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doi:10.2204/iodp.proc.308.108.2006

Biostratigraphy

Calcareous nannofossils and foraminifers were studied in all core catcher samples from Holes U1324B and U1324C plus several samples from Cores 308-U1324B-9H, 14H, 19H, and 27H. Rare to common calcareous nannofossils and foraminifers with good to moderate preservation occur in samples above 350 mbsf, with reduced abundance toward the bottom of the holes.

We identified nannofossil Zone QAZ1 Emiliania huxleyi Acme with Subzones A and B, as well as planktonic foraminifer Zones Z and Y (including Subzones Y1–Y5) (Fig. F16). Nannofossil and planktonic foraminifer data indicate that, similar to Site U1322, the sediment sequence recovered at Site U1324 was deposited during the last 60 k.y. However, the average sedimentation rate of ~10 m/k.y., is 2.5 times higher than that for Site U1322. Benthic foraminifers are dominated by infaunal species that prefer low-oxygen “stress” environments, which is expected for such rapid sedimentation rates.

Calcareous nannofossils

Calcareous nannofossils were encountered in all samples from Holes U1324B and U1324C (Figs. F17, F18, F19, F20). Samples 308-U1324B-7H-2, 30 cm, and 27H-1, 13–18 cm, contain abundant nannofossils. Preservation ranges from good to moderate throughout the holes. Coarse-grained samples typically contain poorly preserved nannofossils with low to barren overall abundance. Nannofossils in samples below 238.65 mbsf in Hole U1324B are less abundant, but it is unclear why. Nannoplankton assemblages contain rare to common in situ and reworked species. E. huxleyi is the dominant in situ species. Other species, Braarudosphaera bigelowii, Calcidiscus leptoporus, Coccolithus pelagicus, Discosphaera tubifera, Gephyrocapsa aperta, Gephyrocapsa caribbeanica, Gephyrocapsa ericsonii, Gephyrocapsa oceanica, Gephyrocapsa sinuosa, Helicosphaera carteri, Helicosphaera wallichii, Pontosphaera multipora, Reticulofenestra productella, Rhabdosphaera clavigera, Rhabdosphaera procera, Scapholithus fossilis, Syracosphaera histrica, Thoracosphaera spp., and Umbilicosphaera sibogae were sporadically encountered. G. oceanica is always more abundant than G. caribbeanica, although both rarely constitute 10% of the total abundance. The majority of reworked species are Cretaceous in age (>99%) and occur throughout the section (Figs. F17, F19, F20). Samples 308-U1324B-1H-CC, 19–24 cm, through 7H-CC, 29–34 cm, contain high numbers of other reworked Mesozoic species.

Generally, species abundances vary significantly from sample to sample. This could be due to cyclic fluctuations in sediment input from turbidity currents in Ursa Basin. This is supported by the relationship that we observed: the more abundant the in situ assemblages, the less abundant the reworked Mesozoic assemblages are, and vice versa (Fig. F17). We found that in situ nannofossils are better preserved in hemipelagic deposits, whereas deposits with higher terrigenous content are richer in reworked Mesozoic assemblages. Lower abundances of nannofossils toward the bottom of the holes point to the possibility that sedimentation rates (Subzone B) were very high in comparison to the upper section (Subzone A).

Based on the nannofossil stratigraphic subdivision of Hine and Weaver (1998), we recognized QAZ1 E. huxleyi Acme Zone in the sediment sequence recovered at Site U1324. The zone can be subdivided into Subzones A and B, which correlates well to Holes U1324B and U1324C.

QAZ1 E. huxleyi Acme Zone

We identified this zone in Samples 308-U1324B-1H-CC, 7–12 cm, through 74X-CC, 36–43 cm, based on the high 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. In situ species have sporadic distribution throughout the holes. Reworked Mesozoic assemblages form >50% of the total nannofossil abundance in most samples from Site U1324. They typically have higher values in MTDs (Fig. F18), lithostratigraphic Subunits IB, ID, IF, IIB, and IID (see “Lithostratigraphy”).

Subzone A

We distinguished Subzone A in samples above 352.74 mbsf of Holes U1324B (308-U1324B-1H-CC through 43H-CC) and U1324C (308-U1324C-1H-CC through 6H-CC). This interval is characterized by higher abundances of both in situ and reworked nannofossils relative to the lower section. The in situ species E. huxleyi has a cyclic distribution throughout Subzone A. Subzone A correlates with lithostratigraphic Unit I between seismic Reflectors S10 and S40-1324.

Subzone B

Subzone B, with low abundances of in situ assemblages, was identified in samples below 357.79 mbsf in Holes U1324B (Samples 308-U1324B-44H-CC through 74X-CC) and U1324C (Samples 308-U1324C-7H-CC through 8H-CC). Reworked Mesozoic assemblages are less abundant relative to Subzone A. Two peaks of reworked Mesozoic assemblages are associated with MTDs. Subzone B correlates with lithostratigraphic Unit II between seismic Reflectors S50-1324 and S60-1324.

Planktonic foraminifers

Planktonic foraminifers are frequent to abundant in samples from core catchers and selected cores above Cores 308-U1324B-29H and 308-U1324C-4H (above ~250 mbsf). Further downhole, planktonic foraminifer abundances are rare to trace and even barren in Samples 308-U1324B-48H-CC, 52X-CC, and 71X-CC through 74X-CC. In most samples, the preservation was good to excellent overall, and only a few specimens exhibit abraded features indicative of reworking. Semiquantitative data for planktonic foraminifers in Hole U1324B are presented in Table T6.

As at other Expedition 308 sites, the planktonic foraminifer assemblage found in samples from Site U1324 is dominated by typical subtropical to temperate taxa. The most abundant species is Globigerinoides ruber (both the pink and white forms), averaging 50% or more. Species found frequently include Globigerinoides sacculifer, Globigerinoides conglobatus, Neogloboquadrina dutertrei, Globorotalia truncatulinoides, Globorotalia inflata (older than 10 ka), Globorotalia crassaformis, Globigerinella siphonifera, Orbulina universa, Globigerina falconensis, and Globigerina quinqueloba. Globorotalia menardii and Globorotalia tumida are found only in Sample 308-U1324B-1H-CC, 19–24 cm, whereas Pulleniatina obliquiloculata not only occurs in the same sample, but a single specimen of this species was also observed in three other widely separated samples from the lower part of the core: Samples 308-U1324B-40H-CC, 31–36 cm, 56X-CC, 36–41 cm, and 61X-CC, 37–42 cm (Fig. F21). Other species, including Globigerina bulloides, Globigerinita glutinata, Hastigerina pelagica, and Globigerinella calida, are very rare and may occur sporadically.

Planktonic foraminifer assemblage Zones Z and Y (including Subzones Y1, Y2, Y3, and Y3–Y6?) (Kennett and Huddlestun, 1972) were identified, suggesting that the sediment recovered at Site U1324 was deposited mainly during the last 60 k.y., during marine isotope Stages (MIS) 1–3 (4?) (Fig. F21). The bottom of the core cannot be dated because of the absence of planktonic foraminifers. The last occurrence datum of G. flexuosa at 68 ka and the temporary last occurrence of consistent P. obliquiloculata at ~65 ka (Kennett and Huddlestun, 1972) were not found at Site U1324. Therefore, the sediments recovered at Site U1324 are likely younger than 65 ka. Abundant planktonic foraminifers found in Samples 308-U1324B-10H-CC, 29–34 cm, and 19H-3, 53–58 cm, may have been influenced by warm interstadial events 2.1 and 3.1, respectively.

Zone Z

This zone is represented only by Sample 308-U1324B-1H-CC, 19–24 cm (Fig. F21). The planktonic foraminifer assemblage is characterized by abundant warm-water species including G. menardii, G. sacculifer, G. crassaformis, P. obliquiloculata, and G. ruber. The absence of the cool-water species G. inflata indicates an age younger than 10.5 ka (Kennett and Huddlestun, 1972).

Zone Y

From Cores 308-U1324B-2H through 74X and 308-U1324C-1H through 8H, planktonic foraminifer assemblages are dominated by G. ruber and G. inflata, and the absence of warm-water species G. menardii (Zones X and Z), G. flexuosa (lower Subzone Y6 and below), and P. obliquiloculata (middle Subzone Y6 and below), collectively suggests that the sediment section spans Subzones Y1–Y6(?). Samples from Core 308-U1324B-43H with very rare planktonic foraminifers are grouped together as representing an interval from the lower part of Subzone Y5 to probably Y6 (Fig. F21), approximately equating to lithostratigraphic Unit II (see “Lithostratigraphy”). Because of the semiquantitative nature of our work on board, the subdivision of Zone Y requires further studies to be confirmed or modified.

Subzone Y1

Samples 308-U1324B-2H-CC, 15–20 cm, and 3H-CC, 19–24 cm, having abundant G. ruber and frequent G. crassaformis and G. siphonifera but reduced G. inflata, are assigned to Subzone Y1. According to Kennett and Huddlestun (1972), Subzone Y1 represents a short interval at the MIS 1/2 transition between ~10.5 and 16 ka.

Subzone Y2

Although G. ruber remains frequent, the cool-water species G. inflata becomes frequent to common, especially in the lower part of the Subzone Y2, in Cores 308-U1324B-13H through 16H. G. crassaformis and G. siphonifera are also consistently present and may become frequent only occasionally. Between Cores 308-U1324B-4H and 9H, G. truncatulinoides is very rare or absent. An increase in the number of planktonic foraminifers, especially G. crassaformis, G. falconensis, and G. conglobatus, in Sample 308-U1324B-10H-CC, 29–34 cm, may signal the influence of warmer climatic conditions such as those during MIS 2.21. Subzone Y2 represents deposition during MIS 2 (Kennett and Huddlestun, 1972) and spans the last 16–24 k.y. based on the timescale of Bassinot et al. (1994).

Subzone Y3

Cores 308-U1324B-17H through 29H are assigned to Subzone Y3 based on the consistent occurrence of many planktonic foraminifers, particularly G. sacculifer and G. inflata without G. menardii and its allied species. G. conglobatus, a species preferring warmer water conditions, occurs in three intervals: from Sample 308-U1324B-19H-2, 52–57 cm, through 19H-3, 53–58 cm; 22H-CC, 29–34 cm, through 25H-CC, 36–41 cm; and 28H-CC, 43–48 cm, through 29H-CC, 50–55 cm. Dominated by subtropical species, the planktonic foraminifer assemblage from Sample 308-U1324B-25H-CC, 36–41 cm, shows all the characteristics of warm conditions within Subzone Y3 (Fig. F21; Table T7). Subzone Y3 represents deposition during the later part of MIS 3 (Kennett and Huddlestun, 1972) and has an age of 24–42 ka based on the timescale of Bassinot et al. (1994).

Subzone Y4

Subzone Y4, characterized by frequent G. inflata and G. falconensis, spans Samples 308-U1324B-30H-CC, 19–24 cm, through 38H-CC, 40–45 cm. G. sacculifer and G. crassaformis are rare, and G. conglobatus is absent. The upper part of Subzone Y4 corresponds to lithostratigraphic Subunit ID (see “Lithostratigraphy”). Subzone Y4 represents deposition in the middle part of MIS 3 (Kennett and Huddlestun, 1972), which lasted from ~42 to 48 ka on the timescale of Bassinot et al. (1994).

Subzone Y5

Samples 308-U1324B-39H-CC, 40–45 cm, through 42H-CC, 31–36 cm, with frequent G. sacculifer and G. crassaformis, can be assigned to Subzone Y5. Other species including G. ruber, G. sacculifer, G. siphonifera, G. inflata, and N. dutertrei are frequent or common. However, rare G. conglobatus occurs only in Sample 308-U1324B-39H-CC, 40–45 cm, whereas a single specimen of P. obliquiloculata is found in Sample 40H-CC, 31–36 cm. Overall, the planktonic foraminifer assemblage from Subzone Y5 supports interpretation of a gradually warming trend across the MIS 3/4 boundary (Kennett and Huddlestun, 1972) at ~48–57 ka (Bassinot et al., 1994).

Subzone Y5–Y6?

Planktonic foraminifers are very rare or barren in samples from Cores 308-U1324B-43H through 74X in lithostratigraphic Unit II, characterized by interbedded thin silt and mud layers and MTDs (see “Lithostratigraphy”). The interval is collectively assigned to Subzone Y5–Y6? pending further studies. The absence of G. menardii (Zone X, 85 ka and older) and G. flexuosa (lower Subzone Y6, 68 ka and older) suggests that the sediment section must be younger than 68 ka. The temporary last occurrence of consistent P. obliquiloculata was ~65 k.y. ago, in Subzone Y6 (Kennett and Huddlestun, 1972; Mallarino et al., in press). Solid evidence of this event was not found at Site U1324, but sporadic P. obliquiloculata occurs in Samples 308-U1324B-56X-CC, 36–41 cm, and 61X-CC, 37–42 cm (as well as 40H-CC, 31–36 cm, mentioned above) and may bear an age close to 65 ka, although we cannot rule out that these single specimens were reworked.

Benthic foraminifers

Benthic foraminifers are rare to common in core catcher samples from Cores 308-U1324B-1H through 43H and 308-U1324C-1H through 6H, above ~350 mbsf, but are very rare or barren further downhole (Fig. F22). Preservation in most samples varies from good to very good except for displaced or reworked specimens. The assemblages include mainly calcareous taxa and only few species and specimens of porcelaneous taxa. The benthic foraminifers generally represent well-known neritic to “deepwater” taxa that prefer oxygen-poor, nutrient-rich environments. The Bolivina-Bulimina assemblage, which was described at Sites U1319 and U1320, is also found at Site U1324 (above 330 mbsf). A Valvulineria assemblage characterized by common occurrences of Valvulineria bradyana is newly identified in Sample 308-U1324B-40H-CC, 31–36 cm. Below 350 mbsf, no assemblages can be recognized because few or no benthic foraminifers are present. Rare Ammonia beccarri, representing reworked inner neritic species, only occurs in Samples 308-U1324B-17H-CC, 33–38 cm, and 18H-CC, 31–36 cm. Semiquantitative data of benthic foraminifers from Hole U1324B are listed in Table T6.

Bolivina-Bulimina assemblage

The Bolivina-Bulimina assemblage was recovered from Cores 308-U1324B-1H through 39H and 308-U1324C-1H through 6H. As already described from Sites U1319 and U1320, this assemblage is characterized by abundant small thin-shelled species including Bolivina spissa, Bolivina spp., Bulimina aculeata, Uvigerina spp., Fursenkoina bradyi, Stainforthia complanata, and Chilostomella ovoidea. Other species that also occur are Globobulimina affinis, Quinqueloculina spp., Gyroidina spp., Cibicidoides spp., Sphaeroidina bulloides, and Oridorsalis tenera. This infauna-dominated assemblage indicates upper slope to lower bathyal depths greater than 400 m with low oxygen content. Fluctuations in the relative abundance of these species, especially above 260 mbsf (Fig. F22), may reflect cyclic changes in sediment load and bottom water circulation.

Valvulineria assemblage

We recognized the Valvulineria assemblage in Sample 308-U1324B-40H-CC, 31–36 cm. Although species of the Bolivina-Bulimina assemblage remain frequent, the common occurrence of Valvulineria bradyana in Sample 308-U1324B-40H-CC, 31–36 cm, warrants a separation. Like many infaunal Bolivina and Bulimina species, the epifaunal Valvulineria spp. are also stress markers (van Hinsbergen et al., 2005), but the latter's epifaunal living mode suggests that the stress conditions were probably not caused by fast sediment loading and low oxygen content, as indicated by infaunal species, but were more likely a result of circulation change in the later part of Subzone Y5, across the MIS 3/4 boundary, as described above.

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 U1324, we took into consideration planktonic foraminifer biostratigraphic data and some magnetostratigraphic tie points (Table T8; Fig. F23). 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 interpretive. The biostratigraphic age constraints are mainly derived from the modified ages for the boundaries between planktonic foraminifer subzones according to the timescale of Bassinot et al. (1994) (see “Biostratigraphy” in the “Methods” chapter). The estimated sedimentation rates (Fig. F23) are 0.86 m/k.y. between the seafloor and 8.65 mbsf (last occurrence [LO] of G. inflata), 10.0 m/k.y. between 8.65 and 163.0 mbsf (Subzone Y2/Y3 boundary), 5.5 m/k.y. between 163.0 and 252.0 mbsf (Subzone Y3/Y4 boundary), and 12.0 m/k.y. between 252.0 and 323.0 mbsf (Subzone Y4/Y5 boundary). The interval between 163.0 and 252.0 mbsf corresponds to lithostratigraphic Unit I, a unit with few MTDs. This can explain the comparatively low sedimentation rates for this interval (5.5 m/k.y.). Below 323.0 mbsf, no biostratigraphic datum was recovered. However, considering that we did not retrieve P. obliquiloculata specimens, we infer that the oldest sediment recovered is younger than the LO of this species (Subzone Y6, ~65 ka for the LO of consistent P. obliquiloculata [Kennett and Huddlestun, 1972; Mallarino et al., in press]). Thus, assuming that the oldest sediment recovered is 65 ka, we estimate a sedimentation rate >25 m/k.y., but this could be even greater (up to 50 m/k.y.) if we assumed that the sediment was deposited starting in Subzone Y6. We note that magnetic tie points 3 (10 mbsf; 26 ka) and 4 (470 mbsf; 52 ka) are in relative good agreement with the proposed age model (Table T8; Fig. F23). More rigorous postcruise work is needed to confirm this interpretation.

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