A single hole (1167A) was drilled at this site (Table T1), and cores from the surface to total depth (0.6-447.5 mbsf; Cores 188-1167A-1H through 49X), with few notable exceptions, produced large residues very dominantly of coarse, poorly sorted, angular sandstone dominated by terrigenous debris, different in appearance from that of coeval material at other Leg 188 sites. The volumes of residues captured on sieves during processing are remarkably uniform throughout the section. Most sediments are gray and obviously clayey during washing, but with high content of sand and coarser fragments. Sample 188-1167A-2H-CC was notably different in being reddish and yielded sand with a high content of iron oxide-coated grains. It is barren of foraminifers. Preservation of assemblages generally is very good and provides material of quality adequate for oxygen/carbon isotope analysis and, where common enough, Sr dating. The water depth, at 1640 m, is close to the CCD in the area (Quilty, 1985; Poisson et al., 1987).
Most samples yield few foraminifers, but over many samples a quite diverse assemblage (Table T6) emerges. The samples are remarkable in lacking siliceous microfossils—diatoms, radiolarians, sponge spicules—that are normal in Antarctic sediments.
Most samples produced foraminifer assemblages with N. pachyderma dominant over other planktonic species, and planktonic content very dominant over benthic. N. pachyderma is consistent with an age of late Miocene or younger (N. pachyderma Zone; AN7 of Berggren 1992, and Berggren et al., 1995). It is, as expected, almost 100% sinistrally coiled, but a few dextral specimens were seen (e.g., Sample 188-1167A-19X-CC). The benthic component of the assemblage usually is dominated by species of Globocassidulina, but with species of Trifarina and Uvigerina an appreciable component, the first section from Leg 188 to contain evidence of a significant infauna. Assemblages with benthic species dominated by globocassidulinids probably represent mid-outer shelf faunas that have moved downslope.
Following identification by the diatomists as containing many "microforaminifera," Samples 188-1167A-5H-3, 34-36 cm, and 25X-CC, 22-23 cm, were sampled separately. They are very different from other samples by virtue of (1) the high abundance of foraminifers (very dominantly planktonic to ~6000 per sample) in a section where they consistently are rare, (2) abundant thin-walled planktonic forms in the fine fraction, and (3) a different benthic assemblage of small species that may represent mid-bathyal conditions consistent with a short interval lacking debris flow activity. The same samples have higher (but still minor) glauconite content. Some N. pachyderma specimens are black through Mn oxide staining and infilling, again consistent with a short hiatus in debris flow activity.
Higher in the hole, benthic species comprise a few percent of each sample and are mainly of calcareous species, especially of globocassidulinds, but with a few uvigerinid species, indicating the existence of an infauna. Variation between samples in foraminiferal content probably reflects variation in the following factors:
It is likely that sedimentation of foraminifers at the site is controlled by (1) periodic, but volumetrically dominant, influx of shelf sediment and (2) short intervals of pelagic conditions when the shelf sediment influx is dramatically reduced. In the former association, benthic species represent outer continental shelf conditions (their source, but redeposited over the shelf edge), and the latter, the in situ mid-upper bathyal fauna in which the calcareous biogenic component of the sediment is correspondingly higher. It is possible, but speculative, 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 approximately constant.
Other than the transport of continental shelf assemblages to the slope, there is little evidence of reworking. A little glauconite is present in a few samples such as Samples 188-1167A-5H-CC, 12X-CC, 25X-1, 128-130 cm, and 27X-CC.
Nowhere at Site 1167 was the horizon with G. puncticulata (Deshayes) identified as it was at Site 1165 in Sample 188-1165-2H-1, 70-75 cm. This may suggest that drilling at Site 1167 has not yet reached this stratigraphic level. Although not recorded by Berggren (1992) as a zonal fauna, it is present at Sites 744, 747, and 1165 and would be expected at Site 1167, which is relatively close to Site 1165.
Because of dissolution effects, there is little value in subdividing the section on the basis of foraminifers alone, and thus a more subjective approach is used to recognize the following intervals:
An alternative approach is to differentiate only two intervals, that above Core 188-1167A-25X and that below. Core 188-1167A-25X seems to mark a very clear boundary with CCD effects almost total below and much reduced above.
The characteristics of the sand in the sandstone changed downhole. The higher samples contain very immature sands with a high content of garnet and other diverse heavy minerals. By Core 188-1167A-25X, the content of heavy minerals has decreased markedly and residues are very clean, white sand. This change with time may reflect an early source of mature, well-washed, well-sorted material carried initially by water, followed by the modern interval of glacially transported, less mature detritus with less opportunity for weathering and sorting to remove the heavy minerals. The change could be from a clean sandstone source (large fragments of clean sandstone are present in the coarse fraction) to one more dependent on the Precambrian shield of East Antarctica, or a change in dominance of the source.
Detrital pyrite (identified as such because it is present in coarse crystals that also occur in fragments of sandstone) is a consistent accessory throughout this section. In Sample 188-1167A-26X-CC, diagenetic pyrite appears differentiated because it is very fine (sometimes framboidal) and occurs as what appear to be pseudomorphs after tubular and globular organic remains. In Sample 188-1167A-40X-CC, pyrite is pseudomorphic after a planktonic foraminifer.
Probably from the Permian Amery Group in the Prince Charles Mountains, traces of black coal are present in virtually all samples.
The absence of opaline siliceous microfossils such as radiolarians, diatoms, and sponge spicules (apart from in the upper section) seems anomalous in the light of their abundance in modern sediments of Prydz Bay. Shell material and echinoderm debris is very seldom encountered and is very rare where present. Bivalve shell fragments occur sporadically, but they are small and not adequate for identification, even to generic level. No bone or teeth were seen. Ostracodes are so rare that no pattern can be detected in their occurrence. In Sample 188-1167A-25X-CC, 22-23 cm, they are represented by rinds of the more insoluble parts of the valves and indicate that their absence may in part be due to dissolution.
Glauconite is present sporadically but is very rare. Its source is unknown, but glauconite is a common component of Paleogene sediments on Mac. Robertson Shelf (Quilty et al., 2000b) and its presence may be associated with downslope (or across shelf) movement from this source.
Thomas (1990), following Corliss and Chen (1988), provided a basis for analyzing the benthic content of assemblages and differentiating epi- and infaunal elements. Unfortunately, recovery was not enough or the effects of dissolution so marked that the approach cannot be employed except on samples near the surface (e.g., Sample 188-1167A-1H-1, 60-64 cm) where the infauna is dominant. The dominance is even more marked in the few deeper samples where the specimen numbers are high enough to consider applying the technique. The principal result of trying to use the approach is to show that dissolution has been active.
Table T7 is a plot of planktonic percentage (percent of the foraminiferal assemblage composed of planktonic species). Whereas generally the figures are consistent with expectations in water this deep, the numbers must be used cautiously because of dissolution effects and the probability of significant downslope movement. The high proportion of barren samples also suggests caution in interpretation.
The modern CCD at the continental shelf edge of Prydz Bay is at ~1500 m (Quilty, 1985; Poisson et al., 1987), and thus dissolution was expected to have a significant negative influence on recovery of calcareous organisms. CCD effects are not visually obvious until Sample 188-1167A-25X-CC, 22-23 cm, when ostracodes and foraminifers both show evidence of dissolution even though abundance still is relatively very high. In ostracodes dissolution is indicated by the presence only of a few rinds of valves and in foraminifers by partial dissolution of layers of the thick tests of N. pachyderma. The CCD may be responsible for some absences where dissolution has been complete, but few samples show much evidence of the effects of the CCD. In Sample 188-1167A-40X-CC, foraminifers are absent but there is a single pyrite pseudomorph after a globigerinid species to indicate that some planktonic specimens were present but have been dissolved. Sample 188-1167A-25R-CC yielded no foraminifers but contains a few pyrite pseudomorphs that may represent infilling of benthic tests. This supports the suggestion that some other absences may be CCD related.
Although visible effects are only rarely obvious, there are other indications that dissolution has been active. Analysis of data from the distribution shown on Table T6 shows clearly that epifaunal species are very rare and that benthics are very dominantly infaunal. Many samples are barren of any foraminifers, but there are intervals in which identical lithology contains useful assemblages, all indicating that any foraminifers exposed on the seafloor were subject to dissolution at some times.
A major consideration in this study has been to determine whether the sediments at this site are in situ or have been brought downslope from the continental shelf by debris flow activity. Throughout the section, assemblages are dominantly of planktonic species and the planktonic percentages are consistent with their being in situ. Above Core 188-1167A-26X, benthic faunas are dominated by Globocassidulina, sub-dominant tapered infaunal species such as T. angulosa and Uvigerina hispidocostata, and subsidiary Ehrenbergina glabra. Agglutinated species are essentially absent. Assemblages with this composition are found in the Shallow Shelf Calcareous Assemblage of Milam and Anderson (1981) in water depths of 230-400 m. They also occur over an almost identical depth range in the Ross Sea, documented by Fillon (1974), who differentiated those faunas from within the Ross Sea from those of the northwestern or ocean open environment. The Milam and Anderson study indicates that the difference means little. Other species occurring sporadically are consistent with assemblages from this depth interval. This suggests that downslope movement is highly likely and that the assemblages (and sediments) have moved some 1000-1100 m vertically and several tens of kilometers horizontally. Planktonic species probably represent the local supply and are likely to be essentially in situ.
Samples 188-1167A-5H-3, 34-36 cm, and 25X-CC, 22-23 cm, contain faunas that, while constituting <1% of the total foraminifers, yield forms not seen elsewhere during studies of Leg 188 material. It is not a shelf assemblage and includes Planulina wuellerstorfi (Schwager). The contrast between this assemblage and the globocassidulinid-dominated shelf assemblages is very marked and suggests strongly that this asemblage is in situ, accumulated in bathyal depths, and is not a product of transport from shallower depths.
Sample 188-1167A-8H-CC provided a diverse, well-preserved assemblage with evidence (in the form of an ostracode and echinoid spines) of a diverse assemblage on the seafloor.