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doi:10.2204/iodp.proc.308.104.2006

Biostratigraphy

Calcareous nannofossils and planktonic foraminifers are rare to common with good preservation except in Samples 308-U1320A-4H-CC, 24–29 cm, 15X-CC, 19–24 cm, and 19X-CC, 42–47 cm, where foraminifers are barren. Below Core 308-U1320A-26X, both nannofossil and foraminifer abundances decline rapidly.

We identified the nannofossil Emiliania huxleyi Zone and a transitional zone, as well as planktonic foraminifer Zones W, X, and Y largely based on analyzing core catcher samples from Hole U1320A (Fig. F10). The results indicate that, similar to Site U1319, sediments recovered at Site U1320 are of late Pleistocene to Holocene age and record deposition over approximately the last 150 k.y. Compared to Site U1319, however, the sedimentary succession at Site U1320 is greatly expanded. Because of incomplete core recovery and rare nannofossils and planktonic foraminifers in some samples, our biostratigraphic data cannot confirm the completeness of the sediment record.

Benthic foraminifers, especially in samples from the lower part of Hole U1320A, are dominated by species living in low-oxygen, nutrient-rich environments. Frequent neritic benthic foraminifers and reworked Cretaceous nannofossils found in samples from the upper 120 m of the hole suggest downslope transport by turbidites.

Calcareous nannofossils

We examined all core catcher samples and additional samples from Sections 308-U1320A-17X-5 and 17X-6 for calcareous nannofossils. All samples yielded rare to abundant nannofossil assemblages, with a general reduction of abundance toward the bottom of the hole. Samples from Core 308-U1320A-17X and above contain very abundant nannofossils, but downhole from Sample 18X-CC, 32–37 cm, their abundance varies from abundant to rare. Preservation is good to moderate throughout the section. Samples with poorly preserved nannofossils are typically coarse grained with low overall abundance. Nannofossil assemblages contain in situ and reworked species. The most dominant in situ species are E. huxleyi, Gephyrocapsa aperta, and Gephyrocapsa ericsonii. Gephyrocapsa oceanica is always more abundant than Gephyrocapsa caribbeanica, although both rarely attain 5% of the total abundance. The majority of reworked species are Cretaceous in age (>99%), and they occur throughout the section (Figs. F11, F12).

Based on the nannofossil stratigraphic subdivision of Hine and Weaver (1998), we were able to recognize two main zones (QAZ1 E. huxleyi Acme and QAZ2 Transitional Zones) for the sediment sequence recovered at Site U1320. Based on the distribution of different groups of nannofossils we can also distinguish Subzones A–C within the QAZ2 Transitional Zone, similar to the results from Site U1319.

QAZ1 E. huxleyi Acme Zone

We identified this zone in Samples 308-U1320A-1H-CC, 7–12 cm, through 17X-5, 5 cm, on the dominant abundance of E. huxleyi (70% or more). According to Berggren et al. (1995), the first occurrence datum of E. huxleyi acme is at 90 ka. Reworked Cretaceous species are more abundant relative to the lower part of the hole (QAZ2 Transitional Zone). Significant reworking of nannofossil assemblages from the QAZ2 Transitional Zone can be traced in Sample 308-U1320A-12X-CC, 15–20 cm. This is interpreted as resulting from increased turbiditic activity at this time.

QAZ2 Transitional Zone

This zone was distinguished in Samples 308-U1320A-17X-5, 145 cm, through 33X-CC, 22–26 cm. It is characterized by the dominance of G. aperta-G. ericsonii. Together, these two species constitute >90% of the overall abundance. Subzones, A, B, and C, are distinguished based on the abundance variations of these two species.

Subzone A

Subzone A, an uppermost subzone of the QAZ2 Transitional Zone, is found in Samples 308-U1320A-17X-5, 145 cm, through 17X-CC, 49–54 cm. It is defined based on an overall abundance of ~100,000 specimens per 100 fields of view of G. aperta-G. ericsonii and the absence of E. huxleyi. G. oceanica-G. caribbeanica complex is common to abundant, as well as Scapholithus fossilis and Umbilicosphaera spp. Section 308-U1320A-17X-5, 145 cm, is now identified as representing the top of the QAZ2 Transitional Zone at 90 ka, which is at odds with the position of ash Layer Y8 ~2.5 m upsection (84 ka) (Mallarino et al., in press). Further scanning electron microscope studies are needed to clarify if small-sized specimens of E. huxleyi are not detected using light microscopes available onboard.

Subzone B

This subzone is distinguished based on the common overall abundance of G. aperta-G. ericsonii. Subzone B is defined in Samples 308-U1320A-18X-CC, 32–37 cm, through 27X-CC, 25–30 cm.

Subzone C

Subzone C is defined by the dominant occurrence of reworked Cretaceous species to >50% of the total abundance. This subzone has rare in situ species including G. aperta-G. ericsonii. Subzone C is defined in Samples 308-U1320A-28X-CC, 38–43 cm, through 33X-CC, 22–26 cm.

All nannofossil stratigraphic subdivisions correlate well with the results from Site U1319.

Planktonic foraminifers

The overall preservation of foraminifers in Hole U1320A is good to excellent. However, planktonic foraminifers are common to abundant only in Samples 308-U1320A-12X-CC, 17X-CC, and 22X-CC through 25X-CC but are rare to trace in other samples. Reworked specimens common in samples from above Core 308-U1320A-14X are considered to be a result of turbidite deposition.

As at Site U1319 and other neighboring sites described recently by Mallarino et al. (in press), the planktonic foraminifer assemblage found in samples from Site U1320 is dominated by Globigerinoides ruber (both the pink and white forms) with lesser amounts of Globigerinoides sacculifer, Globigerinoides conglobatus, Neogloboquadrina dutertrei, Globorotalia truncatulinoides, Globorotalia menardii, Globorotalia tumida, Globorotalia flexuosa (older than 68 ka), Globorotalia inflata (older than 10 ka), Globorotalia crassaformis, Globigerinella siphonifera, Orbulina universa, Globigerina bulloides, Globigerina falconensis, and Pulleniatina obliquiloculata. Other taxa, such as Globigerinita glutinata, Hastigerina pelagica, and Globigerinella calida, are rare.

Planktonic foraminifer assemblage Zones W, X, and Y were identified, suggesting that the sediment recovered from Hole U1320A was deposited mainly during the last 180 k.y. (Fig. F13). Holocene Zone Z, characterized by G. menardii without G. inflata, is not recognized, probably because its components have been diluted in sands from the uppermost core catcher sample from Hole U1320A. The absence of both G. menardii and G. inflata from Sample 308-U1320A-1H-CC, 7–12 cm (4.54 mbsf), may indicate a position near the Zone Y/Z boundary. The interval from Cores 308-U1320A-3H through 14X contains mostly reworked specimens associated with turbidite deposition, thus hampering a proper subdivision of Zone Y. Semiquantitative planktonic foraminifer data are presented in Table T4.

Zone Y

From Cores 308-U1320A-2H through 16X the planktonic foraminifer assemblage is dominated by G. ruber and G. inflata. G. menardii and many other warm-water species found in several samples from this interval are obviously reworked because these samples contain abundant plant debris, mica, and shallow-water benthic foraminifers that indicate a nearshore origin. In addition, the presence of many broken specimens and relatively poor to moderate preservation suggests that these were transported by turbidites. The possibility of reworking diminishes our ability to use the last occurrence of G. flexuosa as a correlation datum at 68 ka. Pending further studies, the base of Zone Y is tentatively placed at Sample 308-U1320A-16X-CC, 35–40 cm (136.1 mbsf). According to lithostratigraphic results, ash Layer Y8 occurs at 143.42 mbsf (see “Lithostratigraphy”).

Zone X

Zone X was recognized only in Sample 308-U1320A-17X-CC, 49–54 cm (145.6 mbsf). It contains abundant warm-water planktonic foraminifers typified by G. tumida, G. flexuosa, G. crassaformis, G. truncatulinoides, G. menardii, G. siphonifera, N. dutertrei, P. obliquiloculata, O. universa, G. ruber, G. sacculifer, G. conglobatus, and G. falconensis. The cool-water species G. inflata is absent from this zone. The downhole distribution of this zone is hampered by a rare assemblage immediately below Core 308-U1320A-17X and by no recovery in Cores 20X (lower part) and 21X (169–185 mbsf). Seismic correlation with Site U1319 suggests that hemipelagic intervals with abundant microfossils may occur in this interval of low core recovery (see “Lithostratigraphy”).

Zone W

Zone W, characterized by G. inflata without G. menardii and other warm-water species, can be assigned to core catcher samples from Core 308-U1320A-18X and below. If the missing interval between 169 and 185 mbsf bears a Zone X warm-water assemblage as mentioned above, then Cores 308-U1320A-18X through 20X may still lie in Zone X because the Subzone X4 assemblage is similar to Zone W in having mainly cool-water species, especially G. inflata. Zone W cannot be subdivided because of rare planktonic foraminifers in all samples from Cores 308-U1320A-24X through 33X.

Benthic foraminifers

Benthic foraminifers are rare to common in core catcher samples from Hole U1320A. Preservation in most samples is good to very good except in displaced or reworked specimens. The assemblages include mainly calcareous taxa and only few species and specimens of agglutinated and other taxa. The benthic foraminifers found generally represent well-known neritic to deepwater taxa that prefer oxygen-poor, nutrient-rich environments. Semiquantitative benthic foraminifer data listed in Table T5 enabled the recognition of three assemblages: a mixed assemblage with deep- and shallow-water species resulting from turbidites, a Laticarinina assemblage representing a normal bathyal association, and a Bolivina-Bulimina assemblage preferring low-oxygen, high-nutrient environments.

Mixed assemblage (within Zone Y, latest Pleistocene)

The coexistence of many inner neritic species and typical bathyal forms was found in samples from Cores 308-U1320A-3H through 14X, a result of frequent turbidite deposits (see “Lithostratigraphy”). The inner neritic species are represented by Ammonia beccarii, Elphidium spp., Florilus spp., and Quinqueloculina spp. A. beccarii and most species of Elphidium live mainly in lagoon to inner shelf environments, and their mixing with Laticarinina pauperata, Sphaeroidina bulloides, Cibicidoides spp., and other bathyal species (as well as warm-water planktonic foraminifers from Zone X) indicates downslope transport. Abundant plant debris also confirms a nearshore origin for the major part of the sediment from this interval.

Laticarinina assemblage (late Pleistocene MIS 5)

This is a well-diversified assemblage found only in Sample 308-U1320A-17X-CC, 49–54 cm, characterized by few or common occurrences of L. pauperata, Cibicidoides wuellerstorfi, Uvigerina hispidicostata, Stilostomella lepidula, Cibicidoides spp., Sigmoilopsis schlumbergeri, Pyrgo spp., Sphaeroidina bulloides, Lenticulina spp., and Pullenia spp. The presence of L. pauperata and P. wuellerstorfi indicate middle to lower bathyal paleodepths greater than 1000 m. The absence of shallow-water forms from this sample also confirms a normal lower bathyal setting. Some deepwater species including L. pauperata in Cores 308-U1320A-6H, 7H, 12X, and 13X appears to have been displaced or reworked in the “mixed assemblage” due to turbidites, as described above.

Bolivina-Bulimina assemblage (late Pleistocene MIS 2–4 and MIS 6)

This is the same assemblage found in the lower part of Site U1319 and is characterized by small thin-shelled species including Bolivina spissa, Bolivina spp., Bulimina aculeata, Uvigerina spp., Fursenkoina bradyi, and Chilostomella ovoidea. Other species that may also occur are Quinqueloculina spp., Gyroidina spp., and Oridorsalis tenera. This infauna-dominated assemblage occurs in samples from Cores 308-U1320A-18X through 33X and indicates upper slope to lower bathyal depths greater than 400 m, where oxygen was low, perhaps due to rapid sediment accumulation.

Fluctuations in the relative abundance of such taxa as Bolivina spp., Bulimina spp., and Fursenkoina bradyi, as observed in samples from Cores 308-U1320A-20X through 29X (Fig. F14), may reflect cyclic changes in sedimentation rates and/or bottom circulation.

Age model and sedimentation rates

The age models developed during the expedition are preliminary. Biostratigraphic dating of Pleistocene sediments is notoriously difficult, and we had to rely on several assumptions to constrain age models and sedimentation rates. In the case of Site U1320, we took into consideration biostratigraphic, lithostratigraphic (ash layer event), and magnetostratigraphic tie points (Table T6). The uppermost 140 m at Site U1320 is composed of sandy sediment and thus lacks reliable magnetostratigraphic tie points. The only reliable chronostratigraphic datums obtained are the last occurrence (LO) of G. inflata at 9.29 mbsf and the presence of ash Layer Y8 at 143.42 (Fig. F15; Table T6). The LO of G. flexuosa was identified at 95.59 mbsf (Fig. F15), but it occurs in a turbidite interval and appears to be reworked. We also note a minor inconsistency between the stratigraphic depth of ash Layer Y8 dated at 84 ka and the depth of the FO of the E. huxleyi acme dated at 90 ka; the younger datum is stratigraphically below the older datum. We argue that this minor discrepancy is due to poor calibration of the age of the E. huxleyi acme, and we decided to use the well-calibrated radiochronologic age of the ash layer instead. Linking the LO of G. inflata with ash Layer Y8 in our age model yields an average sedimentation rate from 143.43 mbsf to 9.29 mbsf of 1.81 m/k.y. (Fig. F15). Below 143.42 mbsf, no reliable markers exist. The lower section of lithostratigraphic Unit II and Unit III, from the lower part of Core 308-U1320A-17X to 16X, appears to be condensed when looking at the magnetostratigraphic tie points, in particular MTP7, with respect to the position of the ash layer (Fig. F15; Table T6). Sedimentation rates in this interval could be as low as 0.2 m/k.y. No stratigraphic datums have been recorded below 148.0 mbsf at Site U1320, but since Helicosphaera inversa is consistently absent we assume that the oldest sediment recovered is younger than the LO of H. inversa (150 ka). If we assume the age of the last recovered sediment to be 150 ka, the minimal sedimentation rate between 148.0 and 299.44 mbsf is 2.6 m/k.y. In view of the many uncertainties outlined above, a more rigorous postcruise study is needed to confirm the preliminary interpretations outlined in this paragraph.