Calcareous nannofossils were generally well preserved and abundant in selected intervals of the Quaternary of lithologic Subunit IA and are consistently abundant and generally well preserved in the Miocene-Oligocene nannofossil oozes of Subunit IB and Unit II. They were abundant and moderately well preserved in the lowermost Oligocene of Unit III, where some taxa show overgrowths.
A relatively pure nannofossil ooze in the top of Section 183-1139A-1R-1 consists of few to very abundant Gephyrocapsa and very abundant Emiliania huxleyi (90% of the assemblage), which indicates the E. huxleyi acme of Gartner (1977) with an age of ~84 ka or less. The second section of the core contained few E. huxleyi but common Gephyrocapsa and is assigned to the lower portion of the E. huxleyi Zone (Zone CN15 or NN21). The remainder of the core, with the exception of the core catcher, yielded few to abundant nannoliths and is assigned to Zone NN20 of Martini.
The core catcher of the first core and all of the second core, except its core catcher, contain Pseudoemiliania lacunosa along with forms sometimes common in abundance that closely resemble E. huxleyi (Pl. P1; figs. 1-4). As P. lacunosa and E. huxleyi should not be present together in the same samples, these minute forms were further examined in the scanning electron microscope (SEM) (Pl. P1; figs. 5, 6). The micrographs reveal that they actually represent other taxa that have been etched in such a way as to mimic E. huxleyi (see further description below in "Effects of Diagenetic Etching on Quaternary Placoliths"). These are indicated in Table T1 by a lower case "m."
The interval in question was assigned as Martini's Zone NN19. The carbonate content of this interval varies considerably, as the nannofossil content may be highly diluted by siliceous microfossils.
Core 183-1139A-3R contains common to very abundant Cyclicargolithus abisectus, Cyclicargolithus floridanus, Coccolithus miopelagicus, Helicosphaera carteri, Helicosphaera granulosa, common Calcidiscus leptoporus/macintyrei, and common Discoaster variabilis, which place it in the combined CN5a/CN3 Zone (middle Miocene). Diatoms are abundant with common Actinocyclus ingens, but no pennates were noted (see "Biostratigraphy" in Shipboard Scientific Party, 2000). This probably indicates that the sample can be assigned to the early part of the middle Miocene (between ~15 and 16 Ma). This age is consistent with the relatively large number of discoasters, which prefer warmer water conditions than would have prevailed at this site after the late-middle Miocene climatic optimum that ended at ~15 Ma (Zachos et al., 2001).
Discoasters are less common in Sample 183-1139A-4R-CC, which is dominated more by coccolithids, cyclicargolithids, and reticulofenestrids. These suggest cooler conditions than in the previous core catcher sample. Climate-induced alternations are common in this part of the section but are not described here in detail. The first evolutionary occurrence (FO) of the genus Calcidiscus in that same sample approximates the base of Zone CP3.
Samples 183-1139A-5R-CC through 8R-CC are mid-early Miocene in age and contain few to abundant Discoaster deflandrei. Samples 183-1139A-9R-CC through 18R-CC lack C. leptoporus/macintyrei and probably belong to the lower Miocene Zones CN2-CN1 in a section greatly expanded by the influx of clays derived from volcanic parent materials (Reusch, this volume). Discoasters are largely absent except for the middle portion of Core 183-1139A-16R; otherwise, the assemblages are overwhelmingly dominated by cool-water reticulofenestrids.
The downhole last occurrence (LO) of Reticulofenestra bisecta marks the nannofossil Miocene/Oligocene boundary at these latitudes and also marks the top of the zone of that name. Its placement is somewhat ambiguous in this section in that its last consistent common occurrence is in Sample 183-1139A-19R-4, 24-25 cm. A higher common occurrence of this taxon in Sample 183-1139A-17R-6, 25-26 cm, however, could be taken as the top of the zone if it is not reworked at that point. Reworking is entirely possible at this locality, as it is in the path of the Antarctic Circumpolar Current, which would have been active by this time (Lawver and Gahagan, 1998). For this reason, a rare occurrence of R. bisecta in Sample 183-1139A-17R-1, 25-26 cm, is considered reworked.
Regardless of where the top of the R. bisecta Zone is placed, this zone is considerably expanded here because of the input of fine clastic sediments as noted above. The base of the zone is placed at Sample 183-1139A-22R-2, 25-26 cm.
Samples 183-1139A-22R-CC through 40R-6, 22-24 cm, contain Chiasmolithus altus and C. abisectus in the virtual absence of Reticulofenestra umbilica; we assigned the samples to the mid-Oligocene C. altus Zone. This is somewhat interpretive because rare to few R. umbilica are sporadically present between Samples 183-1139A-29R-2, 26-27 cm, and 38R-3, 27-27 cm. These and related forms are considered reworked, again presumably under the influence of the then strengthening Antarctic Circumpolar Current. Support for this interpretation is provided by planktonic foraminiferal analysis, which suggests that Core 183-1139-33R can be assigned to the middle Oligocene Zone AP14 with "reasonable confidence" (Shipboard Scientific Party, 2000). Similar support is rendered by the presence of the middle Oligocene diatom Azpetia oligocenica in Sample 183-1139A-32-CC (Shipboard Scientific Party, 2000).
The last common occurrence of the holococcolith Zygrhablithus bijugatus is in Sample 183-1139A-22R-CC, which is also at the top of the C. altus Zone. Helicosphaera bramlettei occurs sporadically in this part of the zone, from Cores 183-1139A-25R to 31R, as do a number of pontosphaerids (particularly Pontosphaera multipora and Pontosphaera versa). A large high-rimmed Pontosphaera sp. was also noted in Sample 183-1139A-30R-CC. D. deflandrei is sporadic but sometimes abundant, whereas Coronocyclus nitescens, also sporadic, may be common.
An unusual presence is Braarudosphaera bigelowii, which is common to abundant as both whole and fragmented specimens in Samples 183-1139A-30R-CC and 31R-1, 26-27 cm. Wei and Thierstein (1991; table 3) recorded a similar presence of this normally neritic taxon in this part of the stratigraphic column in Hole 737B on the Northern Kerguelen Plateau. These presence of these might correspond to the mid-latitude Oligocene Braarudosphaera blooms in the Atlantic (e.g., Parker et al., 1985) and the Indian Ocean off northwest Australia (Siesser et al., 1992).
We noted a single reworked specimen of Isthmolithus recurvus near the bottom of the C. altus Zone in Sample 183-1139A-38R-CC. A few Discoaster tanii (five and six rayed) plus a high abundance of small reticulofenestrids are present in Sample 183-1139A-39R-CC, along with Reticulofenestra daviesii, which ranges to the top of the zone in this section. Few to common D. deflandrei and Helicosphaera perch-nielseniae accompany Sphenolithus in the first two sections of Core 183-1139A-40R, and small, delicate pontospherids that superficially resemble Reticulofenestra oamaruensis are found throughout much of this core; however, the overall assemblage is characteristic of the C. altus Zone. Blackites spinosus is common in Sample 183-1139A-40R-5, 25-27 cm, and in the core catcher of Core 40R.
The color of the sediment changes downhole in the lower part of Section 183-1139A-40R-5 from a greenish to a reddish orange color, but the nannoflora do not change until an apparent disconformity between Samples 183-1139A-40R-6, 20-22 cm, and 40R-6, 30-32 cm, well within the oxidized sediments (see figs. F2, F4, both in Shipboard Scientific Party, 2000). The latter sample and Sample 183-1139A-40R-CC are characterized by abundant Reticulofenestra hillae; common to abundant I. recurvus along with few to common Coccolithus formosus, D. deflandrei, and D. tanii; and common to abundant Clausicoccus fenestratus, C. altus, and C. oamaruensis. We observed no C. abisectus, Discoaster saipanensis, R. oamaruensis, or Reticulofenestra reticulata.
A broad age range for the assemblage described above is represented by the LO of C. formosus (32.8 Ma) and the LO of R. reticulata (35.4 Ma) or the FO of I. recurvus (35.7-36.3 Ma at these latitudes according to Wei, 1992). This assumes that D. saipanensis and R. oamaruensis are not present here because of truncated upper ranges resulting from ecological restriction in these higher latitudes (e.g., Wei, 1992). Nevertheless, this age range spans the Eocene/Oligocene boundary.
The relatively high number of C. fenestratus in Sample 183-1139A-40R-CC, however, suggests essentially an earliest Oligocene age (approximately Subzone CP16a/b) when compared with the Eocene/Oligocene sequence from Hole 511 of Deep Sea Drilling Project Leg 71 on the Falkland Plateau and Hole 737B of ODP Leg 119 on the Northern Kerguelen Plateau (table 1A of Wise, 1983; table 3 of Wei and Thierstein, 1991). At these localities, C. fenestratus is quite rare or absent below the Eocene/Oligocene boundary. An early Oligocene age for the base of Core 183-1139A-40R is also supported by the planktonic foraminiferal fauna, which lacks the definitive upper Eocene high-latitude index taxon Globigerinatheka index (Wise et al., 2002).
Nevertheless, the presence of both C. formosus and I. recurvus below the disconformity and the presence of both C. abisectus and R. umbilica above signals the absence of at least the R. davesii Zone and possibly more. The missing section would be equivalent to at least Subzone CP16, Zone CP17, and lower CP18 Zone.