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doi:10.2204/iodp.proc.303306.104.2006

Biostratigraphy

Recurring laminated diatom-rich sequences are the most prominent feature in all holes drilled at Site U1304 (Fig. F14). Diatom assemblages are dominated by needle-shaped species of the Thalassiothrix-Lionnsloma complex. All other groups investigated (coccoliths, planktonic and benthic foraminifers, radiolarians, and palynomorphs) are present in high to moderate abundance and are well preserved throughout (Tables T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17). Biostratigraphic datums mainly derive from coccoliths and are consistent with datums provided by diatoms, planktonic foraminifers, and dinocysts (Fig. F15). Accordingly, the first occurrence (FO) of encrusted Neogloboquadrina pachyderma (sinistral) indicates that the composite sequence of Site U1304 should have its base in the Olduvai Subchron. The FO of Gephyrocapsa caribbeanica (1.73 Ma) close to the base of the section indicates that the entire Pleistocene is represented. The microfossil assemblages indicate only minor redeposition of sediments at Site U1304.

Preliminary paleoceanographic and paleoclimatologic interpretation of the microflora and microfauna reveals large-amplitude changes in surface water temperature and trophic conditions. Diatom layers were formed during both cold and warm phases according to the species composition of diatom and planktonic foraminifer assemblages (Fig. F16). A shift from dominance of autotrophic to dominance of heterotrophic dinocyst taxa is recorded at ~1.2 Ma. This may suggest a general change in trophic conditions of the surface ocean. The presence of the benthic foraminifer Epistominella exigua documents recurring pulses of fresh organic matter reaching the seafloor (Fig. F16).

Calcareous nannofossils

Calcareous nannofossils were examined in all core catcher samples from Holes U1304A–U1304D (Tables T2, T3, T4, T5). Several additional samples were examined to refine the depth of biostratigraphic datums. All samples are characterized by well to moderately well preserved and abundant calcareous nannofossils except Samples 303-U1304A-3H-CC, 18H-1, 0–1 cm, 19H-CC, and 21H-CC; 303-U1304B-13H-CC; 303-U1304C-4H-CC; and 303-U1304D-14H-CC, 15H-CC, and 20H-CC (Fig. F14). The assemblages are dominated by small-sized coccoliths of Gephyrocapsa spp. and Reticulofenestra spp. Reworked nannofossils from the Cretaceous–Miocene occur throughout the sections.

Ten Quaternary nannofossil datums defined by Sato et al. (1999) are detected in the sequences at Site U1304 (Fig. F15). The FO of Emiliania huxleyi (0.25 Ma) and last occurrence (LO) of Pseudoemiliania lacunosa (0.41 Ma), which are situated in Brunhes Chron, are found in Samples 303-U1304A-4H-CC, 303-U1304B-4H-CC, and 303-U1304C-5H-CC and in Samples 303-U1304A-7H-CC, 303-U1304B-8H-CC, 303-U1304C-8H-CC, and 303-U1304D-2H-CC, respectively (Tables T2, T3, T4, T5). Both the LO of Reticulofenestra asanoi (0.85 Ma) and the FO of Gephyrocapsa parallela (0.95 Ma) are correlated to the interval between the base of the Brunhes Chron and the top of the Jaramillo Subchron of the Matuyama Chron and are detected in Samples 303-U1304A-14H-CC, 303-U1304B-15H-CC, and 303-U1304D-9H-CC, and in Samples 303-U1304A-15H-CC, 303-U1304B-16H-CC, and 303-U1304D-10H-CC, respectively. The FO of R. asanoi (1.16 Ma), situated just below the Jaramillo Subchron, occurs in Samples 303-U1304A-17H-CC, 303-U1304B-18H-CC, and 303-U1304D-12H-CC. Large forms of Gephyrocapsa spp. occur from 1.45 to 1.21 Ma and are detected in Samples 303-U1304A-18H-7, 50–51 cm, to 20H-CC; 303-U1304B-19H-CC to 21H-CC; and 303-U1304D-13H-CC to 15H-CC. The LO of Helicosphaera sellii, which is dated at 1.27 Ma, is found in Samples 303-U1304A-18H-CC, 303-U1304B-20H-CC, and 303-U1304D-15H-CC.

The oldest two nannofossil datums recognized are the FOs of Gephyrocapsa oceanica (1.65 Ma) and G. caribbeanica (1.73 Ma), situated in Samples 303-U1304A-24H-CC, 303-U1304B-24H-CC, and 303-U1304D-19H-CC. Assemblages found in samples below these datums are characterized by small Gephyrocapsa and Calcidiscus macintyrei and by the absence of G. caribbeanica, G. oceanica, and Discoaster brouweri. This suggests that the lowermost samples of Holes U1304A, U1304B, and U1304D correspond to the uppermost Pliocene between 1.97 and 1.73 Ma (Fig. F15).

Planktonic foraminifers

Planktonic foraminifers were examined in all core catcher samples from Holes U1304A–U1304D (Table T6, T7, T8, T9). Diatom-rich sediments were washed, and foraminiferal tests were separated from diatoms by decanting the diatom frustules. A certain proportion of the foraminiferal tests present in the sample were probably lost due to this procedure. Some planktonic foraminiferal assemblages within diatom oozes are dominated by small-sized tests. Within diatom-poor sediments, planktonic foraminifers are abundant (>50% of all particles >63 µm) and well preserved. Planktonic foraminiferal tests were absent only in Sample 303-U1304D-16H-CC.

The dominant planktonic foraminiferal species are N. pachyderma (sinistral), Globorotalia inflata, or Globigerina bulloides. Most of the N. pachyderma (sinistral) tests are encrusted. N. pachyderma (sinistral) is almost continuously present downcore (Fig. F14). The FO of encrusted N. pachyderma (sinistral) lies between Samples 303-U1304A-26H-CC and 303-U1304B-25H-CC and is assigned to the boundary between the N. pachyderma (sinistral) Zone and the G. inflata Zone (Weaver and Clement, 1987) at the top of the Olduvai Subchron (Fig. F14). Tests of N. pachyderma (sinistral), present in minor amounts in Sample 303-U1304D-21H-CC, are not encrusted. Consequently, Sample 303-U1304D-21H-CC is placed in the G. inflata Zone (Weaver and Clement, 1987). The occurrence of the extinct species Neogloboquadrina atlantica and Globigerina decoraperta in a few sections from Holes U1304B and U1304D was possibly caused by sediment reworking. Turborotalita quinqueloba (sinistral and dextral) and N. pachyderma (dextral) are abundant, and Globigerinita glutinata is rare in many sections. Tropical and subtropical species (Globigerinoides ruber, Globigerinoides trilobus, Globigerinella siphonifera, and Orbulina universa) and deep-dwelling species (e.g., Globorotalia truncatulinoides) are sporadically present. All of these species are present in the modern North Atlantic.

An alternation of subpolar/polar species (N. pachyderma [sinistral]) and temperate species (e.g., G. bulloides and N. pachyderma [dextral]) occurs throughout the studied core intervals (Table T6, T7, T8, T9). The variable frequency of G. bulloides and G. inflata suggests changing trophic conditions. The dominance of G. bulloides indicates distinct seasonality and phytoplankton blooms (Schiebel and Hemleben, 2002). Globorotalia inflata is generally considered a “frontal species,” and its dominance during many intervals indicates a front-dominated hydrography (Fig. F16). Tropical species indicate rare intrusions of warm surface waters by the Gulf Stream. G. truncatulinoides is distributed by currents around the subtropical gyres and occasionally to higher latitudes and indicates rare intrusions of subsurface water masses from the south.

Benthic foraminifers

Benthic foraminifers were examined in all core catcher samples from Holes U1304A–U1304D. E. exigua and other small-sized and thin-walled taxa (e.g., Gavelinopsis) are most abundant. Pullenia quinqueloba and Pullenia bulloides are frequent throughout the cores. Occasionally, Globobulimina sp. dominates the benthic foraminiferal assemblage. Agglutinated taxa are rare (Table T10).

The presence of E. exigua suggests flux pulses of fresh organic matter formed during phytoplankton blooms (Gooday, 1993). Globobulimina is characteristic of high-nutrient, low-oxygen conditions (Lutze, 1980; Jorissen et al., 1995). Both species indicate frequent sedimentation of large amounts of surface-derived organic matter to the seafloor throughout the Pleistocene (Fig. F16).

Diatoms

Diatom assemblages were investigated in all core catcher samples and several additional samples from Holes U1304A–U1304D (Tables T11, T12, T13, T14). Diatoms are abundant or common throughout the sedimentary sequence (Fig. F14) and are very well to moderately well preserved. Monospecific diatom layers occur at several depths and are mainly composed of needle-shaped diatoms of the Thalassiothrix-Lioloma complex. Additionally, some thinner diatom layers composed of other genera are observed. Four silicoflagellate species and the siliceous dinoflagellate Actiniscus pentasterias are observed throughout, and sponge spicules occur in some samples.

Four diatom zones following the zonation of Koç et al. (1999) are assigned using three diatom datum events (Fig. F15; Tables T11, T12, T13, T14). 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 found between Samples 303-U1304A-4H-CC and 5H-CC, 303-U1304B-4H-CC and 5H-CC, and 303-U1304C-6H-CC and 7H-CC. The LO of Neodenticula seminae (0.84–0.85 Ma, MIS 21; Koç et al., 1999) lies between Samples 303-U1304A-13H-CC and 14H-CC, 303-U1304B-13H-CC and 14H-CC, and 303-U1304D-8H-CC and 9H-CC. The FO of N. seminae (1.25–1.26 Ma, MIS 37; Koç et al., 1999) is between Samples 303-U1304A-17H-CC and 18H-CC, 303-U1304B-18H-CC and 19H-CC, and 303-U1304D-12H-CC and 13H-CC. Intervals below the N. seminae Zone are placed in the Fragilariopsis reinholdii Zone (1.25 through ~1.89 Ma; Koç et al., 1999).

The diatom association at Site U1304 is rich in species (>95 diatom taxa are recognized). The most striking feature is the presence of thick diatom layers in several horizons, mainly composed of needle-shaped forms of the Thalassiothrix-Lioloma complex (Tables T11, T12, T13, T14). The occurrence of very thick diatom-rich deposits in the open North Atlantic is possibly related to intensified surface circulation leading to the development of frontal systems (Kemp and Baldauf, 1993; Bodén and Backman, 1996). The diatom mats reveal repeated episodes of increased primary production between ~1.8 Ma and the Holocene (Fig. F16).

Several thinner diatom mats (<25 cm) are frequent throughout the sediment sequence at Site U1304. These diatom sequences are almost monospecifically composed of Coscinodiscus spp. (Samples 303-U1304A-5H-4, 84 cm; 21H-3, 124 cm; and 303-U1304B-1H-6, 30 cm), Rhizosolenia spp. (Samples 303-U1304A-23H-1, 148 cm, and 25H-CC), vegetative cells of Chaetoceros concavicornis (Samples 303-U1304A-1H-1, 12 cm; 21H-3, 124 cm; and 303-U1304B-1H-6, 30 cm), and resting spores of Chaetoceros spp. (Sample 303-U1304C-2H-7, 40 cm). The continuous influence of the warm North Atlantic water masses at the site is indicated by the presence of Fragilariopsis doliolus, Coscinodiscus asteromphalus, C. marginatus, T. oestrupii var. oestrupii, and T. oestrupii var. venrickae (Andersen et al., 2004). The influence of colder northern water masses is indicated by the occurrence of Actinocyclus curvatulus, forms of Rhizosolenia hebetata, and vegetative cells of Thalassiosira gravida, which is typical of subarctic and arctic waters (Andersen et al., 2004). The co-occurrence of both warm and cold assemblages suggests the existence of a hydrographic front between subpolar and temperate water masses near Site U1304 during the Pleistocene.

Radiolarians

Radiolarians were examined in all core catcher samples from Holes U1304A and U1304B (Table T15). In general, radiolarians are abundant to common and preservation is good (Table T15). In the diatomaceous oozes, only a few radiolarians are present because of dilution by diatoms (Fig. F14).

Abundant species and species groups are Cycladophora davisiana davisiana, Stylochlamidium venustum, Spongodiscus spp., and Actinomma leptodermum. Pylospira sp., P. octopyle, and Pseudodictyophimus sp. are abundant in the laminated diatomaceous oozes. Several species of theoperids, pterocorythids, artostrobiids, and cannobotryids also occur in these samples, and species diversity is generally high. A. leptodermum is common to abundant in calcareous oozes of the middle part of Holes U1304A and U1304B and is relatively rare in the diatom oozes.

C. davisiana davisiana occurs almost throughout Holes U1304A and U1304B. The sequences are therefore assigned to the Upper Pliocene–Pleistocene C. davisiana davisiana Zone of Goll and Bjørklund (1989).

Palynomorphs

Palynological assemblages were examined in core catcher samples from Holes U1304A and U1304D. Samples were extremely difficult to process because of the abundance of diatoms. Two sampling protocols were used. Sediment samples from Hole U1304A were processed using heavy liquid separations, allowing palynological observations in all samples (Table T16). This preparation may result in underestimation of the palynomorph abundances because the small organic particles might be trapped and entrained within the diatom network. Sediment samples from Hole U1304D were processed without heavy liquid separation. In this case, the abundant diatom remains made palynological examination possible only in samples containing a very high number of dinocysts (Table T17).

Many samples from Holes U1304A and U1304B contain diversified palynological assemblages with extremely abundant dinocysts (Fig. F14). In some samples, the dinocyst concentration is higher than 10⁴ cysts/cm³. Most assemblages are dominated by Brigantedinium spp. cysts, which are produced by heterotrophic Protoperidinium dinoflagellates feeding on diatoms and are common in estuarine and epicontinental domains as well as upwelling areas. Abundant Protoperidinium dinoflagellates in the open ocean constitutes a peculiarity that could be related to the extremely high diatom production (Fig. F14).

The composition of dinocyst assemblages suggests three ecostratigraphic zones (Fig. F15):

1. The upper part of Holes U1304A (Samples 303-U1304A-1H-CC to 9H-CC; i.e., down to ~100 mcd) and U1304D (Samples 303-U1304D-1H-CC to 4H-CC, i.e., down to ~98 mcd) are dominated by Brigantedinium.

2. In the middle part of Hole U1304A (Samples 303-U1304A-10H-CC to 19H-CC) and Hole U1304D, Brigantedinium spp. is often common or abundant but does not always dominate the assemblage. Autotrophic Gonyaulacales dinoflagellates may also be abundant, such as Impagidinium aculeatum, Operculodinium centrocarpum, Spiniferites membranaceus, or Nematosphaeropsis labyrinthea.

3. In the lower part of the Holes U1304A (Samples 303-U1304A-20H-CC to 26H-CC) and U1304B, below ~200 mcd, the dinocyst concentration is lower. The assemblages are dominated by Gonyaulacales I. aculeatum, O. centrocarpum, or Filisphaera filifera, whereas Brigantedinium is a minor component.

In the three zones mentioned above, large changes in concentration and species composition of assemblages suggest large-amplitude variability in sea-surface conditions including temperature, salinity, and nutrient availability.

The dinocyst assemblages contain a few species that are biostratigraphically useful. I. velorum and F. filifera record their LO in the North Atlantic at ~0.4 and 0.7 Ma, respectively (cf. de Vernal et al., 1992). The presence of I. velorum in Sample 303-U1304A-8H-CC suggests an age of at least 0.4 Ma at 89 mcd, and the presence of F. filifera in Sample 12H-CC indicates an age of ~0.7 Ma at 131 mcd (Fig. F15).

The palynological assemblages include small numbers of palynomorphs of terrestrial or terrigenous origin. There are few to common Pinus pollen grains, which are wind transported over very long distances. Reworked palynomorphs are rare in examined samples, except in Sample 303-U130A-15H-CC (Fig. F14).

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