The nannofossil zonation schemes proposed by Martini (1971; modified in Martini and Müller, 1986) and Okada and Bukry (1980) for low latitudes were used as the basic zonal reference in this study. Absolute ages were assigned to all datum levels to facilitate derivation of sedimentation rates and easy comparison with other studies. Most ages were compiled from Berggren et al. (1995a, 1995b), and a few were compiled from other commonly cited sources (Table T2). The positions of biostratigraphic datums and their numerical ages are presented in Table T2.
The calcareous nannofossils recovered from the Quaternary are generally abundant to common and moderately preserved in Hole 1256B and have been affected to different degrees mainly by etching and fragmentation. No significant reworking of nannofossils was apparent, and the sparse occurrence of reworked discoasterids and sphenoliths in a few samples did not hinder the recognition of zonal datums. The assemblages have low diversity, with ~17–21 species, and are characterized by abundant to common Gephyrocapsa, Reticulofenestrata, Calcidiscus, and zonal markers for the specific zones.
Sediments downhole to Sample 206-1256B-1H-2, 40–42 cm (1.90 mbsf), were placed in this zone. The base of this zone is defined by the first occurrence (FO) of Emiliania huxleyi, for which recognition required confirmation under SEM due to its tiny size (2–4 µm).
Samples 206-1256B-1H-3, 40–42 cm (3.40 mbsf), and 1H-4, 40–42 cm (4.90 mbsf), were assigned to Zone NN20, which is a "gap" zone defined by the presence of Gephyrocapsa and absence of E. huxleyi and Pseudoemiliania lacunosa. The barren Sample 206-1256B-1H-3, 115–117 cm (2.65 mbsf), immediately overlying this interval renders the NN21/NN20 boundary uncertain.
This interval lies between the last occurrences (LOs) of P. lacunosa in Sample 206-1256B-1H-4, 115–117 cm (5.65 mbsf), and the apparent LO of Discoaster brouweri in Sample 3H-3, 115–117 cm (19.75 mbsf). Another three datums, the LO of Helicosphaera sellii, LO of Calcidiscus macintyrei, and FO of Gephyrocapsa caribbeanica, which are present in Samples 206-1256B-2H-CC (15.91 mbsf), 3H-1, 115–117 cm (16.75 mbsf), and 3H-1, 40–42 cm (16.00 mbsf), respectively, can be used to refine this interval according to Gartner (1977), Okada and Bukry (1980), and Raffi et al. (1993). Their ages are listed in Table T2 but not described in detail here.
The transition from the Pliocene to Pleistocene at Site 1256 is marked by an interval barren of nannofossils that spans from Samples 206-1256B-3H-2, 40–42 cm (17.50 mbsf), to 3H-3, 40–42 cm (19.00 mbsf), and falls within Zones NN19 and NN18. The Pleistocene/Pliocene boundary is arbitrarily placed at the top of this interval (17.50 mbsf), which closely approximates the lithologic Subunit IA/IB boundary at a sediment color change at Section 206-1256B-3H-2, 38 cm (17.48 mbsf). This boundary, with an age of 1.78 Ma (Berggren et al., 1995b), is above the apparent LO of D. brouweri (1.95 Ma) in Sample 206-1256B-3H-3, 40–42 cm (19.75 mbsf).
Nannofossils recorded from the Pliocene generally have a higher abundance relative to the Quaternary, except that there are several intervals barren of nannofossils and the abundance increases with depth. The preservation of nannofossil assemblages ameliorates with increasing depth and is closely associated with the presence and abundance of diatoms, which release silica to pore waters and thus inhibit dissolution and/or precipitation of calcite during diagenesis (Wise, 1977). The assemblages above Zone NN15 remain low in diversity and contain abundant to common reticulofenestrids, discoasterids, coccoliths, and Calcidiscus. Downhole from Zone NN15, the diversity and abundance of nannofossils increases abruptly, which is caused by the dominance and diversification of reticulofenestrids and sphenoliths.
Samples 206-1256B-3H-3, 115–117 cm (19.75 mbsf), to 3H-5, 115–117 cm (22.75 mbsf), are assigned to Zone NN18. The interval is normally defined as the LO of Discoaster pentaradiatus to the LO of D. brouweri. Assuming a normal stratigraphic order by stratigraphic sequence, the nannofossil-barren intervals at the top and bottom of this zone can be constrained by the overlying and underlying biozones and are assigned to the combined Zones NN19–NN18 and NN18–NN17, respectively; hence, the upper and lower boundaries of Zone NN18 are uncertain. The apparent LO of Ceratolithus rugosus is also recorded in the same sample as D. brouweri (Sample 206-1256B-3H-3, 115–117 cm [19.75 mbsf]).
As mentioned above, the upper boundary of this zone lies in a nannofossil-barren interval and thus remains uncertain. The base of this zone is recorded above Sample 206-1256B-4H-2, 40–42 cm (27.00 mbsf), based on the LO of Discoaster surculus in this sample.
This zone is assigned to Samples 206-1256B-4H-2, 40–42 cm (27.00 mbsf), through 4H-4, 40–42 cm (30.00 mbsf), based on the LO of Reticulofenestra pseudoumbilica in the underlying Sample 4H-4, 115–117 (30.75 mbsf), a species defined by a minimum coccolith length of >7 µm. An alternative zonal marker proposed by Okada and Bukry (1980) for the base of this zone is the LO of Sphenolithus abies/neoabies, which occurs a little higher in stratigraphic position and was recorded in the lowermost sample of this zonal interval. The presence of a barren sample in the middle of this interval does not prohibit placement of the zonal boundaries.
This zone encompasses Samples 206-1256B-4H-4, 115–117 cm (30.75 mbsf), through 5H-1, 115–117 cm (35.75 mbsf), which directly overly the LO of Amaurolithus primus in Sample 5H-2, 40–42 cm (36.50 mbsf), an equivalent zonal marker proposed by Okada and Bukry (1980) to replace the rare Amaurolithus tricorniculatus. The latter only occurs in Sample 206-1256B-5H-2, 115–117 cm (37.25 mbsf).
Sediments between Samples 206-1256B-5H-2, 40–42 cm (36.50 mbsf), and 5H-3, 115–117 cm (38.75 mbsf), an interval directly above the LO of C. rugosus in Sample 5H-4, 40–42 cm (39.50 mbsf), are assigned to this combined zone. Another useful datum, the LO of Ceratolithus acutus, occurs in Sample 206-1256B-5H-2, 115–117 cm (37.25 mbsf). No attempt was made to determine the FO of Discoaster asymmetricus, a datum defining the boundary between Zones NN14 and NN13, because of the intermittent occurrence of this species.
This zone is recorded in the interval from Samples 206-1256B-5H-4, 40–42 cm (39.50 mbsf), to 5H-5, 115–117 cm (41.75 mbsf), where both C. rugosus and Discoaster quinqueramus are absent. The FO of C. acutus falls within this zone, which helps in the recognition of this "gap" zone.
The Pliocene/Miocene boundary in terms of calcareous nannofossils falls within Subzone CN10a or Zone NN12 (Perch-Nielsen, 1985). The FO of C. acutus closely approximates this epoch boundary, which lies just above the extinction of D. quinqueramus, a well-documented datum level in most sections.
Nearly all the Miocene samples examined contain very abundant and diversified nannofossil assemblages. These assemblages are generally moderately to well preserved but deteriorate downhole, due mainly to overgrowth as well as dissolution. Similar to preservation in the overlying Pliocene assemblages, the preservation depends to a great extent on the presence and abundance of diatoms. The assemblages consist of highly diversified and predominant reticulofenestrids, discoasterids, sphenoliths, coccoliths, and Calcidiscus.
This zone covers the stratigraphic range of D. quinqueramus, which was observed between Samples 206-1256B-5H-6, 40–42 cm (42.50 mbsf), and 9H-6, 40–42 cm (80.50 mbsf); in the latter sample, the FO of Discoaster berggrenii was documented. The FO of A. primus, located in Sample 206-1256B-7H-7, 40–42 cm (63.00 mbsf), is used to subdivide this thick zone into upper Subzone NN11b and lower Subzone NN11a. Two other events, the LO and FO of Amaurolithus amplificus in Samples 206-1256B-6H-3, 40–42 cm (47.50 mbsf), and 7H-3, 40–42 cm (57.00 mbsf), respectively, can be used with caution because of its generally rare occurrence to further refine Subzone NN11b.
The LO and FO of Reticulofenestra rotaria, a circular placolith with a relatively large central opening and a diameter of 5–7 µm, recorded in Samples 206-1256B-6H-6, 115–117 cm (52.75 mbsf), through 7H-CC (63.39 mbsf), between Chrons C3An.2n (t) (51.60 mbsf) and C4n.1n (o) (71.56 mbsf) (Shipboard Scientific Party, 2003), have the stratigraphic potential to refine Subzone NN11b. Assuming a constant sedimentation rate, derived independently from biostratigraphy and magnetostratigraphy, the FO is calculated at 7.18 and 7.03 Ma and the LO is calculated at 6.32 and 6.34 Ma, respectively. The difference in the age estimates probably arises from sedimentation rate variation and sample spacing for biostratigraphy; the latter issue in sample spacing could be resolved by further closer sampling. Considering the stratigraphic distribution of R. rotaria, A. primus, and A. amplificus in the sequence (Table T1), the ages for the FO and LO of R. rotaria are assigned to 7.18 and 6.32 Ma, respectively. Taking into consideration the differences in timescales used in both studies, these assignments are reasonably consistent with those calibrated by Wei (1995).
This zone is assigned to Samples 206-1256B-9H-6, 115–117 cm (81.25 mbsf), through 10H-2, 40–42 cm (84.00 mbsf), the base of which is defined by the LO of Discoaster hamatus in Sample 10H-2, 115–117 cm (84.75 mbsf). This datum coincides with the LO of Discoaster bollii in this hole. The presence of Discoaster neorectus aids in identifying this gap zone.
This zone spans the stratigraphic range of D. hamatus and encompasses Samples 206-1256B-10H-2, 115–117 cm (84.75 mbsf), through 12H-2, 115–117 cm (103.75 mbsf). Shipboard observation of this marker in Sample 206-1256B-10H-4, 100 cm (87.60 mbsf), is consistent with this assignment. Because the upper boundary is populated by the rare and sparse occurrence of this species and there exist samples bearing very few or no nannofossils, the precise placement requires much effort. It is worth mentioning that the occurrences of this zonal marker in Samples 206-1256B-9H-4, 40–42 cm (77.50 mbsf), and 9H-4, 115–117 cm (78.25 mbsf), are considered to be reworked, the significance of which will be discussed in "Discussion." The LO of Catinaster coalitus recorded in Sample 206-1256B-11H-3, 115–117 cm (95.75 mbsf), can be used to subdivide this zone. The intermittent occurrence of Discoaster neohamatus in the lower part of its range at this site renders the application of its FO datum impossible.
This zone lies between the FOs of D. hamatus and C. coalitus, the latter datum being observed in Sample 206-1256B-12H-4, 40–42 cm (106.00 mbsf). The LO of Coccolithus miopelagicus approximates the top of this zone.
Zone NN7 is recorded by the FO of C. coalitus downhole to the FO of Discoaster kugleri in Sample 206-1256B-17H-1, 40–42 cm (149.00 mbsf). The preservation of nannofossils deteriorates with depth from the middle of this zone, which makes differentiation among discoaster species difficult. D. kugleri, however, can still be distinguished from Discoaster sanmiguelensis and other discoasterids in overgrown specimens by the diagnostic six short rays and a broad, flat central area. Its LO recorded in Sample 206-1256B-14H-CC (130.05 mbsf), provides a datum to further subdivide this thick interval.
The lowermost biostratigraphic zonal event in the section, the LO of Sphenolithus heteromorphus, the specimens of which are overgrown but recognizable by their diagnostic bright apical spines and butterfly-shaped bases, marks the base of Zone NN6 in Sample 206-1256B-25X-1, 115–117 cm (214.15 mbsf). Here Cyclicargolithus floridanus, the last continuous occurrence of which is in Sample 206-1256B-23X-CC (202.77 mbsf), provides a further datum to subdivide Zone NN6. This additional datum provides a sound biostratigraphic correlation line in equatorial areas (Raffi et al., 1995).
Helicosphaera ampliaperta, the LO of which marks the base of Zone NN5, was absent from all samples including the lowermost Sample 206-1256B-28X-CC (245.83 mbsf). This indicates that the basaltic basement underlying the sedimentary sequence is younger than 15.6 Ma (age of the LO of H. ampliaperta). The rare and sparse, but distinctive, onset of R. pseudoumbilica is in Sample 206-1256B-26X-3, 40–42 cm (226.10 mbsf), and provides a useful datum to assess the age of basement. Assuming a constant sedimentation rate of 39.1 m/m.y., based on datums from below 100 mbsf, a basement age of ~14.5 Ma is implied (Fig. F2), consistent with the assignment to Zone NN5 of the middle Miocene. This age is reasonably consistent with the age of 15.1 Ma inferred from magnetic anomalies (Shipboard Scientific Party, 2003).