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doi:10.2204/iodp.proc.320321.105.2010

Biostratigraphy

At Site U1333, we recovered a 183 m thick sequence of lower Miocene to middle Eocene nannofossil ooze, radiolarian ooze, and radiolarian clays. The uppermost 2 m of clay is barren of calcareous microfossils but contains radiolarians of early Miocene age. Nannofossil ooze is dominant in the thick Oligocene section, and radiolarian clays and nannofossil ooze are dominant in the Eocene. Radiolarians are present through most of the section and are well preserved in the Eocene. They provide a coherent, high-resolution biochronology, and there appears to be a complete sequence of radiolarian zones from Zones RN1 (lower Miocene) to RP13 (middle Eocene). Calcareous nannofossils are present and moderately to well preserved through most of the succession, although there are some short barren intervals around the middle to upper Eocene boundary. The succession comprises an apparently complete sequence of nannofossil zones from the lower Miocene Zone NN1 to the middle Eocene Zone NP15. Nannofossil datum and zonal determinations agree well with radiolarian biostratigraphy. An integrated calcareous and siliceous microfossil biozonation is shown in Figure F9. A detailed age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F10. Planktonic foraminifers are relatively abundant and well preserved from the lowest part of the Miocene to the lower Oligocene and less abundant but moderately preserved in the middle Eocene. They are poorly preserved or absent in the upper Eocene. Benthic foraminifers are present through most of the section and indicate lower bathyal to abyssal paleodepths.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher samples from all three holes and from samples from each core section, predominantly from Hole U1333A. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Nannofossils are abundant in the nannofossil ooze of the Oligocene and are present, and often abundant, through the Eocene and Miocene. Barren intervals occur in the uppermost 2 m (Core 320-U1333B-1H) and the upper middle Eocene (Core 320-U1333A-16X). In nannofossil ooze lithologies, nannofossil preservation is moderate to good. Increased etching is observed in darker cycles that are dominated by radiolarians.

The interval from Samples 320-U1333A-1H-2, 110 cm, to 4H-CC (1.10–38.60 m CSF) consists of brown radiolarian nannofossil clay that contains low diversity but abundant and moderately preserved nannofossil assemblages dominated by Discoaster deflandrei, Cyclicargolithus floridanus, and Triquetrorhabdulus carinatus. Age-diagnostic taxa are rare, but the assemblages are typical of Zone NN1 (lower Miocene–upper Oligocene). The base of S. disbelemnos in Sample 320-U1333A-2H-5, 70 cm (16.20 m CSF), and the occurrence of rare S. delphix in Sample 320-U1333A-2H-CC (19.57 m CSF) bracket the Oligocene/Miocene boundary.

The Oligocene interval, Cores 320-U1333A-5H through 13H, is composed of white nannofossil ooze, which contains abundant nannofossil assemblages that are moderately to well preserved. Nannofossil Zones NP25 through NP22 are recognized using the top and base of Sphenolithus ciperoensis in Samples 320-U1333A-4H-6, 70 cm (36.70 m CSF), and 5H-4, 70 cm (43.20 m CSF), respectively; the top of Reticulofenestra umbilicus in Sample 320-U1333A-11X-CC (101.17 m CSF); and the top of Coccolithus formosus in Sample 320-U1333A-12X-4, 70 cm (105.90 m CSF). The crossover from Triquetrorhabdulus longus to T. carinatus is an intra–Zone NP25 event (24.7 Ma) and occurs between Samples 320-U1333A-3H-CC (28.87 m CSF) and 4H-1, 70 cm (29.20 m CSF). The top of Sphenolithus predistentus occurs in Sample 320-U1333A-5H-4, 70 cm (67.4 m CSF), and confirms the Zone NP24 designation. The base of Sphenolithus distentus is an intra–Zone NP23 datum and occurs in Sample 320-U1333A-9H-3, 50 cm (79.50 m CSF).

The Eocene/Oligocene boundary interval lies between the top of C. formosus (Sample 320-U1333A-12X-4, 70 cm; 105.90 m CSF) and the top of Discoaster saipanensis (Sample 320-U1333A-13X-3, 140 cm; 114.70 m CSF). The boundary interval yields nannofossils throughout and is apparently complete at the resolution provided by nannofossil biostratigraphy. This interval is associated with a lithologic change from white nannofossil ooze to a darker colored brown radiolarian clay.

Eocene nannofossil Zones NP18-20 through NP15 are recognized using the top of Chiasmolithus grandis in Sample 320-U1333A-14X-CC (129.80 m CSF); the top of Chiasmolithus solitus in Sample 320-U1333A-16X-5, 40 cm (145.60 m CSF); and the total range of Nannotetrina fulgens from Samples 320-U1333A-18X-CC (163.86 m CSF) to 20X-1, 73 cm (178.33 m CSF). The following datums were also useful in supporting these zonal determinations: base of Dictyococcites bisectus, total range of Discoaster bifax (Samples 320-U1333A-16X-6, 40 cm, to 18X-3, 88 cm; 147.10–161.62 m CSF), top and base of Nannotetrina, and top and base of Sphenolithus furcatolithoides. Middle Eocene assemblages are also characterized by the presence of common Blackites spines (including Pseudotriquetrorhabdulus inversus of many authors).

Basal sediments are composed of dolostone with green flecks in Holes U1333A and U1333C and dolomite nannofossil ooze in Hole U1333B. Nannofossil assemblages from these sediments are poor to moderately well preserved. The presence of S. furcatolithoides in Sample 320-U1333A-20X-2, 50 cm, suggests an age of Zone NP15 (45.8 Ma or younger), even though N. fulgens is absent from this sample to the base of the hole. The absence of Nannotetrina in this oldest time interval may be due to ecological exclusion; discoasters are very rare in these basal assemblages, indicative of a eutrophic environment, which is consistent with the site's paleolatitude at this time within the equatorial upwelling zone. It is therefore likely that the base of N. fulgens and Nannotetrina are both too high in the core, and the basal sediments have been tentatively assigned to Zone NP15.

Radiolarians

Radiolarian stratigraphy at Site U1333 (Table T4) spans the interval between Zone RN2 (near the base of the lower Miocene) and the upper part of Zone RP13 (middle Eocene) (Tables T5, T6, T7). The first core (Sample 320-U1333A-1-2H, 104–106 cm) recovered lower Miocene radiolarians in a moderately preserved assemblage with no detectable reworked older microfossils. This is very different from Sites U1331 and U1332, at which the uppermost cores were dominated by a highly mixed assemblage of Oligocene through Eocene radiolarians. Preservation in the upper part of Zone RP22 (Core 320-U1333A-3H; upper Oligocene) is generally poor to moderate but improves through the middle part of Zone RP20 (Cores 320-U1333A-4H through 8H), becomes poor again briefly in Core 320-U1333A-9H (Zone RP20), and then remains moderate to good through the remaining Oligocene and Eocene section.

Magnetic susceptibility records from Core 320-U1333A-13X show a "two-step" transition from Eocene to Oligocene sediments, with the base of Chron 13n occurring a few centimeters above the younger of these two steps. This pattern is indicative of an apparently complete Eocene/Oligocene boundary section, similar to that recovered at Site 1218. Initial assessment of radiolarian assemblages across the Eocene/Oligocene boundary interval indicates a significant loss of diversity through this transition. Although a few species from the Eocene carry through to the Oligocene, only one stratigraphic marker species (L. angusta) first appears near the Eocene/Oligocene boundary (Sample 320-U1333A-13X-3, 0–8 cm). Most of the lower Oligocene marker species make their first appearance in the middle part of Core 320-U1333A-12X, a few meters above the younger step in magnetic susceptibility. Between these first occurrences and the last occurrence of the Eocene marker species, there is a zone of relativity low radiolarian diversity, which commonly contains abundant diatoms in the >63 µm fraction.

There is a slight amount of reworked older Eocene species in the lower part of the Oligocene section. With the detailed sample coverage in this interval, the reworked forms clearly show a discontinuous appearance in the samples.

All the radiolarian zones down to Zone RP13 are present with moderate to good preservation of assemblages. Sample 320-U1333A-19X-CC and all of Core 320-U1333A-20X were barren of radiolarians.

Diatoms

Diatoms were examined in core catcher samples from Holes U1333A–U1333C and selected intermediate samples. The examined interval represents the Rocella gelida through Coscinodiscus excavatus Zones of Barron (1985, 2006) and Barron et al. (2004). Diatoms range in abundance from rare to abundant depending on the specific sample. Diatom preservation is variable but generally is poor to moderate.

Samples examined from Cores 320-U1333A-1H, 320-U1333B-1H, and 320-U1333C-1H and Sample 320-U1333C-2H-CC contain rare or no diatoms. No zonal assignment was made. The interval from Samples 320-U1333B-2H-CC through 320-U1333A-3H-2, 100–101 cm, is assigned to the R. gelida Zone based on the occurrence of R. gelida. Specimens of Bogorovia veniamini, Rocella vigilans, and Cavitatus miocenica also occur in Sample 320-U1333B-2H-CC.

The interval from Samples 320-U1333A-3H-4, 100–101 cm, through 5H-CC is assigned to the B. veniamini Zone based on the occurrence of B. veniamini in most samples examined from this interval. Supporting this zonal assignment is the occurrences of Cestodiscus kugleri in Samples 320-U1333A-4H-3, 100–101 cm, and 4H-5, 100–101 cm, and Rossiella symmetrica in Sample 4H-5, 100–101 cm. Other species typical of this interval, but not necessarily present in each sample are R. vigilans, Azpeitia oligocenica, Cestodiscus pulchellus, Coscinodiscus rhombicus, and C. miocenica.

The interval from Samples 320-U1333A-6H-2, 110–111 cm, through 8H-4, 115–116 cm, is assigned to the R. vigilans Zone based on the occurrence of R. vigilans without B. veniamini. Sample 320-U1333A-6H-5, 110–111 cm, is placed in Subzone A of this zone based on the occurrence of Cavitatus jouseanus. Sample 320-U1333A-7H-2, 100–101 cm, is placed into Subzone B based on the occurrence of R. symmetrica without C. jouseanus. The occurrence of Cestodiscus trochus without R. symmetricus in Sample 320-U1333A-7H-CC suggests placement of this sample into Subzone A; however, diatom preservation through this interval is poor to moderate.

Samples 320-U1333A-8H-CC and 9H-CC are assigned to the C. trochus Zone based on the occurrences of C. miocenica in Sample 8H-CC and C. trochus and Cestodiscus robustus in Sample 9H-CC without C. excavatus.

The C. excavatus Zone is represented by the occurrence of C. excavatus in the interval from Samples 320-U1333A-10H-2, 100–101 cm, through 13H-2, 100–101 cm. Typical of this interval is the occurrence of C. trochus, C. excavatus, A. oligocenica, and C. robustus.

Sample 320-U1333A-13H-4, 100–101 cm, and below typically contain rare diatoms or are barren of diatoms. The exceptions are Samples 320-U1333A-16X-CC and 320-U1333B-16X-CC, both of which contain fragments typified by Hemiaulus.

Planktonic foraminifers

Core catchers were sampled from all three holes at Site U1333, and additional samples were taken in Hole U1333A (two per core) to develop a high-resolution biostratigraphy. The early Miocene and much of the Oligocene is characterized by abundant and relatively well to moderately preserved planktonic foraminifers that delineate a sequence of lower Miocene (Zone M1) through Oligocene (Zone O2) zones. The record of planktonic foraminifers at this site indicate a relatively continuous succession of zones that agree well with calcareous nannofossil and radiolarian zonal determinations (Fig. F9). Preservation and abundance of planktonic foraminifers is poor in the earliest Oligocene and latest Eocene with many samples either barren or containing a few poorly preserved specimens. Preservation briefly improves in the late middle Eocene. Depth positions and age estimates of biostratigraphic marker events identified are shown in Table T8. Taxon preservation and occurrences are shown in Table T9.

Cores 320-U1333A-1H and 320-U1331C-2H were assigned to Zone M1a based on the co-occurrence of P. kugleri and Paragloborotalia pseudokugleri and the absence of Globoquadrina dehiscens. Early Miocene assemblages are characterized by the presence of Dentoglobigerina spp. and representatives of the Paragloborotalia semivera–siakensis–mayeri group. The Oligocene/Miocene boundary approximated by the first occurrence of P. kugleri is well constrained at this site and falls between Sample 320-U1333A-2H-2, 38–40 cm (11.38 m CSF), and Sample 2H-CC (19.57 m CSF), in excellent agreement with the estimate from calcareous nannofossils. A poorly preserved and scarce fauna in Sample 320-U1333A-2H-4, 38–40 cm, prevented closer constraints on the boundary. In Holes U1333B and U1333C, P. kugleri was rare or absent, as was Globigerina ciperoensis, the last occurrence of which falls directly above the Oligocene/Miocene boundary, hindering precise determination of the boundary in these holes. Samples 320-U1333A-2H-CC through 6H-CC (18.11–56.20 m CSF) are assigned to Zone O6 based on the last occurrence of Paragloborotalia opima, the sporadic presence of G. ciperoensis and P. pseudokugleri, and the absence of P. kugleri. The assemblage is dominated by paragloborotaliids, including the P. opima–mayeri group, Paragloborotalia semivera, and Paragloborotalia pseudocontinousa.

The presence of Zones O2–O5 is recognized by the top and base of P. opima between Samples 320-U1333A-6H-3, 38–40 cm (50.88 m CSF), and 10H-2, 38–40 cm (87.38 m CSF), respectively (see Table T8 for further details). Differentiation between P. opima and P. nana is based on the size criterion proposed by Bolli and Saunders (1985). In addition, Spezzaferri (1994) noted that P. opima exhibits a more lobulate profile, larger final chamber, and higher arched aperture than observed in P. nana; these criteria were also used as a guide our identifications. Zones O4 and O5 were determined based on the overlapping ranges of P. opima and Globigerina angulisuturalis. Chiloguembelina cubensis was also identified in a single sample (320-U1333A-7H-5, 38–40 cm; 63.38 m CSF) but was not employed as a datum to distinguish Zones O4 and O5 because of its very low abundance and absence from the rest of the samples investigated. The presence of Zone O3 was identified between the base of G. angulisuturalis and the topmost occurrences of Turborotalia ampliapertura in Sample 320-U1333A-8H-CC (76.14 m CSF). However, the last occurrence of T. ampliapertura occurs somewhat shallower in the sedimentary record than expected (Fig. F10) and was too rare in Hole U1333B and U1333C core catchers to define. Furthermore, the topmost occurrence of Subbotina angiporoides in Sample 320-U1333A-9H-2, 52–53 cm (78.02 m CSF), should fall in Zone O3 but here occurs deeper than expected and falls in Zone O2; thus, caution is warranted.

As at the two previous drill sites, samples in the earliest Oligocene–latest Eocene (Cores 320-U1333A-13X through 16X, 320-U1333B-14H through 17H, and 320-U1333C-15H through 17H) are barren of planktonic foraminifers, preventing detection of the Eocene/Oligocene boundary. This barren interval directly coincides with a shift in sediment lithology from carbonate nannofossil ooze to radiolarian ooze. However, the FO of Globoquadrina venezuelana (~108 m CSF) can be used to roughly approximate the Eocene/Oligocene boundary (Wade and Pearson, 2008). The first consistent presence of Catapsydrax unicarus also occurs in Zone 01 following the Eocene/Oligocene boundary in Wade and Pearson (2008) and provides a good approximation of the boundary at ~108 m CSF but occurs much deeper in Hole U1333C (Sample 320-U1333B-13H-CC; ~122.14 m CSF).

Cores 320-U1333A-17X through 20X, 320-U1333B-18X through 20X, and 320-U1333C-18H through 20H are either barren or yield assemblages containing moderately to poorly preserved, dissolution-resistant planktonic foraminifers. The variable planktonic foraminifer abundance and preservation in the middle and late Eocene reflects shifts in the dominant sediment lithology between carbonate nannofossil and radiolarian oozes. The Eocene assemblage comprises parasubbotinids, paragloborotaliids, subbotinids, and broken but distinctive elongate chambers from Clavigerinella eocaenica. Species identified include C. unicavus, Paragloborotalia griffinoides, Parasubbotina griffinae, Parasubbotina varianta, Subbotina corpulenta, Subbotina eocaena, Subbotina hagni, Subbotina linaperta, and Subbotina senni. In samples where preservation is better (e.g., Sample 320-U1333A-20X-2, 42–44; 179.52 m CSF), Acarinina praetopilensis, Acarinina bullbrooki, and other small unidentified acarininids can also be found. The dominance of stratigraphically long-ranging Eocene taxa coupled with the absence of the genera Globigerinatheka and Morozovella makes precise age determination of individual samples problematic. However, the presence of A. bullbrooki in Sample 320-U1333A-20X-2, 42–44 cm (179.52 m CSF), indicates a basement age older than 40.8 Ma (below Zone E12), but better precision was given by the nannofossil biostratigraphic estimates (Zone NP15) (Fig. F9).

On an additional note, high abundances of Clavigerinella spp. are often linked to high-productivity environments (e.g., Coxall et al., 2003), which is consistent with the paleogeographic situation of this site within the high-productivity equatorial belt during the middle–late Eocene. Further evidence for this (although also a by-product of dissolution) is the dominance of globigerinid forms—parasubbotinids, subbotinids, paragloborotaliids—also associated with nutrient-rich surface waters.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in all three holes of Site U1333. Benthic foraminifers occurred almost continuously in calcareous nannofossil ooze of the Oligocene and in radiolarian ooze of the Eocene. The occurrence of benthic foraminifers at this site is shown in Table T10.

In Samples 320-U1333A-1H-CC through 12X-CC (9.95–107.99 m CSF), N. umbonifer, O. umbonatus, C. mundulus, Globocassidulina subglobosa, Gyroidinoides spp., and Pullenia spp. were common and Astrononion echolsi, Nonion havanensis, Siphonodosaria antillea, and Cibicidoides grimsdalei were subordinate. C. mundulus and G. subglobosa were generally common in the lower part of the interval (maximum = 28% and 15%, respectively), whereas N. umbonifer was abundant in the upper part of the interval (maximum = 23%). A similar faunal transition was recognized in Holes U1333B (Samples 320-U1333B-1H-CC through 12H-CC; 7.69–112.32 m CSF) and U1333C (Samples 320-U1333C-2H-CC through 13H-CC; 36.54–75.94 m CSF). In addition, the abundance of N. umbonifer in Hole U1333C also varied continuously in the late Oligocene. Preservation of benthic foraminifer tests is good. These faunal compositions indicate lower bathyal and abyssal paleodepths during the Oligocene and the early Miocene, based on van Morkhoven et al. (1986). The Oligocene and early Miocene fauna at this site are basically similar to those observed at Site U1332 and in previous studies in the eastern equatorial Pacific (DSDP Site 573, Thomas, 1985; Sites 1218 and 1219, Takata and Nomura, 2005). N. umbonifer is a tolerant species to carbonate undersaturation and/or low food supply (e.g., Mackensen et al., 1990; Schmiedl et al., 1997). O. umbonatus, Cibicidoides spp., and G. subglobosa are common oligotrophic taxa in deep water (e.g., Nomura, 1995). This suggests that changes in carbonate undersaturation and/or food supply from the surface ocean may have occured in the late Oligocene.

Samples 320-U1333A-13X-CC through 19X-CC (119.96–174.73 m CSF) contained benthic foraminifers, except for Sample 320-U1333A-15X-CC (139.17 m CSF). O. umbonatus, N. truempyi, Cibicidoides eocanus, C. grimsdalei, G. subglobosa, and S. antillea were common in this interval. In addition, various taxa, such as Abyssamina quadrata, Abyssamina poagi, and Alabamina dissonata, were subordinate in Sample 320-U1333A-19X-CC (174.73 m CSF). Similar occurrences were also recognized in Samples 320-U1333B-13H-CC through 19X-CC (122.15–169.85 m CSF) and 320-U1333C-14H-CC through 20X-CC (117.59–155.10 m CSF). Test preservation of these calcareous foraminifers was generally poor, but well-preserved specimens were sometimes found in the lowermost part of the interval. These fauna suggest lower bathyal to abyssal paleodepth in the middle to late Eocene. Faunal associations of these calcareous taxa in the middle to late Eocene are similar to those recorded at Sites U1331 and U1332 and previous preliminary studies in the eastern equatorial Pacific (Site 1218; Shipboard Scientific Party, 2002b). Calcareous foraminifers at this site were more consistently present compared to those of Sites U1331 and U1332 (Fig. F11) and may be attributed to a shallower water depth at this site than at Sites U1331 and U1332. Common occurrences of N. truempyi and O. umbonatus were recognized in nannofossil Zone NP16 (e.g., Samples 320-U1331B-8H-CC, 320-U1332A-13H-CC, and 16X-CC), roughly coincide with the high-carbonate interval observed (see "Lithostratigraphy" and "Geochemistry") at all three sites, and may correlate to deepening of the calcium carbonate compensation depth in the middle Eocene (Lyle et al., 2005).

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