boundary between Subzones CN9b and CN9a. The lowermost common occurrence of larger Reticulofenestra specimens correlates to the top of the paracme of R. pseudoumbilicus as defined by Rio et al. (1990) and Raffi and Flores (1995).
This interval corresponds to Zones CN8 through CN3. Large specimens of Reticulofenestra are common. There are two subintervals (Fig. 5, Fig. 6).
This interval is characterized by the dominance of medium to large specimens of Reticulofenestra (>5 µm in diameter), whereas small to very small Reticulofenestra specimens (<5 µm in diameter) are very rare to absent (Fig. 5, Fig. 6). The upper boundary of this interval corresponds to the abrupt disappearance of large specimens of Reticulofenestra near the base of Subzone CN8b. This dramatic event is called the base of paracme of R. pseudoumbilicus by Rio et al. (1990) and Raffi and Flores (1995) and can be identified in the three holes studied.
This interval contains small to medium specimens of Reticulofenestra (3-7 µm in diameter). Very small specimens of Reticulofenestra (<3 µm in diameter) are rare (Fig. 5, Fig. 6). The upper boundary of this interval is characterized by the abrupt decrease in the abundance of small specimens of Reticulofenestra near the base of Zone CN6. In the lower part of this interval, large Reticulofenestra specimens are rare.
We have recognized the size variations in specimens of Reticulofenestra in the stratigraphic interval from CN3 to CN12 that can be correlated between the Caribbean sites (Fig. 5, Fig. 6). The patterns of size variations include cyclic changes in the relative abundance of the different size groups as well as changes in the maximum size of the specimens. For example, in Interval III (Zones CN3-CN4), small to medium (<5 µm in diameter) specimens of Reticulofenestra are common. The maximum size of specimens increases upward, exceeding 10 µm in Zone CN5. At the same time, medium to large specimens become the most common size group. This upward increase in maximum size of specimens is a characteristic trend in the evolution of Neogene Reticulofenestra. Interval II seems to include two cycles in the increase in maximum size. The lower cycle corresponds to Intervals IIc to IIb and the upper cycle is the middle part of Intervals IIb to IIa. Interval I is more complicated and contains several short cycles of variations in size superimposed on an upward increase in the maximum size of specimens.
We propose eight datums based on Reticulofenestra size variation that are stratigraphically useful (listed in descending order):