At Site 1139, a thin (19 m) Quaternary section of foraminifer/diatom-bearing nannofossil ooze (lithologic Subunit 1A) is underlain by a greatly expanded 364-m lower upper Miocene to mid-Oligocene calcareous pelagite with generally well-preserved siliceous and calcareous microfaunas and floras (lithologic Subunit IB to Unit III). There is no appreciable chert within this section, which was deposited well above the calcite compensation depth (present water depth = 1415.5 m).
Minimum sedimentation rates are ~18 m/m.y. in the Miocene and 29 m/m.y. in the Oligocene, or 23 m/m.y. for the entire Tertiary pelagic section (Fig. F9). We attribute the high sedimentation rates to high regional pelagic productivity, plus the influx of fine terrigenous clastics derived from the weathering of exposed portions of the volcanic edifice, Skiff Bank, on which the sediments were deposited (see "Lithostratigraphy"). The clastic input colored the normally white calcareous ooze and chalks gray to brownish gray. Only at the bottom of the section, where such input was minimal, are the sediments oxidized to a pinkish color.
An unusual nannofossil Braarudosphaera bloom in the late Oligocene has been reported previously from the southern Kerguelen Plateau (Wei and Thierstein, 1991) and may correlate with other such occurrences of this age in the Atlantic and Indian Oceans. We date a foraminiferal nannofossil chalk at the base of the pelagic section (Section 183-1139-40R-6) as earliest Oligocene in age by nannofossils and foraminifers (within the interval 32.8 to 34.3 Ma; CP16a/b and basal AP13).
Calcareous nannofossils were generally well preserved and abundant in selected intervals only of the Quaternary of lithologic Subunit IA and are consistently abundant and generally well preserved in the Miocene-Pliocene 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.
Diatoms, observed only in smear slides, were abundant and pristine in the Quaternary section. In the middle Miocene at the top of lithologic Subunit IB (Sample 183-1139A-3R-CC), they are abundant and moderately well preserved. Diatoms were not studied systematically below that level, but useful occurrences were noted in some nannofossil smear slides.
A relatively pure nannofossil ooze in Sample 183-1139A-1R-1, 33 cm, consisted of common Gephyrocapsa sp. and Coccolithus pelagicus, but over 90% of the assemblage consisted of small 2- to 3-µm forms presumed to be Emiliania huxleyi, which indicates an age of about 84 ka for this sample. C. pelagicus was only present in the "few" to "common" categories in core-catcher Samples 183-1139A-1R-CC and 2R-CC. We dated these samples by using diatoms, which belonged to the Actinocyclus ingens Zone. In Sample 183-1139A-1R-CC, the nominate species abounds as does Thalassiosira lentigenosa; T. elliptipora are few.
Sample 183-1139A-3R-CC contains abundant Cyclicargolithus abisectus, C. floridanus, Coccolithus miopelagicus, Helicosphaera granulosa, common Calcidiscus leptoporus/macintyrei, and common Discoaster variabilis, which placed it in the combined CN5a/CN3 Zone (middle to latest early Miocene). Diatoms are abundant with common A. ingens, but no pennates were noted in the smear slide; this probably indicates that we can assign this sample to the early part of the middle Miocene (between ~15 and 16 Ma). This age date is consistent with the relatively large number of discoasters, which prefer warmer water conditions than prevailed at this site after that time.
Discoaster veriabilis is rare and cyclicargolithids are few in Sample 183-1139A-4R-CC, which is dominated by coccolithids and reticulofenestrids. These suggest cooler conditions than in the superjacent core. Climate-induced alternations are common in this part of the section but will not be described here in detail because of the wide spacing of the core-catcher samples.
Samples 183-1139A-5R-CC to 8R-CC are mid-early Miocene in age. Of these, Samples 183-1139A-5R-CC and 7R-CC contain few to common Discoaster deflandrei. Samples 183-1139A-9R-CC through 18R-CC lack C. leptoporus/macintyrei, and we assigned them to the lower Miocene Zones CN2-CN1 in a section greatly expanded by the influx of clays derived from volcanic parent materials (see "Lithostratigraphy"). Among the diatoms, we noted Raphidodiscus marylandicus (early Miocene to earliest middle Miocene) in Sample 183-1139A-13R-CC, abundant Atinopthycus undulatus in Sample 183-1139A-15R-CC, and Nitzschia maleinterpretaria in 183-1139A-17R-CC.
The downhole last occurrence (LO) of Reticulofenestra bisecta in Sample 183-1139A-19-CC marks the nannofossil top of the Oligocene at these latitudes and also marks the top of the zone of that name. We also assigned the next two core-catcher samples to this zone, which is expanded here because of the input of fine clastics as described above. We noted Helicosphaera euphrates and D. deflandrei in Sample 183-1139A-20R-CC.
Samples 183-1139A-22R-CC to 40R-6, 22-24 cm, contain Chiasmolithus altus and C. abisectus in the absence of Reticulofenestra umbilica; we assigned the samples to the mid-Oligocene C. altus Zone. The LO of Zygrhablithus bijugatus occurs in Sample 183-1139A-2R-CC, which is near the top of the C. altus Zone. Helicosphaera bramlettei occurs sporadically in this part of the zone (Samples 183-1139A-26R-CC, 27R-CC, and 29R-CC), as do a small number of pontosphaerids (a few Pontosphaera multipora and P. sp. cf. inconspicua in Sample 183-1139A-26R-CC; a large, high-rimmed Pontosphaera sp. is found in Sample 183-1139A-30R-CC). Discoaster deflandrei is rare (Sample 183-1139A-30R-CC) as is Coronocyclus sp. (Sample 183-1139A-29R-CC).
An unusual occurrence is Braarudosphaera bigelowii, which is common as both whole and fragmented specimens in Sample 183-1139A-30R-CC. Wei and Thierstein (1991, table 3) recorded a similar occurrence of this normally neritic taxon in this part of the stratigraphic column in Hole 737B on the NKP. These occurrences might correspond to the rather widespread Oligocene Braarudosphaera blooms in the Atlantic (e.g., Parker et al. 1985) and Indian Ocean off northwest Australia (Siesser et al., 1992).
Diatoms are common in some intervals. We noted Azpetia oligocenica in Sample 183-1139A-32R-CC.
We noted a single reworked specimen of Ismolithus recurvus toward the bottom of the C. altus Zone in Sample 183-1139A-38R-CC. A few Discoaster tanii (five- and six-rayed) plus a flood of small reticulofenestrids are present in Sample 183-1139A-39R-CC, along with Reticulofenestra daviesii, which ranges higher in the zone. A few D. deflandrei and common Helicosphaera perch-nielseniae accompany Sphenolithus moriformis in Sample 183-1139A-40R-2, 25-27 cm, 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 spinosa abounds in Sample 183-1139A-40R-5, 25-27 cm.
The color of the sediment changes in the lower part of Section 183-1139A-40R-5 downhole from a greenish to a reddish orange color, but the nannoflora do not change until an apparent disconformity is crossed between Samples 183-1139A-40R-6, 20-22 cm, and 40R-6, 89-91 cm, well within the oxidized sediments (see "Lithostratigraphy"). 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, 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 last occurrence of C. formosus (32.8 Ma) and the last occurrence of R. reticulata (35.4 Ma) or the first occurrence 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 high abundance of Clausicoccus fenestratus, however, suggests essentially an earliest Oligocene age (~CP16a/b) when compared with the Eocene/Oligocene sequence at Deep Sea Drilling Project Leg 71 Hole 511 on the Falkland Plateau and Ocean Drilling Program Leg 119 Hole 737B on the southern Kerguelen Plateau (Wise, 1983, table 1A; Wei and Thierstein, 1991, table 3, respectively). At these localities, C. fenestratus is quite rare or absent below the Eocene/Oligocene boundary.
Nevertheless, the co-occurrence of Coccolithus formosus and Isthmolithus recurvus below the disconformity and the co-occurrence of 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 CP16c, CP17, and lower CP18.
We recovered relatively abundant and well-preserved assemblages of Quaternary (upper Pleistocene) and upper Neogene planktonic foraminifers from lithostratigraphic Subunits IA and IB. In contrast, middle Miocene to Oligocene assemblages in the nannofossil-bearing clays and claystones of Unit II contain sparse and only moderately well-preserved planktonic foraminifers. Preservation quality and abundance of microfossils increases dramatically in the reddish orange foraminifer nannofossil chalk of Unit III beneath the pelagic section. Individual foraminifers from this short interval exhibit pink iron oxide staining. Planktonic foraminifers are absent from the micritized grainstones and packstones of Unit V. These coarse and generally unconsolidated, shoaled-carbonate sediments contain abundant bryozoan fragments, rare mollusk-shell fragments, and occasional benthic foraminifers.
Planktonic foraminifer assemblages in Samples 183-1139-1R-CC and 2R-CC are dominated by Neogloboquadrina pachyderma and Globigerina bulloides, with less common Globigerina falconensis, Turborotalia quinquelobula, and Globorotalia puncticulata. This assemblage, typical of high-latitude Indian Ocean sites, is composed of long-ranging species (late Miocene to Holocene), which are of little biostratigraphic utility. Large, spherical radiolarians also abound in the Subunit IA diatom oozes.
The cream-beige nannofossil ooze of Unit II exhibits the highest carbonate content for the entire sedimentary succession (see "Organic and Inorganic Geochemistry") but contains fewer planktonic foraminifers than the overlying diatom-ooze. Samples 183-1139-3R-CC and 4R-CC contain generally small but well-preserved G. bulloides, Globigerina woodi, Globorotalia miozea, Globorotalia praescitula, and Globorotalia panda. Neogloboquadrina spp. is absent, indicating that we can place these samples in the middle Miocene. The presence of G. panda is interesting. Berggren (1992) shows the range of this species on the central Kerguelen Plateau restricted to a short interval in the late middle Miocene. At lower latitudes, this species ranges into the lower middle Miocene (Kennett and Srinivasan, 1983). The more northerly position of this site (~ 49°S compared to ~56°S) and implied slightly warmer water conditions may allow a longer biostratigraphic range at Site 1139. Until additional samples are studied, we assign this assemblage to Zones NK4-NK5.
Abundance and preservation of microfossils is poor in the following 315 m of green-gray nannofossil clays and claystones (Unit II) because of dilution by abundant grain aggregates that did not break down during sample washing. Planktonic foraminifers are of limited biostratigraphic use in this section. Samples 183-1139-5R-CC to 39R-CC contain radiolarians and rare benthic and planktonic foraminifers, except for a few intervals where clay content decreases and microfossil abundance increases slightly. Globorotalia zealandica, Paragloborotalia incognita, Catapsydrax unicavus, Globigerina brazieri, Globigerina praebulloides, Globigerina spp. cf. G. labiacrassata, and Tenuitella spp. are sporadic in Samples 183-1139-5R-CC to 17R-CC, indicating an early to middle Miocene age.
After an interval of particularly poor preservation and low microfossil abundance in Cores 183-1139-18R to 29R, we recognize the upper Oligocene zonal marker Globigerina euapertura in Sample 183-1139-30R-CC. Also present in this sample are rare C. unicavus, Globorotalia opima, and Globorotaloides spp. Chiloguembelina cubensis appears to be absent, which suggests that we can place this sample in the AP16-AP15 zonal range.
The next sample with recognizable planktonic foraminifers is Sample 183-1139-32R-CC. Rare, but moderately well-preserved, specimens of Tenuitella gemma, Tenuitella munda, Globrotalia opima, and Globorotaloides spp. occur with C. cubensis. In the absence of the Zone AP13 marker Subbotina angiporoides, we assign this sample to mid-Oligocene Zone AP14. Despite low planktonic foraminifer abundance, we also assign Samples 183-1139-33R-CC and 34R-CC to Zone AP14 with reasonable confidence. The last five cores of Unit II, Cores 183-1139-35R to 39R, are essentially barren of planktonic foraminifers.
Unit III sediments were of dramatically different color and composition and yielded abundant and well-preserved planktonic foraminifers. Common Subbotina angiporoides, Catapsydrax unicavus, C. cubensis, T. gemma, and Subbotina utilusindex in Sample 183-1139-40R-CC, in the absence of Globigerinatheka index, indicate a latest Eocene to early Oligocene age (Zone AP13). We cannot delineate the Eocene/Oligocene boundary using planktonic foraminifers (denoted by the extinction of the Hantkeninidae at low to middle latitudes [Coccioni et al., 1988; Berggren et al., 1995]) because the important index species (Hantkenina spp. and Cribrohantkenina spp.) are not found at high latitudes. The core catcher of Core 183-1139-40R-CC is the last interval above igneous basement that contains datable microfossils. We estimate the maximum age of these to be 34.3-30.3 Ma.