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doi:10.2204/iodp.proc.303306.106.2006

Biostratigraphy

Micropaleontologic analysis of samples from Site U1306 reveals rich assemblages of calcareous, siliceous, and organic-walled microfossils. Coccoliths permit establishment of a biostratigraphic scheme that is complemented by a few datums from diatoms and planktonic foraminifers. According to this scheme, the composite sequence of Site U1306 covers an interval spanning about the last 2.2 m.y. (Fig. F16).

All microfossil groups are present but exhibit large variations in abundances downcore. Apart from the biostratigraphic scheme, the micropaleontologic assemblages also provide insight into the paleoceanographic conditions on the Erik Drift area. The dominant components of each microfossil group reflect cold sea-surface temperatures for most of the time during which the sedimentary sequence was deposited. Preservation is moderate to poor for all groups in the lower part of the sedimentary sequence (below 170 mcd). Reworked nannofossils and reworked palynomorphs of Cretaceous–Miocene age are present throughout the sequence at Site U1306.

Calcareous nannofossils

Calcareous nannofossils were examined in all core catcher samples from Holes U1306A–U1306D (Tables T2, T3, T4, T5). Nannofossils are generally abundant to few and rare to barren in some samples (Fig. F17). Preservation of nannofossils is mostly good to moderate (Fig. F18). Reworked nannofossils from the Cretaceous–Miocene are found throughout the sedimentary sequences of Site U1306. Some particular horizons have a high proportion of reworked nannofossils (e.g., Samples 303-U1306A-3H-CC, 303-U1306B-1H-CC and 13H-CC, 303-U1306C-1H-CC, and 303-U1306D-5H-CC). About 50% of the assemblage in Sample 303-U1306A-3H-CC comprises reworked nannofossils from the Upper Cretaceous. The nannofossil datums found at Site U1306 are listed in Tables T2, T3, T4, and T5. Biostratigraphic correlation among holes is shown in Figure F16. The first occurrence (FO) of Emiliania huxleyi, which defines the Zone NN21/NN20 boundary (0.25 Ma), is placed between Samples 303-U1306A-4H-CC and 5H-CC, 303-U1306B-3H-CC and 5H-CC, 303-U1306C-4H-CC and 5-CC, and 303-U1306D-4H-CC and 5H-CC. The last occurrence (LO) of Pseudoemiliania lacunosa, which indicates the base of Zone NN20 (0.41 Ma), is present in Samples 303-U1306A-8H-CC, 303-U1306B-8H-CC, 303-U1306C-8H-CC, and 303-U1306D-8H-CC. The LO of Reticulofenestra asanoi (0.85 Ma), which is located just below the Brunhes Chron, is placed between Samples 303-U1306A-14H-CC and 15H-CC, 303-U1306B-13H-CC and 15H-CC, 303-U1306C-14H-CC and 15H-CC, and 303-U1306D-14H-CC and 15H-CC. The FO of Gephyrocapsa parallela (0.95 Ma) is correlated to the interval just above the Jaramillo Subchron of the Matuyama Chron and is found between Samples 303-U1306A-15H-CC and 16H-CC, 303-U1306B-15H-CC and 17H-CC, 303-U1306C-15H-CC and 16H-CC, and 303-U1306D-15H-CC and 16H-CC. We therefore suggest an age spanning 0.85–0.95 Ma for the interval between 142 and 154 mcd (Fig. F16).

The sampled intervals 303-U1306A-16H-CC to 20H-CC, 303-U1306B-16H-CC to 19H-CC, and 303-U1306C-16H-CC to 19H-CC are characterized by the presence of R. asanoi and the absence of G. parallela and are assigned to the age interval 0.95–1.16 Ma (the latter age is the FO of R. asanoi) (Fig. F16). Large-sized specimens (>6 µm) of Gephyrocapsa spp. found in Samples 303-U1306A-22H-CC to 24H-CC, 303-U1305B-20H-CC, and 303-U1306C-19H-CC to 20H-CC (between 190 and 235 mcd), give an approximate age of 1.21–1.45 Ma (Fig. F16). The FO of Gephyrocapsa oceanica (1.65 Ma) is found between Samples 303-U1306A-26H-CC and 28H-CC, 303-U1306B-26H-CC and 28H-CC, and 303-U1306C-24H-CC and 26H-CC. The base of the Pleistocene, defined by the FO of Gephyrocapsa caribbeanica (1.73 Ma), is recognized between Samples 303-U1306A-26H-CC and 28H-CC, 303-U1306B-26H-CC and 28H-CC, and 303-U1306C-26H-CC and 27H-CC and is situated just above the Olduvai Subchron (Fig. F16). The assemblages found in samples below the above-mentioned datums are dominated by the presence of Coccolithus pelagicus, small-sized Reticulofenestra spp., P. lacunosa, and Calcidiscus macintyrei and by the absence of G. caribbeanica, G. oceanica, and Discoaster brouweri. This suggests that the lowermost core catcher samples at Site U1306 correspond to the uppermost Pliocene (1.97–1.73 Ma) (Fig. F16).

Planktonic foraminifers

Planktonic foraminifers were examined in all core catcher samples from Holes U1306A–U1306D (Tables T6, T7, T8, T9). The majority of core catcher samples from each hole contain soft sediment that was washed with tap water. All samples below 303-U1306A-D-18H-CC to 20H-CC, however, needed additional H₂O₂ treatment before washing. Planktonic foraminifers are mostly abundant to common within the upper half of all holes (Tables T6, T7, T8, T9). Samples 303-U1306D-8H-CC and 9H-CC contain planktonic foraminiferal sand. Below 175 mcd, many intervals are barren of planktonic foraminifers (Fig. F17). Preservation of tests is good to moderate in the upper half of the sedimentary sequence and mostly poor in the lower half (Fig. F17).

Encrusted Neogloboquadrina pachyderma (sinistral) is the most common taxon at Site U1306. The FO of encrusted N. pachyderma (sinistral), correlated to the top of the Olduvai Subchron (Weaver and Clement, 1987), occurs in Samples 303-U1306A-28H-CC and 303-U1306B-27H-CC. N. pachyderma (dextral) typically occurs with 2%–4% of the sinistrally coiled form. Globigerina bulloides and Turborotalita quinqueloba are occasionally present. These species are all present also in the modern North Atlantic. The presence of the extinct species Neogloboquadrina atlantica and Globigerina decoraperta in Samples 303-U1306A-7H-CC and 303-U1306B-27H-CC, respectively, is probably a result of sediment reworking.

Benthic foraminifers

Benthic foraminifers were examined in all core catcher samples from Holes U1306A–U1306D (Table T10). Benthic foraminifers are abundant in the upper half of the succession. Many sections are barren of benthic foraminifers below 175 mcd. The specimen-rich benthic assemblages are characterized by small-sized species and juvenile specimens. Bolivina translucens is the most frequent species at Site U1306, and small-sized Cassidulina spp., Gavelinopsis praegeri, Melonis spp., and Pullenia spp. are abundant. A fragment of Stilostomella is found in Sample 303-U1306A-28H-CC.

Diatoms

Diatom assemblages were investigated in 113 core catcher samples and 108 smear slides from Holes U1306A–U1306D (Tables T11, T12, T13, T14). Overall diatom abundance is mostly moderate to low below ~180 mcd and improves progressively above that depth (Fig. F17). Spores of Chaetoceros spp. are the main components of the sedimentary diatom assemblage, with other diatoms as secondary contributors representing different surface water conditions. The preservation of diatom valves varies strongly throughout the sedimentary sequence at Site U1306. The quality of preserved valves is mostly moderate between 290 and 337 mcd, poor between 245 and 290 mcd, and good to moderate above 245 mcd (Tables T11, T12, T13, T14; Fig. F18). Three silicoflagellate species (Dictyocha fibula, Distephanus speculum, and Octactis pulchra) and the siliceous dinoflagellate Actiniscus pentasterias occur sporadically (Tables T11, T12, T13, T14). Sponge spicules were observed mostly in samples barren of diatoms and silicoflagellates.

Four Quaternary diatom zones proposed by Koç et al. (1999) are based on three primary diatom events (Fig. F16). The LO of Proboscia curvirostris, which defines the base of the Thalassiosira oestrupii Zone and the top of the P. curvirostris Zone (0.3 Ma, MIS 9; Koç et al., 1999), is located between Samples 303-U1306A-5H-CC and 6H-CC, 303-U1306B-5H-CC and 6H-CC, and 303-U1306C-5H-CC and 6H-CC. The LO of Neodenticula seminae (0.84–0.85 Ma, MIS 21; Koç et al., 1999) appears between Samples 303-U1306A-13H-CC and 14H-CC, 303-U1306B-14H-CC and 15H-CC, 303-U1306C-14H-CC and 15H-CC, and 303-U1306D-13H-CC and 14H-CC. The FO of N. seminae (1.25–1.26 Ma, MIS 37; Koç et al., 1999) occurs between Samples 303-U1306A-19H-CC and 20H-CC, 303-U1306B-19H-CC and 20H-CC, 303-U1306C-19H-CC and 20H-CC, and in Sample 303-U1306D-19H-CC. As a secondary datum, the FO of P. curvirostris (1.52–1.53 Ma; Koç et al., 1999) is tentatively identified in Samples 303-U1306A-23H-CC and 24H-CC, 303-U1306B-22H-CC and 23H-CC, and 303-U1306C-22H-CC and 23H-CC.

A very diverse diatom flora occur downcore in the recovered sedimentary section (~90 marine diatom species and a few freshwater diatoms were identified). The diatom assemblage is mostly dominated by Chaetoceros resting spores (Tables T11, T12, T13, T14). The high relative contribution of Chaetoceros spores at high latitudes reflects high productivity of surface waters (Crosta et al., 1997). The resting spore community is accompanied by several diatom species indicative of different ecological conditions. The main secondary assemblage, composed of spores and vegetative cells of Thalassiosira gravida, Actinocyclus curvatulus, Rhizosolenia hebetata f. semispina, and needle-shaped diatoms of the Thalassiothrix/Lioloma complex, is typical of subarctic and arctic waters (Andersen et al., 2004). Minor influence of warm and saline waters is mirrored by the presence of Coscinodiscus marginatus and varieties of T. oestrupii. The winter sea-ice-associated diatoms Fragilariopsis oceanica and Bacterosira fragilis are present in several samples (Monjanel and Baldauf, 1989).

Radiolarians

Radiolarians were examined in all core catcher samples from Holes U1306A and U1306B. Radiolarians are generally common to rare (Table T15; Fig. F17). Species diversity is lower at Site U1306 than at Site U1305. Cycladophora davisiana davisiana is the dominant species throughout the sequences examined at Site U1306. Actinomma leptodermum occurs abundantly in the upper part of the holes (Table T15). Mostly moderately preserved radiolarians occur throughout the sedimentary sequences (Fig. F18). Since C. davisiana davisiana is present in all core catcher samples of Holes U1306A and U1306B, the sequences are assigned to the Late Pliocene–Pleistocene C. davisiana davisiana Zone of Goll and Bjørklund (1989).

Palynomorphs

Palynological assemblages were examined in all core catcher samples from Holes U1306A and U1306D (Tables T16, T17). All samples contain moderately well to well preserved palynomorphs. Dinocysts are present in variable numbers and are abundant only in a few samples (Fig. F17). Terrestrial palynomorphs, dominated by pollen grains of Pinus and Picea spp., occur in low numbers throughout the sequence at Site U1306. Reworked pre-Neogene palynomorphs are present almost throughout (Fig. F17) and are often common (Tables T16, T17). Microscopic fragments of charcoal are frequently observed in palynological slides.

The dinocyst assemblages are generally characterized by low concentrations and low species diversity. Abundant dinocysts are recorded in only a few samples in which Brigantedinium spp. is the dominant component (Tables T16, T17). Samples 303-U1306D-9H-CC and 11H-CC contain common dinocysts, and the assemblage is dominated by Operculodinium centrocarpum. Both Brigantedinium spp. and O. centrocarpum are among the most ubiquitous Pleistocene dinocyst taxa. Their dominance, the low dinocyst concentration, and the low species diversity suggest that the assemblages mostly reflect cold and/or low-salinity surface waters.

The palynological assemblages contain a few species with biostratigraphic significance: Corrudinium? labradori with an LO at ~0.4 Ma and Filisphaera filifera and Cymatiosphaera ?invaginata with a North Atlantic LO at ~0.7 Ma (de Vernal et al., 1992). The presence of C.? labradori in Sample 303-U1306D-8H-CC suggests an age of ~0.4 Ma at 73 mcd, and the presence of F. filifera in Sample 19H-CC indicates the lowermost core catcher sample of this hole is older than 0.7 Ma.

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