The uppermost sediments at Site 1121 are current-influenced silty clays and sands with frequent manganese nodules. The sediments contain both calcareous and siliceous microfossils of late Pleistocene to late Pliocene age (Figs. F9, F10). The underlying sediments, which represent a late early to late Paleocene age, are nannofossil-diatom oozes and siliceous clays with porcellanite and chert layers in the deeper part (Fig. F9). All major microfossil groups, including calcareous nannofossils, diatoms, radiolarians, and benthic foraminifers, are present and extremely well preserved. The highly diverse assemblages provide an integrated biostratigraphy to combine with the magnetostratigraphy, which for the first time provides a continuous pelagic Paleocene record for the Southwest Pacific Ocean.
A significant hiatus (Fig. F9) between Samples 181-1121A-1H-CC and 181-1121B-2H-CC (8.32-19.4 mbsf) in Site 1121 represents a long age gap between the early late Pliocene and the late Paleocene (~53 Ma). Using our best fit from the constraining datum levels from all four microfossil groups, we conclude that Site 1121 cored a very thin sediment drift between 0 and 3 mbsf. A detailed diatom analysis indicates that the topmost part (0-0.36 mbsf) is of late Pleistocene age (<0.42 Ma), and the interval between 0.36 and 0.90 mbsf is probably of early Pleistocene age (0.65-1.8 Ma). A late Pliocene age (2.2-3.36 Ma) was determined for the short interval between 0.90 and 1.00 mbsf and an early Pliocene age (3.26-3.6 Ma) between 1.00 and 3.00 mbsf.
The stratigraphic sequence recovered at Site 1121 is a nearly complete section of late early to late Paleocene-age sediments (56 to 62 Ma). The sequence (32.72-134.72 mbsf) can be correlated with four nannofossil zones (NP8 to NP4), three radiolarian zones (RP6 to RP4), and one diatom zone, respectively (Fig. F9). The correlation and age assignments between diatoms, radiolarians, calcareous nannofossils, and foraminifers are strongly concordant. From our analysis, the boundary between lower and upper Paleocene is placed at a horizon between Samples 181-1121B-15X-CC and 16X-CC (122.30-127.20 mbsf). Stratigraphically useful datum levels such as FOs of Heliolithus kleinpelli (base of NP6 in Sample 181-1121B-10X-CC), Fasciculithus tympaniformis (base of NP5 in Sample 181-1121B-14X-CC), and Buryella tetradica (base of RP5 in Sample 181-1121B-15X-CC) are also recognized.
The micropaleontological biostratigraphy of Site 1121 is based mostly on the onboard study of core-catcher samples. Hole 1121A samples were used for the uppermost part of the section and Hole 1121B samples for the lower part. Additional samples were taken from within selected cores to address specific age and paleoenvironmental questions. The absolute ages assigned to biostratigraphic datums follow the references listed in Tables T2, T3, T4, T5, all in the "Explanatory Notes" chapter.
The nannofossil assemblages from the uppermost part of Holes 1121A and 1121B (from the core top to Sample 181-1121B-5X-CC) are extremely poor, and several samples are totally barren. We examined many samples from the upper part of the first core, but the scarcity of nannofossils made the sediments difficult to date. However, based upon the occurrence of Gephyrocapsa parallela (originated at about 0.95 Ma), the very top of the section is late Pleistocene in age. Downcore, from Sample 181-1121B-1H-CC to 5X-CC (8.32-23.72 mbsf), a few late Neogene nannofossils occur and indicate an age older than Pleistocene, based upon the absence of typical Pleistocene Gephyrocapsa species.
Nannofossils are generally abundant and moderately preserved in Cores 181-1121B-6X to 17X (Table T2). An apparently complete middle Paleocene section (lower Seladian Stage and upper Danian; Zones NP4 to NP8 of Martini, 1971) was recovered. Species of high latitudes, such as those of Prinsius, Chismolithus, Hornibrookina, and Toweius are abundant. Differential dissolution, however, has resulted in the destruction of the central parts of many specimens, which renders species identification difficult. The stratigraphic distribution of the identifiable species is presented in Table T2.
Datum levels belonging to the Fasciculithus-Bomolithus-Heliolithus group were recognized, which allowed us to correlate Cores 181-1121B- 6X to 17X to NP8-NP4. The topmost two sections of Core 181-1121B-6X are characterized by the unique occurrence of Fasciculithus bobii and Heliolithus redelii. The association of these two species suggests strongly that these two sections are correlated to Zone NP8. The occurrence of Bomolithus elegans in Sample 181-1121B-6X-1, 2 cm, further constrains that the uppermost part of Core 181-1121B-6X is in the middle to lower part of Zone NP8 (Perch-Nielsen, 1985). Furthermore, based upon the first appearance of F. bobii in Sample 181-1121B-6X-CC, we assigned the bottom of Zone NP8 to Core 181-1121B-7X.
The next easily recognized datum is the first occurrence of Heliolithus kleinpellii in Sample 181-1121B-10X-CC, which marks the base of Zone NP6. It is not possible to define the NP7 biozone, because of the lack of the marker species Discoaster mohleri at the base of Zone NP7. Consequently, Cores 181-1121B-7X-CC to 10X-CC are assigned to the Zones NP6-NP7.
The first occurrence of the marker species of the base of Zone NP5, Fasciculithus tympaniformis, was observed in Sample 181-1121B-14X-CC. The first occurrence of the continuous occurrence of Sphenolithus primus in Core 181-1121B-14X also supports the placement of the NP5/NP4 boundary below Core 181-1121B-14X. The occurrence of Fasciculithus janii and F. ulii in the lowermost cores (Cores 181-1121B-15X, 16X, and 17X) is consistent with the known evolutionary succession of this genus (Perch-Nielsen, 1985). The occurrence of these two species also indicates that the bottom of Hole 1121B is in the topmost part of Zone NP4.
The top meter below seafloor (down to Sample 181-1121A-1H-1,18-20 cm) contains rich calcareous, foraminiferal faunas, which become progressively less abundant downhole. Planktonic forms compose over 99% of the foraminifers and are dominated by white Globigerina bulloides, Globorotalia inflata, and small Neogloboquadrina pachyderma, with fewer Globorotalia truncatulinoides and Globigerina quinqueloba. These suggest a late Pleistocene to Holocene age (<0.9 Ma). Horizons within this upper interval (e.g., Sample 181-1121A-1H-1, 0-1 cm) and below (down to Sample 181-1121B-1H-CC) contain a much reduced foraminiferal fauna, lacking small specimens and only containing sparse large, thick-walled, white Globorotalia inflata and Globigerina bulloides, together with isolated Globorotalia truncatulinoides and benthic foraminifers. This relict, current-sorted, and partly dissolved assemblage is still likely to be of late Pleistocene age (<0.9 Ma).
From Samples 181-1121B-1H-CC to 13X-CC, the only microfossils recovered were radiolarian casts, sponge spicules, and fish teeth (e.g., in Sample 181-1121B-5X-CC), with the exception of Sample 181-1121B-3H-CC, which contains an impoverished agglutinated benthic foraminifer assemblage with frequent specimens of Rzehakina epigona and Spiroplectammina spectabilis, and rare Rhabdammina sp. and Tritaxia sp. This assemblage is Paleocene in age, in agreement with the more diverse assemblage, including the same taxa, deeper in the hole. Sample 181-1121B-6X-2, 60-65 cm, proved to be barren.
In Hole 1121B samples, a more diversified Paleocene foraminiferal assemblage was recovered in and below Core 181-1121B-11X (Table T3). The most abundant and best preserved faunas occur in Samples 181-1121B-15X-CC and 16X-CC. Identified taxa include (recorded New Zealand time ranges in brackets, from Hornibrook et al., 1989): Spiroplectammina spectablis (Late Cretaceous-Paleocene), Hormosina ovulum ovulum, Cribrostomoides trinitatensis, Glomospira charoides, Reticulophragmium aff. paupera, Rzehakina epigona (Cretaceous-Paleocene), Rhabdammina sp., Psammosphaera sp., Arenobulimina aff. dorbignyi, Dorothia aff. oxycona, Dorothia spp., Tritaxia sp., Conotrochammina whangaia (Late Cretaceous-Paleocene), Alabamina creta (latest Cretaceous-Paleocene), Allomorphina conica (late Paleocene-middle Eocene), Anomalinoides piripaua (Late Cretaceous-Paleocene), Charltonina acutimargina (latest Cretaceous-Paleocene), Nuttallides carinotruempyi (late Paleocene-middle Eocene), N. florealis (late Paleocene, Sample 181-1121B-16X-CC), Oridorsalis umbonatus, Gavelinella beccariiformis (latest Cretaceous-Paleocene), Pullenia bulloides, Valvulineria teuriensis (latest Cretaceous-Paleocene), and Lenticulina spp.
The assemblage may be assigned a Paleocene, probably middle to late Paleocene age (late Teurian Stage, ~62-55 Ma) or within deep-sea benthic zone CD1 (older than 55.5 Ma, Kaiho et al., 1993).
In Hole 1121B, diatoms are present and well preserved in the top 3 m of Core 181-1121B-1H and throughout Cores 181-1121B-6H to 11X (Table T4; Fig. F10). In the interval between (Core 181-1121B-1H-3 to Core 5H), and below (Core 181-1121B-12X), silica diagenesis has destroyed the original diatom content, and the authigenic zeoliths clinoptilolite and phillipsite are present instead.
The upper 3 m of this site contains Neogene diatom assemblages, in which from the top down a sequence of Pleistocene and then Pliocene species associations can be recognized (Fig. F10), with more rare, clearly reworked Miocene species admixed. The frequent to common occurrence of Hemidiscus karstenii, together with a typical late Pleistocene diatom assemblage, indicates an age younger than 0.42 Ma for the upper 36 cm of the core. Below, the most abundant species are characteristic of the early Pleistocene and Pliocene, such as Thalassiosira elliptipora, T. vulnifica, T. inura, T. kolbei, T. insigna, Simonseniella barboi, Nitzschia weaveri, N. praeinterfrigidaria, and N. interfrigidaria; species that ranged into the early Pleistocene such as Nitzschia barronii and Actinocyclus karstenii also occur. Although the assemblages contain several species that do not occur with overlapping stratigraphic ranges, nevertheless, some of the younger species are delimited to the upper part of this core, so that a sequence to successively older species can be constructed. An example of a relatively young species that occurs only in the upper part of the diatom-containing Neogene is Thalassiosira elliptipora, which ranges from 0.65 to 1.8 Ma. This species is not found below Sample 181-1121B-1H-1, 89-90 cm. It therefore is used as an argument to place the core interval from 36 to 90 cm into the lower Pleistocene.
Another species with an occurrence restricted to the upper part of the section is T. vulnifica (stratigraphic range 2.2-3.26 Ma). It was not found below Sample 181-1121B-1H-1, 100-102 cm, suggesting a late Pliocene age for core between 0.9 and 1 mbsf. Below 1 mbsf, reworked Miocene species like Hemidiscus ovalis, H. karstenii f. 1, Bruniopsis mirabilis, Denticulopsis hustedtii, D. dimorpha, D. meridionalis, and Nitzschia denticuloides are more common in association with an otherwise early late Pliocene-age diatom assemblage. This core interval from 1 to 3 mbsf is placed, because of the absence of T. vulnifica and the presence of the other above-listed Pliocene species, into an age range from 3.26 to 3.6 Ma.
The upper 3 mbsf, interpreted as drift deposits, is thus documenting slow sedimentation rates of 0.1 to 0.5 cm/k.y. in the late Neogene, with the intervals of preserved sediment separated by two hiatuses, and the deposits characterized also by the presence of reworked Miocene microfossils.
The well-preserved, diverse Paleocene diatom assemblage from Cores 181-1121B-6X to 11X fall into the Hemiaulus incurvus Zone of Fourtanier (1991). The marker species that delimits this zone (top Triceratium gombosi; bottom Hemiaulus peripterus) were not found during shipboard analysis. T. gombosi is a very small species, and a good separation of the diatoms from the sediment matrix was not possible during shipboard preparation, but may be possible during shore-based analysis.
The radiolarian biostratigraphy at Site 1121 is based on the examination of 20 core-catcher samples and two core samples (Table T5). The top core sample (181-1121A-1H-1, 0-2 cm) contains rich radiolarian faunas including abundant Spongoplegma antarcticum and Cycladophora pliocenica together with rare Antarctissa denticulata, Antarctissa strelkovi, Lithelius nautiloides (FO 1.93 Ma), and common Eucyrtidium calvertense (LO 1.92 Ma). The radiolarians indicate older than latest Pliocene age (>1.92 Ma). The other core-catcher samples near the top (181-1121A-1H-CC, 181-1121B-1H-CC, 1-3 cm, 181-1121B-1H-CC, 6-8 cm, and 181-1121B-1H-CC) are barren or very rarely contain radiolarians.
Sample 181-1121B-2H-CC contains very rare specimens of Amphipyndax stocki (Early Cretaceous-Paleocene), Myllocercion acineton (Late Cretaceous-Paleocene), and Amphisphaera sp. Sample 181-1121B-3H-CC yields rare but age-diagnostic specimens of Amphisphaera goruna (early Paleocene to early Eocene), Buryella tetradica (FO 61 Ma, Hollis, 1997), Lithomespilus coronatus (Late Cretaceous to early Eocene), and Rhopalocanium aff. ornatum.
Sample 181-1121B-4X-CC includes rare Amphisphaera kina, indicating an early to late Paleocene age. In Sample 181-1121B-5X-CC radiolarians are absent, but fish teeth are commonly observed. These assemblages are late Paleocene in age, which is concordant with the age assignments for the deeper samples.
Radiolarian faunas from Samples 181-1121B-6X-CC to 17X-CC are generally well preserved and highly diversified, with the exception of Samples 181-1121B-12X-CC to 14X-CC, which contain lithologies like chert or hard siliceous hemipelagic limestone. However, these faunas give an excellent, continuous record of an early to late Paleocene oceanic history. Throughout this radiolarian-rich interval, Amphisphaera coronata s.l., Amphisphaera goruna, Bathropyramis sanjoaquinensis s.l., and Spongodiscus sp. are present. In addition, common occurrence of Buryella tetradica, rare to common Buryella cf. tetradica, and rare to common Buryella granulata characterizes this interval. Corythomelissa adunca has a rare to few occurrence, which also characterizes the fossiliferous section (Samples 181-1120B-6X-2, 60-65 cm, to 11X-CC). Abundant Microsciadiacapsa sp. is present in Samples 181-1121B-7X-CC and 11X-CC. Rare to few Buryella foremanae are consistently present in the lowest Sections 181-1121B-15X-CC to 17X-CC.
The first occurrence (FO) of Buryella tetradica, which marks the base of the Buryella tetradica Zone (RP5), is placed between Samples 181-1121B-15X-CC and 16X-CC. According to the Cretaceous-Paleocene radiolarian study in New Zealand by Hollis (1997), the interval 181-1120B-6X-2, 60-65 cm, to 11X-CC (34.8-87.1 mbsf) can be assigned to the Bekoma compechensis (RP6) Zone (58.0-55.5 Ma), interval 11X-CC to 15X-CC (87.1-122 mbsf) to the Buryella tetradica (RP5) Zone (58-61 Ma), and the lowest interval 16X-CC to 17X-CC (122-134 mbsf) to the Buryella foremanae (RP4) Zone (61-63 Ma), respectively (Fig. F9). Although Bekoma compechensis, a nominal species of the Zone RP6, could not be identified, Corythomelissa adunca, which first appears in the lower part of the RP6 Zone (Hollis, 1997), was found in Samples 181-1120B-6X-2, 60-65 cm, to 11X-CC. It is noteworthy that the FO of Buryella cf. tetradica (three-segmented form) is recognized in Sample 181-1121B-15X-CC and then the FO of Buryella tetradica s.s. (four-segmented form) is recorded in Sample 181-1120B-11X-CC.
Site 1121, at the foot of the Campbell Plateau, was drilled in the vicinity of marine magnetic anomaly 32, in 4511 m present-day water depth. The present-day CCD is near 4700 m. Using generalized backtracking by assuming a normal or slightly shallow ocean spreading-ridge crest in Late Cretaceous time (late Campanian), and 1 km of Campanian-Paleocene sediment accumulation, a probable paleo-water depth of ~ 3500 m is indicated at Site 1121 in the mid-Paleocene (Fig. F11). Previous estimates of the Late Cretaceous through Cenozoic CCD in the Pacific (Van Andel, 1975; Kennett, 1982) indicate a Paleocene CCD depth just below 3000 m. Hence, Site 1121, in Paleocene time, probably was at or just below the CCD. These data are in agreement with the finding that Paleocene planktonic foraminifers have been dissolved, whereas Paleocene nannofossils are only partly dissolved, and do occur in smear-slide residues. Agglutinated foraminifers, resistant to dissolution, are common. Apparently, in Paleocene time, ODP Site 1121 was located at a paleo-water depth between the CCD for nannofossils and planktonic foraminifers. As may be seen from Figure F11, Site 1121 was above the CCD in the early Paleocene, in agreement with the fact that calcareous benthic foraminifers, and a relatively high proportion of ataxophragmiid agglutinated taxa with calcareous cemented tests, were preserved in the oldest three cores above the bottom of the hole. The unknown variables are the properties of Cretaceous ocean crust that might produce an anomalously shallow or deep ridge, and the precise location of the CCD in the vicinity of a noncarbonate continental margin (i.e., New Zealand) in a relatively warm ocean.
The young foraminiferal fauna from the upper unit in the hole (Core 181-1121B-1H) is typical of shallow abyssal environments close to the CCD. The upper samples with rich faunas contain a mix of fresh large and small planktonic forms that were rapidly buried and preserved, together with many broken and partly dissolved larger, thicker-walled tests that spent longer on the seafloor before being buried. This dissolution is indicated by the 2:1 ratio of planktonic test fragments/whole tests. Horizons with more depauperate faunas only retain the thicker-walled, larger, dissolution-resistant tests, and these intervals presumably had a lower sedimentation rate.
The impoverished agglutinated foraminiferal assemblage recovered from the Paleocene interval at Site 1121 is part of the cosmopolitan assemblage that is widespread worldwide in bathyal to shallow abyssal continental margins and in abyssal plains from the Late Cretaceous to the Paleogene (Gradstein and Berggren, 1981). The common presence of calcareous benthic foraminifers and of ataxophragmiid agglutinated taxa with calcareous cemented tests in several core-catcher samples indicates limited dissolution of tests below the local CCD. On the other hand, the marine pelagic "snow" from planktonic foraminifers was dissolved before landing on the ocean floor. That common to abundant planktonic forms might have been expected with a deeper CCD, or shallower site setting, is known through drilling of DSDP Site 277 (80% planktonic foraminifers in the upper Paleocene; Hollis et al., 1997), in 1214 m present-day water depth on the western edge of the Campbell Plateau.
In composition, the Site 1121 Paleocene fauna appears to be an impoverished version of the rich and diverse deep-water assemblage of the type Teurian and the Tawanui sections (Whangai facies) on the East Coast of the North Island of New Zealand (Hornibrook et al., 1989; Kaiho et al., 1993). The benthic fauna of Site 1121 has much in common with the impoverished assemblage recorded from the gray siltstone and limestone unit above the ribbon chert unit of middle to late Paleocene age in Marlborough, northern South Island (Strong et al., 1995), and the late Paleocene assemblage from nearby DSDP Site 277, except for the lack of planktonic forms (Hollis et al., 1997).
The Site 1121 Paleocene foraminiferal fauna differs significantly from:
During the late Neogene and Quaternary subantarctic/Antarctic, fully planktonic species were deposited at this site. During the middle Paleocene, the assemblages are dominated by cosmopolitan, neritic species, which may have been transported downslope from the Chatham Plateau area. The abundance of siliceous microfossils, especially of the most abundant group among them, the diatoms, is inversely correlated to the abundance of the authigenic zeolites clinoptilolite and phillipsite (Fig. F12). The interval of well-preserved middle Paleocene diatomaceous clayey silts is flanked above and below by sediments strongly affected by silica diagenesis. Silica diagenesis here has not only resulted in dissolution of biogenic silica and precipitation of zeolites, but also in the formation of chert nodules and chertified layers.
The Pliocene radiolarian fauna from Sample 181-1121A-1H-1, 0-2 cm, consists of diversified species of typical Antarctic/subantarctic environments (e.g., Antarctissa denticulata, Antarctissa strelkovi, and Spongoplegma antarcticum).
The early to late Paleocene radiolarian succession recognized in Hole 1121B has excellent preservation with highly diversified faunas. The succession's thickness (RP6 Zone: 34.8-87.1 mbsf, RP5 Zone: 87.1-122 mbsf, RP4 Zone: 122-134 mbsf) is similar to or greater than the age-equivalent units of the Mead Stream section (RP5 + RP4 Zones: ~50 m thick) and Woodside Creek section (RP5 + RP4 Zones: ~30 m thick) in the Marlborough area of South Island, New Zealand (Hollis, 1997; Strong et al., 1995).