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doi:10.2204/iodp.proc.303306.107.2006

Biostratigraphy

Samples from Site U1307 yield common to rare calcareous, siliceous, and organic-walled microfossils moderate to poor in preservation (Tables T3, T4, T5, T6, T7, T8, T9, T10, T11). Twenty datum events of calcareous nannofossils, diatoms, planktonic foraminifers, and dinocysts provide a preliminary age model at Site U1307 (Fig. F12). According to these events, at Site U1307 we recovered an Upper Pliocene–Pleistocene sequence spanning at least the last 3.4 m.y. An unconformity with ~0.24 m.y. duration, or a highly condensed section, is likely between Samples 303-U1307A-7H-1, 10–11 cm, and 7H-5, 10–11 cm (56.15–62.15 mcd). The hiatus is implied by absence of calcareous nannofossil marker species Gephyrocapsa spp. (large form).

Floral and faunal assemblages of planktonic organisms provide some insight into paleoceanographic conditions. Species abundance is generally low (Fig. F13). Assemblages of all groups are characterized by the presence of species indicative of subpolar to polar conditions during the Pleistocene. In particular, planktonic foraminifer assemblages are characterized by the dominance of Neogloboquadrina pachyderma (sinistral). In the early Late Pliocene (before 2.74 Ma), the sediments contain warm-water species such as the calcareous nannofossil Discoaster asymmetricus, Discoaster brouweri, Discoaster pentaradiatus, Discoaster surculus, and Discoaster tamalis.

Calcareous nannofossils

Calcareous nannofossils were examined in all core catcher samples from Holes U1307A and U1307B. Additional samples from interval 303-U1307A-5H-1, 10–11 cm, to 8H-6, 10–11 cm (1 sample/section), were also studied to determine if an unconformity exists between Samples 303-U1307A-7H-1, 10–11 cm, and 7H-5, 10–11 cm, and other stratigraphic issues in the sequences. Nannofossils are generally few to common in most samples and abundant or barren in a small number of samples. Preservation is poor to moderate and rarely good in some samples, and diversity is comparatively low. However, most of datum planes described by Sato et al. (1999) have been identified in the sedimentary sequence. The nannofossil datum events are listed in Tables T3 and T4, and the correlations between holes and to the magnetostratigraphy are shown in Figure F12.

The first occurrence (FO) of Emiliania huxleyi (0.25 Ma) and the LO of Pseudoemiliania lacunosa (0.41 Ma) occur between Samples 303-U1307A-1H-CC and 2H-CC and 303-U1307B-1H-CC and 3H-CC. Reticulofenestra asanoi, which ranges from 1.16 to 0.85 Ma, is found just below and above the Jaramillo Subchron, between Samples 303-U1307A-5H-1, 10–11 cm, and 6H-1, 10–11 cm, and in Sample 303-U1307B-5H-CC. The FO of Gephyrocapsa parallela (0.95 Ma), which lies just above the Jaramillo Subchron of the Matuyama Chron, is found between Samples 303-U1307A-5H-2, 10–11 cm, and 5H-6, 10–11 cm.

Large-form Gephyrocapsa spp. (>6 m), which usually occurs between 1.45 and 1.21 Ma, was absent throughout the section. This indicates a hiatus or inferred condensed sequence with a duration of ~0.24 m.y. (minimum) between Samples 303-U1307A-7H-1, 10–11 cm, and 7H-5, 10–11 cm. Samples 303-U1307A-7H-5, 10–11 cm, to 9H-CC and 303-U1307B-8H-CC are characterized by the presence of Gephyrocapsa oceanica, Gephyrocapsa caribbeanica, and P. lacunosa and the absence of large-form Gephyrocapsa spp. This indicates an age of 1.45–1.65 Ma. Samples 303-U1307A-10H-CC and 303-U1307B-10H-CC are correlated to the latest Pliocene (1.73–2.74 Ma) because of the absence of G. oceanica, G. caribbeanica, and D. tamalis and rare presence of Reticulofenestra spp. (small). Samples 303-U1307A-15H-CC to 19H-CC and 303-U1307B-14H-CC to 17H-CC contain the Late Pliocene marker species such as D. asymmetricus, D. brouweri, D. pentaradiatus, D. surculus, D. tamalis, Reticulofenestra ampla, and P. lacunosa. Moreover, the occurrence of R. ampla and D. tamalis indicates this interval has an age of 2.74–3.85 Ma in the Late Pliocene. From these observations, we concluded that the Pliocene/Pleistocene boundary is situated between Samples 303-U1307A-9H-CC and 11H-CC and 303-U1307B-8H-CC and 10H-CC. Part of the Lower Pleistocene (1.21–1.45 Ma) may be missing or condensed at this site.

Although the assemblages found in the Quaternary sequences are characterized by low species diversity and by absence or rare occurrence of warm-water species, the lower Upper Pliocene assemblages (2.74–3.85 Ma) contain warm-water species such as D. asymmetricus, D. brouweri, D. pentaradiatus, D. surculus, and D. tamalis. This indicates that this area was strongly influenced by the drastic increase in arctic ice that occurred at 2.74 Ma (Jansen et al., 1988; Whitman and Berger, 1992; Thiede and Myhre, 1996; Sato et al., 2004).

Planktonic foraminifers

Planktonic foraminifers were examined in all core catcher samples from Holes U1307A and U1307B (Tables T5, T6). The upper core catchers of each hole contain soft sediment that was washed with tap water. Sediments of the lower sections were treated with H₂O₂ before washing. Planktonic foraminifers are mostly common to rare in abundance in the upper half of both holes and rare or barren in the lower half (Table T5). Preservation of tests is moderate to poor throughout the cored interval.

The FO of encrusted N. pachyderma (sinistral) generally occurs in the top of the Oludvai Subchron (Weaver and Clement, 1987) and is found in Samples 303-U1307A-10H-CC and 303-U1307B-9H-CC. In the Pleistocene section, N. pachyderma (dextral) typically occurs with 2%–4% of the frequency of N. pachyderma (sinistral). In the Pliocene, N. pachyderma (sinistral) is rare and not encrusted. In Sample 303-U1307A-15H-CC and 303-U1307B-14H-CC and below, Neogloboquadrina atlantica (sinistral) is the most abundant species, which indicates the N. atlantica Zone of the Upper Pliocene (Weaver and Clement, 1987). Globigerina bulloides and Turborotalita quinqueloba are occasionally present.

Benthic foraminifers

Benthic foraminifers were examined in all core catcher samples from Holes U1307A and U1307B (Table T7). Benthic foraminifers are rare in most sections of both holes and abundant only in the middle part (Samples 303-U1307A-11H-CC and 303-U1307B-10H-CC). Small-sized species Bolivina translucens species, small Cassidulina species, and Epistominella exigua are most commonly present.

Diatoms

Diatom assemblages were investigated in 36 core catcher samples and 39 additional samples from Holes U1307A and U1307B (Tables T8, T9). Diatoms are rare to common in the lowermost and uppermost part of the sequence (173–125 and 10–0 mcd, respectively) and mostly rare to barren between 125 and 33 mcd (Fig. F13). The sequence between 33 and 14 mcd is barren of diatoms. The preservation of diatom valves varies greatly throughout the sedimentary sequence. Valves appear better preserved in the lower and upper part of the sequence (163–125 and 65–0 mcd, respectively), whereas they show strong signs of dissolution between 110 and 65 mcd (Tables T8, T9). Silicoflagellates and the siliceous dinoflagellate Actiniscus pentasterias are present only in a few samples below 100 mcd (Tables T8, T9).

Because of low abundance of diatom valves at the site, only two datum events were recognized with low reliability. The LO of Neodenticula seminae (0.84–0.85 Ma, marine isotope Stage [MIS] 21; Koç et al., 1999) is tentatively placed between Samples 303-U1307A-3H-CC and 4H-CC and 303-U1307B-3H-CC and 4H-CC (Fig. F12). The FO of this species (1.25–1.26 Ma, MIS 37; Koç et al., 1999) is implied between Samples 303-U1307A-6H-CC and 7H-CC and 303-U1307B-6H-CC and 7H-CC. These stratigraphic placements do not reflect the true LO and FO of the species.

Seventy-two diatom species were identified at Site U1307 (Tables T8, T9). The diatom assemblage is dominated throughout by resting spores of Chaetoceros, mirroring high-nutrient content and low temperature of surface waters. The Chaetoceros community is accompanied by a few diatom species indicative of different ecological conditions. The main association, composed of spores of Thalassiosira gravida, forms of Rhizosolenia hebetata, Actinocyclus curvatulus, and abundant fragments of the Thalassiothrix/Lioloma complex, indicates subarctic–arctic waters (Andersen et al., 2004). Minor influence of sea-ice cover is revealed by the occurrence of Fragilariopsis oceanica and Bacterosira fragilis.

Radiolarians

Radiolarians were examined in all core catcher samples from Hole U1307A (Table T10). Radiolarian specimens occur in most samples examined. Their preservation is moderate to poor. The abundance of radiolarians is rare to trace because of both dissolution and dilution by detritus. Species diversity is low.

Cycladophora davisiana davisiana occurs in only three core catcher samples of Hole U1307A (Samples 303-U1307A-5H-CC, 6H-CC, and 12H-CC), and these samples are thus assigned to the Upper Pliocene–Pleistocene C. davisiana davisiana Zone of Goll and Bjørklund (1989). In the lower part of the sequence, from Samples 303-U1307A-13H-CC to 19H-CC, there is no occurrence of key species.

Palynomorphs

Palynological assemblages were examined in all core catcher samples from Hole U1307A (Table T11). The concentration of palynomorphs is low in most samples. Dinocysts are present in low numbers down to ~120 mcd. A few samples are barren (303-U1307A-2H-CC, 3H-CC, and 10H-CC). The interval below 120 mcd contains common to abundant marine palynomorphs including dinocysts Cymatiosphaera and incertae sedis, which probably belong to prasinophytes. In all studied samples, Tasmanites occurs below ~120 mcd. Terrestrial palynomorphs are present except in Sample 303-U1307A-2H-CC, which is barren. Pollen of Pinus and Picea are dominant. Pollen grains of angiosperms, spores of pteridophytes, and Sphagnum are also present in significant numbers in a few samples (notably in Sample 303-U1307A-1H-CC). Colonies of the freshwater algae Pediastrum are present in many samples. Reworked pre-Neogene palynomorphs are present in most samples. They are dominated by trilete spores but pollen, dinocyst, and acritarchs also occur. The acritarchs are mainly represented by small-sized species of Pterospermella. Among reworked dinocysts, specimens of the genera Deflandrea and Oligosphaeridinium were identified.

In the upper part of the sedimentary section (Samples 303-U1307A-1H-CC to 13H-CC), the dinocyst assemblages are characterized by moderately poor preservation, low concentrations, and low species diversity. The discontinuity of the sampled record and the presence of reworking prevents interpretations in terms of paleoceanography or biostratigraphy. The lower part of the section (below ~120 mcd), however, shows a different facies. Concentrations are higher and Nematosphaeropsis sp. 1, Cymatiosphaera ?invaginata, and incertae sedis sp. 1 occur in significant numbers. These three groups are characteristic of the Pliocene at Ocean Drilling Program (ODP) Site 646 (cf. de Vernal and Mudie, 1989). Nematosphaeropsis sp. 1 and incertae sedis sp. 1 are small-sized taxa (25–30 and 12–25 µm, respectively). They are not reported elsewhere, possibly because of sieving procedures (often 20 µm) or because these taxa are endemic to the northwestern North Atlantic. In any case, correlations with the palynological record of Site 646 allow some biostratigraphic conclusions, at least on regional scale. The distribution of marine palynomorphs in Hole 646B has been dated using age interpolations between the paleomagnetic reversal boundaries in the same hole (Clement et al., 1989). On these grounds, the LO of Nematosphaeropsis sp. 1 (Sample 303-U1307A-13H-CC) is dated 2.4 Ma and the LO of incertae sedis sp. 1 (Sample 303-U1307A-16H-CC) is dated ~2.6 Ma. In addition, the lowermost sample (Sample 303-U1307A-19H-CC) contains diversified assemblages of Operculodinium ?eirikianum and Barssidinium pliocenum, with LOs at ~3.3 Ma at Site 646.

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