Calcareous nannofossil and planktonic foraminifer biostratigraphies indicate that the 220-m upper Pleistocene to upper Oligocene sediment succession recovered at Site 1236 is essentially complete (Table T7; Figs. F23, F29). The presence of Sphenolithus ciperoensis and Sphenolithus distentus, along with other Oligocene taxa in Samples 202-1236A-23X-CC and 24X-CC (216.1-223.1 mcd), suggests a basal age of 27.5-29.9 Ma for this interval.
Calcareous nannofossils are generally abundant and moderately to well preserved throughout the interval. Planktonic foraminifers are common to abundant down to ~150 mcd (19-20 Ma). Below this depth, foraminiferal abundance decreases and preservation deteriorates markedly. Benthic foraminiferal assemblages document an oligotrophic, pelagic environment from Pliocene to middle Miocene time. Older intervals contain higher numbers of buliminids and nodosariids, reflecting shallower water depth and proximity to the oxygen minimum zone. The presence of poorly preserved larger benthic foraminifers, which have modern equivalents that host algal endosymbionts, indicates redeposition from a nearby carbonate platform during the Oligocene and early Miocene.
Diatoms are only found close to two ash layers at 82.3 and 206.7 mcd and indicate either increased diatom production near the time of ash deposition or anomalous preservation as a result of the presence of ash. A typical low-latitude diatom assemblage with an age between 13.5 and 12.5 Ma is present at 82.3 mcd. The assemblages from the two ash levels also include benthic and neritic diatoms that indicate redeposition from shallow-water environments.
Calcareous nannofossils are generally very abundant and well to moderately preserved at Site 1236 (Table T8; Fig. F23). Virtually no reworked nannofossil specimens were observed. Most of the standard nannofossil zonal markers, as well as some nonstandard markers, were recognized between core sections, which allows a relatively detailed nannofossil biostratigraphy for the entire sequence from the upper Oligocene to upper Pleistocene.
Sample 202-1236A-1H-1, 75 cm (0.78 mcd), contains a late Pleistocene assemblage dominated by Gephyrocapsa spp. The absence of Emiliania huxleyi and Pseudoemiliania lacunosa indicates Zone NN20 (0.24-0.46 Ma). The last occurrence (LO) of P. lacunosa, which defines the top of Zone NN20, is located between Samples 202-1236A-1H-1, 75 cm, and 1H-2, 75 cm (0.78-2.28 mcd). The LO and first occurrence (FO) of Reticulofenestra asanoi and Gephyrocapsa spp. large (>5.5 µm) and the LO of Calcidiscus macintyrei were recognized in the lower part of this core and allow subdivision of Zone NN19.
The topmost Pliocene datum, the LO of Discoaster brouweri, which defines the top of NN18 (1.96 Ma), is located between Samples 202-1236A-2H-1, 76 cm, and 2H-2, 75 cm (7.17-8.68 mcd). The next three standard zonal markers for the late Pliocene have been determined in the lower part of the same core: the LO of Discoaster pentaradiatus (base of Zone NN18; 2.44 Ma), between Samples 202-1236A-2H-3, 76 cm, and 2H-4, 76 cm (10.19-11.7 mcd); the LO of Discoaster surculus (base of Zone NN17; 2.61 Ma), also between Samples 2H-3, 76 cm, and 2H-4, 76 cm (10.19-11.7 mcd); and the LO of Discoaster tamalis (base of Subzone CN12b; 2.76 Ma), between Samples 2H-4, 76 cm, and 2H-5, 76 cm (11.7-13.22 mcd).
The late/early Pliocene boundary in terms of nannofossil biostratigraphy can generally be approximated by the LO of Reticulofenestra pseudoumbilicus, which is placed between Samples 202-1236A-3H-2, 76 cm, and 3H-3, 76 cm (18.1-19.61 mcd). Slightly below this boundary is another useful datum, the FO of P. lacunosa (4.0 Ma), which is located between Samples 202-1236A-3H-3, 76 cm, and 3H-4, 76 cm (19.61-21.12 mcd).
The LO of Triquetrorhabdulus rugosus (5.35 Ma), between Samples 202-1236A-5H-CC and 6H-1, 76 cm (48.17-50.54 mcd), may be an approximation for the Miocene/Pliocene boundary. A more widely used index for the top of the Miocene, Discoaster quinqueramus, was not found at this site. As the range of this species defines Zone NN11 (5.6-8.6 Ma), this means a significant hiatus if this standard zone is applied to this site. However, two nonstandard datums, the top of the R. pseudoumbilicus (>7 µm) "absence interval" and the FO of Amaurolithus primus (normally in this zone), were found to be present in succession, suggesting that a major hiatus probably does not exist here and the absence of D. quinqueramus may be a result of an anomalous biogeographic pattern in this part of the southeastern Pacific.
Abundant Cyclicargolithus floridanus was first encountered in Sample 202-1236A-8H-1, 79 cm (73.87 mcd), just one sample below the LO of Coccolithus miopelagicus and slightly above the LO of Calcidiscus premacintyrei. The relationship of the latter datum with that of the LO of C. floridanus is in a reverse sequence as indicated by Young (1998), and the age of the LO of C. floridanus appears to be over 1 m.y. younger at the site than previously thought.
The presence of Sphenolithus heteromorphus from Samples 202-1236A-9H-3, 76 cm, to 13H-3, 6 cm (85.41-131.8 mcd), indicates an age of 13.57 Ma for the top sample and an age of 18.2 Ma for the bottom sample. Similarly, the presence of Sphenolithus belemnos in Samples 202-1236A-13H-CC through 15H-3, 76 cm (132.9-146.4 mcd), firmly dates the interval as Zone NN3 (18.5-19.2 Ma).
Cores 202-1236A-16H through 22X (154.9-206.7 mcd) generally yielded rare to few poorly preserved nannofossils. However, the darker 20-cm interval from the top of Section 202-1236A-18H-4 contains abundant and moderately preserved nannofossils, which include such age-diagnostic species as Reticulofenestra bisecta, Zygrhabdolithus bijugatus, and S. distentus. This suggests an age of 23.9-24.75 Ma for the interval. No mixture of taxa of discordant ages was observed in any of the samples examined from this interval. Furthermore, the sample immediately below the darker interval and within the white carbonate sediment (Sample 202-1236A-18H-4, 25 cm; 170.7 mcd) also contains the Oligocene taxa R. bisecta and Z. bijugatus. The presence of these taxa indicates that the darker interval is most likely in situ, and it may represent a time interval with substantially less supply of shallower-water carbonate material from nearby carbonate platforms. The abruptness of this change is remarkable.
The last two core catcher samples (Samples 202-1236A-23X-CC and 24X-CC; 216.1-223.1 mcd) contain both S. ciperoensis and S. distentus along with other late Oligocene taxa and, thus, clearly belong to Zone NP24 (27.5-29.9 Ma), which straddles the lower/upper Oligocene boundary.
Planktonic foraminifers are abundant to common down to ~135.3 mcd (Cores 202-1236A-1H through 15H), but abundance decreases in the lower part of Hole 1236A (Table T9; Fig. F23). Preservation is good to that depth but deteriorates markedly below this level, and carbonate cement and recrystallized overgrowths frequently obscure test features. Sample 202-1236A-24X-CC (223.07 mcd) is barren. The FOs and LOs of marker species allow a preliminary biostratigraphy to be established for the entire Pleistocene-Miocene succession recovered at this site. However, Oligocene biozones were not recognized because of the generally poor preservation of planktonic foraminifers within this interval.
The well-preserved Pleistocene planktonic foraminiferal assemblage includes Globigerina bulloides, Globigerina falconensis, Globigerina quinqueloba, Globigerinoides ruber, Globigerinita glutinata, Globorotalia scitula, Globorotalia tosaensis, Globorotalia truncatulinoides, Globorotalia tumida, Orbulina universa, and Neogloboquadrina dutertrei. Sample 202-1236A-1H-CC is placed in the lower Pleistocene Subzone Pt1a of Berggren et al. (1995) (Fig. F12 in the "Explanatory Notes" chapter), based on the presence of Globorotalia inflata, G. tosaensis, and G. truncatulinoides and the absence of Globigerinoides extremus.
The presence of Sphaeroidinellopsis seminula, with G. extremus, G. tosaensis, and Dentoglobigerina altispira, in Sample 202-1236X-2H-CC indicates upper Pliocene Zone Pl4. Samples 202-1236A-3H-CC and 4H-CC are assigned to lower Pliocene Zone Pl1, based on the presence of Globoturborotalia nepenthes, G. tumida, and Globorotalia margaritae.
Samples 202-1236A-5H-CC and 6H-CC contain Globorotalia conomiozea, Globorotalia menardii, Globorotalia plesiotumida, G. nepenthes, D. altispira, O. universa, Sphaeroidinellopsis kochi, and S. seminula, which indicate upper Miocene Zones M14-M13. The upper part of Core 202-1236A-7H is also probably of late Miocene age; however, this could not be determined by shipboard studies as a result of low-resolution sampling. Samples 202-1236A-7H-CC through 11H-CC are characterized by a relatively diverse assemblage including Globoquadrina dehiscens, D. altispira, and S. seminula. The FO of Orbulina suturalis and Praeorbulina sicana can be placed between Samples 202-1236A-9H-CC and 10H-CC and between Samples 11H-CC and 12H-CC, respectively, indicating the middle Miocene Zone M5 for this interval. The FO of G. dehiscens, between Samples 202-1236A-18H-CC and 19H-CC and the LO of Globoturborotalia angulisuturalis between Samples 17H-CC and 18H-CC indicate the lower Miocene Subzone M1b. The absence of some standard middle and early Miocene zonal markers in the shipboard samples examined probably reflects the low sedimentation rates at this site and the low-resolution sampling. The FO of G. angulisuturalis (29.4 Ma) occurs between Samples 202-1236A-20H-CC and 21H-CC. However, because of the poor preservation of the lowermost samples, it is impossible to ascertain whether this is truly the true FO of the taxon.
Benthic foraminifers are rare in the upper part of Hole 1236A, and the benthic/planktonic foraminiferal ratio is extremely low (<1% of total foraminifers) down to 142.67 mbsf (Cores 202-1236A-1H through 14H) (Table T9). Below this depth, the proportion of benthic foraminifers increases to 5% and reaches 20% in Sample 202-1236A-23X-CC (216.07 mcd) (Fig. F23). Sample 202-1236A-24X-CC (223.07 mcd) is barren. Preservation is good to moderate down to 137.9 mbsf (Cores 202-1236A-1H through 15H) but deteriorates below this depth in Hole 1236A.
The Pleistocene-middle Miocene assemblage is relatively diverse and characterizes an oligotrophic pelagic environment. Characteristic taxa are Cibicidoides mundulus, Globocassidulina subglobosa, Gyroidinoides soldanii, Gyroidinoides orbicularis, Laticarinina pauperata, Melonis affinis, Oridorsalis umbonatus, Planulina wuellerstorfi, Pullenia bulloides, Pyrgo murrhina, Rectuvigerina striata, Siphonina tenuicarinata, Stilostomella abyssorum, Stilostomella subspinosa, and Vulvulina spinosa.
A decrease in water depth from middle bathyal in the Pleistocene-middle Miocene to upper bathyal or shelf during the early Miocene and late Oligocene is registered by a change in assemblage composition. Below 175.4 mcd (Cores 202-1236A-18H through 23X), buliminids and nodosariids become more abundant and are present together with shallow-water forms such as Rosalina and Palliolatella. Coarser layers within this interval often show grading (see Fig. F19 and "Lithostratigraphy") and contain large abraded tests of Lenticulina with rusty staining and poorly preserved large symbiont-bearing foraminifers (mainly alveolinids and soritinids). Their presence, together with robust bryozoan fragments, indicates redeposition from a nearby carbonate platform, probably in a cool subtropical climate.
All core catchers and split sections examined for diatoms at Site 1236 were barren, with the exception of Samples 202-1236A-9H-1, 64 cm, and 9H-1, 66 cm (82.3 mcd), where diatoms are frequent, and 22X-CC (206.7 mcd), where their presence is rare. The fact that both these levels are present above ash layers and, in the case of Core 202-1236A-9H, that diatoms disappear 5 cm above the referred ash layer, might be indicative of selective preservation because of increased silica in the sediment pore waters. However, three other ash-layer intervals were examined (intervals 202-1236A-23X-5, 29-31 cm [214.0 mcd]; 8H-6, 110-150 cm [82.0-82.4 mcd]; and 11H-7, 46-57 cm [114.9-115.0 mcd]), and none of them contained diatoms. This points to increased diatom production at the time of deposition of the sediments at 82.3 and 206.7 mcd.
Samples 202-1236A-9H-1, 64 and 66 cm (82.3 mcd), contain a typical middle Miocene low-latitude diatom assemblage including Actinocyclus moronensis, Actinocyclus ellipticus, Rossiela paleacea s.l., Coscinodiscus lewisianus, Denticulopsis simonsenii s.s., and Denticulopsis nicobarica. The presence of A. ellipticus, A. moronensis, and C. lewisianus and the absence of Cestodiscus peplum in this sample assign it to the C. lewisianus Zone, with an age between 13.51 and 12.47 Ma.
For Sample 202-1236A-22X-CC (206.7 mcd), no age assignment can be made based on the rare specimens observed.
Benthic and neritic diatoms are also found in Samples 202-1236A-9H-1, 64 and 66 cm (82.3 mcd), and 22X-CC (206.7 mcd). The presence of benthic and neritic forms suggests shallow-water, possibly shelf, conditions close to the drilled site.