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doi:10.2204/iodp.proc.308.103.2006

Biostratigraphy

Calcareous nannofossils and planktonic foraminifers are generally common to abundant and well preserved in samples from Cores 308-U1319A-1H to 4H and become rare further downhole. Benthic foraminifers are few to common with good preservation in Hole U1319A and are overall far less abundant than planktonic foraminifers in the upper 30 m of Hole U1319A.

Sediments recovered from Site U1319 range in age from Holocene to late Pleistocene, spanning approximately the last 150 k.y. No obvious hiatuses were detected. Nannofossil QAZ1 Emiliania huxleyi Acme Zone and QAZ2 Transitional Zone, as well as planktonic foraminifer Zones W, X, Y, and Z were identified (Fig. F17). The rarity of nannofossils and planktonic foraminifers below ~30 mbsf hampered biostratigraphic resolution in this part of the hole.

Benthic foraminifers identified in Hole U1319A are dominated by species mainly living in low-oxygen, nutrient-rich environments. This is consistent with the muddy lithology described as fine-grained turbidites (see “Lithostratigraphy”). The distinction of a “Laticarinina” assemblage in the 20–30 mbsf interval indicates a normal lower bathyal setting.

Sedimentation rates were relatively low in the uppermost Pleistocene (0.97 m/k.y.) to Holocene (0.15 m/k.y.). Pending further stratigraphic precision, sedimentation rates below 24 mbsf (equivalent to the last ~150–90 k.y.) appear to have been much higher (>2.31 m/k.y.).

Calcareous nannofossils

We examined all core catcher samples and one additional sample from interval 308-U1319A-4H-5, 9–13 cm, for calcareous nannofossils. Quantitative nannofossil data and their general distributions are presented in Figures F18 and F19. All samples yielded rare to very abundant nannofossil assemblages. A general reduction in abundance is seen toward the bottom of the hole. The interval from the seafloor to Sample 308-U1319A-4H-5, 84–88 cm, has a very abundant nannofossil assemblage. Downhole from Sample 308-U1319A-4H-CC, the assemblages vary from abundant to rare. Preservation is good to moderately good throughout the section. Samples with poor preservation are typically coarser 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 and contributes <5% of the total abundance. The great majority of reworked species are Cretaceous in age (>99%), and they occur throughout the cored section.

We divided the cored interval into two main zones, the QAZ1 E. huxleyi Acmeand QAZ2 Transitional Zones based on the nannofossil stratigraphic subdivision of Hine and Weaver (1998).

QAZ1 E. huxleyi Acme Zone

We identified this zone in Samples 308-U1319A-1H-CC and 2H-CC based on the dominant abundance of E. huxleyi (approximately >70% of total abundance) (Fig. F18). According to Berggren et al. (1995), the first occurrence datum of E. huxleyi acme is at 90 ka. During this time, Syracosphaera spp. became more abundant in comparison with the lower interval of the hole, where they varied from abundant to common. Reworked Cretaceous assemblages are abundant in this part of the section.

QAZ2 Transitional Zone

This zone was distinguished in Samples 308-U1319A-3H-CC to 18H-CC (Fig. F18). It is characterized by the dominance of G. aperta-G. ericsonii. Together, these two species form >90% of the overall abundance. Basedon the distribution of different groups of nannofossils we distinguished Subzones A, B, and C within the QAZ1 Transitional Zone.

QAZ1 Subzone A

Subzone A is defined based on the presence of G. aperta-G. ericsonii and the absence of E. huxleyi (Fig. F18). The overall abundance of G. aperta-G. ericsonii may be up to 100,000 specimens per 100 fields of view. The G. oceanica-G. caribbeanica complex is common to abundant and forms the second dominant group of the assemblage. Scapholithus fossilis and Umbilicosphaera spp. are more abundant in this unit. Subzone A is defined in Samples 308-U1319A-3H-CC and 4H-5, 9–13 cm. It is the uppermost subzone of the QAZ2 Transitional Zone.

QAZ2 Subzone B

This subzone is distinguished by the common overall abundance of G. aperta-G. ericsonii, and reworked Cretaceous specimens form <50% of the total abundance. Samples 308-U1319A-4H-CC through 9H-CC were assigned to Subzone B.

QAZ2 Subzone C

Subzone C is defined by the dominance of reworked Cretaceous assemblages that form >50% of the total abundance. It is defined in Samples 308-U1319A-10H-CC to 18H-CC. The other common group of the assemblage is G. aperta-G. ericsonii. This subzone has the lowest abundance of in situ species throughout the entire section.

Planktonic foraminifers

The preservation of foraminifers in Hole U1319A is excellent overall. Semiquantitative planktonic foraminifer data are presented in Table T5. Samples from above Section 308-U1319A-4H-5 contain common to abundant planktonic foraminifers, although they are rarer in Sample 2H-CC, 22–24 cm. Samples from Section 308-U1319A-4H-CC and below contain rare planktonic foraminifers.

The planktonic foraminifer assemblage 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. There are traces of many other taxa, such as Globigerinita glutinata, Hastigerina pelagica, and Globigerinella calida.

The Pleistocene zonation of Ericson and Wollin (1968), based upon the presence or absence of the warm-water species G. menardii and allied species for the Gulf of Mexico, and as modified by Kennett and Huddlestun (1972), was applied to the fossil succession in Hole U1319A. Consequently, planktonic foraminifer assemblage Zones W, X, Y, and Z were identified, suggesting that the sediment recovered from Hole U1319A was deposited during the last 180 k.y. (Fig. F20). These results are similar to the findings of Kohl (1985) from Pigmy Basin and of Mallarino et al. (in press) from the western margin of Brazos-Trinity Basin IV.

Zone Z

This zone is represented only by Sample 308-U1319A-1H-CC, 15–20 cm (Fig. F20). The planktonic foraminifer assemblage is characterized by abundant G. menardii and G. ruber and other warm-water species. It can be distinguished from Zone X by the absence of both G. flexuosa and G. inflata. According to Berggren et al. (1995), the last occurrence datum of G. flexuosa was at 68 ka.

Zone Y

A distinct cool-water assemblage underlies the Zone X sample and can be assigned to Zone Y (Sample 308-U1319A-2H-CC, 22–27 cm) (Fig. F20). This assemblage includes mainly G. inflata, G. ruber, G. crassaformis, and G. siphonifera. G. menardii or its allied species (G. tumida and G. flexuosa) were not recorded from this zone. According to Kennett and Huddlestun (1972) and Martin et al. (1990), Zone Y can be divided into Subzones Y1–Y8 and represents MIS 2 to upper MIS 5 (10.5–90 ka). Just above the Zone X/Y boundary at 22.73 mbsf the distinct 2 cm thick ash Layer Y8 bears an age of ~84 ka (Drexler et al., 1980).

Zone X

Zone X was recognized in Samples 308-U1319A-3H-CC, 17–22 cm, 4H-5, 9–13 cm, and 4H-5, 84–88 cm (Fig. F20). It contains abundant warm-water planktonic foraminifers typified by G. menardii, G. tumida, G. flexuosa, G. crassaformis, G. truncatulinoides. G. siphonifera, N. dutertrei, P. obliquiloculata, Orbulina universa, G. ruber, G. sacculifer, G. conglobatus, and G. falconensis. The cool-water species G. inflata is absent from this zone. According to Kennett and Huddlestun (1972) and Martin et al. (1990), Zone X can be divided into Subzones X1–X5 and represents MIS 5c–5e with an age of 90–129 ka (see “Biostratigraphy” in the “Methods” chapter).

Zone W

Zone W is characterized by the presence of G. inflata and the absence of G. menardii and other warm-water species in a less thriving assemblage (Fig. F20). Sample 308-U1319A-5H-CC, 36–41 cm, and remaining samples downhole were assigned to Zone W. According to Kennett and Huddlestun (1972), Zone W can be divided into Subzones W1 and W2 and represents MIS 6 (129–180 ka). Biostratigraphic subdivision of this interval could not be achieved shipboard because planktonic foraminifers were rare.

Benthic foraminifers

The benthic foraminiferal assemblages studied include mainly calcareous taxa and only few species and specimens of agglutinated taxa. The benthic foraminifers generally represent well-known neritic to “deepwater” taxa that prefer oxygen-poor, nutrient-rich environments (Phleger and Parker, 1951a, 1951b; Poag, 1981; Culver and Buzas, 1983; van Morkhoven et al., 1986). Semiquantitative benthic foraminifer data listed in Table T6 enabled recognition of two assemblages: the Laticarinina assemblage, representing a normal bathyal association, and the Bolivina-Bulimina assemblage, representing an assemblage preferring low-oxygen, high-nutrient environments. The relative abundances of selected species are presented in Figure F21.

Benthic foraminifers are moderately abundant in core catcher samples from Hole U1319A, although they are rare in comparison with planktonic foraminifers in Cores 308-U1319A-1H to 3H. Benthic foraminifer abundance drops significantly in samples from Core 308-U1319A-2H and below Core 10H, in which small, thin-shelled species are common. Preservation in most samples is good to very good, showing no signs of dissolution.

Laticarinina assemblage (Holocene MIS 1 and late Pleistocene MIS 5)

This is a well-diversified assemblage recognized in Cores 308-U1319A-1H, 3H, and 4H, characterized by the few or common occurrences of Laticarinina pauperata, Cibicidoides wuellerstorfi, Uvigerina hispidicostata, Stilostomella lepidula, Cibicidoides spp., Sigmoilopsis schlumbergeri, Pyrgo spp., Sphaeroidina bulloides, Lenticulina spp., and Pullenia spp (Fig. F21). Middle to lower bathyal paleodepths are indicated for this assemblage by the presence of L. pauperata and P. wuellerstorfi, generally found in water depths exceeding 1000 m (van Morkhoven et al., 1986).

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

In Cores 308-U1319A-2H and 308-U1319A-5H through 18X, small thin-shelled species including Bolivina spissa, Bolivina spp., Bulimina aculeata, Uvigerina spp., Fursenkoina bradyi, and Chilostomella ovoidea dominate the assemblage (Fig. F21). Other species that may also occur are Quinqueloculina spp., Gyroidina spp., and Oridorsalis tenera. This assemblage indicates upper slope to lower bathyal depths, with the upper limit near 400 m (van Morkhoven et al., 1986). An infaunal habitat preference for most common species from this assemblage indicates low-oxygen, nutrient-rich environments (Poag, 1981). Rapid sediment accumulation causing stress conditions may also nurture similar assemblages.

Fluctuations in the relative abundance of such taxa as Bolivina spp., Bulimina spp. and Fursenkoina bradyi, observed in some of the samples (Fig. F21), may reflect cyclic changes in sediment input and/or circulation.

Age model and sedimentation rates

The age models developed during the expedition are preliminary. Biostratigraphic dating of Pleistocene sediments is difficult, and we had to rely on several assumptions to constrain age models and sedimentation rates. In the case of Site U1319, we took into consideration biostratigraphic, lithostratigraphic (ash layer events), and magnetostratigraphic tie points (Table T7). The magnetostratigraphic tie points were derived by matching the rock magnetic record with a global δ¹⁸O curve (see “Paleomagnetism” in the “Methods” chapter), and they are thus very interpretative. However, at Site U1319 the trend in paleomagnetic tie points matches the trend obtained by linking biostratigraphic with lithostratigraphic datums (Fig. F22). All of the datums are recorded in the uppermost 30 m. The deepest datum at ~30 mbsf is a magnetic tie point with an age of ~122 ka. Fitting one line through all the data points from the uppermost 30 m yields an estimated sedimentation rate for this interval of ~0.2 m/k.y. (Fig. F22). No stratigraphic datums were recorded below 30 mbsf at this site. However, we make the assumption that since no occurrence of Helicosphaera inversa was recorded, the oldest sediment recovered is younger than the last occurrence (LO) of H. inversa (LO = 150 ka). This assumption implies that sediment below 30 mbsf accumulated at a rate of 5 m/k.y. More extensive postcruise work is needed to confirm this interpretation.

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