Hole 1167A was drilled on the Prydz Bay Trough Mouth Fan in order to penetrate a late Miocene and younger sequence and elucidate the history of the advance and retreat of the ice sheet to and from the Antarctic continental shelf edge.
Foraminifers are the only consistently present microfossil in Hole 1167A. The dominance of Neogloboquadrina pachyderma (Ehrenberg) suggests that the section is late Miocene or younger in age (N. pachyderma Zone or AN7 of Berggren et al., 1995). The foraminifers present in the diamictons indicate that outer continental shelf faunas are being recycled. Clay-rich horizons contain in situ mid-bathyal faunas.
Diatoms indicate that Core 188-1167A-1H is <0.66 Ma (Thalassiosira lentiginosa Zone) in age. Calcareous nannofossils in Sample 188-1167A-5H-3, 35-36 cm, are likely to be mid-Pleistocene in age (Zones CN12-14a of Samtleben, 1980), and those in Sample 188-1167A-25X-CC, 22-23 cm, are early to mid-Pleistocene in age (Zone CN13b).
Biostratigraphic results from Hole 1167A are summarized in Figure F20.
N. pachyderma (Ehrenberg) dominates all foraminferal assemblages recovered from Hole 1167A. Above 217 mbsf, all samples other than Sample 188-1167A-1H-CC contain some foraminifers; below Sample 188-1167A-25X-CC, a high proportion of samples are barren.
Two distinct associations of foraminifers are present in Hole 1167A. The most common association is with very coarse poorly sorted sandstone dominated by angular terrigenous debris. Samples from these intervals yield few foraminifers, but over many samples a diverse array emerges. These faunas are dominated by N. pachyderma (Ehrenberg) with one or two subordinate planktonic species. Higher in the hole, benthics comprise a few percent of each sample. Benthic shelf calcareous species, especially cassidulinids but with a few buliminid species, indicate an infaunal element. Sediments barren of foraminifers could represent times of reduced habitat availability and lower sea level (glacial maxima), and sections with greater foraminiferal content could indicate greater habitat availability during times of low ice cover and higher sea level (glacial minima). Glacial-interglacial variations in the carbonate compensation depth (CCD) may have affected carbonate preservation at Site 1167.
The second association occurs in the few gray clay samples. This lithology yields abundant (99%) planktonic foraminifers dominated by N. pachyderma. Benthic forms are rare but more diverse and typical of the bathyal or slope environment. The presence of echinoid spines suggests that the seafloor supported a more diverse fauna than in other samples.
The two associations represent fundamentally different environments of deposition consistent with the hypothesis of (1) periodic influx of shelf sediment and (2) pelagic conditions when the shelf sediment influx is reduced. In the former association, benthic species represent outer continental shelf conditions (their source, but redeposited over the shelf edge). In the second case, the calcareous biogenic component of the sediment is higher and represents an in situ mid- to upper bathyal fauna. It is possible that the variation in numbers of planktonic species is a proxy for the difference in rate of influx of sediment. This assumes that the rate of production of planktonic species is roughly constant.
Other than the transport of continental shelf faunas to the slope, there is little evidence of reworking. A little glauconite is present in Samples 188-1167A-5H-CC, 12X-CC, and 27X-CC. Sample 188-1167A-1H-CC was notably different, being reddish in color and yielding sand with a high content of iron oxide-coated grains. It is the only sample examined above Core 188-1167A-25X that is barren of foraminifers.
Samples 188-1167A-5H-3, 34-35 cm, and 25X-CC, 22-23 cm, contain abundant and well-preserved foraminifers; these assemblages are dominantly planktonic (to ~6000 per sample) and contain a diverse, mid-bathyal benthic fauna that is very different from that in other samples from Hole 1167A.
The modern CCD at the continental shelf edge of Prydz Bay is at ~1500 m (Quilty, 1985); thus, dissolution was expected to be pervasive at Site 1167. However, dissolution effects are not obvious until Sample 188-1167A-25X-CC, 22-23 cm. Here, both foraminifers and ostracods show evidence of dissolution even though abundance is still relatively high. Ostracods are represented by a few valve rinds and foraminifers by partial dissolution of layers of the thick tests of N. pachyderma. In Sample 188-1167A-40X-CC, foraminifers are absent but there is a single pyrite pseudomorph after a species of Globigerina to indicate that some planktonic specimens were present but have subsequently been dissolved. Sample 188-1167A-25X-CC yielded no foraminifers but contains a few pyrite pseudomorphs that may represent infilling of benthic tests. Calcite dissolution may be diagenetic or CCD related.
Using Table T4 for guidance, three intervals can be roughly delineated on the basis of foraminiferal abundance. Samples 188-1167A-2H-CC through 19X-CC yielded faunas that are poor to good, with the exception of the excellent preservation of a bathyal fauna in Sample 188-1167A-5H-3, 34-36 cm. Samples 188-1167A-20X-CC through 31X-CC are barren or contain very poor faunas similar to the mid-bathyal fauna of Sample 188-1167A-25X-CC, 22-23 cm. The interval 188-1167A-32X-CC through 49X-CC contains no excellent samples but provided enough specimens for Sr dating.
N. pachyderma is the dominant planktonic species throughout Hole 1167A down to Sample 188-1167A-48X-CC, consistent with an age of late Miocene or younger (N. pachyderma Zone or AN7 of Berggren, 1992, and Berggren et al., 1995). The Pliocene-lowermost Pleistocene interval rich in other species such as Globorotalia puncticulata (Deshayes) recognized in Hole 1165B could not be identified in Hole 1167A. This may suggest that drilling in Hole 1167A did not reach this stratigraphic level. Although not recorded by Berggren (1992) as a zonal fauna, it is present in Sites 747 (Ocean Drilling Program [ODP] Leg 120) and 1165 and would be expected at Site 1167.
Samples 188-1167A-5H-3, 34-36 cm, and 25X-CC, 22-23 cm, are particularly noteworthy. In contrast to other samples in this interval, these samples yielded rich faunas dominated by N. pachyderma but with one or two other planktonic species well represented (with a total planktonic component well over 98% of the total foraminifer fauna). Samples have been set aside for Sr dating and for detailed taxonomic study of a Globorotalia (Tenuitella) sp. that is common in the fauna. It also has a small benthic component of environmental significance. These assemblages are characteristic of the slope faunas.
Specimens of N. pachyderma in Sample 188-1167A-37X-CC are unusual in being very small and compact, in contrast to samples above this level, which are large, less compact, and generally more abundant. N. pachyderma is, as expected, almost 100% sinistrally coiled, although a few dextral specimens were seen (e.g., in Sample 188-1167A-19X-CC).
Eight samples with adequate numbers of N. pachyderma were set aside for postcruise Sr dating. It is expected that further samples will be obtainable when all samples have been processed. There is ample material in most samples for oxygen/carbon isotope studies.
The benthic component of Hole 1167A samples is dominated by species of Globocassidulina and common buliminids and is considered to be an allochthonous fauna. This is the only Leg 188 section to contain evidence of a significant infauna, perhaps reflecting high nutrient conditions near a zone of upwelling. Upwelling here may simply be a function of the flow of Circumpolar Deep Water into Prydz Bay from the deeper ocean. These faunas probably represent outer shelf faunas that have been moved downslope.
Globocassidulina biora (Crespin) is a very characteristic Antarctic species; its first occurrence (FO) may, when documented fully, provide a useful chronostratigraphic marker. From preliminary observations, the FO of this taxon is noted in Sample 188-1167A-19X-CC.
Samples 188-1167A-5H-3, 34-36 cm, and 25X-CC, 22-23 cm, contain faunas that, although constituting <1% of the total foraminiferal fauna, yield forms not seen elsewhere during Leg 188. This assemblage is not a shelf fauna and includes Planulina wuellerstorfi (Schwager). The contrast between this fauna and the globocassidulinid-dominated shelf faunas is very marked and suggests that this fauna is in situ and not a product of transport from shallower depths.
Sample 188-1167A-8H-CC provided a well-preserved fauna with evidence (ostracods and echinoid spines) of a diverse fauna on the seafloor.
Only a few samples examined from Cores 188-1167A-1H through 49X contained calcareous nannofossils. Sample 188-1167A-5H-3, 35-36 cm, was taken from a fine-grained muddy interval at the base of a sandy diamictite. The sample contains few moderately to well-preserved nannofossils, including Gephyrocapsa sp. morphotypes with central bars not preserved. One specimen of Gephyrocapsa ericsonii was noted with a bar intact, and a few small Pseudoemiliania lacunosa were noted. The assemblage also included rare Calcidiscus leptoporus and few fragments of the calcareous dinocyst, Thoracosphaera. A reworked specimen of the Late Cretaceous species Gartnerago obliquum was observed, consistent with the suggestion by Quilty et al. (1999) of the presence of uppermost Cretaceous marine sediments in the region.
The presence of P. lacunosa along with Gephyrocapsa places this sample in nannofossil Zones CN12-CN14a of late Pliocene to mid-Pleistocene age (Fig. F20); however, the small G. ericsonii morphotypes probably indicate a younger mid-Pleistocene age (Zone 14a?) (Samtleben, 1980).
Within Core 188-1167A-25X, several centimeter-scale green to brown claystone intervals were sampled and contained rare to few moderately to well-preserved calcareous nannofossils. Notably, Sample 188-1167A-25X-CC, 22-23 cm, contained P. lacunosa, Gephyrocapsa spp., C. leptoporus, Coccolithus pelagicus, and rare large forms (5.5 µm) of Gephyrocapsa caribbeanica. A similar assemblage of rare nannofossils, without the large Gephyrocapsa, was also noted in Sample 188-1167A-27X-CC. The presence of G. caribbeanica and P. lacunosa indicates an early to mid-Pleistocene age (Zone CN13b) for this interval.
Modern calcareous nannoplankton do not thrive in surface waters south of the Antarctic Divergence (>62°S) (Findlay, 1998), and few occurrences of nannofossils have been reported from Quaternary sediments of the Antarctic region. Previous drilling on the Kerguelen Plateau has revealed depauperate Pleistocene assemblages of similar composition to those noted here (Wei and Thierstein, 1991; Wei and Wise, 1992). Comparable Quaternary nannofossil assemblages were also noted in sediments of the Antarctic Peninsula Pacific margin (Barker, Camerlenghi, Acton, et al., 1999).
Particularly interesting at Site 1167 is the presence of the calcareous dinoflagellate Thoracosphaera, which was not previously noted in Quaternary Antarctic sediments until Villa and Wise (1998) reported rare specimens in shelf sediments of this age from the Ross Sea region (Cape Roberts Project). Because no other nannofossils were noted in their assemblages and age control is limited at both localities, it is difficult to determine whether the Thoracosphaera-bearing sediments of Site 1167 are correlative. Regardless, Villa and Wise (1998) point out that the presence of Thoracosphaera may indicate warmer conditions and/or result from its ability to develop cysts in response to rapidly changing conditions, as is noted for the Quaternary. The presence of calcareous nannofossils along with Thoracosphaera at Site 1167 likely suggests warmer sea-surface temperatures at this locality at various times throughout the Quaternary.
Diatoms are absent from the entire section of Hole 1167A, except for limited intervals within Core 188-1167A-1H and Sample 188-1167A-36X-CC (313.30 mbsf). Between the top of Core 188-1167A-1H (Sample 188-1167A-1H-1, 1-2 cm; 0.01 mbsf) and its base (Sample 188-1167A-1H-CC; 5.02 mbsf), only extant diatoms are present. This interval is placed within the T. lentiginosa Zone based on the absence of Actinocyclus ingens (last occurrence [LO] = 0.66 Ma). Diatom abundance and preservation decrease down through Core 188-1167A-1H, and they are entirely absent in Sample 188-1167A-2H-CC. A broken specimen of A. ingens was observed in Sample 188-1167A-1H-CC and is interpreted as reworked.
At depths below 5.02 mbsf, only one diatom specimen was observed. A slightly recrystallized specimen of Denticulopsis dimorpha (FO = 12.2 Ma; last common occurrence = 10.7 Ma) was noted in Sample 188-1167A-36X-CC (313.30 mbsf) and is interpreted to be reworked. Calcareous nannofossil and foraminifer biostratigraphy indicate a Pliocene-Pleistocene age for this interval.
The absence of diatoms through almost all of Hole 1167A is noteworthy, given that there are well-preserved and abundant in situ planktonic foraminiferal assemblages in several intervals. Fine-grained intervals in Core 188-1167A-25X, for example, were sampled thoroughly, and no siliceous microfossils were observed (whole frustules, fragments, or otherwise). The presence of common planktonic foraminifers and rare nannofossils in these intervals would suggest, however, that phytoplankton primary production occurred. The lack of diatoms from the foraminifer-rich sediment samples could result from numerous processes, such as (1) lightly silicified biocoenosis that was not preserved in the sediments because of dissolution in the water column and/or at the sediment/water interface or (2) the dominance of nonsiliceous phytoplankton communities that may have outcompeted, or filled a niche unfavorable to, the diatoms. Water-current winnowing is not considered a factor because of the presence of clay-dominated sediments and rare calcareous nannofossils, which are smaller than most diatoms and more susceptible to winnowing.
Hole 1167A is dominated by coarse-sediment lithofacies. Massively bedded sands and sandy diamicts present through lithostratigraphic Unit II are interpreted as representing sediment-gravity flows (see "Lithostratigraphy"). The absence of diatoms in these intervals is interpreted (at least partially) to result from the dilution of biosiliceous particles by rapid accumulation of terrigenous material.
A radiolarian fauna was found only in Sample 188-1167A-1H-CC. This sample contains rare, well-preserved radiolarians, but it is not clear whether it should be assigned to the Chi Zone (1.9-0.83 Ma) or the Psi Zone (0.83-0.46 Ma) of Lazarus (1992). Triceraspyris antarctica (Haecker) and Lithelius nautilodes Popofsky are both present in this sample; these taxa are reported to have a FO at the base of the Chi Zone (Lazarus, 1992). Missing from the sample, however, are Cycladophora pliocenica (Hays) Lombari and Lazarus, which has a LO in the middle of the Chi Zone, and Pterocanium c. trilobum (Haeckel), which has a LO at the top of the Chi Zone (or bottom of Psi Zone). This suggests that this sample could be assigned to the Psi Zone. Also present in this sample are Spongotrochus? glacialis, Antarctissa denticulata, Antarctissa cyclindrica, and Phorticum clevei, all high-latitude species consistent with assignment to the Chi or Psi Zones.
The radiolarian fauna in Sample 188-1167A-1H-CC on the continental slope are very similar to those noted in Sample 188-1166A-1R-2, 70-72 cm, from the continental shelf site. At both sites, all core-catcher samples were processed and examined for radiolarians, but both sites were nearly barren below these uppermost samples. At Site 1167, it appears that the radiolarians were never deposited, as no traces were found in any samples below the top level; at Site 1166, however, some evidence was seen of rare, poorly preserved radiolarians in Core 188-1166A-13R.
At Hole 1167A, it was expected that siliceous microfossils would provide chronostratigraphic control, but except for Core 188-1167A-1H, where they indicate an age younger than 0.66 Ma, they were essentially absent. Likewise, calcareous nannofossils provided little control other than for Sample 188-1167A-25X-CC, 22-23 cm, which is assigned an age of early to mid-Pleistocene.
Foraminifers are present in most samples, but the low diversity of long-ranging planktonic taxa allows an age assessment only of late Miocene or younger. These foraminifers do, however, provide at least eight samples for postcruise Sr dating. Thus, assessment of ages through the section depends on finalizing paleomagnetic analyses and determination of Sr dates. The value of foraminifers also lies in their use for reconstruction of paleoenvironments. Two faunal associations—mid-bathyal (in situ) and outer continental shelf (recycled)—are recognized, and their distribution is consistent with changes in lithology.
Chronostratigraphic control in Hole 1167A is of insufficient resolution to warrant construction of an age-depth plot and interpretation of sedimentation rates. Assuming that the section is still Pleistocene at the base of the hole, the sedimentation rate may be as high as ~400 m/m.y. Improved age control may emerge as Sr dating and refined of paleomagnetic data are integrated.