From Site 1120, drilled on the western part of the Campbell Plateau, sediments were recovered from ~550 m water depth. The sediments are foraminifer-bearing nannofossil oozes, and calcareous microfossils (including Bolboforma) are well preserved with diverse assemblages, which provide a wealth of information. Through the upper and middle Miocene part of the section, moreover, siliceous microfossils are present in sufficient numbers to provide paleoceanographic information, as well as first and last appearance datums for age control. Because of the paleolatitudinal position of the site, both subantarctic species and cosmopolitan species were used to date the sediments.
The Pleistocene-Pliocene stratigraphic sequence recovered is very thin. The Pliocene is probably missing completely because of hiatuses caused by intensified flow of water masses across the Campbell Plateau. The upper 2.5-3 meters composite depth (mcd) are late Pleistocene in age (mostly younger than 0.24 Ma). Down to ~10-12 mcd, a middle Pleistocene age was determined (0.9-1.2 Ma). A large part of the upper and lower Pleistocene thus is missing.
A 219-m-long lower to upper Miocene section was recovered from interval 181-1120B-2H-CC to 181-1120D-9X. A detailed age control is provided through the Miocene at this site by numerous datum levels from calcareous and siliceous microfossils (Table T3). As the site is located near the northern occurrence of subantarctic species, their last and/or first occurrence datums may deviate from the ages published in the literature. Unfortunately, magnetostratigraphy, which would allow us to check whether any datum levels are time transgressive, is not available for this site (see "Paleomagnetism"). Using the published ages (compare Tables T2, T3, T4, and T5, all in the "Explanatory Notes" chapter), and depending on which species are selected for the age-depth plot, either a number of short hiatuses can be detected (Fig. F6; see also "Age Models and Sedimentation Rates") or a more-or-less complete section from ~5.5-22 Ma results, with only one possible hiatus (detected by calcareous nannofossil analysis, but not by foraminifer biostratigraphy) between 168.7 and 171.2 mbsf, where sediment representing ~1.5-3.0 m.y. is missing. Sedimentation rates in the upper and middle Miocene are 10-20 m/m.y.
Benthic foraminifers indicate that this site's paleoenvironment has been middle to upper bathyal throughout the Neogene-Quaternary. Characteristics of all planktonic microfossil groups show that, during the Pleistocene, surface waters and intermediate water masses above the Campbell Plateau were cold. For the Miocene, the calcareous microfossils reflect alternations between warmer and colder conditions. The planktonic foraminifers and calcareous nannofossils show, in addition, a general trend toward colder planktonic faunas and floras from the early/middle Miocene to the late Miocene. For radiolarians, an opposite trend is observed: although subantarctic species are present in the lower and middle Miocene, they are lacking in the upper Miocene.
At this site, the abundance of diatoms cannot simply be related to paleoproductivity but is clearly also influenced by diagenetic processes. This is evident from the inverse correlation between diatom abundance and the abundance of the authigenically formed zeolite, clinoptilolite, in the acid-insoluble residue (Fig. F7). The sediments from the lower Miocene and uppermost Miocene are strongly affected by silica diagenesis, with the exception of sediments at ~184 mbsf core depth, which probably represent an altered tephra.
The biostratigraphy of Site 1120 is largely based on the onboard study of core-catcher samples. Samples from Holes 1120A, 1120B, and 1120C were used for the uppermost part of the section, samples from Hole 1120B for the bulk of the sequence and of Hole 1120D for the lowest part. Additional samples were taken from within selected cores to address specific questions concerning age and paleoenvironment. The absolute ages assigned to biostratigraphic datums follow the references listed in the "Explanatory Notes" chapter (Tables T2, T3, T4, T5).
More than 45 nannofossil species were recognized and recorded at Site 1120 (Table T4). Nannofossils are generally well preserved, showing both cool subtropical/temperate and subantarctic assemblages.
Samples collected in the uppermost portion of the first core (from the mud-line to Sample 181-1120A-1H-2, 28-29 cm) yield very abundant and well-preserved nannofossil assemblages, characterized by abundant Calcidiscus leptoporus, Gephyrocapsa spp. (small), Helicosphaera carteri, Emiliania huxleyi, Gephyrocapsa spp. (medium), Reticulofenestra spp. (small), and a few Coccolithus pelagicus, indicating an age of latest Pleistocene (<0.24 Ma).
A drastic change in nannofossil assemblage occurs between Samples 181-1120A-1H-2, 86 cm, and 1H-3, 30-32 cm). The nannofossil assemblages from Samples 181-1120A-1H-3, 30-32 cm, to 1H-3, 130-132 cm, are characterized by the dominance of small-sized Gephyrocapsa, and disappearance of medium-sized Gephyrocapsa. In association with the presence of Pseudoemiliania lacunosa, this interval is correlatable to the middle Pleistocene "small Gephyrocapsa Zone," with an estimated age of 0.9-1.2 Ma. This drastic change in assemblage composition requires a hiatus between the depth of 2.35 and 3.3 mbsf occupying most of the upper Pleistocene (~0.25-0.9 Ma).
Late Miocene nannofossil assemblages occur from Sample 181-1120B-2H-CC downward. The assemblages are characterized by abundant Dictyococcites antarcticus, Reticulofenestra pseudoumbilica/gelida, medium-sized Reticulofenestra (~5 µm), and Sphenolithus moriformis. The absence of Pliocene assemblages indicates a major hiatus between 4.3 and 12.8 mbsf.
Below 12.8 mbsf to the bottom of this site (Core 181-1120D-9H, 221.86 mbsf), the nannofossil assemblages are well preserved and indicate a more or less continuous record of Miocene sediment deposition. As a result of the paucity of subtropical species (e.g., the genera Amaurolithus and Discoaster) in the upper part of this site (above Core 181-1120B-12X), the correlation to the standard nannofossil zonation is difficult. The only datum level we adopted for this upper part of the Miocene was the last occurrence of Calcidiscus miopelagicus, dated at 10.4 Ma (Raffi and Flores, 1995). For the intervals below 92 mbsf with more warm-water species present, 11 datum levels were recognized. The depths and ages of these datum levels are summarized in Table T5. An age-depth curve was constructed accordingly (see "Age Models and Sedimentation Rates"), which indicates that there is probably a continuous sedimentary sequence deposited between 23 and 11 Ma. However, the occurrence of the age-diagnostic species, Sphenolithus heteromorphus (with a range of 18.5-3.5 Ma, Raffi and Flores, 1995) in only a short interval (177-166.9 mbsf), suggests that the top of the lower Miocene and the lowest part of the middle Miocene might be missing. It is also possible that the short occurrence of this species results merely from the sporadic occurrence of this species in the subantarctic area, and we may have recorded only the acme. The hiatus, if it exists, would be short, no more than 3 m.y. duration. Assuming that there is no hiatus at ~170 mbsf (see "Age Models and Sedimentation Rates"), we calculated that the sedimentation rate of the upper and middle Miocene is ~15 m/m.y.
For the lower Miocene interval between 167 and 200 mbsf, we have recognized four bioevents, namely, the FO of Calcidiscus premacintyrei, the LO of Sphenolithus dissimilis, the FO of Geminilithella rotula, and the FO of Calcidiscus leptoporus. However, the estimated ages given by these four events are conflicting.
The age of the lowest part of the Hole 1120D is constrained by two age markers, Ilselithina fusa and Discoaster druggi. We found two specimens of Ilselithina fusa in Sample 181-1120D-8X-2, 138 cm, which has an estimated LO at 22.5 Ma (Gartner, 1992). On the other hand, the lowermost portion of the core, from 213.5 to 221.86 mbsf, contains Discoaster druggi, an early Miocene indicator with its FO at 23.2 Ma (Berggren et al., 1995). This means that the age of the lowermost part of the core (215 to 221.86 mbsf) is bracketed between 22.5 and 23.2 Ma. The sedimentation rate of the interval between 200-222 mbsf is estimated to be 5.5 m/m.y. Given the uncertain reliability of the datum levels adopted and the coarse resolution, we are not sure whether this interval is a condensed section or whether hiatuses exist.
Foraminiferal assemblages are generally rich and well preserved, with planktonic forms composing ~95% of the total (Table T6). Throughout the section, the assemblages are dominated by Globigerina and Globorotalia taxa, with Globigerinoides and Globoquadrina being sporadically present in the lower half of the section.
From the highest sample down to Sample 181-1120A-1H-2, 85-87 cm, Globorotalia truncatulinoides and abundant Globorotalia inflata are present. This top interval is assigned an age of no older than 0.7 Ma (late Pleistocene, Castlecliffian-Haweran Stages, Wc-Wq), based on the local age ranges of these two species in this cool region.
Samples 181-1120A-1H-3, 130-132 cm, 181-1120A-1H-CC, 181-1120B-1H-CC, 181-1120C-1H-CC, and 181-1120B-2H-4, 90-95 cm, are rich in left-coiling, encrusted Neogloboquadrina pachyderma, Globigerina quinqueloba, G. bulloides, Globorotalia puncticuloides, G. crassaformis crassacarina, and small 3- to 3.5-chambered Globorotalia inflata; G. crassula and G. scitula are rare. The assemblage is younger than 2.5 Ma, according to the biochronology shown in Table T3 in the "Explanatory Notes" chapter, and is assumed to be stratigraphically below the local FO of G. truncatulinoides, discussed in "Foraminifers" in "Biostratigraphy" in the "Site 1119" chapter. The presence of Plectofrondicularia advena, Orthomorphina jedlitschkai, and Haeuslerella pliocenica (all of which became extinct during the Stilostomella Event) in Sample 181-1120B-2H-4, 90-95 cm, confirms that this interval is older than ~0.8 Ma. Tentatively, an age is assigned of latest Pliocene to early Pleistocene (0.8-2.5 Ma, Nukumaruan Stage, Wn).
Pliocene strata appear to be lacking, with Sample 181-1120B-2H-CC assigned a latest Miocene age (Kapitean Stage, Tk). The evidence stems from the finding of Globorotalia margaritae evoluta, a rather flat type, and Neogloboquadrina dutertrei, without G. puncticulata or G. crassaformis. Populations of the Globorotalia miozea lineage have forms spanning the range of Globorotalia sphericomiozea and Globorotalia miotumida, suggesting an age ~5.6 Ma (Table T2 in the "Explanatory Notes" chapter). The benthic foraminifers include large Haeuslerella pliocenica (late Miocene to early Pleistocene).
Samples 181-1120C-2H-CC and 181-1120B-3H-CC have populations of Globorotalia juanai, which give a latest Miocene age of 5.2-6.6 Ma (Kapitean Stage, Tk). Samples 181-1120B-4H-CC to 8H-CC contain no short-ranged taxa that provide precise age estimates. The presence of common, random to left-coiling Globorotalia miotumida (~13.2-5.6 Ma) is consistent with a late Miocene age inferred from foraminifer events above and below this interval. Common Neogloboquadrina pachyderma (common above 9.2 Ma) occur in Cores 181-1120B-3H and 4H. The last occurrence of Rectuvigerina ongleyi (~8 Ma), in Sample 181-1120B-3H-CC, is probably not a reliable datum as this benthic species is clearly facies controlled. Samples 181-1120B-4H-3, 35-37 cm, and 4H-3, 129-132 cm, are dominated by a high spiro-conical form of Globorotalia resembling G. margaritae (FO in New Zealand ~5.2 Ma), but having a fine pustular ornamentation on the tests (reminiscent of G. hirsuta) and weaker keels than typical forms. This acme is unusual in that it does not appear to have been recorded in the region previously. More study of the taxonomy of the populations in the samples is required before we can be confident of an age assignment.
Sample 181-1120B-9X-CC contains a right-coiling population of Globorotalia miotumida (Kaiti coiling event, 10.7-10.9 Ma), together with typical Globorotalia panda (LO 10.3 Ma), Orbulina suturalis (LO ~10.5 Ma), and rare Globoquadrina dehiscens (LO 9.9 Ma), indicating an early late Miocene age (10.7-10.9, early Tongaporutuan Stage). Sample 181-1120B-10X-3, 90-95 cm, again has the more usual left-coiling population of Globorotalia miotumida.
Samples from 181-1120B-12X-CC to 19X-CC are assigned a middle Miocene age on the basis of their planktonic foraminifers. Cores 181-1120B-12X to 17X contain Orbulina suturalis (FO 15.1 Ma), Globorotalia conica, G. amuria (LO for both 13 Ma), and common Globorotalia praemenardii (15.8-13.2 Ma), giving an age of 15.1-13.2 Ma (Lillburnian Stage). Praeorbulina circularis and Praeorbulina glomerosa (15.6-14.8 Ma) occur in Samples 181-1120B-17X-CC and 18X-4, 90-95 cm. No taxa of the Orbulina lineage occur below this level.
Samples from 181-1120B-18X, 181-1120B-19X and 181-1120D-2X-1, 50-55 cm (depth equivalent to 181-1120B-20X-1, 35 cm) are immediately below the lower/middle Miocene boundary. They contain common Globorotalia miozea, isolated G. zealandica, common Sphaeroidinella disjuncta, and rare Globigerinoides trilobus; an odd occurrence is Dentoglobigerina altispira s.s.; Orbulina lineage taxa were not observed. In New Zealand biostratigraphy, the assemblage is upper Altonian (16.7-16.3 Ma). In contrast, Sample 181-1120D-2X-6, 126-132 cm (depth approximately equivalent to 181-1120B-20X-CC) has no G. miozea, but contains abundant G. praescitula and G. trilobus, rare Catapsydrax sp., common "high-spired" "Globoquadrina" venezuelana, Globigerina praebulloides, Globigerina woodi s.s., but no G. zealandica nor G. incognita. This sample may be assigned to the early or middle Altonian Stage (early Miocene), presumably the latter, based on the presence of common G. zealandica in Sample 181-1120D-3X-CC. Thus, there is no foraminiferal evidence to support any hiatus as implied by nannofossils at this level.
The foraminiferal assemblage indicates an early Miocene age from Sample 181-1120B-20X-CC to the bottom of the section (Sample 181-1120D-9X-CC). The time ranges of the planktonic foraminifers allow us to subdivide this into three intervals. Samples 181-1120B-20X-CC to 181-1120D-4X-CC contain common Globorotalia zealandica (acme zone 18.5-16.7 Ma, middle Altonian Stage), Globigerinoides trilobus (FO ~19 Ma) and largely lack Globoquadrina dehiscens. The uppermost sample in this interval (181-1120B-20X-CC) contains a population with forms spanning the morphotype ranges of Globorotalia praescitula and G. miozea, indicating an age close to 16.7 Ma. Samples 181-1120D-3X-CC and 5X-CC contain small Globorotalia praescitula (FO 19 Ma) and Globorotalia incognita (LO 18.5 Ma), giving an age of 19-18.5 Ma (early Altonian Stage). In all samples from 181-1120D-6X-CC and below, the planktonic assemblage is dominated by large globigerines and Globoquadrina dehiscens. The presence of rich populations of small Globorotalia incognita (FO 21.6 Ma) in Samples 181-1120D-6X-CC to 9X-CC indicates an age within the range of 21.6-19 Ma (Otaian Stage) for the bottom of the hole.
Blooms of bolboformid taxa are present in lower upper Miocene samples (Cores 181-1120B-5H to 10X, Table T6). Specimen abundance is high and species diversity low. The spiny form Bolboforma pentaspinosa (regional time range ~7-11.5 Ma) occurs from Samples 181-1120B-5H-CC to 10X-CC. Bolboforma subfragoris (regional time range 10.5-11.5 Ma) occurs between Samples 181-1120B-7H-CC and 10X-CC. The ages are compatible with those from other microfossil groups.
Diatoms are present in the upper and middle Miocene (Table T7). Preservation is moderate to poor. As not all of the biostratigraphic marker species used for the Antarctic-subantarctic region are present, no zones were identified. But, where possible, the datums of biostratigraphically relevant species were determined. Five datums provided good age control and are in agreement with the datums from calcareous nannofossil analysis and planktonic foraminifers identified at this site. The species available for biostratigraphic age determinations were Hemiaulus triangulus (stratigraphic range 5.3-5.6 Ma), Hemidiscus karstenii f.1 (LO 5.7 Ma), and Denticulopsis dimorpha (10.6-12.2 Ma). Other datum levels such as the base of H. karstenii f. 1 could not be used, because of poor preservation of diatoms, which did not allow recognition of this species with certainty, and Simonseniella barboi was present but occurred too inconsistently to be useful stratigraphically.
Reworking of Paleogene diatoms was encountered in Samples 181-1120B-6H-CC, 12X-CC, 6X-CC, and 17X-CC. Some of the reworked species (e.g., Cestodiscus reticulatus and Pyxilla prolongata), have a relatively short stratigraphic range and appear to be reworked from lower Oligocene sediments.
Radiolarian biostratigraphy at Site 1120 is based on the examination of 32 core-catcher samples and six core samples (Table T8). Radiolarian faunas are generally abundant from the top to the lower part of the section (Samples 181-1120A-1H-CC to 181-1120B-18X-CC, 0-158.5 mbsf). Although radiolarians are almost absent or very rare in the lowest section (181-1120B-19H-CC and below, 167 to 222 mbsf), two samples (181-1120B-21X-CC and 181-1120D-3X-CC) contain rare to common radiolarians that give age information.
Cyrtocapsella japonica is consistently present in varying numbers in the upper to lower part of the section (181-1120B-6H-CC to 16X-CC). Cyrtocapsella tetrapera occurs continuously throughout the lower half of the section from Sample 181-1120B-11X-CC to 181-1120D-3X-CC, and its occurrence represents an early to middle Miocene age.
A single specimen of Theocorythium vetulum (LO 1.2-1.3 Ma; Alexandrovich, 1989) and abundant Lithelius nautiloides (FO 1.93 Ma) are present in Sample 181-1120A-1H-CC. Furthermore, the few to common occurrences of Antarctissa denticulata, A. strelkovi, A. longa, and of abundant Lithelius nautiloides in Sample 181-1120A-1H-CC give this sample an Antarctic-subantarctic character.
Samples 181-1120B-6H-CC to 8H-CC yield common to very abundant Cyrtocapsella japonica (LO 10.11 Ma, acme 10.2 Ma) and rare to few Eucyrtidium calvertense, which indicate an early late Miocene age.
Sample 181-1120B-10X-CC contains common Cyrtocapsella japonica, rare Cycladophora bicornis amphora, and few Lithopera neotera, together with rare Cycladophora humerous (LO 10.5 Ma). An evolutionary transition event from Lithopera neotera to Lithopera bacca is known to occur within the Diartus petterssoni Zone (RN6, 8.77-11.95 Ma, Sanfilippo and Nigrini, 1998). Although the stratigraphic marker species of the genera Diartus and Didymocyrtis are rare in this section, they indicate for Sample 181-1129B-10X-CC a late middle to early late Miocene age.
Sample 181-1120B-11X-CC yields a single specimen of Dendrospyris megalocephalis (FO 12.68 Ma) and abundant Cyrtocapsella tetrapera. Below, in Samples 181-1120B-12X-CC and 15X-CC Cyrtocapsella tetrapera is rare.
In Sample 181-1120B-16X-CC, very abundant Cyrtocapsella tetrapera (LCO 12.6 Ma) and rare Eucyrtidium inflatum (LO 12.3) are present, indicating a late middle Miocene age. Common to abundant Eucyrtidium punctatum (FO 17.02 Ma) are present in Samples 181-1120B-17X-CC, 18X-CC, and 181-1120D-3X-CC. This interval of the lower part of the section fits within the early to middle Miocene Eucyrtidium punctatum Zone of Lazarus (1992) and Abelmann (1992). The fauna, including Eucyrtidium punctatum, is characteristic of Antarctic-subantarctic waters.
A major change in the flora occurs between Cores 181-1120B-11X and 12X (between 86.55 and 99.1 mbsf). Below this level, assemblages are more diverse, containing larger sized placoliths and helicoliths. The most conspicuous are those of Calcidiscus premacintyrei, Cyclicargolithus floridanus, and Helicosphaera carteri. Also present with more frequent occurrence is Sphenolithus moriformis. Above this middle Miocene (~12 Ma) change in nannofossil flora, there are virtually no discoasterids, whereas below this level discoasterids occur quite frequently. Based upon the more frequent occurrence of discoasterids and sphenoliths in the lower part of the sequence (Cores 181-1120B-15X to 181-1120D-9X, corresponding to 139-222 mbsf), we suggest that before 14 Ma, there were warmer water masses over this site than after that time. This climatic change, as indicated by the floral change, coincides well with major glaciation in Antarctica at ~14 Ma (Shackleton and Kennett, 1975; Margolis, 1975).
There was insufficient time for detailed paleoenvironmental analysis of the benthic foraminiferal faunas during onboard studies. Throughout Site 1120, the samples contain rich benthic foraminiferal assemblages of rather similar overall composition, typical of middle to upper bathyal depths, differing little from the Holocene fauna at this site. These include common Sigmoilopsis schlumbergeri, Bolivina affiliata, Bulimina marginata, Cassidulina carinata, Ehrenbergina mestayeri, Globocassidulina subglobosa, Laevidentalina spp., Laticarinina altocamerata, L. pauperata, Lenticulina mammiligera, Melonis barleeanum, Oridorsalis umbonatus, Pullenia quinqueloba, P. bulloides, Planulina wuellerstorfi, Sphaeroidina bulloides, Trifarina bradyi, Uvigerina spp., Cibicidoides spp., Karreriella spp., Cassidulina sp., and Eggerella brady.
The planktonic foraminiferal assemblages exhibit compositional differences related to near-surface paleo-water-mass temperatures. For example, the upper Miocene Sample 181-1120B-3H-CC contains a colder water mass assemblage, with dominant Globigerina bulloides, G. umbilicata, and Neogloboquadrina pachyderma, together with rare to common Globigerina praebulloides, Globigerina woodi woodi, Orbulina universa, Globorotalia scitula, G. miotumida (no preferential coiling), and Paragloborotalia continuosa. Samples 181-1120B-4H-3, 35-37 cm, and 4H-3, 129-132 cm, contain a comparable, but more temperate, water mass assemblage, with frequent Globorotalia cf. margaritae and rare Paragloborotalia mayeri.
Diatoms are present in Hole 1120B from 15 to ~150 mbsf core depth with varying abundance. The diatom assemblages at this site consist exclusively of planktonic diatoms with cosmopolitan species and species characteristic of the subantarctic belt. The diversity of the assemblages is low with species of the genera Actinocyclus, Azpeitia, Coscinodiscus, Denticulopsis, Thalassionema dominating the assemblages, and additional characteristic neritic species occurring (e.g., of the genera Stephanopyxis, Paralia, and Pseudopodosira).
At this site, the abundance of diatoms is not predominantly controlled by primary production but is strongly influenced by silica dissolution and diagenesis. This is shown by the inverse correlation between diatom abundance and the abundance of the authigenic zeolite clinoptilolite in the sediments (Fig. F7). This silicate is obviously formed here out of the high silica concentrations in the pore water after dissolution of siliceous microfossils. Only in one interval (Samples 181-1120B-21X-CC and 181-1120D-3X-CC) in the lower Miocene does silica diagenesis deviate in its character. This sample is practically free of zeolites, but has a few diatoms preserved in the acid-insoluble residue together with clay minerals. Because there is a well-known relation between the occurrence of tephra and siliceous microfossils, which mutually protect each other, this sediment is interpreted as an altered tephra.
The early Pleistocene radiolarian fauna from the Sample 181-1120A-1H-CC, which is characterized by occurrences of southern high-latitude species (e.g., Antarctissa strelkovi, Antarctissa denticulata, Antarctissa longa, and Lithelius nautiloides), shows strong Antarctic/subantarctic affinity. Also the lower and middle Miocene faunas at Holes 1120B and 1120D from 148 to 185 mbsf (Samples 181-1120B-17X-CC to 21X-CC, and 181-1120D-3X-CC) are of Antarctic/subantarctic affinity. Whereas the radiolarians of middle and late Miocene at this site are quite unique in their species composition, they are not Antarctic/subantarctic assemblages. They rather show enhancement of cosmopolitan species, including Cyrtocapsella japonica, which may allow these assemblages to be correlated with Northern Hemisphere mid-latitude faunas.