Previous   |    Next

doi:10.2204/iodp.proc.314315316.136.2009

Biostratigraphy

Three holes were drilled at Site C0008 (Holes C0008A, C0008B, and C0008C). The biostratigraphy determined for Site C0008 was mainly based on examination of calcareous nannofossils and radiolarians from Holes C0008A and C0008C.

Calcareous nannofossils

All core catcher samples plus additional samples from some critical intervals in the vicinity of zonal boundaries were examined for calcareous nannofossils at Site C0008. Calcareous nannofossils are generally abundant and moderately preserved above 100 m CSF in Holes C0008A and C0008C; they are in low abundance and moderately to mostly poorly preserved below that depth in both holes (Tables T7, T8).

In Hole C0008A, 16 nannofossil biostratigraphic events are recognized (Table T9). The uppermost Sample 316-C0008A-1H-CC contains few Emiliania huxleyi and abundant Gephyrocapsa spp. (>3.5 µm), indicating the transition (X) of Gephyrocapsa spp. (>3.5 µm) to E. huxleyi (<0.063 Ma) within Zone NN21. Samples from 316-C0008A-2H-CC through 31X-CC are assigned to Zone NN19, based on the presence of Pseudoemiliania lacunosa and absence of reliable occurrence of Discoaster brouweri (marker of Zone NN18). The top of Zone NN19 is not marked by the last occurrence (LO) of P. lacunosa, as Sample 316-C0008A-2H-CC contains Reticulofenestra asanoi but lacks Gephyrocapsa spp. (≥4 µm), indicating an age interval of Zone NN19, between 1.04 Ma (reentrance [RE] of Gephyrocapsa spp. [≥4 µm]) and 1.078 Ma (first consistent occurrence [FCO] of R. asanoi). There should be low sedimentation rates or a discontinuity between these two samples. The subdivision of Zone NN19 is somewhat difficult as the event sequence of the LO of Gephyrocapsa spp. (>5.5 µm) LO of Helicosphaera sellii, FCO of Gephyrocapsa spp. (>5.5 µm), and LO of Calcidiscus macintyrei (≥11 µm) repeats (Table T9; Fig. F15). The age reversal occurs above and below Sample 316-C0008A-9H-CC. The radiolarian record in Hole C0008A suggests the age reversal occurs below Sample 316-C0008A-9H-CC in agreement with the nannofossil record. As seismic results showed a continuous normal sequence in this hole, it is hard to explain the record at the present time. Considering the location of Hole C0008A on the unstable slope, slumping might be one possible cause for the sequence disturbance. On the other hand, the possibility exists that determinations of microfossil events were affected by reworked fossils. The latter possibility will be examined in postcruise study. The lowest event of Zone NN19 is the first occurrence (FO) of Gephyrocapsa spp. (>3.5 µm) at 238.91 m CSF. Pliocene Zones NN18, NN17, and NN16 are recognized in the lower sequence from 254.10 to 282.35 m CSF in Hole C0008A (Table T9; Fig. F15). The occurrence of Sphenolithus spp. at 282.35 m CSF is taken as its LO event for the lowest possible event in Zone NN16. No reliable data were obtained below 282.35 m CSF because of poor recovery. The lowermost sample from 329.36 m CSF contains abundant Reticulofenestra pseudoumbilicus (>7 µm), dominant Sphenolithus spp., and a few Discoaster quinqueramus (marker of Zone NN11), indicating an age of Miocene Zone NN11. This sample may be a mudclast (lithoclast) fallen from one of the mudstone gravels encountered higher in the borehole. No other samples of Miocene age were identified during shipboard study, however. Postcruise study will clarify the association of this lowermost sample.

A total of six Pleistocene nannofossil events were determined for Hole C0008C (Table T10). Sample 316-C0008C-1H-CC contains dominant E. huxleyi, indicating the sample is younger than the transition of Gephyrocapsa spp. (>3.5 µm) to E. huxleyi, and is assigned to Zone NN21 (<0.063 Ma). Sample 316-C0008C-2H-CC contains Gephyrocapsa spp. (>5.5 µm) and abundant Gephyrocapsa lumina (an earlier Pleistocene species) in the Gephyrocapsa medium II group, and the occurrence of the Gephyrocapsa spp. (>5.5 µm) is determined as its LO event. As a result, Sample 316-C0008C-2H-CC is assigned to Zone NN19. A ~1 m.y. difference between Samples 316-C0008C-1H-CC and 2H-CC implies low sedimentation rate or discontinuity between these two samples (Fig. F16). Zone NN19 sediments in Hole C0008C are divided by the following events: the LO of H. sellii, FCO of Gephyrocapsa spp. (>5.5 µm), LO of C. macintyrei (≥11 µm), and FO of Gephyrocapsa spp. (>3.5 µm). These events are comparable to the same events below Sample 316-C0008A-9H-CC in Hole C0008A. The sporadic occurrence of the Pliocene Zone NN18 marker D. brouweri in the Pleistocene sequence is clearly due to reworking. Therefore, the entire sequence recovered in Hole C0008C is assigned to the Pleistocene.

In addition, the nannofossil record at Site C0008 is notable. Sample 316-C0008A-19H-CC in Hole C0008A and Samples 316-C0008C-15H-CC and 16H-CC in Hole C0008C contain a number of small forms (~2.5–3 µm) that are morphologically very similar to E. huxleyi (marker of Zone NN21), and their occurrence in these samples is marked with a “?” (Tables T7, T8). These samples are assigned to Zone NN19 according to the presence of Gephyrocapsa spp. large (>5.5 µm) and abundant G. lumina (an earlier Pleistocene species) at the present time. A detailed study on this small form is necessary to refine biostratigraphy of Site C0008 during postcruise study, including examination of more samples and study with a scanning electron microscope (SEM).

Other microfossil groups

Radiolarians, diatoms, and foraminifers are common to abundant and moderately to poorly preserved throughout the upper part of Holes C0008A, C0008B, and C0008C. Sediments retrieved below Core 316-C0008A-30X are barren of radiolarians but contain a few silty horizons with abundant foraminifers, mainly benthic. All core catchers from both holes and a few additional discrete samples (Hole C0008A only) have been processed for radiolarian content. When foraminifers were abundant, samples were also checked for marker species.

Radiolarians are common to abundant and moderately to poorly preserved in the upper part of the sequence in Hole C0008A (until Section 316-C0008A-5H-CC). Preservation and abundance decreases but radiolarians still occur sparsely to Core 316-C0008A-30X. Two zones are identified. Sample 316-C0008A-1H-CC and possibly 2H-CC are included in the Botryostrobus aquilonaris Zone (Fig. F17). In Sample 316-C0008A-3H-CC, the occurrence of Eucyrtidium matuyamai and Stylatractus universus assigns that sample to the Eucyrtidium matuyamai Zone, implying an age gap within the S. universus interval. This gap correlates with low sedimentation rates or a discontinuity between Cores 316-C0008A-1H-CC and 2H-CC, clearly shown by the calcareous nannofossil age determination. The interval to Sample 316-C0008A-30X-CC was also attributed to the E. matuyamai Zone, although the marker species only occurs to Core 316-C0008A-25X-CC, but species occurring only in the Pleistocene were found to Core 316-C0008A-30X (e.g., Spongaster tetras).

Between Cores 316-C0008A-27X and 30X, layers with abundant foraminifers are observed. Most of these foraminifers are benthic with very few small planktonic taxa. In Cores 316-C0008A-31X through 316-C0008A-40X, no radiolarians are found.

In Hole C0008C (including Section 316-C0008B-1H-CC), radiolarians are common to abundant and moderately to poorly preserved in the upper part of the sequence (until Section 316-C0008C-15H-CC). Preservation and abundance decreases downhole, but radiolarians still occur sparsely to Core 316-C0008C-24X. The Botryostrobus aquilonaris Zone is identified to Sample 316-C0008A-2H-CC. The occurrence of E. matuyamai in the interval from Sample 316-C0008C-3H-CC through Sample 316-C0008C-24H-CC indicates an age range between 1.05 and 1.98 Ma for this interval (25.37–166.90 m CSF) and implies an age gap spanning at least the entire S. universus Zone between 15.48 and 25.37 m CSF.

Summary

Two microfossil groups (from core catcher samples), calcareous nannofossils and radiolarians, were analyzed for biostratigraphy at Site C0008. Moderately preserved and abundant calcareous nannofossils and radiolarians were seen in samples from the upper part of the sequences in Holes C0008A and C0008C, whereas they are relatively poorly preserved in the lower part of the sequences in both holes.

In Hole C0008A, calcareous nannofossil zones from Pleistocene Zone NN21 through Pliocene Zone NN16 were recognized and Pleistocene radiolarians B. aquilonaris and E. matuyamai zones were determined. Records from both of these microfossil groups suggest a discontinuity between Samples 316-C0008A-1H-CC and 3H-CC and a possible age reversal or disturbance between Samples 9H-CC and 10H-CC.

In Hole C0008C, calcareous nannofossil zones from Pleistocene Zones NN21 through NN19 were recognized and Pleistocene radiolarians B. aquilonaris and E. matuyamai zones were determined. Records from both of these microfossil groups suggest a discontinuity between Samples 316-C0008A-1H-CC and 3H-CC.

Top of page   |    Previous   |    Next