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doi:10.2204/iodp.proc.303306.103.2006

Biostratigraphy

Samples from Sites U1302 and U1303 were examined for their micropaleontologic content and revealed rich assemblages of calcareous, siliceous, and organic-walled microfossils (Figs. F15, F16). Coccoliths are abundant and well preserved in most samples and permit establishment of biostratigraphic schemes that are complemented by diatoms and palynologic datums. According to these schemes, the composite sequence of Site U1302 is younger than 1.16 Ma but most likely spans the last 0.95 m.y. (Fig. F17). The composite sequence of Site U1303 probably corresponds to an interval spanning approximately the last 0.85 m.y. (Fig. F18).

Beyond the biostratigraphic schemes, the micropaleontologic assemblages provide insight into paleoclimatologic and paleoceanographic conditions. In particular, the relative abundance of the planktonic foraminifer Neogloboquadrina pachyderma (sinistral) and some dinocyst assemblages allow identification of glacial and interglacial conditions from some core catcher samples (Figs. F19, F20).

Calcareous nannofossils

Site U1302

Calcareous nannofossils were examined in all core catcher samples from Holes U1302A–U1302E (Tables T2, T3, T4, T5, T6). Additional samples were examined from Cores 303-U1302B-7H and 8H and 303-U1302C-8H at different depths (see Tables T3, T4) to verify the precise depth occurrences of two marker taxa (Emiliania huxleyi and Pseudoemiliania lacunosa). All samples are characterized by well-preserved and abundant calcareous nannofossils, except Samples 303-U1302A-4H-CC and 303-U1302C-5H-CC. The assemblages are dominated by small-sized coccoliths such as Gephyrocapsa spp. and Reticulofenestra spp. Reworked nannofossils of Cretaceous–Miocene age are found throughout the sedimentary sequence.

E. huxleyi, the marker species of Zone NN21 (0.25–0 Ma), occurs commonly in intervals 303-U1302A-1H-CC to 3H-CC, 303-U1302B-1H-CC to 2H-CC, and 303-U1302C-1H-CC to 3H-CC. The last occurrence (LO) of P. lacunosa, which marks the uppermost part of Zone NN19 (0.41 Ma), is detected in Samples 303-U1302A-6H-CC, 303-U1302B-7H-CC, and 303-U1302C-7H-CC (Tables T2, T3, T4).

Lower in the holes, Reticulofenestra asanoi is a characteristic feature. Its FO and LO are recorded at 1.16 and 0.85 Ma, respectively. It is present in Samples 303-U1302A-11H-CC through 13H-CC, 303-U1302B-10H-CC through 11H-CC, and 303-U1302C-11H-CC (Tables T2, T3, T4). The lowest occurrence of Gephyrocapsa parallela, which indicates an age of 0.95 Ma, is found in Sample 303-U1302A-12H-CC. We thus suggest an age spanning 0.85–0.95 Ma for the depth interval between 111 and 128.2 mcd (Fig. F17). Sample 303-U1302A-13H-CC (bottom of the sequence), characterized by occurrences of R. asanoi and by the absence of G. parallela, is assigned an age interval between 0.95 and 1.16 Ma. These results indicate that the sequence recovered at Site U1302 corresponds to an interval composed between 0 and <1.16 Ma, encompassing most of the last 950 k.y. (Fig. F17).

Site U1303

The calcareous nannofossil assemblage found at this site is similar to that observed at Site U1302. We examined calcareous nannofossils in all core catcher samples from Holes U1303A and U1303B (see Tables T7, T8). The assemblages are also characterized by well-preserved and abundantly occurring coccoliths, except Sample 303-U1303A-6H-CC, and by the occurrence of nannofossils reworked from the Cretaceous to the Miocene.

As at Site U1303, there is a co-occurrence of E. huxleyi and P. lacunosa (Sample 303-U1303B-8H-CC). Sample 303-U1303A-10H-CC is characterized by the LO of R. asanoi, which is dated 0.85 Ma. The FO of G. parallela (0.95 Ma) is not found in the samples, suggesting an age younger than 0.95 Ma at the base of Site U1303 (Fig. F18).

Planktonic foraminifers

Planktonic foraminifers were examined in all core catcher samples from Holes U1302A–U1302C, U1303A, and U1303B (Tables T9, T10, T11, T12, T13). The upper three core catchers of each hole contain soft sediment that was treated with tap water. Sediments of the deeper core catchers were soaked in H₂O₂ solution, which may have resulted in a higher degree of test fragmentation. Planktonic foraminiferal tests are abundant (>50% of all particles >63 µm) in most samples investigated (Tables T9, T10, T11, T12). Planktonic foraminiferal tests are rare in Samples 303-U1302C-5H-CC, 303-U1303A-9H-CC, and 303-U1303B-7H-CC and barren in Sample 303-U1303A-6H-CC. Test preservation is very good in the uppermost core catchers and generally decreases downhole. In Sample 303-U1303B-7H-CC, the test preservation is poor. A dwarfed assemblage characterized by many very small tests occurs in Samples 303-U1302B-5H-CC and 303-U1302C-7H-CC.

The dominant planktonic foraminiferal species are N. pachyderma, Globigerina bulloides, and Globorotalia inflata in most samples (Tables T9, T10, T11, T12). At both sites, N. pachyderma (sinistral) is abundant continuously down to the bottom of all holes (Fig. F19) and most of the tests are encrusted. Its occurrence indicates that the entire sequence belongs to the N. pachyderma (sinistral) Zone in the Pleistocene–Upper Pliocene (Weaver and Clement, 1987). Other abundant species include Turborotalita quinqueloba (sinistral and dextral) and N. pachyderma (dextral). Specimens of Globigerinita glutinata are rare in many of the samples investigated. All of these species are present in the modern North Atlantic. The only extinct species recovered is Globigerina decoraperta, whose LO was at 1.77 Ma. It occurs in Samples 303-U1302C-6H-CC, 8H-CC, and 11H-CC, 303-U1303A-7H-CC, and 303-U1303A-5H-CC, possibly due to remobilization of older sediments.

The species composition of most planktonic foraminiferal assemblages indicates that surface water conditions were largely subpolar/polar to temperate. The frequent occurrence of G. inflata at Sites U1302 and U1303 suggests that the hydrography of the Orphan Knoll area was dominated by fronts. The occurrence of tropical species (Globigerinoides ruber, Globigerinoides trilobus, Globigerinella siphonifera, and Orbulina universa) indicates occasional intrusion of warm surface waters, like the modern ones with warm-core eddies from the Gulf Stream entering in the northwestern North Atlantic. The presence of adult specimens of the deep-dwelling species Globorotalia truncatulinoides indicates subduction of intermediate to deepwater masses from the south.

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1302A–U1302C, U1303A, and U1303B. All investigated sediments contain benthic foraminiferal tests (Table T14). Preservation of benthic foraminiferal tests is good, suggesting limited reworking of the sediments. Uvigerina spp. (peregrine group) and Epistominella exigua are the most abundant taxa. The only agglutinated species found is Eggerelloides bradyi.

Dominance of E. exigua suggests bottom water bathed by the Northeast Atlantic Deep Water. U. peregrina is frequent in Samples 303-U1302A-2H-CC, 303-U1302B-2H-CC, and 303-U1303B-1H-CC and 3H-CC. These samples also contain the highest frequency of the polar planktonic foraminifer N. pachyderma (sinistral). The occurrence of U. peregrina may indicate reduced North Atlantic Deep Water formation and the presence of oxygen-poor Southern Source Waters at Sites U1302 and U1303 during cold phases (Bilodeau et al., 1994).

Diatoms

Diatoms were examined on smear slides prepared from all core catchers and additional samples from Sites U1302 and U1303 (Tables T15, T16, T17, T18, T19, T20, T21). Overall diatom abundance is mostly low (barren to rare) at both sites but becomes common to abundant in a few samples. Diatom preservation is poor almost throughout. The silicoflagellate Dictyocha fibula and the siliceous dinoflagellate Actiniscus pentasterias are sporadically present. Sponge spicules are abundant in several samples at both sites.

Two diatom zones, Thalassiosira oestrupii Zone and Proboscia curvirostris Zone, are recognized (Koç et al., 1999). The stratigraphic marker P. curvirostris (LO = 0.30 Ma) is observed in several horizons at both sites (Tables T15, T16, T17, T20, T21; Figs. F17, F18) and is therefore assigned to the P. curvirostris Zone (Figs. F17, F18; 0.60–0.3 Ma) (Koç et al. 1999). Thalassiosira jouseae, found only in Samples 303-U1302A-8H-CC, 303-U1303B-5H, 2–65 cm, 303-U1303B-7H, 3–90 cm, and 303-U1303B-8H-CC gives a very similar age (LO = 0.30 Ma). The occurrence of Fragilariopsis reinholdii (LO = 0.62 Ma) in Sample 303-U1303A-8H-CC suggests that the interval below 75 mcd in Holes U1302B and U1302B belongs to the lower P. curvirostris Zone (0.85 to ~0.60 Ma). We cannot date other samples because of extremely rare and poorly preserved diatoms.

The major constituent of the diatom flora is Lioloma sp. This needle-shaped diatom dominates the assemblage in almost every sample where diatoms are either common or abundant (Tables T15, T16, T17, T20, T21). The dominance of Lioloma sp. may be related to enhanced surface circulation leading to the development of frontal systems and facilitating the concentration of diatom cells in the plankton and, consequently, greater downward flux through the water column (Kemp et al., 1995, and references therein). Entire valves of Lioloma sp. are very rare, reflecting bad preservation conditions of diatoms. A species-rich pelagic warm-water-related flora also occurs throughout. The main constituents of this tropical/subtropical association are Fragilariopsis doliolus and several species of Azpeitia, accompanied by some Coscinodiscus spp., Roperia tesselata, Thalassiosira ferelineata, and Thalassiosira lineata. Secondary species in each horizon are Actinocyclus curvatulus, T. oestrupii var. oestrupii, and T. oestrupii var. venrickae.

The diatom flora is enriched by resting spores of Chaetoceros, which are indicative of high productivity in the upper water column. In addition, coastal-associated tycopelagic A. curvatulus, Thalassionema nitzschioides var. nitzschioides, and Paralia sulcata occur in several horizons. Typical subpolar species (Rhizosolenia hebetata f. hiemalis, Thalassiosira gravida, and Thalassiosira trifulta) occur throughout as trace components.

Radiolarians

Radiolarians were examined in all core catcher samples from Sites U1302 and U1303 (Tables T22, T23). In most samples, radiolarians are barren to rare and their preservation is moderate to poor. A few samples contain common to abundant radiolarians, notably in the middle part of Holes U1302A (Sample 303-U1302A-7H-CC), U1302B (Samples 303-U1302B-8H-CC and 9H-CC), and U1302C (Samples 303-U1302C-4H-CC and 8H-CC) and in the lower part of Holes U1303A (Sample 303-U1303A-7H-CC) and U1303B (Samples 303-U1303B-8H-CC and 9H-CC) (Tables T22, T23). In these samples, the diversity of species is high.

The most abundant species are Actinomma leptodermum and Cycladophora davisiana davisiana. The stratigraphically diagnostic C. davisiana davisiana occurs in the middle part of Holes U1302A (Samples 303-U1302A-5H-CC and 7H-CC) and U1302C (Samples 303-U1302C-4H-CC and 8H-CC) and in the middle to upper part of Hole U1302B (Samples 303-U1302B-1H-CC, 3H-CC, and 5H-CC to 9H-CC). This species also occurs in Samples 303-U1302D-2H-CC and 303-U1302E-1H-CC. The assemblages of these samples thus belong to the Upper Pliocene–Pleistocene C. davisiana davisiana Zone of Goll and Bjørklund (1989). C. davisiana davisiana is also characteristic of many samples from Holes U1303A (Samples 303-U13013A-3H-CC, 5H-CC, 7H-CC, and 8H-CC) and U1303B (Samples 303-U1303B-4H-CC, 5H-CC, 8H-CC, and 9H-CC), indicating the C. davisiana davisiana Zone.

Palynomorphs

Palynologic assemblages were examined in core catcher samples from Holes U1302A, U1302B, and U1303A (Tables T24, T25, T26). In general, samples were difficult to process because of abundant detrital and biogenic silica. Nevertheless, well-preserved pollen grains and dinoflagellate cysts were recovered in most samples. The species composition of assemblages is modern, suggesting Pleistocene age of all analyzed samples.

The pollen content is largely dominated by Pinus and/or Picea, which are related to atmospheric input from boreal forest of the adjacent northeast North American continent. Variable pollen concentrations suggest changes in the density of the forest cover in the source area and/or changes in the strength and pattern of atmospheric trajectories.

The dinocyst concentration is extremely variable, ranging from <10 cysts/cm³ to >10³ cysts/cm³. Such amplitude of variations of more than three orders of magnitude suggests varying dinoflagellate productivity and sea-surface conditions through geologic time. Relatively high dinocyst concentrations characterize Quaternary interglacial and interstadial intervals in the northern North Atlantic, whereas generally low concentrations characterize glacial stages (de Vernal and Mudie, 1992).

The dinocyst assemblages show relatively low species diversity, probably partly due to the insufficient onboard sample preparation. Large changes in both species composition and dominant species from one sample to another indicate changing conditions in surface waters. Reworked palynomorphs of Paleozoic–Paleogene ages are a characteristic feature of many core catcher samples, even dominating the assemblages in some samples, indicating abundant detrital inputs that originate from older sedimentary formations.

Hole U1302A

Only seven core catcher samples from Hole U1302A were processed (Table T24). Five of these samples revealed more abundant reworked palynomorphs than dinocysts (Samples 1H-CC, 4H-CC, 6H-CC, 10H-CC, and 12H-CC), suggesting that detrital input dominated over pelagic flux during sedimentation at these depths. Samples 3H-CC and 8H-CC contain moderately high dinocyst content, reflecting interglacial-type conditions when pelagic flux dominates over detrital input.

Hole U1302B

All core catcher samples from Hole U1302B were prepared and examined for their palynologic content (Table T25). Three of the samples revealed more abundant reworked palynomorphs than dinocysts (Samples 3H-CC, 7H-CC, and 11H-CC), indicating important detrital inputs from Paleozoic–Paleogene formations and limited dinocyst fluxes. Sample 7H-CC contains abundant microscopic charcoal fragments. Samples 6H-CC, 8H-CC, and 9H-CC contain relatively rich dinocyst assemblages dominated by Brigantedinium, which suggests low-salinity conditions in surface waters. Samples 1H-CC and 5H-CC are characterized by rich assemblages with both neritic and oceanic components.

Hole U1303A

All core catcher samples from Hole U1303A were prepared and examined for their palynologic content (Table T26). Two samples revealed no dinocysts (Samples 6H-CC and 9H-CC) but contained reworked palynomorphs. Samples 5H-CC and 8H-CC possibly include palynomorphs that might be useful as biostratigraphic markers. In Sample 5H-CC, Filisphaera filifera, (last-appearance datum [LAD] in MIS 17) was recovered. However, this sample contains very rare and poorly preserved dinocysts, which may indicate sedimentary redeposition. In Sample 8H-CC, which contains a rich palynologic assemblage, the presence of specimens belonging to the prasinophyte Cymatiosphaera ?invaginata (LAD in MIS 17) may suggest an age of ≥0.7 Ma. Samples 3H-CC, 7H-CC, and 8H-CC are characterized by dinocyst assemblages typical of temperate waters and relatively low sea-surface salinity. These samples likely indicate interglacial stages.

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