BIOSTRATIGRAPHY

The sedimentary sequence recovered from the three holes cored at Site 1239 consists of a 560-m-thick interval of Quaternary-middle Miocene sediments. A relatively well constrained biostratigraphy is provided by all three microfossil groups (Table T9). The biostratigraphic sequence is similar to that at Site 1238, except that a major hiatus was clearly identified by three different microfossil groups between Samples 202-1239A-51X-CC (519.3 mcd) and 52X-CC (527.7 mcd) (Fig. F37). The time interval of the hiatus is tentatively estimated by the calcareous nannofossil biostratigraphy as ~7.8-13.6 Ma. The presence of the middle Miocene planktonic foraminifer index species Globorotalia peripheroronda and Globorotalia praemenardii in Sample 202-1239X-55X-CC suggests that the basal age at Site 1239 is between 14.6 and 14.9 Ma.

The abundance and preservation of calcareous microfossils at Site 1239 are similar to those at Site 1238 (Fig. F21). Calcareous nannofossils are abundant and well preserved within the top ~100 mcd, but both abundance and preservation decline slightly below this horizon. Planktonic foraminifers are common within the top ~100 mcd but decrease rapidly at greater depths. Benthic foraminifers, which are common to abundant below ~100 mcd, generally indicate high carbon fluxes at the seafloor, probably derived at this location from upwelling driven productivity. Diatoms are also present throughout the sedimentary section, although their abundance is relatively low in the Holocene-mid-Pleistocene and lower Pliocene-uppermost Miocene intervals in comparison to Site 1238.

As at Site 1238, Site 1239 yielded abundant and well to moderately preserved calcareous nannofossils in all samples examined (Table T10). Because of the high abundance of siliceous microfossils, which indicates eutrophic conditions in most of the sequence, some traditional index nannofossil species, particularly some discoaster and amaurolith species, are rare or absent. Persistent reworking of nannofossils in the upper upper Miocene-lower Pliocene is suggested by the presence of older species in demonstrably younger sediments. This hampered the stratigraphic resolution to some extent. Nevertheless, a moderate-resolution nannofossil biostratigraphy has been established, which suggests a relatively complete section for the top ~450 mcd (0-8 Ma), a major hiatus encompassing the ~8- to 13-Ma interval, and a basal sediment age of ~15 Ma at the site.

Virtually, all the well-known Pleistocene nannofossil events have been located within an uncertainty of about 1.5 m, allowing a relatively high resolution biostratigraphy for the Pleistocene. The nannofossil events recognized are the beginning of the acme of Emiliania huxleyi (0.08 Ma) between Samples 202-1239A-1H-2, 75 cm (2.26 mcd), and 1H-3, 40 cm (2.92 mcd); the first occurrence (FO) of E. huxleyi (0.26 Ma) between Samples 2H-3, 75 cm (7.3 mcd), and 2H-4, 75 cm (8.8 mcd); the last occurrence (LO) of Pseudoemiliania lacunosa (0.46 Ma) between Samples 3H-1, 75 cm (15.05 mcd), and 3H-2, 75 cm (16.56 mcd); the LO of Reticulofenestra asanoi (0.88 Ma) between Samples 5H-1, 75 cm (36.4 mcd), and 5H-2, 75 cm (37.9 mcd); the reentry of medium Gephyrocapsa spp. (1.02 Ma) between Samples 6H-5, 75 cm (52.77 mcd), and 6H-6, 75 cm (54.27 mcd); the FO of R. asanoi (1.08 Ma) between Samples 6H-7, 40 cm (55.42 mcd), and 6H-CC, 35 cm (56.1 mcd); the LO of large Gephyrocapsa spp. (1.24 Ma) between Samples 7H-4, 75 cm (61.4 mcd), and 7H-5, 75 cm (62.9 mcd); the FO of large Gephyrocapsa spp. (1.45 Ma) between Samples 8H-CC, 31 cm (76.41 mcd), and 9H-1, 75 cm (77.95 mcd); the LO of Calcidiscus macintyrei (1.59 Ma) between Samples 9H-7, 40 cm (86.66 mcd), and 9H-CC (87.26 mcd); and the FO of medium Gephyrocapsa spp. (1.67 Ma) between Samples 9H-CC (87.26 mcd), and 10H-1, 75 cm (87.95 mcd).

The youngest Pliocene nannofossil datum, the LO of Discoaster brouweri (1.96 Ma), placed between Samples 202-1239A-11H-2, 75 cm (100.1 mcd), and 11H-3, 75 cm (101.6 mcd), may not be very reliable because of some reworking of nannofossils in this interval. The next three older discoaster datums of the late Pliocene (the LOs of Discoaster pentaradiatus, Discoaster surculus, and Discoaster tamalis) were not recognized because of the rare abundance or absence of those species. Rare to few Reticulofenestra pseudoumbilicus and Sphenolithus spp. were found from Cores 202-1239A-16H through 29X, but considerable stratigraphic overlap occurs with P. lacunosa, indicating that all these specimens of R. pseudoumbilicus and Sphenolithus spp. are reworked. The first common occurrences of the two taxa were found in Sample 202-1239A-30X-CC, 1 cm (298 mcd), downhole, and thus, the LOs of Sphenolithus spp. (3.65 Ma) and R. pseudoumbilicus (3.8 Ma) are placed between Samples 29X-CC (288.09 mcd) and 30X-CC, 1 cm (297.99 mcd).

As at Site 1238, the youngest Miocene datum, the LO of Discoaster quinqueramus (5.56 Ma), could not be used at this site because of persistent reworking of nannofossils in the upper upper Miocene (Table T10). However, two first occurrence datums have been reasonably well determined in this interval: the FO of Ceratolithus cristatus (5.04 Ma) between Samples 202-1239A-38X-2, 75 cm (375.3 mcd), and 38X-3, 75 cm (376.8 mcd), and the FO of Ceratolithus armatus (= Ceratolithus acutus) (5.34 Ma) between Samples 41X-CC (414.4 mcd) and 42X-1, 75 cm (415.8 mcd).

A usually reliable late Miocene datum, the top of the R. pseudoumbilicus (>7 µm) absence interval (6.8 Ma), is located between Samples 202-1239A-47X-6, 75 cm (475.7 mcd), and 47X-7, 40 cm (476.8 mcd). A major hiatus was identified between Samples 202-1239A-51X-CC (519.3 mcd) and 52X-CC (527.7 mcd), as the upper sample contains D. surculus and D. loeblichii but no R. pseudoumbilicus and thus has an age of 6.8-7.8 Ma, whereas the lower sample contains Sphenolithus heteromorphus and Cyclicargolithus floridanus and thus has an age of 13.6-18.2 Ma. The next several samples to the deepest sample from the hole—Sample 202-1239A-55X-CC (557 mcd)—all contain the same assemblage with the presence of S. heteromorphus and thus all have the same age range of 13.6-18.2 Ma.

Planktonic foraminifers are abundant and well preserved in sediment from the upper part of Hole 1239A (mudline to Sample 202-1239A-10H-CC; 0-97.2 mcd). Between Cores 202-1239A-11H and 40X (107.7-404.0 mcd), abundance and preservation vary markedly, and the relative proportion of radiolarians and benthic foraminifers in the residue generally increases downhole (Table T11; Fig. F21). Between Cores 202-1239A-41X and 51X (414.4-519.3 mcd), radiolarians are generally abundant, and planktonic foraminifers are extremely rare, representing ~1% or less of total foraminifers. Abundance increases slightly in the lowermost samples (Cores 202-1239A-52X through 55X; 527.7-557.0 mcd); however, preservation deteriorates significantly within this interval and recrystallized overgrowths frequently obscure test features.

Diversity is overall quite low, and assemblages tend to be dominated by upwelling taxa such as Neogloboquadrina dutertrei or Neogloboquadrina pachyderma. Standard marker species are present in samples from throughout the upper part of Hole 1239A and can be used to establish a preliminary biostratigraphy for the Pleistocene to upper Pliocene interval (Cores 202-1239A-1H through 12H; 3.4-13.0 mcd). However, few datums are available between Cores 202-1239A-13H and 51X (128.7-519.3 mcd) to interpret and constrain the biostratigraphy of lower Pliocene to upper Miocene sediments. The foraminiferal assemblage within this interval contains a significant proportion of blackened tests with mud infills that indicate some reworking. The presence of middle Miocene index species in the lowermost samples (Samples 202-1239A-52X-CC through 55X-CC (527.7-557.0 mcd) indicates a major hiatus between Samples 51X-CC and 52X-CC (519.3-527.7 mcd). An age between 14.6 and 14.9 Ma is attributed to the base of Hole 1239A.

The well-preserved Pleistocene to late Pliocene planktonic foraminiferal assemblage includes Globigerina bulloides, Globigerina quinqueloba, Globigerinita glutinata, Globigerinoides ruber, Globorotalia menardii, Globorotalia scitula, Globorotalia tumida, N. dutertrei, N. pachyderma, Orbulina universa, and Sphaeroidinella dehiscens. The LO of G. ruber pink (0.12 Ma) is placed between Samples 202-1239A-1H-CC and 2H-CC (3.4-13.0 mcd), and the overlying section can be assigned to the upper Pleistocene Subzone Pt1b of Berggren et al. (1995) (Fig. F12 in the "Explanatory Notes" chapter). The FO of G. ruber pink (0.40 Ma) is recognized between Samples 202-1239A-2H-CC and 3H-CC (13.0-24.3 mcd).

The boundary between upper Pleistocene Subzone Pt1b and lower Pleistocene Subzone Pt1a is marked by the LO of Globorotalia tosaensis (0.65 Ma; Zone Pl5), which can be placed between Samples 202-1239A-4H-CC and 5H-CC (34.6-45.7 mcd). The LO of representatives of the benthic genus Stilostomella, which occurred at ~0.65 Ma (Hayward, 2001), provides an additional datum to approximate this boundary between Samples 202-1239A-4H-CC and 5H-CC (34.6-45.7 mcd). The LO of Neogloboquadrina acostaensis (1.58 Ma), which can be identified between Samples 202-1239A-8H-CC and 9H-CC (76.4-87.3 mcd), indicates Subzone Pt1a.

The Pliocene/Pleistocene boundary could not be recognized, due to the absence of the standard zonal markers. However, the LO of Globorotalia exilis (2.15 Ma) between Samples 202-1239A-11H-CC and 12H-CC (107.7-118.3 mcd) indicates upper Pliocene Zone Pl6, and the LO of Globorotalia limbata (2.38 Ma) between Samples 11H-CC and 12H-CC (107.7-118.3 mcd) identifies Zone Pl5. The LO of Globorotalia margaritae (3.58 Ma) between Samples 202-1239A-20X-CC and 21X-CC (199.4-205.0 mcd) marks the base of upper Pliocene Zone Pl3. Two useful markers for lower Pliocene Zone Pl2 are the LO of Pulleniatina primalis (3.65 Ma) and the LO of G. margaritae common (3.96 Ma) between Samples 202-1239A-26H-CC and 27H-CC (257.3-267.4 mcd) and Samples 27H-CC and 28H-CC (267.4-277.8 mcd), respectively. The FO of Globorotalia puncticulata (4.50 Ma), identified between Samples 202-1239A-34X-CC and 35X-CC (340.9-347.6 mcd), and the LO of Globoquadrina dehiscens (5.49 Ma), placed between Samples 39X-CC and 40X-CC (391.3-404.0 mcd), indicate Subzones Pl1b and Pl1a, respectively.

The Pliocene/Miocene boundary is marked by the FO of G. tumida (5.82 Ma) between Samples 202-1239A-40X-CC and 41X-CC. The FO of G. margaritae (6.09 Ma), which corresponds to uppermost Miocene Zone M14, can be placed between Samples 202-1239A-46H-CC and 47H-CC (466.70-477.23 mcd). The FO of Globorotalia conglobatus (6.20 Ma) between Samples 202-1239A-48X-CC and 49X-CC (487.7-498.2 mcd) identifies the upper Miocene Subzone M13b. Several datums provide useful age control for the lower part of Hole 1239A, indicating a major hiatus (~4 m.y.) followed by much lower sedimentation rates during the late middle Miocene. The LO and FO of Globorotalia fohsi s.l. (11.68 and 13.42 Ma, respectively), which occur between Samples 202-1239A-51X-CC and 52X-CC (519.29-527.69 mcd) and between Samples 52X-CC and 53X-CC (527.7-534.9 mcd), respectively, mark the base and top of middle Miocene Zone M9. The LO of G. peripheroronda (14.6 Ma) between Samples 202-1239A-52X-CC and 53X-CC (527.7-534.9 mcd) corresponds to Zone M7. The base of this zone is identified by the FO of Globorotalia peripheroacuta (14.8 Ma). This species is present in Samples 202-1239A-53X-CC and 54X-CC (534.9-549.10 mcd). However, it was not possible to ascertain its FO from the low-resolution shipboard study because of the poor preservation and scarcity of foraminifers in the samples investigated. The FO of G. praemenardii (14.9 Ma), present in Sample 202-1239A-55X-CC (557.0 mcd), is not determined, as it is present below the lowermost core catcher sample from Hole 1239A.

The abundance and preservation of benthic foraminifers vary markedly in Hole 1239A (Fig. F21). The percentage of benthic foraminifers relative to total foraminifers is initially low (1% or less), and preservation is good between the mudline and Sample 202-1239A-10H-CC (0-97.2 mcd). Between Cores 202-1239A-11H and 40X (107.7-404.0 mcd), the proportion of benthic foraminifers fluctuates between 10% and 65% of the total assemblage, except in Sample 202-1239A-14H-CC, which contains ~1% benthic foraminifers. Benthic foraminifers dominate in Cores 202-1239A-41X to 51X (414.4-519.3 mcd), representing ~99% of total foraminifers. Preservation is moderate overall in the interval between Cores 202-1239A-11H and 51X. Abundance decreases (10%-15%) and preservation deteriorates in the lowermost samples (Cores 202-1239A-52X through 55X; 527.7-557.0 mcd), where recrystallized overgrowths frequently obscure test features.

The assemblage is characterized by Bolivina seminuda, Bulimina mexicana, Cibicidoides mundulus, Globocassidulina subglobosa, Globobulimina affinis, Globobulimina pyrula, Gyroidinoides soldanii, Laticarinina pauperata, Melonis affinis, Melonis pompilioides, Oridorsalis umbonatus, Planulina wuellerstorfi, Pleurostomella brevis, Plectofrondicularia vaughani, Pullenia bulloides, Pyrgo murrhina, Pyrgo serrata, and Uvigerina peregrina. The assemblage generally contains a high proportion of Bolivina, Bulimina, Globobulimina, and Uvigerina, which are indicators for high carbon fluxes at the seafloor, probably derived at this location from upwelling-driven productivity. Preliminary shipboard study of core catcher samples revealed marked fluctuations in abundance and assemblage composition, particularly in the relative proportion of high-productivity indicators. However, it was not possible to quantify such changes on the ship or to evaluate to what extent they express variability of the upwelling system and of circulation patterns or merely reflect a preservation bias.

All core catcher samples from Hole 1239A were analyzed as well as smear slides of some additional layers from the split cores. In contrast to Site 1238, where diatoms were generally common to abundant, diatom abundance at Site 1239 is relatively low in the Holocene to mid- Pleistocene and lower Pliocene to uppermost Miocene (Fig. F21; Table T12). Diatom abundance decreases rapidly below Sample 202-1239A-51X-CC (519.3 mcd), and Sample 55X-CC (557.0 mcd) was barren of diatoms. Diatom assemblages observed in most samples are either dominated by Thalassiothrix and Thalassionema species or by Azpeitia nodulifer species. Diatoms recovered from Site 1239 represent a continuous stratigraphic interval from the Holocene Fragilariopsis doliolus Zone through the Nitzschia miocenica Zone of the late Miocene in Core 202-1239A-51X (Table T9; Fig. F21). The early middle Miocene diatom species Crucidenticula nicobarica has been observed, without any late Miocene diatoms, within Cores 202-1239A-52X and 54X. This observation supports the existence of a hiatus within Core 202-1239A-52X, as suggested by the calcareous nannofossil and foraminiferal biostratigraphy.

The generally low abundance of some index species and the frequent reworking of older diatoms into the lower Pliocene-upper Miocene sediments compromises the stratigraphic resolution to some extent and suggests the need for much more detailed shore-based work.

The mid-Pleistocene diatom datums, including the LO of Nitzschia reinholdii (0.62 Ma) and the LO of Nitzschia fossilis (0.70 Ma), are difficult to place because of the rare and sporadic presence of these species. Reliable Pleistocene diatom datums recognized included the following: the LO of Rhizosolenia matuyamai (1.05 Ma) between Sample 202-1239A-5H-4, 75 cm, and 5H-6, 75 cm (40.9-43.9 mcd); the FO of R. matuyamai (1.18 Ma) between Samples 6H-CC and 7H-1, 75 cm (56.1-56.9 mcd); and the FO of F. doliolus (2.00 Ma) between Samples 10H-CC and 11H-2, 75 cm (97.2-100.0 mcd). Rhizosolenia praebergonii var. robusta, which has its LO at 1.73 Ma, was not observed within the Pleistocene section at Site 1238.

The late Pliocene diatom datums recognized are the LO of Thalassiosira convexa s.l. (2.41 Ma) between Samples 202-1239A-14H-3, 75 cm, and 14H-4, 75 cm (132.5-134.0 mcd); the LO of Nitzschia jouseae (2.77 Ma) between Samples 17H-2, 75 cm, and 17H-3, 75 cm (161.2-162.7 mcd); and the FO of R. praebergonii s.l. (3.17 Ma) between Samples 22X-5, 75 cm, and 22X-7, 40 cm (212.6-215.2 mcd). The FO of T. convexa var. convexa (3.81 Ma) is present between Samples 202-1239A-29X-6, 40 cm, and 29X-7, 40 cm (286.5-287.5 mcd), and the FO of N. jouseae (5.12 Ma) is placed between Samples 36X-4, 75 cm, and 36X-5, 75 cm (357.4-358.9 mcd).

For the late Miocene, given the observed reworking, the stratigraphic model is based mainly on FO datums, but LOs have also been considered when they are in accordance with the FO definitions and have been tentatively placed as follows: the LO of Thalassiosira miocenica (5.84 Ma) is between Samples 202-1139A-45X-CC and 46X-CC (456.5-466.7 mcd); the FO of T. miocenica (6.55 Ma) is between Samples 49X-CC and 50X-CC (498.2-508.2 mcd); the FO of T. convexa (6.57 Ma) is between Samples 49X-CC and 50X-CC (498.2-508.2 mcd); and the FO of N. miocenica (7.30 Ma) is between Samples 50X-CC and 51X-CC (508.8-519.3 mcd).

Diatom abundance decreases and preservation deteriorates between the lower Pliocene and the upper Miocene section, relative to the upper Pliocene to lower Pleistocene section.

Sample 202-1239A-51X-CC (519.3 mcd) contains a monospecific diatom assemblage dominated by Thalassiothrix and Thalassiosira. The presence of N. cylindrica in this sample suggests that this sample is younger than the FO of this species (8.07 Ma). Middle Miocene diatoms, such as C. nicobarica, Thalassiosira tappanae, and Denticulopsis hyalina, were observed in Cores 202-1239A-52X and 53X (~520-535 mcd). The presence of these species and the absence of Denticulopsis simonsenii indicate an age between 13.51 and 15.0 Ma for these samples.