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doi:10.2204/iodp.proc.320321.109.2010

Biostratigraphy

A 450 m thick succession of Pleistocene to upper Oligocene sediment was recovered at Site U1337. Planktonic foraminifers are rare to abundant with poor to good preservation throughout most of the succession but are absent or extremely rare from Samples 321-U1337A-18H-CC through 24X-CC and 36X-4, 90–92 cm, through 37X-CC. Biozones PT1b to O6 are recognized, with the exception of Zones PL4, M12, and M3. Calcareous nannofossils at Site U1337 are moderately to poorly preserved, and some samples with high biosilica content are barren. Nannofossil Zones NN1 to NN21 are present, indicating an apparently complete sequence. The radiolarian stratigraphy at Site U1337 spans the interval from the uppermost part of Zone RN16–RN17 (upper Pleistocene) to RN1 (lower Miocene). The radiolarian assemblages of Pleistocene to upper Miocene tend to have good preservation, whereas middle to lower Miocene assemblages show moderate preservation. Below the uppermost portion of Zone RN1, sediments are barren of radiolarians. The high-resolution diatom stratigraphy at Site U1337 spans the interval from the Fragilariopsis doliolus Zone (upper Pleistocene) to the lowermost part of the C. elegans Zone (lower Miocene). The diatom assemblage is generally well to moderately preserved throughout the recovered section; however, there are several intervals in which valve preservation becomes moderate to poor. The nannofossil, foraminifer, radiolarian, and diatom datums and zonal schemes generally agree, though some discrepancies occur in the lowest part of the core. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F13. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F14. Benthic foraminifers occur continuously throughout the succession recovered in Hole U1337A and show good to moderate preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths. Marked variations in downcore abundance and species distribution may reflect major changes in global climate linked to fluctuations in ice volume and reorganization of Pacific Ocean circulation during the Neogene.

Calcareous nannofossils

Calcareous nannofossil bioevents from all holes at Site U1337 are presented in Table T3. Most bioevents are determined to within a core section (1.5 m). Hereafter, all mention of nannofossil bioevents refers to this table. Qualitative estimates of assemblage preservation and relative abundances at the species level are presented in range chart format (Table T4). When subsampling split cores, the most biosilica-rich layers were avoided, therefore implying that the abundances and preservation of samples shown in Table T4 are biased to reflect layers that are richer in calcareous fossils. Nannofossil bioevents from Holes U1337B–U1337D are listed in Table T3. The discussion below mainly refers to Hole U1337A.

Core 321-U1337A-1H consists only of the core catcher sample, containing a relatively diverse assemblage with abundant Gephyrocapsa oceanica and common Gephyrocapsa omega. This sample, and several samples below, contains abundant occurrences of small Gephyrocapsa (<3 µm), which are here referred to as Gephyrocapsa ericsonii but likely represent more than one species. A few specimens of Emiliania huxleyi are observed in an assemblage dominated by abundant gephyrocapsids, placing this sample in Zone NN21. Rare reworked Miocene nannofossils include Discoaster bellus, Discoaster deflandrei, Reticulofenestra pseudoumbilicus, and Sphenolithus heteromorphus.

Small gephyrocapsids are dominant in Sample 321-U1337A-2H-CC. The top of Pseudoemiliania lacunosa occurs in the lower part of Core 321-U1337A-2H. In Hole U1337B, this event occurs between Samples 321-U1337B-1H-4, 101 cm, and 1H-CC. A major change in the abundance and preservation of calcareous nannofossils occurs between Samples 321-U1337A-3H-5, 115–116 cm, and 3H-CC, progressing from abundant and moderately preserved assemblages in the upper sample to less abundant (common) and poorly preserved assemblages in the lower sample. Placoliths are strongly dissolved, resulting in disjointed shields and missing central areas. The appearance of common Reticulofenestra asanoi and the disappearance of large Gephyrocapsa (>5.5 µm) occur in the lower part Core 321-U1337A-3H, indicating an approximate age of 1.1–1.3 Ma for this part of the core.

The Pliocene/Pleistocene boundary is placed in the middle part of Core 321-U1337A-4H, between the successive extinctions of Calcidiscus macintyrei (above) and Discoaster brouweri (below). Sample 321-U1337A-4H-CC is characterized by an upper Pliocene assemblage including common D. brouweri and rare to few Discoaster triradiatus, placing it in Zone NN18. The proportion of these taxa implies that the sample was deposited before the onset of the "acme" interval of the latter species in upper Zone NN18 (Fig. F2A in the "Methods" chapter). Other taxa in this sample are Calcidiscus leptoporus, Ceratolithus cristatus, Ceratolithus rugosus, Coccolithus pelagicus, Helicosphaera carteri, Helicosphaera sellii, Pontosphaera japonica, Pontosphaera multipora, P. lacunosa, small (<5 µm) reticulofenestrids, and very small (<3 µm) reticulofenestrid/dictyococcid placoliths. The tops of Zones NN17 (Discoaster pentaradiatus) and NN16 (Discoaster surculus) occur within Core 321-U1337A-5H. The diatom-rich Sample 321-U1337A-5H-3, 96 cm, lacked discoasters. The uppermost occurrence of Discoaster tamalis is recorded in a low-diversity assemblage within Sample U1337A-5H-CC, impoverished by (selective) calcite dissolution. Rare occurrences of discoasters in the overlying diatom-rich sediments in Core 321-U1337A-5H make it difficult to say whether or not the core catcher sample represents the true extinction of D. tamalis.

The lower/middle Pliocene boundary occurs in the upper part of Core 321-U1337A-7H, based on the extinction of R. pseudoumbilicus. Several biosilica-rich samples in the upper part Core 321-U1337A-7H are virtually barren of calcareous nannofossils. Early Pliocene assemblages are characterized by the presence of C. rugosus, R. pseudoumbilicus, Sphenolithus abies, and about a dozen other taxa, including tiny reticulofenestrids/dictyococcitids that are abundant or dominant in some samples. Furthermore, the distinct but rarely reported Discoaster altus is common in Sample 321-U1337A-8H-4, 10 cm, the range of which appears to have biostratigraphic potential in the tropical Pacific Ocean. The Miocene/Pliocene boundary is well constrained in the lower part of Core 321-U1337A-10H, located above the first occurrence of Ceratolithus acutus. Zone NN12 is present in Cores 321-U1337A-10H and 11H.

Nicklithus amplificus is observed in Cores 321-U1337A-12H and 13H, with a base occurrence in Section 321-U1337A-14H-1. It has a well-defined top and basal occurrence within Zone NN11 despite the poor to moderate preservation. The assemblages in these cores are characterized by abundant reticulofenestrids and, in Sample 321-U1337A-13H-CC, discoasters such as Discoaster berggrenii and Discoaster quinqueramus.

In Core 321-U1337A-14H the carbonate content increases and contains massive abundances of small placoliths representing a mixture of reticulofenestrid forms having either closed central areas (Dictyococcites) or minute pores/central openings (Reticulofenestra). The top of the paracme interval of R. pseudoumbilicus and the base of Amaurolithus spp. both occur in Core 321-U1337A-15H. The preservation deteriorates in Core 321-U1337A-16H, where the nannofossil assemblage shows low diversity and is characterized by robust placolith species and overgrown discoasters. The Zone NN11/NN10 boundary is observed in the lower part of Core 321-U1337A-17H, defined by the base of D. berggrenii. Discoaster assemblages in samples from a few meters below the evolutionary appearance of D. berggrenii are characterized by abundant D. bellus as the dominant member of the genus. This composition changes upcore to a dominance of six-rayed forms belonging to the Discoaster variabilis group. D. berggrenii appears in this latter discoaster assemblage. The large (>20 µm) Discoaster neorectus is common in several samples 3–4 m above the first occurrence of D. berggrenii. The bottom of the paracme interval of R. pseudoumbilicus occurs in a short interval characterized by a major change in preservation, from moderately preserved assemblages above to poorly preserved below in which most placoliths have been dissolved (see Core 321-U1337D-17H).

The core catcher samples of Cores 321-U1337A-18H and 19H contain well-preserved discoasters and poorly preserved placoliths, which are dissolved and fragmented. In Sample 321-U1337A-19H-CC, some of the larger Calcidiscus specimens are referred to Calcidiscus tropicus, which is slightly smaller than Calcidicus macintyrei and possesses a larger central pore or opening. The D. variabilis plexus shows much variability, including large forms such as Discoaster variabilis pansus.

Diatom mats composed of needle-shaped Thalassiothrix and Lioloma occur abundantly in Cores 321-U1337A-20H through 24X. Calcareous nannofossil assemblages are often impoverished in terms of diversity and show poorly preserved assemblages in the most diatom-rich sediments. In terms of nannofossil biostratigraphy, diatom mats are concentrated in an interval from the upper half of Zone NN7 (below the top of Coccolithus miopelagicus but above the top of common D. kugleri) through lower Zone NN10 (shortly above the top of Discoaster hamatus). It is difficult to divide this interval rich in centimeter- to decimeter-thick diatom mats because of rare occurrences of D. hamatus and the Catinaster taxa, used for recognition of Zones NN9 and NN8.

C. miopelagicus, Catinaster coalitus, and D. hamatus provide a series of bioevents within 0.5 m.y. in the early late Miocene based on the compilation by Lourens et al. (2004), which is adopted for the Neogene Pacific Equatorial Age Transect (Fig. F2B in the "Methods" chapter). The age estimates of these three taxa were derived from orbital tuning of ODP Leg 154 cores in the western equatorial Atlantic Ocean (see Raffi et al., 2006). In that region, the C. miopelagicus extinction occurs nearly 140 k.y. prior to the first appearance of C. coalitus. Raffi et al. (2006) indicate a 0.4 m.y. younger age estimate (10.60 Ma) for the extinction of C. miopelagicus from the eastern equatorial Pacific Ocean based on linear interpolation between geomagnetic reversal boundaries in Leg 138 sites, which is adopted here.

At Site U1337, C. miopelagicus has a range that extends into the range of C. coalitus. Primarily because of better core recovery, these taxa could be more tightly constrained in Holes U1337C and U1337D than in Hole U1337A (Table T3). In Hole U1337C, the C. miopelagicus and D. hamatus events occur within a 2.90 m (still unresolved) interval from Samples 321-U1337C-6H-CC to 7H-2, whereas in Hole U1337D, they occur within a 2.14 m interval (unresolved; Samples 321-U1337D-21H-CC to 22H-2) on the spliced depth scale (CCSF-A; see "Stratigraphic correlation and composite section"). A 2.14 m interval represents <100 k.y. in this part of the section. Raffi et al. (2006) estimated a 50 k.y. difference between these two events from Leg 138 Sites in the eastern equatorial Pacific Ocean, which is consistent with the observations made at Site U1337.

Sample 321-U1337A-22X-1, 53 cm, contains rare Discoaster neohamatus, a six-rayed species present in upper Zone NN9 and in lower Zone NN10. Better preserved intervals from Cores 321-U1337B-19H and 20H have frequent occurrences of D. neohamatus. Despite the well-preserved and diverse discoaster assemblages in these intervals, Catinaster species are rare. This genus ranges throughout Zones NN8 and NN9. Sample 321-U1337A-24X-CC contains less biosilica and better preservation of calcareous nannofossil placolith taxa. Another sign of the improved calcareous preservation is that discoasters begin to be overgrown by secondary calcite, a typical phenomenon in carbonate-rich Miocene nannofossil oozes. Common D. kugleri are observed from the upper part of Core 321-U1337A-25X to upper 26X, marking the lower part of Zone NN7.

Core 321-U1337A-26X contains several bioevents which occur in upper Zone NN6, including the tops of Cyclicargolithus floridanus, Coronocyclus nitescens, and Calcidiscus premacintyrei in Sections 321-U1337A-26X-3 to 26X-5. Of the three taxa, C. premacintyrei is the most abundant species, making it easier to recognize its uppermost occurrence. In contrast, C. floridanus and C. nitescens are rare in the upper parts of their ranges. In Cores 321-U1337A-27X through 30X, preservation is predominantly poor but varies based upon the silica content in surrounding sediments. Only one section with alternating bands of diatom mats and more calcareous sediment from Core 321-U1337A-27X was recovered. The calcareous intervals contain some of the best preserved material from the Miocene, an example of which is a 22 µm diameter C. miopelagicus coccosphere observed in Sample 321-U1337D-27X-1, 15 cm. The assemblage in Sample 321-U1337A-27X-CC shows a dominance of small (<3 µm) Dictyococcites spp., together with other placoliths and discoasters. The latter group displays little sign of overgrowth. This is in stark contrast to discoasters in Sample 321-U1337A-29X-CC, most of which cannot be identified at the species level because of severe overgrowth. The top common occurrence of C. floridanus in lower Zone NN6 is placed at the base of Core 321-U1337A-30X. There is not a distinct change in the abundance of this species. Rather, the abundance gradually increases downcore, making this bioevent unreliable at this location.

Zone NN5 occurs in Cores 321-U1337A-31X through 34X as defined by the extinctions of S. heteromorphus (for top of zone) and Helicosphaera ampliaperta (for base of zone). This interval is moderately preserved and contains abundant, diverse nannofossils with common to abundant Sphenolithus, Reticulofenesta, and Coccolithus specimens. The amount of calcite overgrowth on the nannofossils in these samples increases with the elevated carbonate content of the sediments. H. ampliaperta is a distinct although rare species. Despite its low abundances, the highest occurrence could be placed between Sections 321-U1337A-34X-4 and 34X-5.

Cores 321-U1337A-35X and 36X have similar assemblage compositions to those of Cores 31X and 34X, and preservation is moderate to poor. S. hetermorphus is abundant in this interval, and Sample 321-U1337A-36X-CC shows unusually high abundances of Sphenolithus moriformis and S. heteromorphus. The top common occurrence of D. deflandrei is observed in Samples 321-U1337A-35X-3 through 35X-5. Reticulofenestra daviesii has its highest occurrence in the lower part of Core 321-U1337A-37X. This species may prove to be a useful biostratigraphic marker in this region because of its consistent presence from the upper Oligocene and distinct highest occurrence. Zone NN4 encompasses the interval between Sections 321-U1337A-34X-5 and 39X-2. The latter section marks the highest observed occurrence of Sphenolithus belemnos and hence the top of Zone NN3. The Zone NN3/NN2 boundary coincides with the Zone NN4/NN3 boundary between Samples 321-U1337A-38X-CC and 39X-1, 40 cm, because weakly birefringent specimens of Triquetrorhabdulus carinatus occur throughout Core 321-U1337A-39X. If assuming that these forms represent an indigenous presence, it follows that Zone NN3 does not exist in the paleoequatorial Pacific Ocean because of the near-simultaneous disappearances of both T. carinatus and S. belemnos. This biostratigraphic order is not unique for Site U1337 because these events occur within 1.5 m at Site U1336 (see "Biostratigraphy" in the "Site U1336" chapter), suggesting that reworking of T. carinatus is probably not the cause as the two sites are located 360 nmi apart. Rather, it appears as if the range of T. carinatus extends to the top of Zone NN3, implying that the nonexistent Zone NN3 at Sites U1336 and U1337 does not represent condensation and/or the presence of an erosional hiatus.

Sample 321-U1337A-39X-2, 40 cm, shows the highest observed occurrence of a thickly calcified sphenolith species which is characterized also by its large size (7–18 µm) and dark reddish, brownish, and blueish colors between crossed nicols. This morphovariant is considered to belong to the Sphenolithus moriformis group and is present from Core 321-U1337A-39X to the sediment/basalt contact in Core 48X. Its uppermost occurrence may have potential as a tropical Pacific Ocean biostratigraphic marker in the lower Miocene. A similar morphovariant has been described from the middle–upper Miocene (Zones NN7–NN11) in the South Atlantic (Haq and Berggren, 1978), termed Sphenolithus grandis. For the time being, we refer this large morphovariant from Site U1337 to the S. grandis concept. The largest specimens (up to 18 µm) are observed in the lower lower Miocene (Sample 321-U1337A-47X-1, 75 cm). Sample 321-U1337A-39X-CC contains the highest observed occurrence of the large, albeit discontinuous and rare, Discoaster druggii, which occurs sporadically down to Sample 321-U1337A-47X-4, 80 cm. This sample may thus be taken to mark the Zone NN1/NN2 boundary.

Helicosphaerid abundance varies markedly in the sediments retrieved from Hole U1337A, although this group is consistently a minor component of the assemblages. The most abundant (1 specimen per 1–10 fields of view at 1000×) helicosphaerid species in Sample 321-U1337A-43X-CC is Helicosphaera carteri, in contrast to the next core catcher sample (Sample 321-U1337A-44X-CC), where Helicosphaera euphratis is the most abundant representative of this genus. Investigation of a few samples within Core 321-U1337A-44X did not provide statistically meaningful data about which of the two taxa was more abundant. We therefore place the crossover event of these two taxa between Samples 321-U1337A-43X-CC and 44X-CC.

Reticulofenestrids and D. deflandrei are common to abundant in Cores 321-U1337A-46X through 48X. Preservation is moderate and nannofossils are abundant. Top common T. carinatus occurs distinctly between Samples 321-U1337A-47X-2, 75 cm, and 47X-3, 75 cm. The Oligocene/Miocene boundary (23.03 Ma) is within Core 321-U1337A-48X, judging from the presence of Sphenolithus delphix in Samples 321-U1337A-48X-3, 55 cm, to 48X-4, 15 cm. The extinction of this short-ranged taxon has an age estimate of ~23.1 Ma, <100 k.y. prior to the Oligocene/Miocene boundary. The sediment from the lowermost occurrence of D. druggii to the sediment/basalt contact belongs to Zone NN1, indicated by the presence of S. delphix shortly above the basalt and the absence of Sphenolithus ciperoensis. The chalk immediately in contact with the basalt contains poorly preserved nannofossils. A smear slide produced from a calcite vein toward the top of the basalt recovered from Hole U1337D shows rare, poorly preserved nannofossils, which evidently settled into the fissures of the basalt crust.

Handling of the Reticulofenestra/Dictyococcites problem in Hole U1337A

Reticulofenestra and Dictyococcites are two Cenozoic genera that belong to an extant evolutionary lineage. Other genera in this lineage are Pseudoemiliania, Gephyrocapsa, and Emiliania. These three genera, as well as Dictyococcites, all share the basic reticulofenestrid character, but each of them has a distinct morphologic quality that has rendered them their own taxonomic status at the generic level. In the case of Dictyococcites, this quality is its closed central area. Reticulofenestra has, per definition, a central area that is open (Hay et al., 1967). In a discussion of reticulofenestrid taxonomy, Young (1999) expressed the opinion that forms with closed central areas (Dictyococcites) are likely "ecophenotypes of no stratigraphic value (Young, 1990), so they are not recommended." That is, he discarded the genus Dictyococcites. However, this genus has several species, some of which are used biostratigraphically (e.g., Dictyococcites bisectus). Moreover, we do not consider potential biostratigraphic usage as a necessary prerequisite in calcareous nannofossil taxonomy. We therefore consider Dictyococcites to be a valid genus, no less valid than other genera in the reticulofenestrid lineage.

Members of both Dictyococcites and Reticulofenestra are common to abundant in most samples investigated at Site U1337. We distinguish five Reticulofenestra/Dictyococcites morphotypes and/or morphotype groups. R. asanoi (circular to subcircular form having a restricted stratigraphic range within the Pleistocene) and R. pseudoumbilicus (≥7 µm, following Rio et al., 1990; includes the variant Reticulofenestra gelida, which has a small central opening) are recognized at the species level together with Dictyococcites antarcticus (>3 µm). We group all medium-sized (3–7 µm) reticulofenestrids into the Reticulofenestra haqii concept (includes the variant Reticulfenestra minutula, which has a large central opening). Most Neogene samples contain small (≤3 µm) reticulofenestrid placoliths varying in abundance from rare to dominant. Most Pliocene–Pleistocene specimens of these minute placoliths have a small central opening, which make them referable to Reticulofenestra minuta. Many of the Miocene specimens, however, appear to have closed central areas or only a median slit in the closed central areas. These two morphotypes are difficult to distinguish from each other using light microscopy. We have therefore grouped all small (≤3 µm) reticulofenestrid placoliths into the category "small Dictyococcites," both having closed and open central areas. At Site U1337, the open-center forms appear to be more common in the Pliocene–Pleistocene section, whereas the closed-center forms dominate in the Miocene. Scanning electron microscopy is required in order to reliably determine the Neogene distribution of these small reticulofenestrid placoliths, which likely contain at least two species.

Perch-Nielsen (1985) remarked that the "link(s) between the typical Eocene and Oligocene Reticulofenestra species and the species assigned to this genus in the Neogene have not been established satisfactorily." At Site U1337, however, we notice a short overlap in the stratigraphic ranges between the Paleogene species R. daviesii and the Neogene species R. pseudoumbilicus. Furthermore, as observed at Site U1337, if the central plug in R. daviesii is removed by an evolutionary development among the reticulofenestrids, the result will be R. pseudoumbilicus morphotypes. The relative abundance plot (Fig. F15) supports the idea that R. pseudoumbilicus evolved from R. daviesii, thereby establishing a link between the Paleogene and Neogene reticulofenestrid lineages. This transition occurred in Zone NN4.

Radiolarians

The radiolarian stratigraphy at Site U1337 (Table T5) spans the interval from the uppermost part of Zone RN16–RN17 (upper Pleistocene) to RN1 (lower Miocene), which generally agrees with planktonic foraminifer, calcareous nannofossil, diatom, and radiolarian biostratigraphies (Fig. F13). Below the uppermost portion of Zone RN1, the sediments are barren of radiolarians. The Pleistocene to upper Miocene radiolarian assemblages tend to have good preservation, whereas middle to lower Miocene assemblages show moderate preservation (Tables T6, T7, T8, T9). Reworked individuals from the lower to middle Miocene rarely occur in the upper Miocene to Pleistocene sediments.

The topmost part of Site U1337 corresponds to Pleistocene Zone RN16–RN17. The base of Zone RN16–RN17 is placed between Samples 321U1337A-1H-CC and 2H-CC, as indicated by the top of Stylatractus universus. Core 321-U1337A-2H contains the base of Collosphaera tuberosa and the top of Anthocyrtidium angulae, which indicate the top of Zones RN14 and RN13, respectively. The top of A. angulae occurs between Samples 321-U1337B-1H-CC and 2H-CC and between Samples 321-U1337D-1H-CC and 2H-CC, marking the base of Zone RN14.

The base of Lamprocyrtis nigriniae is observed between Samples 321-U1337B-3H-CC and 4H-CC and between 321-U1337C-1H-CC and 2H-CC. This datum was not observed in Hole U1337A. The top of Theocyrtidium vetulum is between Samples 321-U1337A-2H-CC and 3H-CC and Samples 321-U1337B-1H-CC and 2H-CC. The base occurrences of Theocyrtidium tracherium and A. angulae are both distinguished between Samples 321-U1337A-3H-CC and 4H-CC and between 321-U1337B-3H-CC and 4H-CC. The top of Pterocanium prismaticum also occurs in these samples in both holes and between Samples 321-U1337D-2H-CC and 3H-CC. This event defines the base of Zone RN13.

The top of Didymocyrtis avita and Stichocorys peregrina are observed between Samples 321-U1337A-4H-CC and 5H-CC and Samples 321-U1337B-4H-CC and 5H-CC. In Hole U1337D, the top of S. peregrina is recognized between Samples 321-U1337D-2H-CC and 3H-CC. The top of S. peregrina marks the boundary between Zones RN12 and RN11. Cycladophora davisiana is present in Zones RN12 to RN16–RN17 at Site U1337 but does not show enough abundance to confidently establish its base occurrence. The ranges of Lamprocyrtis neoheteroporus and Lamprocyrtis heteroporus are not determined because of their sporadic and discontinuous occurrences in Holes U1337A and U1337B.

The top of Phormostichoartus fistula is recognized in only Hole U1337B between Samples 321-U1337B-4H-CC and 5H-CC. This datum defines the boundary between Subzones RN11b and RN11a. This datum is not observed in Hole U1337A. The top of Lychnodictyum audax is calibrated to Subzone RN11a. However, this species has its top occurrence in Zone RN9 at Site U1337.

The top of Phormostichoartus doliolum is found between Samples 321-U1337A-6H-CC and 7H-CC, Samples 321-U1337B-6H-CC and 7H-CC, and Samples 321-U1337D-6H-CC and 7H-CC. This species marks the top of Zone RN10. The base of Anphirhopalum ypsilon is observed between Samples 321-U1337A-7H-CC and 8H-CC and Samples 321-U1337B-5H-CC and 6H-CC.

The top of Zone RN9 is indicated by the base of Didymocyrtis penultima, which occurs between Samples 321-U1337A-8H-CC and 9H-CC and Samples 321-U1337B-7H-CC and 8H-CC. Nephrospyris renilla occurs in Zones RN9 to RN14 in both Holes U1337A and U1337B but not in sufficient abundance to establish its exact range. The tops of both Solenosphaera omnitubus and Spongaster berminghami are observed between Samples 321-U1337A-10H-CC and 11H-CC and Samples 321-U1337B-9H-CC and 10H-CC. The base of Didymocyrtis avita is between Samples 321-U1337A-11X-CC and 12H-CC and Samples 321-U1337B-12X-CC and 13H-CC. The top of Didymocyrtis antipenultima occurs between Samples 321-U1337A-12H-CC and 13H-CC, Samples 321-U1337B-12H-CC and 13H-CC, and Samples 321-U1337D-6H-CC and 7H-CC. The base of Didymocyrtis tetrathalamus and the top of Calocycletta caepa are between Samples 321-U1337A-13H-CC and 14H-CC and Samples 321-U1337B-13H-CC and 14H-CC. Stichocorys johnsoni is rare in Samples 321-U1337A-13H-CC to 16H-CC and 321-U1337B-16H-CC. The abundance of this taxon was insufficient to determine its highest and lowest datums.

The evolutionary transition from Stichocorys delmontensis to S. peregrina, which defines the boundary between Zones RN9 and RN8, occurs between Samples 321-U1337A-13H-CC and 14H-CC, Samples 321-U1337B-12H-CC and 13H-CC, and Samples 321-U1337D-13H-CC and 14H-CC. The base occurrences of Theocyrtidium vetulum and Splenosphaera omnitubus are located between Samples 321-U1337A-14H-CC and 15H-CC but are not clearly distinguished in Hole U1337B because of the rare occurrences of these two taxa.

The top of Diartus hughesi is between Samples 321-U1337A-15H-CC and 16H-CC, between 321-U1337B-15H-CC and 16H-CC, and between 321-U1337D-14H-CC and 15H-CC. The boundary between Zones RN8 and RN7 is distinguished in this level. The top of Didymocyrtis laticonus also occurs between Samples 321-U1337A-15H-CC and 16H-CC and between 321-U1337B-16H-CC and 17H-CC. The base of D. penultima is detected between Samples 321-U1337A-16H-CC and 17H-CC and Samples 321-U1337B-16H-CC and 17H-CC. The top of Botryostrobus miralestensis occurs between Samples 321-U1337A-17H-CC and 18H-CC and Samples 321-U1337B-16H-CC and 17H-CC. The base of S. berminghami could not be recognized because this species occurs so rarely in the lowermost part of its range.

The top of Zone RN6 is marked by the evolutionary transition of Diartus pettersoni to D. hughesi between Samples 321-U1337A-18H-CC and 19H-CC, Samples 321-U1337B-18H-CC and 19H-CC, and Samples 321-U1337D-17H-CC and 18H-CC. The top of Stichocorys wolffii was not clearly found at Site U1337 because of its sparse presence. The base of D. antipenultima is found between Samples 321-U1337A-19H-CC and 20H-CC and Samples 321-U1337B-18H-CC and 19H-CC. The top events of both Cyrtocapsera japonica and Carpocanopsis cristata are observed between Samples 321-U1337A-22X-CC and 23H-CC. In Hole U1337B, the top occurrence of C. japonica occurs between Samples 321-U1337B-19H-CC and 20H-CC, and the top of C. cristata is between Samples 321-U1337B-21H-CC and 22H-CC. In Hole U1337C, the top of C. japonica occurs between Samples 321-U1337C-4H-CC and 5H-CC. The top of C. cristata was not observed in Hole U1337C. Lithopera neotera occurs in Zones RN5 and RN6 in Holes U1337A and U1337B, but it is so rare that it is difficult to precisely decide the range of this species. The top occurrences of Cyrtocapsera cornuta and Cyrtocapsera tetrapera are located between Samples 321-U1337A-25X-CC and 26H-CC, Samples 321-U1337B-24H-CC and 25H-CC, and Samples 321-U1337C-10H-CC and 11X-CC.

The base of D. pettersoni occurs between Samples 321-U1337A-27X-CC and 28H-CC, Samples 321-U1337C-11X-CC and 12X-CC, and Samples 321-U1337D-27H-CC and 28H-CC, indicating the top of Zone RN5. Rare specimens of Lithopera renzae occur within Zone RN5 in Holes U1337A and U1337C, but it is not possible to delineate and occurs so rarely that we could not decide the range of this species. The top of Calocycletta robusta is between Samples 321-U1337A-29X-CC and 30H-CC and 321-U1337C-15X-CC and 16X-CC. The top occurrences of Acrocubus octopyle and Liriospyris parkerae are between Samples 321-U1337A-30X-CC and 31H-CC. In Hole U1337B, the top of L. parkerae was recognized between Samples 321-U1337C-15X-CC and 16X-CC, whereas the top of A. octopyle was not determined because of its rare occurrence. Between Samples 321-U1337A-33X-CC and 34H-CC and Samples 321-U1337C-17X-CC and 18X-CC, the top occurrences of Didymocyrtis violina, Calocycletta virginis, Calocycletta costata, and Dorcadospyris dentata are observed. The top events of Liriospyris stauropora and Didymocyrtis prismatica and the base of L. parkerae are detected between Samples 321-U1337A-33X-CC and 34H-CC and Samples 321-U1337C-19X-CC and 20X-CC. Didymocyrtis mammifera and Dorcadospiris forcipata are present in Holes U1337A and U1337B, but they occur so rarely that their ranges could not be determined precisely.

The zonal boundary between Zones RN5 and RN4, defined by the evolutionary transition from D. dentata to Dorcadospyris alata, is difficult to recognize at Site U1337 because of the absence of D. alata. However, the boundary between Zones RN5 and RN4 is considered to occur between Samples 321-U1337A-33X-CC and 34X-CC and Samples 321-U1337C-11X-CC and 12X-CC using secondary datums as follows. Samples 321-U1337A-33X-CC and 33X-CC and 321-U1337D-33X-CC clearly belong to Zone RN5 because these samples did not contain Calocycletta costata, which disappears in the lowermost portion of Zone RN5. D. prismatica occurs in Samples 321-U1337A-34X-CC, 321-U1337C-20X-CC, and 321-U1337D-34X-CC and belongs to Zone RN4.

The top of Carpocanopsios cingulata is found between Samples 321-U1337A-35X-CC and 36X-CC. The base of Zone RN4 is identified by the base of C. costata. This event and the base of D. dentata were found between Samples 321-U1337A-38X-CC and 39X-CC and Samples 321-U1337C-23X-CC and 24X-CC. In Hole U1337D, the base of C. costata is recognized between Samples 321-U1337D-39X-CC and 40X-CC. Dorcadospuris forcipata is rare in Samples 321-U1337A-39X-CC to 43X-CC.

The base of S. wolffii (base of Zone RN3) is observed between Samples 321-U1337A-40X-CC and 41H-CC, Samples 321-U1337C-25X-CC and 26X-CC, and Samples 321-U1337D-40X-CC and 41X-CC. The top of Dorcadospiris scambos is found between Samples 321-U1337A-40X-CC and 41H-CC but is not found in Hole U1337C. The base of S. delmontensis is between Samples 321-U1337A-44X-CC and 45X-CC and Samples 321-U1337C-29X-CC and 30X-CC. The top of Lophocyrtis delmontensis is constrained between Samples 321-U1337A-45X-CC and 46H-CC.

With the top of Theocyrtis anossa, the boundary between Zones RN2 and RN1 is recognized between Samples 321-U1337A-45X-CC and 46X-CC.

Diatoms

High-resolution biostratigraphy was performed on core catcher and additional samples (mostly two per core) from Hole U1337A. The diatom stratigraphy at Site U1337 (Fig. F13; Tables T10, T11) spans the interval from the F. doliolus Zone (upper Pleistocene) in Sample 321-U1337A-1H-CC to the lowermost part of the C. elegans Zone (lower Miocene) in Sample 43X-CC (Fig. F13). The diatom assemblage is generally well to moderately preserved throughout the recovered section; however, there are several intervals in which valve preservation becomes moderate to poor. Intervals with moderate to poor diatom preservation are most commonly found between Samples 321-U1337A-16H-CC and 18H-CC (middle upper Miocene); Samples 321-U1337A-29X-3, 44–45 cm, through 31X-2, 46–47 cm (middle Miocene); and Cores 321-U1337A-33X through 37X (lower Miocene to upper Miocene). This intermittent decrease in preservation is not clearly associated with the occurrence of turbidites or with reworking of older microfossils. The diverse diatom assemblage observed in samples from Site U1337 consists of species typical of the low-latitude eastern equatorial Pacific Ocean, including Actinocyclus ellipticus, several species of Azpeitia and Coscinodiscus, Hemidiscus cuneiformis, F. doliolus, Nitzschia fossilis, Fragilariopsis reinholdii, a few varieties of Thalassionema nitzschioides, and several species of Thalassiosira as well as Thalassiothrix spp. (Table T10).

The topmost Sample 321-U1337A-1H-CC and 2H-4, 121–122 cm, are assigned to the Pleistocene F. doliolus Zone. Samples 321-U1337A-2H-6, 50–51 cm, through 4H-4, 55–56 cm, are within the F. reinholdii Zone. The majority of the Pliocene section of Hole U1337A is divided into the Rhizosolenia praebergonii and Nitzschia jouseae Zones (Fig. F13). The boundary between the R. praebergonii and N. jouseae Zones is tentatively located between Samples 321-U1337A-8H-CC and 9H-1, 60–70 cm, because the base occurrence of R. praebergonii and the top occurrence of A. ellipticus f. lanceolata cannot be found. Similarly, it was not possible to fully subdivide the R. praebergonii Zone because of the absence of the age-diagnostic taxon Thalassiosira convexa within the high-resolution samples examined between Samples 321-U1337A-4H-CC and 8H-CC.

Samples 321-U1337A-9H-1, 69–70 cm, through 10H-3, 87–88 cm, are assigned to the N. jouseae Zone. Stratigraphic events observed in this early Pliocene zone include the top occurrence of Fragilariopsis cylindrica (Sample 321-U1337A-10H-3, 87–88 cm) and the base occurrences of Thalassiosira lineata (Sample 10H-3, 87–88 cm) and Thalassiosira oestrupii var. venrickae (Sample 9H-CC). The base of the N. jouseae Zone is defined by the base occurrence of N. jouseae in Sample 321-U1337A-10H-3, 87–88 cm. For most of the Pliocene, diatom abundance is high and preservation varies from moderate to good.

In the expanded Miocene section, it becomes clear that he presence of some of the Miocene species used in the diatom stratigraphy is discontinuous (Table T10). This has been noticed in sediments of Miocene age recovered during Leg 138 (Baldauf and Iwai, 1995). The discontinuous occurrences may reflect large variations in the abundances of these species with time and changing ecologic conditions. However, there is also the possibility that some of these intermittent disappearances of a species may reflect genetic changes in the lineage that give rise to either "iterative evolution" or changing ecological preferences. These questions will be addressed during postcruise research. The upper boundary of the late Miocene T. convexa Zone is defined by base occurrence of N. jouseae in Sample 321-U1337A-10H-3, 87–88 cm. The base of the T. convexa Zone is given by the base occurrence of Thalassiosira miocenia in Sample 321-U1337A-14H-2, 58–59 cm. The lower boundary of the Nitzschia miocenica Zone is placed between Samples 321-U1337A-15H-4, 108–109 cm, and 15H-CC on the base occurrence of N. miocenica in Sample 15H-4, 108–109 cm. The base occurrence of Thalassiosira eccentrica occurs in Sample 321-U1337A-14H-4, 90–91 cm.

Samples 321-U1337A-15H-CC through 17H-CC are placed in the Nitzschia porteri Zone based on the base occurrence of N. miocenica and the top occurrences of Thalassiosira grunowii and Thalassiosira yabei. Several secondary datum events, such as the top occurrences of Thalassiosira burckliana and Rosiella paleaceae, occur in the N. porteri Zone. The placement of Subzones A and B, defined by the top occurrence of Thalassiosira praeconvexa cannot be determined. The N. porteri/T. yabei Zone boundary occurs between Samples 321-U1337A-17H-CC and 18H-3, 139–140 cm. Similar to the N. porteri Zone, no exact placement of Subzones A and B within the T. yabei Zone can be determined. Given the absence of Actinocyclus moronensis, the top of the A. moronensis cannot be determined. The A. moronensis Zone is tentatively allocated to Sample 321-U1337A-23X-2, 124–125 cm. The problematic differentiation of the A. moronensis Zone suggests the low biostratigraphic potential of this diatom. As a result, the exact position of some of the datums within the Craspedodiscus coscinodiscus Zone immediately below the A. moronensis Zone might be slightly biased.

The C. coscinodiscus and Coscinodiscus gigas var. diorama Zones are represented by the base occurrence of N. porteri in Sample 321-U1337A-26X-5, 37–38 cm, the base occurrence of H. cuneiformis in Sample 24X-CC, and the top occurrences of Crucidenticula nicobarica in Sample 25X-2, 42–43 cm, and Coscinodiscus lewisianus in Sample 26X-5, 37–38 cm. The C. gigas var. diorama Zone is tentatively assigned to Samples 321-U1337A-26X-CC and 27X-CC.

Samples 321-U1337A-28X-4, 75–75 cm, through 30X-CC are tentatively assigned to the C. lewisianus Zone. Recognized in this zone are the base occurrences of Azpeitia apiculata, Thalassiosira tappanae, T. yabei, and Triceratium cinnamomeum. The Cestodiscus peplum Zone occurs between the base of the C. lewisianus Zone in Samples 321-U1337A-28X-4, 74–75 cm, and 35X-6, 54–55 cm, which is almost coincidental with the middle/lower Miocene boundary.

The boundary between the C. peplum and Crucidenticula nicobarica Zones is tentatively placed between Samples 321-U1337A-35X-CC and 36X-CC. The C. nicobarica Zone is characterized by several diatom events. Two base occurrences are observed: C. nicobarica in Sample 321-U1337A-38X-CC, and Raphidodiscus marylandicus in Sample 38X-2, 92–93 cm. The top occurrences of R. marylandicus and Coscinodiscus lewisianus var. similis are observed in Sample 321-U1337A-37X-4, 88–89 cm. Diatom abundance is predominantly low through the C. nicobarica Zone, and valve preservation is moderate to poor.

The final two diatom zones found in Hole U1337A, Triceratium pileus and C. elegans, are weakly constrained, mainly because of low diatom abundance. Both zones are characterized by having the lowest diatom species diversity found in Hole U1337A. The boundary between the T. pileus and C. elegans Zones is tentatively located in Samples 321-U1337A-40X-CC and 41X-CC. Similarly, the lower boundary of the C. elegans Zone cannot be identified, and this zonal placement is tentative. Diatoms are absent from the remainder of the samples examined from the lowermost portion of the recovered sequence (Cores 321-U1337A-44X through 47X).

The continuous occurrence of diatoms throughout the upper 410 m CSF makes Site U1337 an ideal type section for the description of stratigraphic events as well as hydrographic and climatic changes spanning from the early Miocene through the Pleistocene in the low-latitude Pacific Ocean. Although earlier authors took an expansive approach toward establishing ages for all of the diatom datums recovered at DSDP and ODP sites along the equatorial Pacific Ocean (e.g., Baldauf, 1985; Baldauf and Iwai, 1995; Barron, 1985a, 1985b, 2006; Barron et al., 2004), onboard analysis of Site U1337 samples shows that some of the diatom datums appear to be more reliable than others. In some cases, differences in the levels of the first or last appearance of a species may be caused by variation in taxonomic interpretation, but more often these variations are due either to real differences in the species ranges at different localities in the tropical Pacific Ocean or to extremely low species abundances at Site U1337.

Occurrence of laminated diatom oozes

A striking feature of sediments from Site U1337 is the occurrence of sequences of laminated diatom oozes (LDOs) intercalated with massive diatom oozes. The LDOs are mainly composed of a few species of the needle-shaped diatom Thalassiothrix. Entire valves are rarely seen, but fragments exceeding 0.5 mm are abundant and well preserved. Each lamination might represent a single productivity event but are not necessarily annual. Compared to laminated sediments from marginal marine basins (Kemp et al., 2006), the examined Site U1337 intervals lack significant terrigenous input and are almost entirely biogenic in origin. The preservation of undisturbed LDOs can be ascribed to the strength of the meshwork of entangled Thalassiothrix frustules. In earlier studies of similar laminated Thalassiothrix oozes from the equatorial Pacific Ocean (Kemp and Baldauf, 1993; Kemp et al., 1995), it was suggested that the frustule strength was sufficient to make the meshworks impenetrable for benthos and thus suppress sediment mixing by physical means. The appearance of LDOs in the open ocean requires conditions favorable for physical accumulation at the surface rather than high productivity (Yoder et al., 2002; Kemp et al., 1995). Grigorov et al. (2002), however, suggested that the contributions of both physical accumulation and enhanced productivity at the front zone might be equally important to the high densities of Thalassiothrix spp.

According to the model proposed by Goldman (1993), autumn/early winter deposition, coinciding with breakdown of summer stratification, is represented by lamina-forming species of Thalassiothrix, which grow at depth in low-light conditions. Both Yoder et al. (2002) and Kemp et al. (1995) argued that these Neogene mat deposits originated from the sinking of surface diatoms generated by tropical instability wave activity. At present, the ecology of Thalassiothrix spp. has not been well studied, either in the field or in culture.

Planktonic foraminifers

Planktonic foraminifer assemblages at Site U1337 are typical of tropical-subtropical eutrophic environments. Assemblages are highly variable with intervals barren of planktonic foraminifers, intervals of poor preservation dominated by the large dissolution-resistant forms (e.g., Sample 321-U1337A-36X-CC), and intervals with moderate to good preservation with diverse assemblages (e.g., Sample 43X-3, 79–82 cm).

A high-resolution planktonic foraminifer biostratigraphy was generated at Site U1337 using core catchers and supplemented by additional samples (usually two per core) from Hole U1337A only. The sedimentary succession at this site ranges from Subzone PT1b (Pleistocene) to Zone O6 (upper Oligocene) (Fig. F13; Table T12), which generally agrees with calcareous nannofossil, diatom, and radiolarian biostratigraphies (Fig. F13). Preservation and abundance of planktonic foraminifers is extremely variable, ranging from poor to good preservation and specimens are commonly infilled. Planktonic foraminifer tests can account for >95% of the total residue in each sample, but in some intervals planktonic foraminifers are rare or absent and assemblages are dominated by dissolution-resistant forms. In such cases it was not possible to identify the biozone. Taxon relative abundances and estimates of assemblage preservation are presented in range chart format (Table T13). A number of primary and secondary markers were absent in the Site U1337 samples or had insufficient abundances to provide robust stratigraphic control. These included Globorotalia birnageae, Globorotalia lenguaensis, Globorotalia (Hirsutella) cibaoensis, Globorotalia (Menardella) exilis, Globorotalia (Truncorotalia) truncatulinoides, Globoturborotalita decoraperta, and Turborotalita humilis.

The topmost part of Hole U1337A corresponds to Pleistocene Subzone PT1b. The youngest sample (321-U1337A-1H-CC) yields Globigerinoides ruber (pink); therefore, the top of the morphotype could be placed above the horizon. This bioevent has been determined at 0.12 Ma in the Indian and Pacific Oceans by oxygen isotope stratigraphy (Thompson et al., 1979), and its converted age to the PEAT timescale is also 0.12 Ma. The top of G. (Truncorotalia) tosaensis is located between Samples 321-U1337A-2H-4, 134–136 cm, and 2H-6, 64–66 cm, defining the boundary between Subzones PT1b and PT1a. The top of Globigerinoides fistulosus occurs between Samples 321-U1337A-4H-3, 94–96 cm, and 4H-5, 9–11 cm, indicating the base of Subzone PT1a. Above the top of G. fistulosus, the change in dominant coiling direction of Pulleniatina spp. from sinistral to dextral is observed between Samples 321-U1337A-3H-5, 115–116 cm, and 3H-CC. This group was mainly composed of dextrally coiled individuals throughout their stratigraphic range, with two significant intervals of sinistrally coiled population. The first interval was from the late Miocene to the early Pliocene and the second was the late Pliocene to the earliest Pleistocene (e.g., Bolli and Saunders, 1985). The latter coiling change from sinistral to dextral has been also recognized from sequences in the northwestern Pacific realm above the Olduvai Subchron (Oda, 1977) and is concordant with the present result. The late Miocene to the early Pliocene coiling change is discussed below. Globigerinoides extremus was sporadically present from Sample 321-U1337A-6H-2, 16–18 cm, through 15H-CC but not in sufficient abundance to establish its top occurrence. The top occurrence of Globorotalia pseudomiocenica (indicating the base of Zone PL6) is between Samples 321-U1337A-4H-CC and 5H-2, 111–113 cm. The top of Globorotalia (Menardella) multicamerata also occurs at this level in Hole U1337A. The base of G. fistulosus is detected between Samples 321-U1337A-6H-2, 16–18 cm, and 6H-6, 129–131 cm. This bioevent has been correlated to the base of the Mammoth Subchron (Berggren et al., 1995). Globoturborotalita woodi was found intermittently in Samples 321-U1337A-15H-4, 104–106 cm, through 40X-2, 56–58 cm; however, the abundance of this taxon was insufficient to determine its top occurrence.

Dentoglobigerina altispira is found between Samples 321-U1337A-6H-2, 16–18 cm, and 39X-CC. The top horizons of D. altispira and Sphaeroidinellopsis seminulina are both found between Samples 321-U1337A-6H-2, 16–18 cm, and 6H-6, 129–131 cm, so we were unable to differentiate Zone PL4; further samples between Samples 321-U1337A-6H-2, 16–18 cm, and 6H-6, 129–131 cm, are required to constrain this zone. The zonal boundary between Zones PL3 and PL2 is also hard to recognize because the index species Globorotalia (Hirsutella) margaritae occurs in only one sample (321-U1337A-7H-4, 98–100 cm) at the present site. The change in coiling direction from sinistral to dextral in Pulleniatina spp. occurs between Samples 321-U1337A-7H-CC and 8H-2, 119–121 cm. This event can be correlated with the coiling change of the group with an astronomically tuned age of 4.08 Ma within Zone PL2 (Lourens et al., 2004). Therefore, at least the latter sample can be assigned to Zone PL2.

Globoturborotalita nepenthes is rare in the assemblages, but the top occurrence is constrained between Samples 321-U1337A-10H-1, 26–28 cm, and 10H-3, 82–84 cm. We were unable to divide Subzones PL1a and PL1b because of the absence of Globorotalia (Hirsutella) cibaoensis. The base of Sphaeroidinella dehiscens sensu lato was found between Samples 321-U1337A-9H-CC and 10H-1, 26–28 cm. The base of Zone PL1, as indicated by the base of Globorotalia tumida, occurs between Samples 321-U1337A-11H-2, 43–45 cm, and 11H-5, 144–146 cm. Below the base of G. tumida, the top of Globoquadrina dehiscens is observed between Samples 321-U1337A-12H-CC and 13H-1, 24–26 cm. The numerical age of this datum has been reported as 5.8 Ma (Berggren et al., 1995), with a recalibrated age to the PEAT timescale of 5.9 Ma, which demonstrates no discrepancy with the stratigraphy at Site U1337.

The base of Subzone M13b is well constrained by the base of Globorotalia plesiotumida between Samples 321-U1337A-16H-2, 32–34 cm, and 16H-4, 24–26 cm. However, Globorotalia lenguaensis was not observed in any of the samples, so we were unable to differentiate between Zone M14 and Subzone M13b.

Between Samples 321-U1337A-18H-5, 83–85 cm, and 24X-4, 28–30 cm, planktonic foraminifers are rare or absent, so we were unable to determine the base of Subzone M13a and Zone M12. Assemblages between Samples 321-U1337A-24X-CC and 5X-5, 36–38 cm, contain both Paragloborotalia mayeri and G. nepenthes, indicating Zone M11. The base of G. nepenthes and thus Zone M11 occurs between Samples 321-U1337A-25X-5, 36–38 cm, and 25X-CC. The top of Globigerinoides subquadratus is placed between Samples 321-U1337A-25X-3, 35–37 cm, and 25X-5, 36–38 cm. It is concordant with the astronomically tuned biohorizon (11.54 Ma) (Lourens et al., 2004).

Zone M10/N13 is represented between Samples 321-U1337A-25X-CC and 26X-5, 40–42 cm, as indicated by the short stratigraphic interval between the base of G. nepenthes and the top of Globorotalia (Fohsella) fohsi sensu lato. The base of G. (Fohsella) fohsi sensu lato is between Samples 321-U1337A-28X-CC and 29X-3, 38–40 cm. The top of Zone N10/M7 is constrained between Samples 321-U1337A-31X-2, 49–51 cm, and 31X-4, 51–53 cm. The base of this zone, as indicated by the base of Globorotalia (Fohsella) peripheroacuta, is between Samples 321-U1337A-33X-3, 39–41 cm, and 33X-5, 39–41 cm. The secondary marker subspecies Globorotalia (Fohsella) fohsi robusta was not detected in any of the samples. Clavatorella bermudezi is rare at Site U1337 but provides a good secondary marker for this interval with the top occurrence between Samples 321-U1337A-31X-4, 51–53 cm, and 31X-CC and the base occurrence between Samples 321-U1337A-33X-CC and 34X-3, 63–65 cm.

The Praeorbulina-Orbulina lineage provides a number of biostratigraphic datums through the early middle Miocene. The base of Orbulina spp. (base of Zone M6/N9) is between Samples 321-U1337A-33X-CC and 34X-3, 63–65 cm. The base of Praeorbulina glomerosa allows differentiation of Subzones M5b and M5a and is placed between Samples 321-U1337A-34X-6, 63–65 cm, and 34X-CC, although this species is rare. Praeorbulina sicana occurs in samples upsection from Sample 321-U1337A-36X-2, 90–92 cm; however, the interval immediately below is barren of planktonic foraminifers, so we are unable to constrain the base of Zone M5.

Paragloborotaliids are abundant and diverse in Hole U1337A. The coiling direction ratio of Paragloborotalia spp. changes from random to exclusively sinistral between Samples 321-U1337A-34X-3, 63–65 cm, and 34X-6, 63–65 cm. The coiling change in this group has been reported around the boundary of early and middle Miocene by several authors (Bolli and Saunders, 1985; Winter and Pearson, 2001; Abdul Aziz et al., 2007). Abdul Aziz et al. (2007) found that randomly coiled Paragloborotalia siakensis occur below the base of P. glomerosa, whereas sinistrally coiled P. siakensis dominate above that horizon. This suggests that the dominant coiling direction change of Paragloborotalia from random to sinistral can be placed around the boundary between Subzones M5a and M5b. Our results from Site U1337 are consistent with Abdul Aziz et al. (2007) and confirm the coiling change as a robust biostratigraphic event.

The top of Catapsydrax dissimilis is constrained between Samples 321-U1337A-39X-CC and 40X-2, 56–58 cm. This bioevent agrees well with the nannofossil, diatom, and radiolarian stratigraphies of this interval. Zone M4 is recognized as the interval between the top of C. dissimilis and the base of P. sicana. However, we are not able to constrain the base of Globigerinatella insueta and therefore can not recognize Zone M3. Catapsydrax unicavus is abundant from the base of the hole to Sample 321-U1337A-36X-2, 90–92 cm, and then rare occurrences continue to Sample 31X-CC (Zone N10/M7). The abundance of this taxon in the lower part of the hole may in part reflect dissolution.

Globoquadrina binaiensis is found from the base of Hole U1337A to Sample 321-U1337A-43X-3, 79–82 cm. However, in younger sediments we also find frequent to rare specimens resembling G. binaiensis with an inflated, arched, and cut-away final chamber and distinctly flattened apertural face but with four, rather than three, chambers in the final whorl. G. binaiensis specimens with four chambers have also been noted by Spezzaferri (1994). We follow the distinction used by Chaisson and Leckie (1993) and distinguish G. binaiensis sensu stricto as forms with three chambers in the final whorl. We refer to these four-chambered specimens as Globoquadrina cf. binaiensis. These specimens range into younger sediments (Sample 321-U1337A-36X-2, 90–92 cm; Subzone M5a). Specimens of G. binaiensis intergraded with G. dehiscens; however, our G. cf. binaiensis do not possess the quadrate, angular chambers that are distinctive in G. dehiscens (Bolli and Saunders, 1985).

There are discrepancies in the order of the biostratigraphic events at Site U1337 through the early Miocene, suggesting either reworking of marker taxa or contamination. The top of Subzone M1b is defined by the top of Paragloborotalia kugleri and is preceded over a short stratigraphic interval by the top of Paragloborotalia pseudokugleri. We apply a very restricted species concept for P. kugleri, including only forms with a pitched periphery, 7–8 chambers in the final whorl, and curved sutures on the spiral side. We find the top of P. kugleri between Samples 321-U1337A-45X-4, 74–76 cm, and 45X-CC and the top of P. pseudokugleri between Samples 321-U1337A-44X-6, 0–2 cm, and 44X-CC. This upper range of P. kugleri and P. pseudokugleri are at odds with Berggren et al. (1995), as we find the extinction of P. kugleri lower than the top of P. pseudokugleri and coincident with the base of G. dehiscens. Although the extinction of P. kugleri is considered a reliable biohorizon, several authors have commented on the sporadic occurrence and rarity of P. kugleri toward the top of its range (e.g., Mancin et al., 2003). Quantitative foraminifer analysis on a number of equatorial and northwest Pacific Ocean sites (DSDP Sites 55, 71, 77B, and 292) by Keller (1981) also previously revealed an abrupt decline in the abundance of P. kugleri concurrent with the base of G. dehiscens. Pearson and Wade (2009) note that the holotype of P. pseudokugleri is a highly developed specimen, with seven chambers in the final whorl. We also find rare and sporadic occurrences of Paragloborotalia cf. kugleri and Paragloborotalia cf. pseudokugleri, with the top of these taxa in Samples 321-U1337A-40X-4, 55–57 cm, and 39X-CC, respectively (higher than the top occurrence of C. dissimilis and G. binaiensis). These specimens have a more rounded periphery but do not fit into the taxonomic concepts of P. mayeri, P. siakensis, Globorotalia (Fohsella) peripheroronda, or Globorotalia birnageae. We were therefore unable to differentiate Subzone M1b, so we have assigned the interval from the base of G. dehiscens to the top of G. binaiensis sensu stricto as Subzone M1b-M2. The base of G. dehiscens, indicating the Subzone M1a/M1b boundary, is constrained between Samples 321-U1337A-45X-4, 74–76 cm, and 45X-CC.

The Oligocene/Miocene boundary occurs within the last core of Hole U1337A, just above basement. We record the base of Zone M1 by the lowest occurrence of P. kugleri between Samples 321-U1337A-48X-2, 85–87 cm, and 48X-4, 112–114 cm. This is concurrent with the base of "Globigerina" primordius. The top of Globigerina ciperoensis is between Samples 321-U1337A-47X-4, 100–102 cm, and 47X-CC.

The genus Globigerinoides is recognized by its sutural supplementary apertures of the spiral side of the test; the oldest taxon traditionally ascribed to this genus is Globigerinoides primordius. Consistent with the findings of Spezzaferri and Premoli Silva (1991), at Site U1337 there are forms that are identical in morphology to G. primordius but without supplementary apertures. Furthermore, detailed scanning electron microscope studies of well-preserved specimens from Trinidad (Pearson and Wade, 2009) show G. primordius to have a "bulloides-type" rather than a "sacculifer-type" wall typical of true Globigerinoides. We therefore refer to these specimens as "Globigerina" primordius pending further study.

Variations in assemblage composition are seen throughout the hole, reflecting both preservational and evolutionary changes and fluctuations in the water column structure (Fig. F16). Globoquadrinids and dentoglobigerinids dominate the assemblages from the base of the hole to Sample 321-U1337A-24X-CC (Zones O6–M11). Globoquadrina venezuelana and Dentoglobigerina larmeui are prominent components of the assemblages from Samples 321U1337A-11H-2, 43–45 cm, and 32X-4, 45–47 cm, respectively, to the base of the hole. Dentoglobigerina tripartita sensu stricto is also very high in abundance from Zone O6 to the base of Zone N10/M7. Globigerinoides increase in abundance from Sample 321-U1337A-34X-CC, and keeled globorotaliids dominate the Pleistocene assemblages (Fig. F16). From Sample 321-U1337A-19H-4, 95–97 cm, Globigerinoides sacculifer becomes a regular component of the assemblages. Peak abundances of Globigerinoides spp. occur within Subzone M13a, with abundant Globigerinoides immaturus, Globigerinoides obliquus, Globigerinoides quadrilobatus, G. sacculifer, and frequent Globigerinoides trilobus. Neogloboquadrina dutertrei is abundant from the top of the hole to Sample 321-U1337A-4H-3, 94–96 cm. Paragloborotaliids are extremely abundant and diverse from the base of the hole to Sample 321-U1337A-38X-CC (Zone M4). Taxa include Paragloborotalia continuosa, Paragloborotalia nana, Paragloborotalia pseudocontinuosa, Paragloborotalia semivera, P. mayeri, and P. siakensis. Above Sample 321-U1337A-35X-6, 50–52 cm, to Sample 24X-CC paragloborotaliids are restricted to three species: P. continuosa, P. mayeri, and P. siakensis. Tenuitellids including Teniutella angustiumbilicata, Teniutella gemma, and Teniutella praestainforthi are rare but recorded sporadically from Sample 321-U1337A-13H-1, 24–26 cm, to Sample 48X-CC, ranging from Zones M14–M13b to O6. The occurrences of T. gemma extend beyond Zone O6, as suggested by Huber et al. (2006), into Miocene Zone M4, which is consistent with the findings of Spezzaferri (1994).

Benthic foraminifers

Benthic foraminifers occur continuously throughout the ~450 m thick Pleistocene to upper Oligocene succession recovered at Site U1337, although abundances vary markedly downcore depending on the silica-carbonate content of the sediment. Assemblages are predominantly composed of calcareous taxa, and agglutinated forms are rare. Benthic foraminifers were examined in core catcher samples from Hole U1337A, supplemented by samples from Hole U1337A sections (two per core) after cores were split. Mudline samples recovered in Holes U1337B–U1337D were also investigated. Large samples with an average volume of ~50 cm³ were processed from all core catchers to obtain quantitative estimates of benthic foraminifer distribution patterns downcore. Smaller 10 cm³ samples were additionally investigated from core sections to provide realistic estimates of species availability for shore-based geochemical and paleontological studies, although these small samples do not yield statistically significant numbers of specimens. Core catcher samples are overall more representative of lithologic variability, as they include siliceous-rich layers. In contrast, core samples were preferentially selected from carbonate-rich intervals, thus introducing a certain amount of bias within the core sample data set, where siliceous intervals are underrepresented.

To assess assemblage composition and variability downhole, all specimens from the >250 µm fraction were picked from core catcher and core samples and mounted onto slides prior to identification and counting. The distribution of benthic foraminifers was additionally checked in the 150–250 µm fraction to ensure that assemblages in the >250 µm fraction were representative and that small species such as phytodetritus feeders were not overlooked. A total of 92 benthic foraminifer taxa were identified. Census counts from core catcher and core section samples are presented in Table T14. Figure F17 summarizes the downcore distribution of the more common benthic foraminifer taxa in core catcher samples from Hole U1337A. Common taxa include Astrononion echolsi, Cibicidoides mundulus, Cibicidoides grimsdalei, Cibicidoides wuellerstorfi, Eggerella bradyi, Fissurina spp., Gyroidinoides soldanii, Laticarinina pauperata, Oridorsalis umbonatus, Pyrgo murrhina, Pyrgo serrata, Pullenia bulloides, and Siphonodosaria abyssorum. Preservation is good overall but deteriorates somewhat in the lower ~100 m of Hole U1337A (below Core 321-U1337A-37X; ~350 m CFS). Fish teeth and ostracodes are intermittently present throughout the succession. Composition of the upper Oligocene to Pleistocene benthic foraminifer assemblage at Site U1337 indicates lower bathyal to abyssal paleodepths. However, marked variations in the downcore distribution of benthic foraminifers probably relate to fundamental changes in global climate during the Neogene associated with major ice-volume fluctuations and reorganization of oceanic circulation.

Mudline samples from Holes U1337B–U1337D were gently washed in order to preserve fragile agglutinated specimens with extremely low fossilization potential. All mudline samples examined revealed a high degree of dissolution in the planktonic foraminifer assemblage, in particular among globorotalids. Rare benthic foraminifers are generally well preserved and consist predominantly of agglutinated forms. Calcareous taxa include C. wuellestorfi, Eobulimina exilis, Melonis sphaeroides, Osangularia plummerae, P. murrhina, and Fissurina spp. Agglutinated taxa are mainly tubular forms with organic cement, such as the branching species Rhizammina algaeformis, which typically agglutinates a variety of planktonic foraminifer tests. These highly fragile taxa occur in all mudline samples but are best preserved in Hole U1337B, suggesting sediment recovery close to the sediment/water interface. The finely agglutinated involute species Cyclammina pusilla, characteristic of lower bathyal to abyssal depths (Hess and Kuhnt, 2005), was also found in the mudline sample from Hole U1337A.

The Pleistocene to late Miocene assemblage within the biosiliceous-rich interval in the upper part of Hole U1337A (Cores 321-U1337A-1H through 12H; 0.14–110.40 m CSF) generally contain relatively low numbers of benthic foraminifers, except for Sample 321-U1337A-1H-CC (0.14 m CSF), which has a more diversified and abundant assemblage, suggesting improved ventilation and relatively high food influx at this abyssal seafloor location. The occurrence of the deep infaunal dweller Chilostomella oolina in Cores 321-U1337A-1H and 4H (0.14–34.44 m CSF) is particularly striking, as this species is more typical of shallower settings with high organic export flux and/or restricted ventilation (Janninck et al., 1998; Jorissen and Rohling, 2000; W. Kuhnt, unpubl. data). In fact, marked variations in the abundance of organic flux–sensitive taxa such as C. oolina, G. subglosa, P. murrhina, and P. serrata suggest substantial changes in equatorial Pacific Ocean surface productivity throughout the Pleistocene to late Miocene, possibly associated with glacial–interglacial climate fluctuations. However, the low resolution of our shipboard benthic foraminifer data set prevents detection of variability on this scale.

A generally diversified and abundant assemblage is found in Cores 321-U1337A-13H through 17H (119.84–157.99 m CSF), coinciding with an interval of overall improved carbonate preservation (see "Geochemistry") and decreased biosiliceous sedimentation at ~8–6 Ma. This assemblage is characterized by relatively high numbers of epifaunal or near-surface dwellers including C. mundulus, C. bradyi, L. pauperata, O. umbonatus, P. murrhina, P. serrata, and Quinqueloculina spp., which suggest a marked improvement in deep-ocean ventilation and relatively high export flux reaching the seafloor at this abyssal location. The lowest occurrence of Osangularia plummerae in Hole U1337A also occurs within this interval (Sample 321-U1337A-16X-CC; 148.52 m CSF). This interval follows a prominent and extended period of low carbonate deposition in the eastern Pacific Ocean at ~11–9 Ma in the early late Miocene, often referred to as the "carbonate crash" (Lyle et al., 1995; Farrell et al., 1995). This transition to higher carbonate deposition marks a fundamental change in Pacific Ocean circulation and productivity regime, possibly associated with changing rates of deepwater production and/or Northern Hemisphere ice development (Zachos et al., 2008).

Core catcher samples from Cores 321-U1337A-18H through 24X (167.50–218.98 m CSF), which are effectively barren of planktonic foraminifers, contain an impoverished benthic foraminifer assemblage. Abundances increase slightly in layers with higher carbonate content, which were selectively sampled for shipboard foraminifer investigation. However, the benthic to planktonic ratio remains at ~99% even in these enriched carbonate samples. Diatom mats with Thalassiothrix and Lioloma are abundant within this biosiliceous-rich interval, which corresponds to the early late Miocene carbonate crash in the eastern Pacific Ocean.

The late middle Miocene assemblage in Cores 321-U1337A-25X through 30X (232.87–280.49 m CSF) is overall diverse and abundant, although marked fluctuations in abundance and composition are evident, which may reflect cyclic changes in surface productivity and circulation. This interval consists of alternating siliceous-rich laminated layers and more carbonate bands, which show some rhythmicity, although this is difficult to ascertain in Hole U1337A because of patchy core recovery. The benthic to planktonic ratio within this interval shows marked oscillations, suggesting substantial variations in carbonate preservation, which may be linked to changes in the CCD.

Cores 321-U1337A-31X through 34X (289.47–318.36 m CSF) contain the most diversified and abundant benthic foraminifer assemblage found in Hole U1337A. This middle Miocene assemblage is characterized by epifaunal species such as C. mundulus, C. wuellerstorfi, L. pauperata, and mobile, shallow infaunal dwellers living close to the sediment/water interface, such as Cibicidoides bradyi, C. grimsdalei, Cibicidoides robertsonianus, G. soldanii, G. subglobosa, O. umbonatus, and Pyrgo spp. The first common occurrence of C. wuellerstorfi occurs within this assemblage (Sample 321-U1337-34X-CC; 318.36 m CSF). The relatively high numbers, high diversity, and low benthic to planktonic ratio indicate well-ventilated bottom water with enhanced carbonate preservation at the seafloor in agreement with CaCO₃ values (see "Geochemistry"). An extended interval of enhanced CaCO₃ deposition is widely recognized at ~16–13 Ma in the eastern equatorial Pacific Ocean (Lyle, 2003). Interestingly, most of this interval corresponds to the episode of major global cooling at ~15–13 Ma, when ice sheets expanded in Antarctica toward the end of the "Monterey carbon isotope excursion." A substantial improvement in deepwater ventilation and deepening of the CCD was also recorded at ODP Site 1237 in the southeastern subtropical Pacific Ocean, implying widespread reorganization in South Pacific Ocean deepwater circulation following middle Miocene ice growth (Holbourn et al., 2005; 2007).

The lower middle to upper lower Miocene assemblage in Cores 321-U1337A-35X through 37X (327.23–347.64 m CSF) exhibits marked changes in abundance and diversity. Sample 321-U1337A-37X-CC (347.64 m CSF), which corresponds to a siliceous-rich interval, contains only rare benthic foraminifers. In contrast, Sample 321-U1337A-36X-CC (338.15 m CSF) is characterized by high numbers of infaunal taxa (>40%) including Siphodonosaria abyssorum and Siphodonosaria spp. and has a high benthic to planktonic ratio (99%), suggesting somewhat reduced bottom water ventilation. The assemblage in Sample 321-U1337A-35X-CC (327.23 m CSF), which is more diversified with a benthic to planktonic ratio of 5%, closely resembles the overlying middle Miocene assemblage in Cores 321-U1337A-31X through 34X (289.47–318.36 m CSF), although abundance is lower. This interval with highly variable benthic foraminifer distribution corresponds to a period of global warmth during the early part of the Monterey carbon isotope excursion, often referred to as the mid-Miocene climatic optimum.

The lower Miocene to uppermost Oligocene assemblage in Cores 321-U1337A-38X through 48X (352.65–449.32 m CSF) shows relatively low benthic to planktonic ratios and high diversity, although abundances are substantially lower than during peak carbonate preservation in the middle Miocene. Common taxa include C. mundulus, C. grimsdalei, G. soldanii, G. subglobosa, and O. umbonatus. The assemblage composition and overall higher CaCO₃ values in this interval (see "Geochemistry") indicate relatively well ventilated bottom water and a deep CCD. However, higher resolution core samples reveal higher variability in assemblage composition and abundance, which may be related to fluctuations in the silica-carbonate content of the sediment and/or oceanographic changes. Shore-based studies will provide an opportunity to investigate these variations and unravel the links with global climatic events.

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