Previous   |    Next

doi:10.2204/iodp.proc.303306.105.2006

Biostratigraphy

Onboard micropaleontologic analyses using core catcher samples allows a biochronologic model for Site U1305. We give a preliminary age model based on 13 datum events derived from calcareous nannofossils, diatoms, and dinocysts (Fig. F16). The sediments at Site U1305 likely cover the latest Pliocene and the whole Pleistocene, as determined by the first occurrence (FO) of the nannofossil Gephyrocapsa caribbeanica (1.73 Ma) close to the base of the studied section.

Floral and faunal assemblages of planktonic organisms are used as indicators of the paleoceanographic conditions during the Pleistocene. All groups investigated onboard show high to moderate abundances and good preservation (Tables T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15). Assemblages of all organisms indicate subpolar to polar conditions (Tables T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15). The planktonic foraminifer assemblage in particular is dominated by Neogloboquadrina pachyderma (sinistral) indicating polar conditions. In general, abundances of the microfossils are higher in the upper 150 mcd of the sequence than below (Fig. F17).

Calcareous nannofossils

Calcareous nannofossils were examined in core catcher samples from Holes U1305A–U1305C (Tables T3, T4, T5). The assemblages, which indicate the latest Pliocene–Quaternary, are characterized by good to poor preservation. The species diversity is high, and reworked nannofossils from the Cretaceous–Miocene are found throughout the sedimentary sequence. The correlations between holes and to the magnetostratigraphy are shown in Figure F16.

The FO of Emiliania huxleyi, which defines the Zone NN21/NN20 boundary (0.25 Ma), is placed between Samples 303-U1305A-3H-CC and 5H-CC, 303-U1305B-4H-CC and 5H-CC, and 303-U1305C-5H-CC and 6H-CC. The last occurrence (LO) of Pseudoemiliania lacunosa, which defines the base of Zone NN20 (0.41 Ma), was found in Samples 303-U1305A-8H-CC, 303-U1305B-8H-CC, and 303-U1305C-9H-CC. Both the LO of Reticulofenestra asanoi (0.85 Ma) and the FO of Gephyrocapsa parallela (0.95 Ma), which correspond to the interval between the base of the Brunhes Chron and the top of the Jaramillo Subchron of the Matuyama Chron, are detected in Samples 303-U1305A-14H-CC, 303-U1305B-15H-CC, and 303-U1305C-15H-CC and 303-U1305A-15H-CC, 303-U1305B-16H-CC, and 303-U1305C-16H-CC. We thus suggest an age spanning 0.85–0.95 Ma for the depth interval between 140.34 and 150.61 mcd (Fig. F16). The intervals from Samples 303-U1305A-15H-CC to 18H-CC, 303-U1305B-16H-CC to 20H-CC, and 303-U1305C-16H-CC to 20H-CC are characterized by the presence of R. asanoi and the absence of G. parallela, so they are assigned an age between 0.95 and 1.16 Ma (FO of R. asanoi). The large form of Gephyrocapsa spp. (>6 µm) was found from Samples 303-U1305A-20H-CC to 22H-CC, 303-U1305B-22H-CC to 24H-CC, and 303-U1305C-21H-CC to 24H-CC, indicating an age ranging between 1.21 and 1.45 Ma. The FO of Gephyrocapsa oceanica, which indicates an age of 1.65 Ma, is found between Samples 303-U1305A-27H-CC and 28H-CC and 303-U1305C-29H-CC and 30H-CC. The base of the Pleistocene, which is defined by the FO of G. caribbeanica (1.73 Ma), is recognized between Samples 303-U1305A-28H-CC and 30H-CC and 303-U1305C-29H-CC and 31H-CC and is situated just above the Olduvai Subchron (285.6–306.25 mcd) (Fig. F16). The assemblages found in samples below the datums are characterized by dominance of small Gephyrocapsa spp. and Calcidiscus macintyrei and the absence of G. caribbeanica, G. oceanica, and Discoaster brouweri. This suggests that the lowermost samples of Holes U1305A and U1305C correspond to the uppermost Pliocene between 1.97 and 1.73 Ma (Fig. F16).

Planktonic foraminifers

Planktonic foraminifers were examined in all core catcher samples from Holes U1305A–U1305C (Tables T6, T7, T8). Sediments below Samples 303-U1305A-14H-CC, 303-U1305B-11H-CC, and 303-U1305C-8H-CC are compact and were soaked in H₂O₂ solution before washing. Planktonic foraminiferal tests are common to abundant (>10% of all particles >63 µm) within the upper half of the cored interval. Below Samples 303-U1305A-14H-CC, 303-U1305B-14H-CC, and 303-U1305C-14H-CC, planktonic foraminifers are mostly common to rare and barren in several samples from all holes (Table T6, T7, T8). Preservation of tests is good in the upper half of the cores and often poor in the lower half.

N. pachyderma (sinistral) is the most frequent species downcore at Site U1305, with most of the tests being encrusted. Consequently, the sediments of Site U1305 are assigned to the N. pachyderma (sinistral) Zone in the Pleistocene (Weaver and Clement, 1987). Globigerina bulloides is occasionally dominant during the late Pleistocene. Turborotalita quinqueloba (sinistral and dextral) and N. pachyderma (dextral) are abundant in some sections. Globigerinita glutinata and Globorotalia inflata are present as rare species in many of the sediments. All of these species are present also in the modern North Atlantic. The minor presence of the extinct species Neogloboquadrina atlantica and Globigerina decoraperta in a few sections from Holes U1305A and U1305B is possibly caused by sediment reworking.

The planktonic foraminiferal fauna of many samples is characterized by a bimodal size distribution of very small tests (<100 µm) and large tests (>200 µm). The small test-size fraction includes small-sized species (e.g., Globigerinita uvula) as well as juvenile tests of larger species. The small tests are thin walled and well preserved (glassy) throughout the core interval.

An observed bimodal test-size distribution of planktonic foraminifers is possibly due to expatriation from northern areas by strong subsurface to bottom currents. All small-sized specimens are cold-water species (G. uvula, N. pachyderma, and T. quinqueloba). Following the death of foraminifers, small-sized tests settle through the water column with low velocity (<100 m per day) (Schiebel and Hemleben, 2000; Schiebel, 2002). Settling through the water column, tests can be transported by currents over several hundred kilometers (Siegel and Deuser, 1997; von Gyldenfeld et al., 2000). Finally, the small tests settle on the seafloor as allochthonous particles and mix with the autochthonous specimens. Test sink velocity of large-sized specimens is several hundred meters per day (up to 1500 m per day), which does not allow large-distance expatriation. Large-sized tests are possibly produced close to their location of sedimentation at the seafloor.

Benthic foraminifers

Benthic foraminifers were examined in all core catcher samples from Holes U1305A–U1305C (Table T9). Small-sized and thin-walled taxa (e.g., Gavelinopsis, Nonionella, and small Cassidulina spp.) are most abundant. Large Pullenia and Melonis specimens are frequent throughout the cores and are occasionally present in large numbers. Stainforthia concava is most frequent in the middle part of the holes (Table T9). Oridorsalis umbonatus and Nuttallides umbonifera occur in the lower part of the holes (Samples 11-CC and below). Agglutinated taxa are rare (Table T9).

Bimodal test-size distribution of benthic foraminifers may be caused by resedimentation. However, small and thin-walled tests are well preserved and could not be transported over long distances without damage. Maximum numbers of benthic foraminifers occur synchronously to the small-sized planktonic foraminiferal assemblages, the latter possibly indicating bottom currents. The same currents may have delivered organic material as a food source for the benthic community and enabled enhanced reproduction rates and benthic foraminiferal faunas that are rich in individuals.

Diatoms

Diatom assemblages were investigated in all core catcher samples from Holes U1305A–U1305C (Tables T10, T11, T12). Diatoms are abundant to common in the upper 200 mcd of the sedimentary sequence and appear well to moderately well preserved, and their abundance decreases and preservation becomes poorer below that depth (Fig. F17). The silicoflagellates Dictyocha fibula and Distephanus speculum and the siliceous dinoflagellate Actiniscus pentasterias occur in a few samples (Tables T10, T11, T12).

Four Quaternary diatom zones proposed by Koç et al. (1999) are determined based on three diatom datum events (Fig. F16; Tables T10, T11, T12). 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) occurs between Samples 303-U1305A-4H-CC and 5H-CC, 303-U1305B-4H-CC and 5H-CC, and 303-U1305C-5H-CC and 6H-CC. The LO of Neodenticula seminae (0.84–0.85 Ma, MIS 21; Koç et al., 1999) appears to be present between Samples 303-U1305A-11H-CC and 12H-CC, 303-U1305B-12H-CC and 13H-CC, and 303-U1305C-13H-CC and 15H-CC. The FO of N. seminae (1.25–1.26 Ma, MIS 37; Koç et al., 1999) is detected between Samples 303-U1305A-19H-CC and 21H-CC, 303-U1305B-20H-CC and 22H-CC, and 303-U1305C-13H-CC and 21H-CC.

The diatom assemblage is dominated by resting spores of Chaetoceros spp. throughout (Tables T10, T11, T12). The high relative contribution of Chaetoceros spores in deep-basin sediments from high latitudes has been proposed to be related either to high productivity of surface waters or winnowing by bottom currents (Crosta et al., 1997). An assemblage composed of the spores of Thalassiosira gravida and the needle-shaped diatoms of the Thalassiothrix-Lioloma complex, accompanied by Actinocyclus curvatulus, Rhizosolenia hebetate f. semispina, and the vegetative cell of T. gravida, is typical of subarctic and arctic waters (Andersen et al., 2004). Influence of the warm and saline waters of the North Atlantic Current is indicated by the presence of Fragilariopsis doliolus, Coscinodiscus marginatus, and varieties of T. oestrupii (Andersen et al., 2004). The sea-ice-related diatom Fragilariopsis oceanica is also present in some samples. Preservation of diatoms is generally poor deeper than 200 mcd in the sediments (Tables T10, T11, T12).

Radiolarians

Radiolarians were examined in all core catcher samples from Holes U1305A and U1305B (Table T13). In the upper part of the succession (Samples 303-U1305A-5H-CC to 11H-CC and 303-U1305B-1H-CC to 9H-CC), radiolarian preservation is good and species diversity is high. The preservation changes from moderate to poor downcore. Total radiolarian abundance varies from trace to abundant because of dilution by detrital grains and diatoms. Throughout the sequences examined, the most abundant species is Cycladophora davisiana davisiana. Actinomma leptodermum, Stylochlamidium venustum, and/or Spongodiscus spp. are also dominant in several samples.

C. davisiana davisiana occurs in most of the core catcher samples of Holes U1305A and U1305B. The sequences of these holes therefore can be 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 Hole U1305A and in 16 core catcher samples from Hole U1305C (Tables T14; T15). Most samples contain abundant dinocysts (Fig. F17), with the exception of the lowest part of the sequence (below ~275 mcd). Terrestrial palynomorphs, dominated by Pinus pollen grains, occur in low numbers throughout the sequences. Reworked palynomorphs also occur in low numbers except in Samples 303-U1305A-16H-CC, 26H-CC, 29H-CC, and 30H-CC, in which they are common. Microscopic fragments of charcoal are frequently observed in palynological slides, especially in the lower half of the sequence.

The dominant components of the dinocyst assemblages are Brigantedinium spp., Operculodinium centrocarpum, and Nematospharopsis labyrinthea. Bitectatodinium tepikiense and Spiniferites spp. are common in many samples. These taxa are also the most abundant taxa in the Pleistocene sediments of nearby Site 646 (cf. de Vernal and Mudie, 1992). At Site 646, high concentration of dinocysts (>1000 cysts/cm³) and dominance of O. centrocarpum or N. labyrinthea, together with high species diversity, characterize interglacial stages. Accordingly, Samples 303-U1305A-6H-CC, 8H-CC, 11H-CC, 12H-CC, 14H-CC, and 22H-CC and 303-U1305C-1H-CC, 3H-CC, 7H-CC, and 15H-CC probably belong to interglacial stages.

The dinocyst assemblages contain only Filisphaera filifera. The LO of this species in the northern North Atlantic is dated ~0.7 Ma (cf. de Vernal et al., 1992). The occurrence of a few specimens of the species in Sample 303-U1304A-11H-CC suggests an age of ~0.7 Ma at ~119 mcd.

Top of page   |    Previous   |    Next