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doi:10.2204/iodp.proc.314315316.124.2009

Biostratigraphy

Preliminary biostratigraphy for Site C0002 is based on calcareous nannofossils and planktonic foraminifers. The complete nannofossil stratigraphy was established during the expedition, but ship-board planktonic foraminiferal biostratigraphic results were only generated for the upper 773 m CSF of Hole C0002B because of the early scheduled departure of the foraminifer biostratigrapher before the end of Expedition 315. The remaining examination of planktonic foraminifers was shore-based. Biostratigraphy for Holes C0002C and C0002D is exclusively based on calcareous nannofossils.

Preservation and abundance of calcareous microfossils vary throughout the sequence. Despite differential preservation of nannofossil and foraminifers in the studied samples, a general common preservational pattern of well- to moderately preserved microfossil assemblages in the upper part of the site (478.28–930.10 m CSF) and moderate to poor preservation below 930.10 m CSF is observed. Microfossil abundances vary strongly. Nannofossil and foraminifer age estimates are in good agreement for Site C0002 (Fig. F14). Retrieved sediments provide a discontinuous sedimentary record from the Pleistocene to the late Miocene. The succession of calcareous microfossil events delineates intervals of different sedimentation rates. Assemblages recovered from the upper 848.38 m CSF (lithologic Units I and II) are Pleistocene in age. The spacing of biostratigraphic events is wide, revealing high sedimentation rates between 400 and 800 m/m.y. In contrast, the sequence belonging to lithologic Unit III comprises a suite of Pliocene biostratigraphic events within a short depth interval (848.38–923.71 m CSF). Here, sedimentation rates are much lower, ranging from 18 to 30 m/m.y. Some samples even contain several events of different age. These horizons can be interpreted as results of low sediment rates or tectonic processes. In sediments of lithologic Unit IV only one nannofossil event was recognized, assigning this unit a late Miocene age.

Calcareous nannofossils

Shipboard nannofossil stratigraphy for this site is based on the recognition of the 22 events reported in Table T7. Nannofossils are continuously present throughout the sequence (Tables T8, T9). Although nannofossils significantly decrease in abundance downhole, a good biostratigraphic framework was established for the entire succession. In samples from the uppermost 923.71 m CSF of Site C0002, nannofossils are abundant to common with moderately to well-preserved assemblages, with the exception of an interval from 555.27 to 613.76 m CSF where nannofossil abundances are low and specimens are moderately preserved. Below 923.71 m CSF, nannofossil content of samples mainly belonging to lithologic Unit IV decreases drastically with only poorly or moderately preserved specimens.

Core catcher samples recovered from the upper 158.90 m of Holes C0002C and C0002D (Table T9) provide the biostratigraphic framework of the uppermost 203.57 m CSF for this site. Spot-coring started from Core 315-C0002D-15X. For these cores only minimum or maximum age datums can be provided.

The only sample recovered from Hole C0002C (Sample 315-C0002C-1H-CC, 0–5 cm) contains dominant Emiliania huxleyi over Gephyrocapsa spp. (>3.5 µm). Therefore, cored material from 6.47 m CSF is younger than 0.063 Ma. However, the uppermost core catcher sample from Hole C0002D (Sample 315-C0002D-1H-CC, 27–32 cm) at 5.85 m CSF yields dominant Gephyrocapsa spp. (>3.5 µm) over E. huxleyi, indicating a minimum age of 0.063 Ma for the retrieved sediment. It is also possible that differences in abundance of the species mentioned are ecologically controlled. The first occurrence (FO) of E. huxleyi, which also defines the base of Zone NN21, is found between Samples 315-C0002D-4H-CC, 0–5 cm, and 5H-CC, 0–5 cm, at a mean depth of 38.81 m CSF. Within the same interval, the last occurrence (LO) of Pseudoemiliania lacunosa in Sample 315-C0002D-5H-CC, 0–5 cm, at 43.52 m CSF is recorded. This event marks the base of Zone NN20 and provides a minimum age of 0.436 Ma. None of the studied samples could be placed in nannofossil Zone NN20. Either Zone NN20 lies in the core sections above the core catcher or it is not present because of a hiatus. Sample 315-C0002D-11H-CC, 0–5 cm (100.70 m CSF), contains Reticulofenestra asanoi. The last consistent occurrence (LCO) of this species indicates a minimum age for this sample of 0.9 Ma. The presence of R. asanoi along with common Gephyrocapsa spp. (≥4 µm) provides a maximum age of 1.04 Ma for Sample 315-C0002D-15X-CC, 19–24 cm, based on their reentrance. R. asanoi is commonly present in the lowermost sample, 315-C0002D-18H-CC, 0–5 cm, at 203.57 m CSF. The bottom sediments of Hole C0002D are younger than 1.078 Ma based on the first consistent occurrence of this species.

Cores retrieved from the upper part of Hole C0002B from 470 to 848.38 m CSF yield abundant and very well preserved calcareous nannofossils, assigning lithologic Unit II a Pleistocene age (Table T8). Within the Pleistocene assemblages several events could be identified. The uppermost core catcher sample from Hole C0002B contains Gephyrocapsa spp. (>5.5 µm). Hence, their LO indicates an age of at least 1.24 Ma for Sample 315-C0002B-1R-CC, 0–23.5 cm (478.28 m CSF). The LO of Helicosphaera sellii was found between Samples 315-C0002B-2R-CC, 0–5 cm, and 3R-CC, 11.5–16.5 cm (485.98–495.37 m CSF). The LCO of Gephyrocapsa spp. (>5.5 µm) is recorded between Samples 315-C0002B-2R-CC, 0–5 cm, and 3R-CC, 11.5–16.5 cm (485.98–495.37 m CSF). Low sedimentation rates are assumed for the interval between Samples 315-C0002B-40R-CC, 0–5 cm, and 48R-CC, 0–5 cm (837.56–916.77 m CSF). They are delineated by several nannofossil events. The Pliocene/Pleistocene boundary is placed between Samples 315-C0002B-40R-CC, 0–5 cm, and 42R-CC, 0–5 cm (837.56–859.11 m CSF), based on two nannofossil events occurring above and below the epoch boundary. In cores above the boundary, the FO of Gephyrocapsa spp. (>3.5 µm) is detected at 842.97 m CSF, indicating an age of 1.67 Ma between Samples 315-C0002B-40R-CC, 0–5 cm, and 41R-CC, 0–5 cm (837.56–848.38 m CSF). The LO of Discoaster brouweri (2.06 Ma), which defines the base of nannofossil Zone NN19 that approximates the Pliocene/Pleistocene boundary, is recorded at 853.75 m CSF between Samples 315-C0002B-41R-CC, 0–5 cm, and 42R-CC, 0–5 cm. The base of Zone NN18 is determined by the LO of Discoaster pentaradiatus between 2.39 and 2.51 Ma. This event is located between Samples 315-C0002B-42R-CC, 0–5 cm (859.11 m CSF), and 43R-CC, 0–5 cm (870.52 m CSF). In the same interval, the LO of Discoaster tamalis, which provides an age datum of 2.87 Ma, is recorded. The LO of Discoaster surculus, which marks the base of Zone NN17 at 2.52 Ma, is found at a mean depth of 891.78 m CSF, between Samples 315-C0002B-45R-CC, 0–5 cm, and 46R-CC, 0–5 cm. The LO of Sphenolithus spp. at 3.65 Ma is placed between Samples 315-C0002B-47R-CC, 0–5 cm, and 48R-CC, 0–5 cm, at a mean depth of 911.84 m CSF. The interval between Samples 315-C0002B-48R-CC, 0–5 cm (916.77 m CSF), and 49R-CC, 0–5 cm (923.71 m CSF), contains several nannofossil events, suggesting either sediment starvation or an unconformity. Here the LO of Reticulofenestra pseudoumbilicus (>7 µm), the LO of Ceratolithus acutus, and the FO of Ceratolithus rugosus, which marks the base of Zone NN13, are detected, spanning a maximum time interval of 3.79–5.12 m.y. The LO of Ceratolithus acutus at 5.04 Ma, which approximates the Miocene/Pliocene boundary, was not discovered in the studied samples. The LO of Discoaster quinqueramus at 5.59 Ma is located between Samples 315-C0002B-49R-CC, 13–18 cm (923.71 m CSF), and 50R-CC, 0–5 cm (930.10 m CSF). This event is assigned to the uppermost Miocene. Here, a marked change in nannofossil abundance and preservation is observed. Below 930.10 m CSF, nannofossil abundance varies from rare to few with mostly poorly preserved specimens. The lowermost event detected, the LO of Nicklithus amplificus, provides an additional datum level of 5.9 Ma between Samples 315-C0002B-65R-CC, 0–5 cm (1049.38 m CSF), and 66R-CC, 6–11 cm (1052.40 m CSF). This observation indicates an age of at least 5.9 Ma for the bottom of the hole. The concomitant presence of R. pseudoumbilicus (>7 µm) further constrains the time window for this interval to a maximum age of 7.1 Ma.

Planktonic foraminifers

All Hole C0002B core catcher samples from 478.28 to 1052.40 m CSF were examined in onboard and shore-based studies. Fossil preservation of planktonic foraminifers shows changes from poor (or even barren) to good throughout the examined sequence. Besides changes in foraminiferal preservation and abundance, there is also a prominent shift in assemblage composition observable. It occurs at the Unit III/IV boundary. Assemblages within Units II and III are mainly composed of temperate to cosmopolitan genera (Neogloboquadrina, Globigerina, and Globoconella) associated with tropical to subtropical genera (Globigerinoides, Globorotalia, and Pulleniatina). In contrast, the lower assemblage from Unit IV is characterized by cold-temperate to cosmopolitan taxa, including Neogloboquadrina pachyderma and Globigerina bulloides.

Fourteen biohorizons are recognized from Hole C0002B (Table T10). Because of the sparse and patchy occurrence of a number of biostratigraphically important key species, the exact positioning of datum events remains questionable and requires the postcruise analysis of additional samples. Stratigraphic distribution of selected species is shown in Table T11. Globoturborotalita obliquus sporadically appears in the sequence. Its uppermost occurrence is recorded in Sample 315-C0002B-2R-CC, 0–5 cm. Therefore, the LO of the species (1.30 Ma) should be located above 485.98 m CSF. Neogloboquadrina asanoi exhibits a relatively high abundance in Sample 315-C0002B-42R-CC, 0–5 cm. The LO of the species (1.8 Ma) is placed between Samples 315-C0002B-41R-CC, 0–5cm, and 42R-CC, 0–5cm (848.38–859.11 m CSF). Truncorotalia truncatulinoides yields sparsely in the upper part of the sequence, and the FO of this species (1.93 Ma) is suggested between Samples 315-C0002B-31R-CC, 0–5 cm, and 315-C0002B-32R-CC, 0–5 cm (753.86–767.20 m CSF). This biohorizon defines the lower boundary of Zone N.22. The FO of T. truncatulinoides is placed >100 m above the LO of N. asanoi at the present site. The reverse occurrence of these biohorizons was also observed at Site C0001. The delay of this event in the study area may be caused by ecological factors.

The FO of Globoconella inflata modern form (2.3–2.5 Ma) is clearly recognized between Samples 315-C0002B-42R-CC, 0–5 cm, and 43R-CC, 0–5 cm (859.11–870.52 m CSF). The FO of Truncorotalia tosaensis (3.35 Ma) defining the base of Zone N.21 is observed in the same interval as G. inflata. However, the precise position of the biohorizon could be located further downhole because of the sporadic occurrence of the species. Three samples from Unit IV contain Dentoglobigerina altispira, and the LO of this species (3.47 Ma) is supposed to be above Sample 315-C0002B-49R-CC, 13–18 cm (923.71 m CSF). The LO of Sphaeroidinellopsis seminulina sensu Gradstein et al. (2004) (S. seminulina and S. subdehiscens of the present author) (3.59 Ma) is detected between Samples 315-C0002B-47R-CC, 0–5 cm, and 48R-CC, 0–5 cm (906.91–916.77 m CSF). Hirsutella margaritae occurs only in two samples from Unit IV. The LO of this species (3.85 Ma) is located above Sample 315-C0002B-52R-CC, 0–5 cm (949.06 m CSF). Pulleniatina spp., consisting of Pulleniatina primalis and Pulleniatina obliquiloculata, show the change in coiling direction from sinistral to dextral (SD) between Samples 315-C0002B-46R-CC, 0–5 cm, and 49R-CC, 13–18 cm (897.36–923.71 m CSF). This coiling change corresponds to the older event SD1 (4.08 Ma). The younger event SD2 (1.7–1.8 Ma) cannot be identified in the present site, probably because of the sparse occurrence of Pulleniatina spp. around the lower part of Unit II. The FO of Truncorotalia crassaformis (4.31 Ma) is recognized between Samples 315-C0002B-48R-CC, 0–5 cm, and 49R-CC, 13–18 cm (916.77–923.71 m CSF). The LO of Globoturborotalita nepenthes (4.37 Ma) is found between Sample 315-C0002B-51R-CC, 0–5 cm, and 52R-CC, 0–5 cm (946.24–949.06 m CSF). Because of the sparse occurrence of Globorotalia tumida and Globigerinoides conglobatus in the studied samples, their FO datum levels are tentatively placed. The FO of G. tumida is suggested between Samples 315-C0002B-49R-CC, 13–18 cm, and 50R-CC, 0–5 cm, at a mean depth of 926.90 m CSF, providing a datum level of 5.57 Ma. The FO of G. conglobatus at 6.2 Ma is recorded between Samples 315-C0002B-57R-CC, 0–5 cm, and 58R-CC, 0–5 cm (996.35–1007.50 m CSF). The lowermost sample of the present site (315-C0002B-66R-CC, 6–11 cm; 1052.40 m CSF) yields P. primalis. The presence of the species indicates the maximum age of the bottom of the hole to be 6.6 Ma based on its FO, which determines the base of Subzone N.17b.

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