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doi:10.2204/iodp.proc.308.106.2006

Biostratigraphy

We studied calcareous nannofossils and foraminifers in all core catcher samples from Hole U1322B, one sample from Section 308-U1322C-1H-CC, and three core catcher samples from Hole U1322D plus additional samples from Cores 308-U1322B-1H and 15H. Calcareous nannofossils and foraminifers are rare to very abundant with good to moderate preservation in most samples, with reduced abundance toward the bottom of the holes.

Nannofossil Zone QAZ1 Emiliania huxleyi Acme with Subzones A (including intervals A1 through A5) and B was identified, as well as planktonic foraminifer Zones Z and Y (including Subzones Y1–Y6) (Fig. F16). Nannofossil and planktonic foraminifer data indicate that, similar to Site U1324, the sediment sequence recovered at Site U1322 was essentially deposited during the last 60 k.y., with an average sedimentation rate of ~4 m/k.y. The encountered nannofossil and planktonic foraminifer zones correlate well with those at Site U1324. Several benthic foraminifer assemblages were recognized: Cibicidoides-Globobulimina, Bolivina-Bulimina, Bulimina-Globobulimina, and Nonionella-Epistominella. The predominance of infaunal benthic foraminifers suggests low-oxygenated “stress” environments due to rapid sedimentation. The deltaic Nonionella-Epistominella assemblage, containing many reworked inner neritic species from the lower part of the section, represents downslope transport from the Mississippi Delta.

Calcareous nannofossils

Calcareous nannofossils were encountered in all samples from Holes U1322B, U1322C, and U1322D (Figs. F17, F18, F19). Nannofossils are extremely abundant in Sample 308-U1322B-7H-1, 100 cm, abundant in Sample 3H-CC, 6–16 cm, and common in other samples above 201.68 mbsf. Nannofossils in samples below 201.68 mbsf in Hole U1322B are rare, which may be related to MTDs in this part of the section. Preservation in the clays and muds ranges from good to moderate throughout the drilled holes. Coarse-grained samples typically contain poorly preserved nannofossils with low to barren overall abundance. Nannofossil assemblages contain rare to common in situ and reworked species. E. huxleyi is the most dominant in situ species. Other species, Calcidiscus leptoporus, Ceratolithus cristatus, Ceratolithus telesmus, Discosphaera tubifera, Florosphaera profunda, Gephyrocapsa caribbeanica, Gephyrocapsa oceanica, Helicosphaera carteri, Helicosphaera wallichii, Pontosphaera multipora, Reticulofenestra productella, Rhabdosphaera clavigera, Rhabdosphaera procera, Scapholithus fossilis, Syracosphaera histrica, Thoracosphaera heimii, Thoracosphaera saxea, Umbilicosphaera sibogae, Braarudosphaera bigelowii, and Coccolithus pelagicus, were sporadically encountered. G. oceanica is typically more abundant than G. caribbeanica, although both rarely constitute ~20% of the total abundance. The reworked species are Cretaceous in age and occur throughout the section (Figs. F17, F18). Samples 308-U1322B-2H-CC through 7H-CC contain a persistent high number of reworked Mesozoic species.

Similar to Site U1324, species abundances vary significantly from sample to sample. This might be due to cyclic fluctuations in sediment input from the Mississippi River. In general, the more abundant the in situ assemblages become, the less the reworked Mesozoic assemblages are found, and vice versa (Fig. F17). We found that hemipelagic environments preserved more in situ nannofossils, whereas environments containing more terrigenous material resulted in less preserved nannofossils and increased reworked Mesozoic assemblages.

Based on the nannofossil stratigraphic subdivision of Hine and Weaver (1998), we recognized the QAZ1 E. huxleyi Acme Zone at Site U1322. The zone was subdivided into Subzones A and B. Nannofossil intervals A1–A5 are distinguished (Fig. F20).

QAZ1 E. huxleyi Acme Zone

We identified this zone in Samples 308-U1322B-1H-CC through 29H-CC, based on the dominant abundance of E. huxleyi (50% 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 a significant percentage of the total nannofossil abundance in most samples from Site U1322. They typically have higher values in deformed zones (Fig. F17), for example, lithostratigraphic Subunit ID below 210.33 mbsf (see “Lithostratigraphy”).

Subzone A

We distinguished Subzone A in samples from above 201.68 mbsf in Holes U1322B (Samples 308-U1322B-1H-CC through 24H-CC), U1322C (Sample 308-U1322C-1H-CC), and U1322D (Samples 308-U1322D-1H-CC through 3H-CC). This interval is characterized by higher abundances of in situ and reworked nannofossils relative to Subzone B. The in situ species E. huxleyi shows cyclic distribution throughout Subzone A. It correlates with lithostratigraphic Unit I and the upper part of Unit II, above seismic Reflector S50-1322 (Fig. F16). Subzone A at Site U1322 corresponds to Subzone A at Site U1324. Within Subzone A, five intervals, A1–A5, were distinguished based on abundance variations in nannofossils. These intervals provide additional datums for correlation between Sites U1322 and U1324 (Fig. F20).

• Interval A1 is distinguished in the uppermost part of the drilled section (Sections 308-U1322B-1H-CC and 308-U1322C-1H-CC). It is characterized by nannofossil ooze dominated by E. huxleyi and relatively few reworked Mesozoic species (<500 specimens per 100 fields of view).
• Interval A2 is identified between 13.81 and 61.26 mbsf (Sections 308-U1322B-2H-CC through 7H-CC and 308-U1322D-2H-CC). It is characterized by abundant reworked Mesozoic species with >500 specimens per 100 fields of view. E. huxleyi is abundant in the upper part of Interval A2 and becomes rare toward the bottom.
• Interval A3 lies between 61.26 and 125.75 mbsf (Sections 308-U1322B-8H-CC through 15H-1, 95–99 cm, and 308-U1322D-3H-CC). The main features of Interval A3 are fewer (<500 specimens per 100 fields of view) and variable reworked Mesozoic species relative to Interval A2 and cyclic distribution of E. huxleyi varying from barren to abundant. At the very bottom of Interval A3, reworked Mesozoic species become frequent (500–1000 specimens per 100 fields of view). At the base of Interval A3, the in situ and reworked Mesozoic species both show their lowest abundance values.
• Interval A4 is a short section between 125.75 and 142.03 mbsf (Sections 308-U1322B-15H-CC and 16H-CC). It is defined between the lowest total abundance and the maxima in a relative increase of the G. caribbeanica-G. oceanica group. Interval A4 has higher values of in situ nannofossil species relative to reworked Mesozoic species.
• Interval A5 is between 142.03 and 201.68 mbsf (Sections 308-U1322B-17H-CC through 24H-CC). The top boundary of Interval A5 is the top of the relative increase of the G. caribbeanica-G. oceanica group, and its base is marked by a decrease of in situ nannofossils. In situ nannofossils within Interval A5 always form >50% of the total abundance.

Subzone B

Subzone B, containing low abundances of in situ assemblages, was identified in samples below 210.68 mbsf in Hole U1322B (Sections 308-U1322B-25H-CC through 29H-CC). Sudden decreases of in situ nannofossils and increases in reworked Mesozoic assemblages are the main characteristics of this subzone, and two peaks of reworked Mesozoic assemblages are associated with MTDs. Subzone B correlates with the lower part of lithostratigraphic Unit II between seismic Reflectors S50-1322 and S60-1322. Subzone B at Site U1322 can be correlated with the upper part of Subzone B at Site U1324 (Fig. F20).

Correlation of the distinguished nannofossil subzones and intervals between Sites U1322 and U1324 is presented in Figure F20. Sediments at Site U1322 above 142.03 mbsf correlate well with sediments from Site U1324 above 162.00 mbsf. This confirms the fact that the upper part of Site U1322 was deposited at a similar sedimentation rate as sediments at Site U1324 above 162.00 mbsf. Sediments below 142.03 mbsf at Site U1322 are condensed relative to those below 162.00 mbsf at Site U1324, making correlation difficult.

Planktonic foraminifers

Planktonic foraminifers are rare to abundant in core catcher samples from Hole U1322B. In Cores 308-U1322B-5H through 13H and 22H through 29H, interpreted as an MTD, planktonic foraminifers are less abundant. In most samples, the preservation of planktonic foraminifers was good to excellent overall, and only few specimens exhibit abrasion features indicative of reworking. Semiquantitative data of planktonic foraminifers from Hole U1322B are presented in Table T8.

As at the other Expedition 308 sites, the planktonic foraminifer assemblage found in samples from Site U1322 is dominated by Globigerinoides ruber (both the pink and white forms) and other subtropical to temperate taxa. Warm-water species Globorotalia menardii and its associated forms occur only in Sample 308-U1322B-1H-CC, 5–10 cm, indicating Holocene Zone Z (Fig. F21). The absence of these species downhole suggests that the sediment section between Cores 308-U1322B-2H and 29H belongs to Zone Y, which is younger than 90 ka (see “Biostratigraphy” in the “Methods” chapter). The last occurrence (LO) datum of Globorotalia flexuosa at 68 ka (Joyce et al., 1990), which marks the base of the Blue Unit (Winker and Booth, 2000), was not found. Several specimens of Pulleniatina obliquiloculata found in Cores 308-U1322B-17H through 19H may represent either in situ deposition from warm spills or reworking from the previous warm interglacial MIS 5. However, the suitability of these specimens to mark the temporary LO of P. obliquiloculata at ~65 ka (Kennett and Huddlestun, 1972; Mallarino et al., in press) is doubtful because these specimens are not concurrent with G. menardii in these samples or samples further downsection to indicate near Zone X deposition. Therefore, the absence of these key datum levels suggests that the sediment sequence recovered from Holes U1322B and U1322C was likely all younger than 60 ka, which is in good agreement with the 68 ka age of the base of the Blue Unit determined by Winker and Booth (2000).

Zone Z

This zone is represented only by Sample 308-1322B-1H-CC, 5–10 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 cool-water species G. inflata present in this interval appears to have been reworked, as the species became absent from the Gulf of Mexico about 10.5 k.y. ago and rarely concurred with G. menardii in younger intervals (Kennett and Huddlestun, 1972).

Zone Y

From Cores 308-U1322B-2H through 29H, planktonic foraminifer assemblages are dominated by G. ruber and G. inflata and occasionally G. falconensis and G. crassaformis. The absence of warm-water species such as G. menardii (Zones X and Z) and G. flexuosa (lower Subzone Y6 and below) suggests that the sediment section spans Subzones Y1 through probably the later part of Y6 (Fig. F21). Together with abundance variations of species, these arrays of evidence helped the recognition of Subzones Y1, Y2, Y3–Y5, and Y5–Y6? (Fig. F21).

Subzone Y1

Samples 308-U1322B-2H-CC, 17–27 cm, and 3H-CC, 6–16 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 an interval of meltwater deposition at the MIS 1/2 transition at ~10.5–16 ka.

Subzone Y2

Subzone Y2 spans Cores 308-U1322B-4H through 14H, or lithostratigraphic Subunits IB–ID, dominated by MTDs (see “Lithostratigraphy”). Subzone Y2 is characterized by increased abundance of the cool-water species G. inflata, especially from the upper and lower parts of the subzone, respectively, in Samples 308-U1322B-4H-CC, 29–34 cm, 5H-CC, 24–29 cm, and 12H-CC, 28–33 cm, through 14H-CC, 43–48 cm. G. crassaformis and G. siphonifera are also consistently present and may occasionally become frequent. Between Cores 308-U1322B-4H and 8H, G. sacculifer is virtually absent, suggesting a period of severe cooling during MIS 2. According to Kennett and Huddlestun (1972), Subzone Y2 represents deposition during MIS 2, which lasted from ~24 to 16 ka (Bassinot et al., 1994).

Subzones Y3–Y5

The consistent occurrence of G. sacculifer, G. ruber, G. crassaformis, G. falconensis, and G. inflata without G. menardii and its associated species in Cores 308-U1322B-15H through 21H is considered to represent Subzones Y3–Y5. Because these subzones could not be separated using the data collected on board, the completeness of these subzones is not clear. Several specimens of the warm-water species P. obliquiloculata occur in three samples: 308-1322B-17H-CC, 40–45 cm, 18H-CC, 31–36 cm, and 19H-CC, 28–33 cm. They appear to represent in situ deposition during warm spills rather than reworking from earlier interglacials because G. menardii and other species commonly associated with P. obliquiloculata in Zone X and earlier warm intervals are missing. It is noteworthy that the samples yielding these specimens are not from the deformed intervals in the upper part of lithostratigraphic Unit II (Fig. F21) (see “Lithostratigraphy”). According to Kennett and Huddlestun (1972), Subzones Y3, Y4, and Y5 represent deposition during MIS 3 at ~57–24 ka based on the timescale of Bassinot et al. (1994).

Subzone Y5/Y6?

Planktonic foraminifers are rare in samples from Cores 308-U1322B-22H through 29H, from seismic Reflector S40-1322 to the bottom of Hole U1322B in the lower part of lithostratigraphic Unit II characterized by 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 Zone 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 at ~65 ka in Subzone Y6 (Kennett and Huddlestun, 1972; Mallarino et al., in press). Several specimens of P. obliquiloculata found in three samples from Subzones Y3–Y5 (as mentioned above) may indicate an age close to 65 ka. However, the absence of such associated species as G. menardii and G. flexuosa from the Y3–Y5 interval, as well as the absence of all the three species from this Y5/Y6 interval, suggests an age younger than 65 ka for the lower sediment section at Site U1322. According to Winker and Shipp (2002), the G. flexuosa datum lies close to the base of the Blue Unit, which is ~200 m below the bottom of Holes U1322B and U1324B (see “Background and objectives” and “Lithostratigraphy” and also “Background and objectives” and “Lithostratigraphy” in the “Site 1324” chapter).

Benthic foraminifers

Benthic foraminifers are rare to abundant in core catcher samples from Hole U1322B, becoming more abundant than planktonic foraminifers in Cores 308-U1322B-23H through 27H (Fig. F22). Preservation of benthic foraminifers in most samples varies from good to very good except for displaced or reworked specimens. The assemblages include many calcareous taxa and a few porcellaneous and agglutinated species. Species with an infaunal living mode are dominant, especially those preferring oxygen-poor, nutrient-rich environments. The Bolivina-Bulimina assemblage, which was described at Sites U1319 and U1320, is also found at Site U1322 (above 330 mbsf). A Bulimina-Globobulimina assemblage, including the two subassemblages Cibicidoides-Globobulimina and Bulimina-Globobulimina and a Nonionella-Epistominella assemblage similar to those found in present-day sediments from the lower Mississippi Delta and upper slope are also recognized. Semiquantitative data of benthic foraminifers from Hole U1322B are listed in Table T9.

Bolivina-Bulimina assemblage

The Bolivina-Bulimina assemblage was recognized in samples from Cores 308-U1322B-4H through 14H. 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, Fursenkoina bradyi, Cassidulina laevigata, Stainforthia complanata, and Chilostomella ovoidea. Epifaunal species such as Gyroidina spp. and Cibicidoides spp. are extremely rare or absent. This infauna-dominated assemblage indicates upper slope to lower bathyal depths greater than 400 m with low oxygen content due to rapid sediment loading. The predominance of Bolivina and Bulimina between 90 and 125 mbsf corresponds to the deformed section in lithostratigraphic Subunit ID (Fig. F22) (see “Lithostratigraphy”).

Bulimina-Globobulimina assemblage

This assemblage is characterized by common occurrences of many species found in the Bolivina-Bulimina assemblage plus Globobulimina affinis and Uvigerina hispidocostata. The large thin-shelled G. affinis is very distinctive. The assemblage can be divided into two subassemblages: Bulimina-Globobulimina and Cibicidoides-Globobulimina, as detailed below.

The Bulimina-Globobulimina subassemblage is found in samples from Cores 308-U1322B-15H through 21H. Apart from those described above, rare to frequent Bulimina mexicana, Oridosalis spp., Valvulineria bradyana, and F. bradyi may also be present. This subassemblage represents a benthic foraminifer association under lower-stress environments than the Bolivina-Bulimina assemblage.

The Cibicidoides-Globobulimina subassemblage spans Cores 308-U1322B-3H through 5H. Unlike the Bulimina-Globobulimina subassemblage, this subassemblage contains more frequent epifaunal taxa including Cibicidoides, Oridosalis, Gyroidina, and Hoeglundina, but Fursenkoina is virtually absent. It represents a transitional benthic foraminifer association toward a normal bathyal environment.

Nonionella-Epistominella assemblage

We recognized the Nonionella-Epistominella assemblage in samples from Cores 308-U1322B-22H through 29H, characterized by numerous tiny Nonionella basiloba and Epistominella exigua with a test ~100 µm across. Small-sized Bolivina, Stainforthia, and Cassidulina and large-sized Lenticulina range from rare to frequent in this assemblage. Although relatively rare, reworked Ammonia beccarii, Elphidium spp., and Quinqueloculina spp. from inner neritic settings occur throughout the interval. Similar assemblages exist today along the outer margin of the Mississippi Delta, close to the shelf edge (Poag, 1981). The presence of this assemblage from the bottom part of Hole U1322B suggests strong downslope transport, probably related to the Southwest Pass Canyon or sediment gravity flows directly from the north during MIS 4 (see “Background and objectives”).

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 U1322, we took into consideration planktonic foraminifer biostratigraphic data only (Table T10; Fig. F23). The age constraints are mainly derived from the modified ages for the boundaries between planktonic foraminifer subzones according to Bassinot et al. (1994) (see “Biostratigraphy” in the “Methods” chapter). The estimated sedimentation rates (Fig. F23) are 0.89 m/k.y. between the seafloor and 13.81 mbsf (last occurrence [LO] of G. inflata), 3.2 m/k.y. between 13.81 and 32.92 mbsf (Subzone Y2/Y3 boundary), and 1.8 m/k.y. between 125.75 and 189.0 mbsf (Subzone Y5/Y6 boundary). The interval between 32.92 and 125.75 corresponds to the lower part of lithostratigraphic Subunit IA–ID, which is characterized by frequent MTDs. This can explain the extremely high sedimentation rates in this interval (~12 m/k.y.). Below 189.0 mbsf, no biostratigraphic datum was recovered. However, considering that we did not retrieve P. Obliquiloculata and G. flexuosa specimens, we infer that the oldest sediment recovered is younger than the LO of these two species (Subzone Y6; ~65 ka for the LO of consistent P. obliquiloculata [Kennett and Huddlestun, 1972; Mallarino et al., in press] and 68 ka for the LO of G. flexuosa [Joyce et al., 1990]). According to Winker and Booth (2000), the LO of G. flexuosa lies close to the base of the Blue Unit, which is ~200 m below the bottom of Hole U1322B (see “Lithostratigraphy”). Thus, assuming that the oldest sediment recovered in Hole U1322B is ~60 ka or older, we estimate a sedimentation rate between 189.0 and 234.46 mbsf of 16 m/k.y. or greater. More rigorous postcruise work is needed to confirm this interpretation.

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