Site 1125, drilled on the northern slopes of the Chatham Rise in 1360-m water depth, contained rich Neogene planktonic faunas and floras with calcareous nannofossils, foraminifers, and radiolarians. The benthic foraminifers comprise a diverse lower bathyal fauna. The calcareous microfaunas and -floras are well preserved, with recrystallization of the calcareous microfossils below Core 181-1125B-47X. Radiolarians are well preserved down to Core 181-1125B-54X. Diatoms are very scarce in the upper 160 mbsf and are missing because of dissolution below Core 181-1125B-53X. The preservation deteriorates strongly below Core 181-1125B-46X.
Rich diatom floras indicating high primary productivity, presumably upwelling conditions, are present in the upper Miocene (160 to 430 mbsf). High productivity is also indicated by the dominance of small coccoliths throughout the lower Pliocene and upper Miocene from 120 to 430 mbsf.
Reworking of older forms is less common compared to other sites. Single reworked valves of diatoms occur only sporadically. At least half of the samples investigated for calcareous nannofossils contain minor amounts of reworked specimens.
The biostratigraphy of Site 1125, as determined by the four major microfossil groups, is summarized in Figure F8. The 75 datums determined from these microfossil groups allow the construction of a detailed age-depth curve (compare Fig. F16) and demonstrate very high sedimentation rates, in excess of 200 m/m.y., for levels near the Miocene/Pliocene boundary (6.0 to 5.0 Ma).
The following major intervals have been determined by consensus results from all groups:
For the late Miocene interval, the following age-depth relationships have been determined:
- 5.7 Ma at ~290 mbsf;
- 6.6 Ma at ~360 mbsf;
- 8.0 Ma at 400 mbsf;
- 9.4 Ma at ~435 mbsf;
- 10.0 Ma at ~475 mbsf; and
- 10.7 Ma at ~548 mbsf.
The biostratigraphy of Site 1125 is mostly based on the study of core-catcher samples. Samples from Hole 1125A were used for the upper part of the section, and samples from Hole 1125B 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, and T5, all in the "Explanatory Notes" chapter.
Calcareous nannofossil biostratigraphy of Site 1125 is based upon examination of core-catcher samples taken from Cores 181-1125A-1H to 22H and Cores 181-1125B-23X to 58X. Several additional samples from within cores in the upper part of the section were also examined to increase resolution. Nannofossils are abundant and well preserved throughout the sequence, except for the lowermost portion, where nannofossil assemblages show signs of dissolution and overgrowth (Table T3). Fourteen datum levels were recognized (Table T4). Most of the age markers are in the upper Pliocene-Pleistocene section (0 to 94.85 mbsf), whereas the major part of the sequence belonging to the upper Miocene and lower Pliocene (95 to 547 mbsf) is poorly resolved.
The first sample (181-1125A-1H-CC; 4.21 mbsf) is definitely older than 0.15 Ma, based upon the presence of Helicosphaera inversa. This sample is dominated by the medium-sized Gephyrocapsa oceanica and Gephyrocapsa caribbeanica. There are a few specimens that may look similar to Emiliania huxleyi. But, because of their small size and scarcity, we are not confident in determining whether this sample indeed contains Emiliania huxleyi and, therefore, is still within Zone NN21, and, in other words, younger than 0.24 Ma. The occurrence of abundant, typical Pseudoemiliania lacunosa (first occurrence [FO] 0.42 Ma) in the upper part of the next core, Sample 181-1125A-2H-2, 54 cm (4.84 mbsf), however, suggests that the topmost Pleistocene is missing, or, alternatively, the top part of the sequence is very condensed and has a sedimentation rate of less than 1 cm/k.y. Judging from the clay-rich, hemipelagite lithology of this section, and the overall high sedimentation rates (~6 cm/k.y.) in the lower part of the sequence, the latter explanation is less likely.
Several age-diagnostic markers were found for the Pleistocene section. In Sample 181-1125A-3H-CC (23.6 mbsf) two short-ranged species were found: Reticulofenestra asanoi (last occurrence [LO] 0.85 Ma) and Gephyrocapsa parallela (FO 0.95 Ma). The coexistence of these two species constrains the age of this sample as between 0.85 to 0.95 Ma. Helicosphaera sellii occurs in the next sample (181-1125A-4H-CC; 32.83 mbsf), indicating an age older than 1.26 Ma.
In Sample 181-1125A-5H-3, 150 cm (35.8 mbsf), only a few medium-sized Gephyrocapsa were found and they disappear downsection. With the absence of any discoasterids, this sample is estimated to be very close to the origin of the medium-sized Gephyrocapsa at ~1.67 Ma. In the core-catcher of the same core (181-1125A-5H-CC; 42.58 mbsf), a few Discoaster brouweri are present without the companionship of other Discoaster species. This indicates that this sample is very close to the extinction level of Discoaster, dated at 1.96 Ma. The Pliocene/Pleistocene boundary (1.81 Ma) is therefore between these two samples at ~37.5 mbsf.
Part of the uppermost Pliocene is missing, as suggested by the occurrence of Discoaster tamalis (LO 2.76 Ma) in the next sample (181-1125A-6H-CC; 51.57 mbsf) (Fig. F9). The sporadic occurrence of discoasterids downcore during the Pliocene at this site precludes the usage of other Discoaster data. The occurrence of abundant small-sized Gephyrocapsa (2-3.5 µm in size) together with Reticulofenestra pseudoumbilicus-gelida in Sample 181-1125A-9H-CC (80.76 mbsf) in the Pliocene is somewhat unusual. The hemipelagic setting and perhaps the high biological productivity associated with the Subtropical Convergence at this site have probably combined to yield this assemblage. As shown by Rio's (1982) detailed study, this is the so-called "mid-Pliocene small Gephyrocapsa bloom," which occurred below the base of the Gauss Chron and higher than the Cochiti Event, namely between 3.58 and 4.18 Ma. The FO of Pseudoemiliania lacunosa (4.0 Ma) in the next sample (Sample 181-1125A-10H-CC; 89.91 mbsf) is consistent with this age assignment.
Discoasterids are present in the section to the bottom of the hole, but major key species are either missing or sporadic. We can only tentatively use the LO of D. quinqueramus in Sample 181-1125B-31X-CC (288.28 mbsf) as an age marker to date the latest Miocene. Within the long sequence downward to the bottom, only the persistent occurrence of Minylitha convallis from Samples 181-1125B-40X-CC to 45X-CC (379.02 mbsf to 427.81 mbsf) provides good age control. The sedimentation rate for this interval, calculated based upon the LO (7.73 Ma) and FO (9.43 Ma) of this species, is ~34 m/m.y.
The presence of Discoaster bellus in Sample 181-1125A-54X-CC (512.73 mbsf) suggests that this interval is at least younger than its first occurrence at 10.5 Ma (in the lower part of magnetic Chron 5n.2n) (Gartner, 1990). The next few samples down to the bottom of the hole contain moderately preserved nannofossil assemblages in which only a few badly preserved discoasterids were found. Therefore, the apparent FO of D. bellus at Sample 181-1125B-54X-CC may not be a genuine first appearance. Nevertheless, the absence of Coccolithus miopelagicus (LO 10.94 Ma) from the lowest part of the sequence does suggest that the bottom of the hole is younger than 10.94 Ma, which agrees with the age assignments based upon Minylitha convallis and Discoaster bellus. However, the bottom of the core is probably only a few meters away from the middle/late Miocene boundary (11.2 Ma)
In summary, nannofossil biochronology indicates that the upper Pliocene and Pleistocene is incomplete and has been truncated by at least two hiatuses, one in the upper Pliocene (2 to 3 m.y. missing?) and the other on the very top of the sequence (topmost Pleistocene missing). The lack of proper age markers for the lower Pliocene and upper Miocene hampers dating the lower part of the sequence in detail. The boundary between the Pliocene and Miocene (5.32 Ma) is estimated to be at ~240 mbsf (Fig. F9). The lower half of the sequence is represented by a thick sequence of upper Miocene. The average sedimentation rate for the late Miocene and early Pliocene is calculated to be 62 m/m.y. based upon two relatively reliable datum levels, the FOs of Pseudoemiliania lacunosa (4.0 Ma) and Minylitha convallis (9.43 Ma) (Fig. F9).
Foraminiferal faunas throughout most of Site 1125 (Tables T5 and T6) are abundant to moderately abundant and are generally well preserved above ~400 m. Evidence of dissolution is only present in a few samples in the upper part of the hole. Recrystallization and flattening of some tests is evident in only the lower samples (Sample 181-1125B-46X-CC and below).
Samples 181-1125A-1H-CC to 5H-CC (0-42.6 mbsf) are late Pliocene and Quaternary in age (0-2.6 Ma; Nukumaruan, Castlecliffian, and Haweran Stages), based on the presence throughout of sparse keeled Globorotalia crassaformis forms (G. crassula, G. crassacarina). Accurate subdivision within this interval is difficult because of the diachronous ranges of the common Globorotalia species between the tropical-subtropical and subantarctic regions. All samples in this interval contain a mixture of common Globorotalia inflata and Globorotalia puncticuloides (the latter has a recorded LO in this region of ~0.6 Ma), suggesting that the first core may contain a condensed section or hiatus and that the remainder of the interval is 0.6-2.6 Ma. The following datums are observed within this interval.
The first occurrence of Globorotalia truncatulinoides is in Sample 181-1125A-3H-CC (FO ~2 Ma in the tropics-subtropics; ~0.8 Ma in the subantarctic). The last occurrence of Stilostomella and Plectofrondicularia advena (0.6-0.8 Ma) also occurs in Sample 181-1125A-2H-CC. These suggest that Sample 181-1124C-1125A-2H-CC is older than ~0.6 Ma.
Sample 181-1125A-4H-CC contains Globigerinoides extremus, which ranges up into planktonic foraminifer zone PL6, late Pliocene. Its LO varies between oceans but it may range up to ~1.8 Ma.
Sample 181-1125A-5H-CC contains the LO of Globorotalia inflata triangula (~2 Ma) and the FO of Globorotalia crassula (2.6 Ma). There appears to be a condensed section or hiatus within Core 181-1125A-6H of ~0.5 m.y.
Samples 181-1125A-6H-2, 130-135 cm, to 7H-CC (45.1-61.8 mbsf) contain the dextral coiling forms of unkeeled Globorotalia crassaformis (2.1-3.0 Ma). When coupled with the absence of Globorotalia crassula (FO 2.6 Ma) and presence of Globorotalia tosaensis (FO 3.0 Ma), this suggests an age of 2.6-3.0 Ma (late Pliocene, Mangapanian Stage) for this interval.
Samples 181-1125A-8H-CC to 11H-CC (70.4-99.8 mbsf) are late Pliocene (3.0-3.6 Ma, Waipipian Stage), based on the presence of Globorotalia crassaconica (LO 3.0 Ma), sinistral unkeeled Globorotalia crassaformis (last common occurrence [LCO] 3.0 Ma), Globorotalia puncticuloides (FO 3.6 Ma), Globorotalia inflata (FO 3.7 Ma), and Globorotalia inflata triangula (FO 3.6 Ma).
Samples 181-1125A-13H-CC to 181-1125B-25X-CC (118.8-234.7 mbsf) are early Pliocene in age (3.7-5.2 Ma, Opoitian Stage), based on the presence throughout of the zone species Globorotalia puncticulata (3.7-5.2 Ma) and Globorotalia pliozea (LO 3.6 Ma). This lower Pliocene interval can be divided into upper (3.7-4.7 Ma) and lower sections (4.7-5.2 Ma) at Sample 181-1125A-17H-CC, which contains the FO of Globorotalia crassaconica (FO 4.7 Ma) and the LO of Globorotalia mons (LO 4.8 Ma).
Sample 181-1125B-25X-CC (234.7 mbsf) lies on the Kapitean/Opoitian Stage boundary, just above the Miocene/Pliocene boundary, because it contains the LO of Globorotalia sphericomiozea (LO 5.2 Ma) and FO of Globorotalia crassaformis (FO 5.2 Ma).
Samples from 181-1125B-25X-CC to 29X-CC (234.7-274 mbsf) are latest Miocene in age (5.2-5.6 Ma), based on the presence of Globorotalia sphericomiozea (5.2-5.6 Ma) and the following accompanying species, which assist in subdividing this short interval:
Samples 181-1125B-31X-CC to 45X-CC (288-427.8 mbsf) are middle late Miocene in age (5.6-9.9 Ma), based on the abundant presence of sinistral Globorotalia miotumida (LO ~5.4 Ma) and the absence of Globoquadrina dehiscens (LO 9.9 Ma). Samples 181-1125B-37X-CC and above contain Globorotalia juanai (FO 6.6 Ma), providing a useful datum within this interval.
Sample 181-1125B-37X-CC has a temperate aspect with Globorotalia explicationis and G. menardii. Also present in the sample are Bolboforma spp. and common Neogloboquadrina pachyderma. A similar assemblage, but without G. juanai and with Catapsydrax parvulus, occurs in Sample 181-1125B-39X-CC.
Samples 181-1125B-41X-CC and lower are much more indurated ("chalky") than are the sediments above, with poor preservation, which persists to total depth in the hole. The assemblages in Samples 181-1125B-41X-CC to 46X-CC include Globorotalia partimlabiata, Catapsydrax parvulus, Globigerinoides quadriloba, G. decoraperta, common to abundant Globorotalia miotumida (both encrusted and nonencrusted forms), Zeaglobigerina nepenthes, Neogloboquadrina pachyderma, and Globigerina quinqueloba (often frequent). In this assemblage N. pachyderma (first common occurrence [FCO] 9.2 Ma, FO 11.3 Ma) is rare, but we are unsure whether this should be interpreted as an age older than 9.2 Ma or not. Its rarity may be an effect of dilution of the planktonic foraminiferal tests by abundant clays, since the late Miocene sedimentation rate is in excess of 100 m/m.y. (see "Age Models and Sedimentation Rates").
The bolboformids, Bolboforma aff. metzmacheri (8.5-10.5 Ma) and B. pentaspinosa (7-11.5 Ma), occur in Sample 181-1125B-45X-CC, which indicates that this level is probably older than 8.5 Ma.
Samples 181-1125B-48X-CC to 57X-CC (450.8-542.2 mbsf) are 9.9-10.7 Ma (late early Tongaporutuan Stage), based on the co-occurrence throughout of Globoquadrina dehiscens (LO 9.9 Ma), and common, sinistral-dominated Globorotalia miotumida (FO of sinistral form 10.7 Ma). G. dehiscens only becomes common in Sample 181-1125B-52X-CC and could possibly represent the calibrated datum at 9.9 Ma. Sporadic specimens referable to Paragloborotalia mayeri (LCO 11.25 Ma) occur in Samples 181-1125B-48X-CC and 57X-CC. However, with the weight of other evidence, we consider that these records must lie above the last consistent occurrence datum of this species.
Sample 181-1125B-57X-CC contains the LO of Globorotalia panda (LO 10.3 Ma). At the bottom of the hole (548.2 mbsf), Sample 181-1125B-58X-CC contains more dextral than sinistral specimens of Globorotalia miotumida (LO 10.7 Ma, FO 10.9 Ma), indicating an age not much older than 10.7 Ma. Consistent with this is the presence of common Zeaglobigerina nepenthes (FO 11.8 Ma) and small Globorotalia scitula (FO 11.5 Ma). Hence the hole bottomed in beds of early late Miocene age (~10.7-10.9 Ma).
We list below a summary of foraminiferal ages at Site 1125, in terms of the New Zealand stage classification, and local chronological calibration of these stages, according to Table T2 in the "Explanatory Notes" chapter.
Diatoms are present throughout the Neogene section recovered at this site on the upper slope of the northern Chatham Rise, except for the lower five cores (Cores 181-1125B-54X to 58X). Their preservation deteriorates and species diversity decreases from 440 mbsf (Core 181-1125B-47X) downward. In the upper 160 m of the profile, diatoms are too scarce to provide reliable stratigraphic information. But from Core 181-1125A-18H downward, that is, in the lowermost Pliocene to upper Miocene sediments, diatoms are abundant and the assemblages diverse. Silicoflagellate occurrence shows a similar pattern. The following species provide reliable datums in this interval:
Hemidiscus triangularis, LO 5.3 Ma, 181-1125B-24X-CC; Hemidiscus triangularis, FO 5.6 Ma, 181-1125B-35X-CC; Nitzschia miocenica, LO 5.7 Ma, 181-1125B-35X-CC; Hemidiscus ovalis, LO 5.7 Ma, 181-1125B-38X-CC; Hemidiscus ovalis, FO 7.9 Ma, 181-1125B-45X-CC; and Denticulopsis hustedtii, LO ?8.4 Ma, 181-1125B-47X-CC.
Within the Messinian interval of high primary productivity and high sedimentation rates, a dissolution event is documented in Core 181-1125B-27X in the diatom assemblages, which may indicate a hiatus, although it can only represent a relatively short time interval.
With silicoflagellates the Miocene/Pliocene boundary has to be placed between Cores 181-1125B-25X and 32X (Fig. F8). In Sample 181-1125B-25X-CC and above, Mesocena quadrangula occurs, whereas the presence of Mesocena hexalitha in Sample 181-1125B-32X-CC and the consistent occurrence of Mesocena diodon borderlandensis below proves a Miocene age from at least this core downward.
Occasional benthic diatoms are found, but hardly any older, reworked diatom valves or silicoflagellates were encountered.
Radiolarian biostratigraphy at Site 1125 is based on the examination of 59 core-catcher samples. Radiolarian faunas are generally abundant and very well preserved throughout the section, except in the lowest part of the section (Samples 181-1125B-54X-CC to 58X-CC; 512.73-548.22 mbsf). The radiolarian faunas at Site 1125 are dominated mostly by cosmopolitan and middle latitude species associated with a few subtropical species. In addition, the faunas are characterized by the absence of Antarctic/subantarctic species (e.g., various species of Antarctissa, Lithelius nautiloides, and Saccospyris antarctica).
Sample 181-1125A-3H-CC (23.6 mbsf) contains a single specimen of Stylatractus universus associated with Eucyrtidium calvertense, Lamprocyrtis heteroporos (LO 1.8 Ma), Theocorythium trachelium (FO 1.6-1.7 Ma), and Theocorythium vetulum (LO 1.2-1.3 Ma). Although the age for the sample is estimated to be early Pleistocene (1.7-1.8 Ma), this age is rather older than the age indicated by other microfossils. Reworking is possible. This, as well as other factors, may be the reason that the top two core-catcher samples yielded only very rare or few radiolarians without age-diagnostic species. Rare specimens of Eucyrtidium matuyamai (LO 1.0 Ma and FO 2.0 Ma in Morley and Nigrini, 1995) occur in Sample 181-1125A-4H-CC (32.83 mbsf).
In Sample 181-1125A-6H-CC (51.57 mbsf), the radiolarian fauna includes few Stichocorys peregrina, Lamprocyrtis neoheteroporos, Axoprunum angelium, and Theocorys redondoensis. The last occurrence of Sphaeropyle langii is recorded in Sample 181-1125A-9H-CC (80.76 mbsf). In Sample 181-1125A-10H-CC (89.91 mbsf), the last occurrence of Lychnodictyum audax (LO 3.7 Ma) indicates the sample is of early late Pliocene age.
Sample 181-1125B-28X-CC (264.44 mbsf) contains a diversified radiolarian fauna, including Lychnocanoma parallelipes (LO 5.6 Ma), Dictyophimus aff. splendence, and Didymocyrtis penultima. This assemblage indicates a latest Miocene age (5.6 Ma) for the sample.
Dissolution of foraminiferal faunas by corrosive bottom waters is only sporadically apparent, as determined by the presence of fragmented planktonic foraminiferal tests, in some darker (colder) intervals in the upper Pliocene to Holocene section. Elsewhere the assemblages contain rich, oceanic planktonic (>95% of foraminifers) faunas and diverse, lower to mid-bathyal benthic foraminiferal faunas. Many samples from the interval younger than ~8 Ma contain abundant, small Neogloboquadrina pachyderma and Globigerina bulloides, typical of cool temperate waters. The larger size fraction of the fauna is typically dominated by warm temperate Globorotalia forms (e.g., G. inflata, G. puncticuloides, G. puncticulata, G. mons, G. pliozea, and G. miotumida), although their abundance varies. More detailed quantitative work on the faunas is likely to provide better paleoenvironmental interpretations.
The diatom assemblages are oceanic and dominated by species of the genera Thalassionema and Thalassiothrix, indicating high productivity. Temperate to subtropical species prevail.
High productivity or upwelling conditions are indicated especially for the time interval between 5.0 and 6.0 Ma. But also in the sediments above and below, with considerably lower sedimentation rates, these same species are dominant. But here they do not indicate high productivity because the overall abundance of diatoms and the diversity of the diatom assemblages is low. This indicates silica dissolution. Winnowing is not a probable explanation as calcareous nannofossils abound in these sediments.