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doi:10.2204/iodp.proc.303306.108.2006

Biostratigraphy

Core catcher samples at Site U1308 reveal species-rich assemblages of calcareous, siliceous, and organic-walled microfossils. Preservation of all microfossil groups is very good to moderate throughout the cored interval. Biostratigraphic datum events mainly derive from coccoliths and are consistent with datum events provided by planktonic foraminifers, diatoms, and dinocysts (Fig. F12). According to these datum events, we recovered an upper Miocene–Quaternary sequence at Site U1308.

Floral and faunal assemblages of planktonic organisms reflect changing paleoclimate and paleoceanographic conditions since the late Miocene. Upper Miocene and lower Upper Pliocene sediments (before 2.74 Ma) contain more warm-water nannofossil species than uppermost Pliocene and Pleistocene sediments. The floral changes can be associated with the onset of northern hemisphere glaciation (NHG). Seasonal changes in bioproductivity and the movement of hydrographic fronts are documented by microfossils at this site.

Calcareous nannofossils

Calcareous nannofossils were examined in all core catcher samples from Holes U1308A–U1308F (Tables T2, T3, T4, T5, T6, T7). Moderate to well-preserved nannofossils are abundant in almost all samples and species diversity is high. Most of the Late Pliocene–Quaternary datum planes described by Sato et al. (1999) and the upper Miocene–Lower Pliocene markers defined by Martini (1971) have been identified in the sedimentary sequence. Correlation of nannofossil datums between the holes and to magnetostratigraphy are shown in Figure F12.

Emiliania huxleyi indicates the uppermost Quaternary Zone NN21 (0–0.25 Ma) and is found in Samples 303-U1308A-1H-CC, 303-U1308B-1H-CC and 2H-CC, 303-U1308C-1H-CC, 303-U1308D-1H-CC, 303-U1308E-1H-CC, and 303-U1308F-1H-CC. The last occurrence (LO) of Pseudoemiliania lacunosa, which shows the top of Zone NN19 (0.41 Ma), is recognized in Sample 3H-CC in all holes except U1308D. Both the LO of Reticulofenestra asanoi (0.85 Ma) and the first occurrence (FO) of Gephyrocapsa parallela (0.95 Ma), situated between the base of the Brunhes Chron and the top of the Jaramillo Subchron, are recognized between Samples 303-U1308A-5H-CC and 6H-CC and 6H-CC and 7H-CC, respectively; 303-U1308C-5H-CC and 6H-CC and 6H-CC and 7H-CC, respectively; 303-U1308E-5H-CC and 6H-CC and 6H-CC and 7H-CC, respectively; and 303-U1308F-5H-CC and 6H-CC and 6H-CC and 7H-CC, respectively. In Hole U1308B, both datums are situated in the same interval between Samples 303-U1308B-6H-CC and 7H-CC. The FO of R. asanoi (1.16 Ma), which lies just below the Jaramillo Subchron, is found between Samples 8H-CC and 9H-CC in all holes. Large-form Gephyrocapsa spp. (>6 µm) appear between 1.45 and 1.21 Ma and occur in Samples 303-U1308A-9H-CC, 303-U1308B-9H-CC and 10H-CC, 303-U1308C-9H-CC, 303-U1308E-9H-CC, and 303-U1308F-9H-CC and 10H-CC. Samples 303-U1308A-10H-CC to 12H-CC, 303-U1308B-11H-CC to 12H-CC, 303-U1308C-10H-C to 11H-CC, 303-U1308E-10H-CC to 11H-CC, and 303-U1308F-11H-CC to 12H-CC are characterized by the presence of Gephyrocapsa oceanica, Gephyrocapsa caribbeanica, and P. lacunosa and the absence of large forms of Gephyrocapsa spp. Accordingly, these intervals are assigned an age between 1.45 and 1.65 Ma. The assemblages below Samples 303-U1308A-12H-CC, 303-U1308B-12H-CC, 303-U1308C-12H-CC, 303-U1308E-11H-CC, and 303-U1308F-12H-CC contain Reticulofenestra spp. (small) and P. lacunosa and lack G. oceanica and G. caribbeanica. Therefore, the Pliocene/Pleistocene boundary defined by the FO of G. caribbeanica is situated between Samples 303-U1308A-12H-CC and 13H-CC, 303-U1308B-12H-CC and 13H-CC, 303-U1308C-12H-CC and 13H-CC, 303-U1308E-11H-CC and 12H-CC, and 303-U1308F-12H-CC and 13H-CC.

The Pliocene assemblages at Site U1308 are characterized by the presence of discoasters. The LOs of Discoaster brouweri, Discoaster pentaradiatus, and Discoaster surculus at 1.97, 2.38, and 2.54 Ma, respectively, are situated in the intervals between Samples 303-U1308A-15H-CC and 19H-CC, 303-U1308B-15H-CC and 19H-CC, 303-U1308C-15H-CC and 19H-CC, 303-U1308E-15H-CC and 19H-CC, and 303-U1308F-15H-CC and 19H-CC. Discoaster tamalis and Reticulofenestra ampla disappeared at 2.74 and 2.78 Ma in the upper part of the Gauss Chron, synchronous with drastic growth of the ice sheet in the Arctic Ocean and final closure of the Isthmus of Panama. The LO of these two species lies between Samples 303-U1308A-19H-CC and 21H-CC, 303-U1308B-20H-CC and 21H-CC, 303-U1308C-20H-CC and 21H-CC, 303-U1308E-20H-CC and 21H-CC, and 303-U1308F-19H-CC and 20H-CC. Below, the assemblages are characterized by abundant occurrences of the warm-water Discoaster spp. and by high species diversity. Reticulofenestra pseudoumbilicus and Sphenolithus abies have their LO in the Middle Pliocene (3.85 Ma) and are found in and below Samples 303-U1308A-28H-CC and 303-U1308C-28H-CC. The LO of Amaurolithus spp. (4.50 Ma) and FO of Ceratolithus rugosus (4.7 Ma), which are Early Pliocene datums recognized in low- to mid-latitude regions, are found in Samples 303-U1308A-29H-CC and 31H-CC, respectively. Discoaster quinqueramus, which has its LO at the Miocene/Pliocene boundary (5.6 Ma), is found in Samples 303-U1308A-35H-CC and 36H-CC. This indicates that the Miocene/Pliocene boundary is situated between Samples 303-U1308A-34H-CC and 35H-CC. Therefore, the bottom of Hole U1308A does correlate with the uppermost Miocene.

The assemblages are characterized by the occurrence of warmer water species compared to other sites of Expedition 303. In particular, the assemblages found in samples older than 2.74 Ma are characterized by high species diversity in all holes. The floral change at Site U1308 can be attributed to the influence of drastic ice sheet growth in the Arctic Ocean and final closure of the Isthmus of Panama (Jansen et al., 1988; Whitman and Berger, 1992; Thiede and Myhre, 1996; Kameo and Sato, 2000; Sato et al., 2004).

Planktonic foraminifers

Planktonic foraminifers were examined in all core catcher samples from Holes U1308A–U1308F (Tables T8, T9, T10, T11, T12). The core catchers from the upper cores of each hole contain soft sediment that was washed with tap water. Sediments of the middle sections (generally Sections 20H-CC to 25H-CC of all holes) were treated with H₂O₂ and occasionally with Calgon solution for several minutes before washing. Planktonic foraminifers are abundant from bottom to top in all holes (Table T8). Preservation of tests is very good to moderate throughout the sections.

The FO of encrusted N. pachyderma (sinistral), situated at the top of the Oludvai Subchron (Weaver and Clement, 1987), is unequivocal in Samples 303-U1308A-12H-CC, 303-U1308C-12H-CC, 303-U1308E-12H-CC, and 303-U1308F-12H-CC. In addition, N. pachyderma (sinistral) is dominant in Sample 303-U1308A-36H-CC, which may therefore be placed in the lower part of the Gilbert Chron (Weaver and Clement, 1987). The FO of Globorotalia inflata is observed in Samples 303-U1308A-15H-CC, 303-U1308B-15H-CC, 303-U1308C-15H-CC, 303-U1308E-14H-CC, and 303-U1308F-15H-CC. The LO of Negloboquadrina atlantica at the base of Matuyama Chron (Weaver and Clement, 1987) is recognized in Sample 303-U1308A-18H-CC, 303-U1308B-18H-CC, 303-U1308C-19H-CC, 303-U1308E-18H-CC, and 303-U1308F-18H-CC. The FO of Globorotalia puncticulata, placed in the Nunivak Subchron (Weaver and Clement, 1987), is found in Samples 303-U1308A-28H-CC and 303-U1308C-29H-CC. According to planktonic foraminifers, we assign a time span from upper Miocene/Lower Pliocene–Quaternary for the recovered sediments (Fig. F12).

At Site U1308, the deep-dwelling species G. truncatulinoides occurs close to its northern distribution limit for the North Atlantic (Ericson et al., 1954; Prell et al., 1999) and is a rare component of the studied faunas. The subtropical to temperate globorotalid species Globorotalia crassaformis, Globorotalia margaritae, and Globorotalia conomiozea are rare (Table T8) and were therefore not considered for biostratigraphy.

Sinistrally coiled N. pachyderma occurs frequently throughout the upper Samples 303-U1308A-1H-CC to 12H-CC, indicating polar conditions. Along with N. pachyderma (sinistral), Globigerina bulloides and Globorotalia inflata are abundant. The presence of dextrally coiled Globorotalia truncatulinoides in Samples 303-U1308A-1H-CC to 3H-CC and sinistrally coiled specimens in Samples 303-U1308A-6H-CC to 8H-CC indicates that mesobathyal water masses of southern origin were also present at Site U1308 during glacial stages.

Core catcher samples of Hole U1308A are characterized by a small-sized planktonic foraminiferal fauna. In contrast, core catcher samples of Holes U1308B, U1308C, U1308E, and U1308F contain more large-sized, subtropical species and less N. pachyderma (sinistral). The warm-water fauna frequently contains species such as Globigerinella siphonifera, Globigerinella calida, Globigerinoides ruber (white), Orbulina universa, and occasionally Globigerinoides sacculifer. During the Pliocene and Pleistocene, G. puncticulata and G. inflata are abundant and may be indicative of hydrographic fronts at Site U1308. Both glacial and interglacial faunas display significant seasonality, with Globigerina bulloides as a typical species following spring blooms of phytoplankton, and dextrally coiled N. pachyderma dominating the fauna during summer (Schiebel and Hemleben, 2000).

Benthic foraminifers

Benthic foraminifers were examined in all core catcher samples from Holes U1308A–U1308F (Tables T13, T14, T15, T16, T17). Benthic foraminifers are rare in Sections 1H-CC to 16H-CC and 24H-CC to 29H-CC and more frequent in Sections 17H-CC to 23H-CC (175–240 mcd) of all holes. Benthic foraminifers are most abundant in Samples 303-U1308A-30H-CC to 36H-CC (297–355.6 mcd). Rotaliid species are frequent at Site U1308, whereas miliolid and agglutinated species are rare. Oridorsalis umbonatus is the most frequent benthic foraminiferal species at Site U1308. In contrast to Sites U1302–U1307, Globocassidulina is frequent at Site U1308. Cibicidoides wuellerstorfi is present in all holes of Site U1308.

Diatoms

Diatom assemblages were investigated in 133 core catcher samples and 107 smear slides from Holes U1308A–U1308F (Tables T18, T19, T20, T21, T22). The only core catcher sample from Hole U1308D was barren of diatoms. Diatoms are generally rare to common within the upper sequence (0–255 mcd) (Fig. F13) during the Late Pliocene and Pleistocene. The lower part of the sequence between 255 and 355 mcd is barren of diatoms. The valves are moderately preserved throughout the sequence above 255 mcd. Five silicoflagellate species and the siliceous dinoflagellate Actiniscus pentasterias are present almost throughout the sequence above 255 mcd.

Although datum placements are inadequately defined because of a discontinuity in diatom occurrences, the four diatom zones used here, previously proposed by Baldauf (1987) for Site 609, are implied by two diatom datum events at Site U1308 (Fig. F12; Tables T18, T19, T20, T21, T22). The LO of Fragilariopsis reinholdii, which defines the base of the Fragilariopsis doliolus Zone and the top of the F. reinholdii Zone at 0.48–0.45 Ma (based on Cande and Kent, 1995, and others), is apparent above Samples 303-U1308A-6H-CC, 303-U1308B-4H-CC, 303-U1308C-4H-CC, 303-U1308E-5H-CC, and 303-U1308F-5H-CC (56–33 mcd). The FO of F. doliolus (1.9 Ma), which defines the bottom of the F. reinholdii Zone and the top of the Alveolus marinus Zone, is unclear in each hole because of poor diatom abundance in the interval. The LO of Thalassiosira convexa var. convexa (0.84–0.85 Ma) appears to occur above Samples 303-U1308A-17H-CC, 303-U1308C-19H-CC, and 303-U1308F-19H-CC (201–177 mcd) and gives a secondary datum (2.58–2.68 Ma) within the A. marinus Zone. The LO of Fragilariopsis jouseae (2.66–2.83 Ma), which defines the bottom of the F. jouseae Zone and the top of the T. convexa Zone, appears to be above Samples 303-U1308A-28H-CC and 22H-CC and 303-U1308B-21H-CC (285–220 mcd). Below Samples 303-U1308A-29H-CC and 303-U1308B-28H-CC (285–290 mcd), the calcareous-rich lithofacies is barren of diatoms.

A very diverse diatom flora is present in the sedimentary section above 255 mcd. Around 110 marine diatom species were identified (Tables T18, T19, T20, T21, T22). As with previous observations at Site 609 (Baldauf, 1987), the dominant diatoms indicate warm temperate, highly saline waters. The main components of the warm-water assemblage are varieties of Thalassiosira oestrupii, Thalassiosira ferelineata, Thalassiosira lineata, Fragilariopsis fossilis, F. reinholdii, Azpeitia spp., Hemidiscus cuneiformis, and A. marinus. The tropical/subtropical diatom community is accompanied by a rich assemblage of resting spores of Chaetoceros, which reflects high productivity of surface waters (Crosta et al., 1997). Minor influence of cold water masses is suggested by the occurrence of Actinocyclus curvatulus and the needle-shaped diatoms of the Thalassiothrix/Lioloma complex (Andersen et al., 2004).

Radiolarians

Radiolarians were examined in all core catcher samples from Holes U1308A and U1308C (Table T23). Radiolarians are abundant from the middle part of the sequences (154–280 mcd) and are rare or absent in the upper and the lower parts. Species diversity is generally higher than at Sites U1302–U1307. Abundant groups throughout the sequence are spongodiscids, litheliids, and actinommids. Cycladophora davisiana davisiana is abundant in the upper part of the sequence (Samples 303-U1308A-6H-CC and 7H-CC) and becomes rare in the middle part. Radiolarian assemblages are characterized by abundant spongodiscids and litheliids with continuous occurrence of Axoprunum angelinum(?) between Samples 303-U1308A-15H-CC and 28H-CC and 303-U1308C-14H-CC and 27H-CC. Below the intervals, radiolarians are poorly preserved and occur sporadically.

Two diagnostic datum events useful for biostratigraphy are observed. The FO of C. davisiana davisiana (2.7 Ma) (Goll and Bjørklund, 1989) is observed between Samples 303-U1308A-20H-CC and 21H-CC and 303-U1308C-19H-CC and 20H-CC. The FO of Spongaster ?tetras (3.0–3.2 Ma) (Goll and Bjørklund, 1989) is located between Samples 303-U1308A-24H-CC and 25H-CC and 303-U1308C-23H-CC and 24H-CC.

Palynomorphs

Palynological assemblages were examined in core catcher samples from Holes U1308A and U1308B (Tables T24, T25). The concentration of terrestrial palynomorphs is low in most samples (Fig. F13), which is expected because the site location is distant from a land vegetation source. The pollen abundance is usually <100 grains/cm³. It is dominated by Pinus and can be related to minor input through atmospheric or ocean circulation. Reworked palynomorphs are rare except in a few samples of the upper 200 m of the sequence where they might be associated with IRD. Reworked palynomorphs are absent in the lower part of the sequence (below 200 mcd). Dinocysts are common to abundant (10²–10⁴ cysts/cm³) in most samples of the upper 200 mcd of the sequence, whereas their concentration is usually lower (<10² cysts/cm³) below 200 mcd. The dinocyst assemblages are largely dominated by Gonyaulacoid taxa, commonly related to autotrophic productivity. Heterotrophic taxa, such as Brigantedinium, occur sporadically and mainly in the upper 145 mcd of the sequence. The species diversity of dinocyst assemblages is high throughout. The dominant taxa are Operculodinium centrocarpum, Nematosphaeropsis labyrinthea, Impagidinium aculeatum, and Spiniferites mirabilis-hyperhacanthus. Several Impagidinium spp. are present. The most common are Impagidinium paradoxum, Impagidinium patulum, Impagidinium velorum, and Impagidinium? pallidum. In general, the dinocyst assemblages suggest cool to warm temperate conditions in surface waters, depending upon the proportion of the warm-water components (I. aculeatum, I. patulum, I. paradoxum, and S. mirabilis-hyperhacanthus) versus that of more ubiquitous taxa (O. centrocarpum and N. labyrinthea). Subpolar taxa, such as I. pallidum and Sphenlolithus elongates, are present in many samples.

The sequence contains many extinct dinocyst taxa that might help to establish a biostratigraphic framework. However, many of these taxa were only recently described and their chronostratigraphic correlations are poorly constrained. This is the case for Operculodinium tegillatum, Pyxidinopsis vesiculata, and Ataxiodinium zevenboomii (Head, 1997; Head and Norris, 2003). Many observed specimens belong to taxa difficult to identify at a species level without systematic observations, such as cysts provisionally attributed to Batiacasphaera spp., Impagidinium spp., and Achomosphaera spp. Nevertheless, the overall dinocyst assemblage allows tentative biostratigraphic considerations. The occurrence of I. velorum in Sample 303-U1308A-3H-CC suggests an age of at least 0.4 Ma at 28 mcd, and the occurrence of Filifera filisfera in Sample 303-U1308A-6H-CC suggests sediments older than 0.7 Ma at 62 mcd (cf. de Vernal et al., 1992). The overall assemblage below Sample 303-U1308A-13H-CC (133 mcd) has a Pliocene character, notably because of the presence of A. zevenboomii, Batiacasphaera spp., and other extinct taxa. The co-occurrence of Invertocysta lacrymosa, Nematosphaeropsis sp. 1, and incertae sedis sp. 1 in Sample 303-U1308A-20H-CC (202 mcd) indicates an age of ~2.7–2.8 Ma (de Vernal and Mudie 1989; Louwye et al., 2004), which is consistent with the occurrence of Operculodinium ?eirikianum based on correlation with DSDP Site 611 (cf. Mudie, 1987). Finally, the significant occurrence of Corrudinium devernaliae in Sample 303-U1308A-29H-CC at 290 mcd suggests an age of at least 3.7 Ma (Head and Norris, 2003).

Bolboforma

Bolboforma occurred in Sections 303-U1308A-35H-CC and 303-U1308A-36H-CC (346.6–355.6 mcd). In Sample 303-U1308A-35H-CC, Bolboforma are rare. They are hemispherical with a flat base. The outer wall has a fine hexagonal structure, and there is an opening on a short smooth neck. Bolboforma are frequent in Sample 303-U1308A-36H-CC. The most abundant larger species (maximum diameter = 200–400 m) is conical with a flat base. The outer wall has a coarse reticulate structure, which often forms hexagons or runs as simple costae toward the simple round opening. The species is possibly Bolboforma reticulata, which has been reported from Hole 609B as abundant in the upper Zone NN11 of the upper Miocene (Pallant and Kaminski, 1989). Other species of Bolboforma in Section 303-U1308A-36H-CC are

• Globular, hispid, with an opening on a short neck;

• Globular with a fine hexagonal outer structure; and

• Onion-shaped with a small flat base, finely striated, with an aperture on a conical elevation.

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