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doi:10.2204/iodp.proc.303306.112.2006

Biostratigraphy

Core catcher samples from Holes U1313A through U1313D contain rich assemblages of calcareous nannofossils and planktonic foraminifers that are well to moderately well preserved. A succession of calcareous nannofossil and planktonic foraminifer events provide a reliable biostratigraphic framework that, in the upper part, is supported by the siliceous plankton biostratigraphic zones for much of the cored intervals (Table T4).

According to the age of biostratigraphic events recognized in this study, Site U1313 provided a sedimentary sequence encompassing the latest Miocene–Holocene (Figs. F14, F15, F16, F17). The oldest sediments are tentatively dated at 6.0 Ma based on the last occurrence (LO) of the calcareous nannofossil Amaurolithus amplificus in Samples 306-U1313A-32H-CC and 306-U1313C-32H-CC and the possible first abundant occurrence (FaO) of the planktonic foraminifer Globorotalia margaritae in Sample 306-U1313A-33H-CC. Sediments from the Pliocene and Pleistocene contain abundant planktonic foraminifers in relation to calcareous nannofossils, whereas planktonic foraminifer abundances in upper Miocene sediments are reduced relative to nannofossils. Specimens of planktonic and benthic foraminifers are small in the lowermost 50 m (upper Miocene), and most are of similar size. This sorting and the lack of large individuals seem to indicate the foraminifers have been transported and deposited in the deepest part of the basin where this site is located. The co-occurrence of frequent fragments of large, heavily calcified benthic foraminifers with mostly unbroken planktonic foraminifers of similar size indicates deposition in a current-influenced environment. The relatively high proportion of fragments of benthic shells also indicates transport from an area of intensive carbonate dissolution.

Comparison of age-depth plots from Holes U1313A through U1313D show very similar, consistent sedimentation rates of 4.1–4.5 cm/k.y. throughout the Pliocene and Pleistocene (Figs. F14, F15, F16, F17). Although the stratigraphic framework for the late Miocene is less constrained, sedimentation rates in the late Miocene appear to be much higher (13–14 cm/k.y.).

Calcareous nannofossils

We examined all core catcher samples from Holes U1313A through U1313D for calcareous nannofossils. Additional samples were taken from Cores 306-U1313A-3H and 6H, as well as 306-U1313B-2H through 12H to refine the biostratigraphy. All samples yielded abundant to very abundant nannofossil assemblages. Preservation was generally good to moderately good throughout the Pliocene and Pleistocene sediments. Upper Miocene calcareous nannofossils were moderately to moderately well preserved, with some discoasters exhibiting significant overgrowth. Gephyrocapsids, small reticulofenestrids, and Pseudoemiliania species dominate Pleistocene assemblages, whereas Pliocene and Miocene assemblages are dominated by reticulofenestrids of all sizes. Very rare reworked nannofossils occur in all holes, especially within the Pleistocene and upper Pliocene sediments.

The sections recovered at Site U1313 yielded Pleistocene, Pliocene, and upper Miocene assemblages (Tables T5, T6, T7, T8, T9). Higher sedimentation rates than those at Site U1312 allowed us to identify the 12 Pleistocene nannofossil datums defined by Sato et al. (1999) in most holes at Site U1313. Most core catchers yielded a new datum, so we took subsamples from many of the Pleistocene cores in Hole U1313B to further resolve the biostratigraphy. The LO of Helicosphaera inversa (0.16 Ma) occurs in Samples 306-U1313A-1H-CC, 306-U1313C-1H-CC, and 306-U1313D-1H-CC; however, the single specimen in Hole U1313C may be reworked, and therefore the LO of H. inversa is placed in Sample 306-U1313C-2H-CC of that hole. The LO of this species also occurs in Sample 306-U1313B-2H-3, 10–11 cm. The first occurrence (FO) of Emiliania huxleyi (0.25 Ma), which marks the base of Martini's (1971) Zone NN21, is present in Samples 306-U1313A-3H-2, top, 306-U1313B-2H-CC, 306-U1313C-2H-CC, and 306-U1313D-1H-CC. This event was problematic because E. huxleyi is difficult to recognize under the light microscope. In Hole U1313D, Zone NN20, which spans the time interval between the LO of Pseudoemiliania lacunosa and the FO of E. huxleyi, occurs in Sample 306-U1313D-2H-CC. This zone was not identified in any other holes at this site, due in part to the difficulty of determining the presence or absence of E. huxleyi.

The LO of P. lacunosa (0.41 Ma), which defines the top of Zone NN19, is found in Samples 306-U1313C-3H-CC and 306-U1313D-3H-CC. Subsamples taken from Holes U1313A and U1313B yielded the LO of P. lacunosa in Samples 306-U1313A-3H-2, top, and 306-U1313B-3H-3, 84–87 cm. The FO of H. inversa (0.51 Ma) is present in Samples 306-U1312A-3H-2, top, 306-U1312B-3H-3, 84–87 cm, 306-U1313C-2H-CC, and 306-U1313D-2H-CC. The absence of H. inversa in conjunction with the FO of P. lacunosa in Samples 306-U1313C-3H-CC and 306-U1313D-3H-CC indicates the uppermost part of Zone NN19 (between 0.41 and 0.51 Ma) occurs somewhere within Core 3H in those holes. The LO of Reticulofenestra asanoi also occurs in Samples 306-U1313A-3H-CC, 306-U1313B-4H-3, 102–105 cm, 306-U1313C-4H-CC, and 306-U1313D-4H-CC. Additionally, the FO of Gephyrocapsa parallela (0.95 Ma) occurs in Samples 306-U1313A-4H-CC, 306-U1313B-5H-4, 18–20 cm, 306-U1313C-5H-CC, and 306-U1313D-4H-CC. The FO of R. asanoi (1.16 Ma) occurs in Samples 306-U1313A-5H-CC, 306-U1313B-5H-CC, 306-U1313C-5H-CC, and 306-U1313A-5H-CC.

The LO of large Gephyrocapsa spp. (1.21 Ma) occurs in Samples 306-U1313A-6H-6, 57 cm, 306-U1313B-6H-CC, and 306-U1313C-6H-CC. This species, which ranges from 1.21 to 1.45 Ma, was not found in any of the core catchers from Hole U1313D, and so its entire range must fall within one of the cores in that hole. The LO of Helicosphaera sellii (1.27 Ma) was difficult to pick with certainty as very rare reworked occurrences are found within the upper Pleistocene sections of Holes U1313A, U1313B, and U1313C. We placed the event in Samples 306-U1313A-7H-CC, 306-U1313B-7H-4, 60–63 cm, and 306-U1313C-7H-CC. This event appears slightly higher in Hole U1313D (Sample 306-U1313D-6H-CC). The FO of large Gephyrocapsa spp. (1.45 Ma) occurs in Samples 306-U1313A-7H-CC, 306-U1313B-7H-CC, and 306-U1313C-8H-CC.

The FOs of Gephyrocapsa oceanica (1.65 Ma) and Gephyrocapsa caribbeanica (1.73 Ma) occur together in all holes at Site U1313. We use the latter species to approximate the Pliocene/Pleistocene boundary. These events occur in Samples 306-U1313A-8H-CC, 306-U1313B-9H-1, 83–90 cm, 306-U1313C-9H-CC, and 306-U1313D-8H-CC. A rare occurrence of G. oceanica co-occurs with Discoaster brouweri in Sample 306-U1313A-10H-CC. This pattern of very rare Gephyrocapsa spp. (>4 µm) occurrences below the Pliocene/Pleistocene boundary is consistent with results from Site U1312 and reports of similar occurrences from Blake Ridge, northwest Atlantic (Okada, 2000). Thus, the FOs of G. oceanica and G. caribbeanica are placed at the base of consistent occurrences of the species, and it is this horizon we use to approximate the boundary.

Pleistocene sediments at Site U1313 contain very rare reworked material throughout much of the interval. Reticulofenestra pseudoumbilicus and, to a lesser extent, Sphenolithus abies are the most frequent reworked species, although only one or two specimens are typically found in any sample. Some discoasters also occur sporadically throughout this section. Since we only looked at one or two samples per core, the reworked material in those samples made picking the LOs of some of the Pliocene events difficult, as discussed below.

Five Pliocene events dated by Sato et al. (1999), one by Shackleton et al. (1995), and one by Backman and Raffi (1997) occur within the sedimentary section at Site U1313. The LO of D. brouweri (1.97 Ma), which marks the top of Zone NN18, occurs in Samples 306-U1313A-10H-CC, 306-U1313B-9H-CC, 306-U1313C-10H-CC, and 306-U1313D-9H-CC. Very rare reworked occurrences of D. brouweri are found in Holes U1313A and U1313B, making it difficult to accurately identify this extinction event in these holes. Discoaster pentaradiatus also occurs sporadically above its LO in several holes. The LO of D. pentaradiatus (2.38 Ma), which marks the top of Zone NN17, is located in Samples 306-U1313A-11H-CC, 306-U1313B-12H-1, 80–87 cm (along with single reworked specimens of Discoaster surculus and Discoaster tamalis), 306-U1313C-12H-CC, and 306-U1313D-11H-CC. The LO of D. surculus (2.54 Ma), which marks the top of Zone NN16, occurs in Samples 306-U1313A-12H-CC, 306-U1313B-12H-CC, 306-U1313C-12H-CC, and 306-U1313D-12H-CC. In Hole U1313C, this event is placed at the same horizon as the LO of D. pentaradiatus. These events may be separated within Core 306-U1313C-12H, or the occurrence of D. surculus in the core catcher sample may be reworked. The LO of D. tamalis (2.74 Ma) occurs in Samples 306-U1313A-13H-CC, 306-U1313B-13H-CC, 306-U1313C-12H-CC, and 306-U1313D-13H-CC. The occurrence of this species higher in Hole U1313C may also be attributed to reworking, as very rare reworked occurrences are found in samples from all holes except Hole U1313A.

Most core catcher samples from the lowermost Pleistocene and uppermost Pliocene contain very rare reworked discoasters. Often these specimens are fragmented, whereas the in situ material is well preserved. Some of the additional samples from Hole U1313B did not contain reworked material; therefore examination of more closely-spaced samples should result in more precise placement of discoaster LOs within the upper Pliocene.

The intervals from Samples 306-U1313A-13H-CC through 17H-CC, 306-U1313B-13H-CC through 17H-CC, 306-U1313C-12H-CC through 18H-CC, and 306-U1313D-13H-CC through total depth are characterized by the occurrence of D. tamalis and very rare reworked occurrences of R. pseudoumbilicus (>7 µm) in nearly every sample, indicating an age between 2.74 and 3.85 Ma. The persistent nature of reworked R. pseudoumbilicus made it very difficult to identify the true LO of this species, which occurs at 3.85 Ma and marks the top of Zone NN15. We placed this event at the top of consistent co-occurrences of R. pseudoumbilicus and S. abies, which occurs in Samples 306-U1313A-17H-CC, 306-U1313B-17H-CC, and 306-U1313C-18H-CC.

Two datums from the lower Pliocene are identified at Site U1313, although both are somewhat problematic. The LO of Amaurolithus primus, which approximates the top of Zone NN14, is dated at 4.56 Ma in the eastern equatorial Pacific by Shackleton et al. (1995). This event, which is equivalent to the LO of the genus Amaurolithus, occurs very consistently within the holes at this site and is located in Samples 306-U1313A-18H-CC, 306-U1313B-18H-CC, and 306-U1313C-19H-CC. The planktonic foraminifer datums from equivalent horizons yield a younger age of 3.81 Ma (Table T10). There are two possibilities: the amauroliths are consistently reworked above their true LO at this site or the age of the LO of Amaurolithus in the northwest Atlantic is younger than that documented in the eastern equatorial Pacific. As a result, we cannot rely on this event for biostratigraphy at this site.

Ceratolith species are generally rare but consistent at Site U1313. Several questionable occurrences of Ceratolithus acutus occur in Holes U1313A and U1313B; however, these occurrences are not persistent enough to use the LO of C. acutus, which occurs at 5.046 Ma, as a datum. The FO of Ceratolithus rugosus, which is considered an overgrown form of the species Ceratolithus cristatus (Bergen, 1984; Young, 1998), can be identified at this site. Backman and Raffi (1997) date the FO of C. rugosus to 5.089 Ma. This event approximates the top of Zone NN12 and occurs in Samples 306-U1313A-23H-CC, 306-U1313B-21H-CC, and 306-U1313C-22H-CC.

Two nannofossil events dated by Backman and Raffi (1997) are identified from the upper Miocene sediments at Site U1313. The LO of Discoaster quinqueramus (5.537 Ma), which we use to approximate the Miocene/Pliocene boundary (5.332 Ma), is difficult to identify at this site. Discoasters within the lowermost Pliocene and uppermost Miocene sediments are generally overgrown, making it difficult to identify the LO of D. quinqueramus. We placed the event at the top of definitive specimens of D. quinqueramus; however, the true LO could be higher. This event, which marks the top of Subzone NN11D, occurs in Samples 306-U1313A-25H-CC, 306-U1313B-25H-CC, and 306-U1313C-25H-CC. The LO of A. amplificus (5.999 Ma), which marks the top of Subzone NN11C, comes in very consistently near the bottom of Holes U1313A and U1313C in Samples 306-U1313A-32H-CC and 306-U1313C-32H-CC. Several occurrences of this species in Samples 306-U1313B-26H-CC and 306-U1313B-27H-CC were initially interpreted as the LO of this species in Hole U1313B. The consistent occurrences at the base of Holes U1313A and U1313C and the absence of the species below Core 306-U1313B-27H in Hole U1313B have led us to reinterpret the A. amplificus occurrences uphole in Hole U1313B as reworked (Fig. F15; Table T6). Thus, the base of Hole U1313B may be just above the true LO of A. amplificus.

Planktonic foraminifers

We studied the planktonic foraminifer assemblages in all core catchers from Holes U1313A through U1313D (Tables T11, T12, T13, T14). In addition, the washout from the top of Cores 306-U1313B-1H and 306-U1313C-1H, for which only the >150 µm fraction is available, was examined. All samples were washed with tap water. Planktonic foraminifers dominate the sand fraction in all core catchers and are mainly moderately to well preserved. The foraminifer assemblage in the core catcher samples consists of 11 to 20 different species. Subtropical to tropical species are present in nearly every sample. Some core catcher samples are from glacial or stadial periods, as they contain small amounts of IRD.

The interhole variability in the occurrence of the stratigraphic events is highlighted in Table T10. All age model–relevant events are also placed into the context of the other microfossil group biostratigraphic events in Table T4.

The FaO of Neogloboquadrina pachyderma (sinistral), dated at 1.78 Ma (Weaver and Clement 1987), is probably related to the advection of cold waters to mid-latitudes of the North Atlantic because this polar species only reaches higher abundances in the Pleistocene under full glacial conditions. Consequently, its abundance in the core catcher samples depends on the time interval recovered in the core catcher, and only in Sample 306-U1313A-8H-CC does it seem to occur in the correct stratigraphic position (Table T10). Encrusted N. pachyderma (sinistral) appears for the first time in Samples 306-U1313B-8H-CC and 306-U1313C-8H-CC only in rare abundances.

An incursion of large-sized (>250 µm), heavily encrusted Neogloboquadrina atlantica (dextral) is observed in the lower Pleistocene. The onset of this incursion occurs in Samples 306-U1313A-9H-CC, 306-U1313B-9H-CC, 306-U1313C-9H-CC, and 306-U1313D-9H-CC and coincides with the bottom of the Olduvai polarity chron (1.95 Ma). The incursion is present until Samples 306-U1313A-7H-CC, 306-U1313B-7H-CC, 306-U1313C-7H-CC, and 306-U1313D-6H-CC. Using a preliminary age model for Site U1313, these core catcher samples are dated between 1.52 and 1.4 Ma (see “Stratigraphic correlation”). The youngest LO reported for this species is 1.84 Ma at ODP Site 644 (Spiegler and Jansen, 1989), but the great resemblance between the specimens found at Site U1313 and those used by Berggren (1972) to define the species clearly identifies them as N. atlantica. The incursion of this subpolar species is contemporaneous with an increase in IRD deposition (see “Lithostratigraphy”) and therefore clearly indicates the advance of cold northern source waters as far south as Site U1313 in the early Pleistocene.

The FO of Globorotalia truncatulinoides occurs one or two cores above the FO of Globorotalia inflata in all four holes. This suggests that these FOs most likely correlate with the more abundant occurrences of this species observed at 1.92 Ma in DSDP Site 607 by Spencer-Cervato and Thierstein (1997), and not with the very first trace occurrences dated at 2.14 Ma by the same authors. The FO of G. inflata, dated at 2.09 Ma in the North Atlantic and Mediterranean (Weaver and Clement, 1987), occurs in Samples 306-U1313A-10H-CC, 306-U1313B-10H-CC, 306-U1313C-9H-CC, and 306-U1313B-10H-CC.

As with the FO of G. inflata, the LO of Globorotalia puncticulata also occurs at the same level in Holes U1313A, U1313B, and U1313D (Sample 12H-CC) and one core higher in Hole U1313C (Sample 11H-CC). This event, dated at 2.41 Ma, was calibrated to the astronomical timescale in the Mediterranean (Lourens et al., 1996). The LOs of Globorotalia miocenica and N. atlantica (sinistral) (at 2.4 Ma) nearly coincide with that of G. puncticulata (Weaver and Clement, 1987). The three biostratigraphic events, however, never occur in the same core catcher sample, but in Holes U1313B and U1313D two of these events coincide in Samples 306-U1313B-12H-CC and 306-U1313D-12H-CC. The scatter in the biostratigraphic events is easily explained because G. miocenica, whose abundance is rare, lived in tropical waters and N. atlantica (sinistral) in temperate to polar waters. From time to time, N. atlantica invaded the mid-latitude waters of the North Atlantic and Mediterranean and changed its coiling direction several times from the late Miocene to the Pleistocene. The LO of N. atlantica (sinistral) occurs in Samples 306-U1313B-12H-CC, 306-U1313C-12H-CC, and 306-U1313D-11H-CC. This event was dated in the North Atlantic at 2.4 Ma (Weaver and Clement, 1987) and probably coincides with the disappearance of the species in the Mediterranean (Lourens et al., 1996).

An interval with relatively abundant Globorotalia hirsuta is found in all holes from approximately Samples 15H-CC to 18H-CC. These specimens were previously identified by Weaver and Clement (1987) as Globorotalia cf. crassula. The highly convex dorsal side, however, led us to relate these forms to G. hirsuta, which evolved from G. margaritae in the late Pliocene (Kennett and Srinivasan, 1983). These forms become a significant component of the assemblages after the extinction of G. margaritae and disappear at ~3.1–3.2 Ma (Weaver and Clement, 1987). After its temporal disappearance, this species is not present again until the late Pleistocene and Holocene. Its disappearance is approximately coetaneous with the LO of Sphaeroidinellopsis seminulina (3.19 Ma) (Weaver and Clement, 1987; Lourens et al., 1996). The LO of S. seminulina occurs in Samples 306-U1313A-15H-CC, 306-U1313B-15H-CC, and 306-U1313C-15H-CC, but is not observed in Hole U1313D.

Similar to records from the Mediterranean Sea, a well-defined gap in the presence of G. puncticulata (Lourens et al., 1996) is observed in all holes at Site U1313. In the Mediterranean, this gap occurs between 3.31 and 3.57 Ma (Lourens et al., 1996). In Holes U1313A and U1313C, the reappearance of G. puncticulata occurs in the same core catcher samples as the LO of S. seminulina or the disappearance of G. hirsuta (Samples 306-U1313A-15H-CC and 306-U1313C-15H-CC).

The next biostratigraphic marker horizons are the LO and the last common occurrence of G. margaritae, which take place in consecutive core catcher samples in the three deeper holes, (i.e., Samples 306-U1313A-18H-CC and 19H-CC, 306-U1313B-18H-CC and 19H-CC, and 306-U1313C-19H-CC and 20H-CC). This event, reported by Weaver and Clement (1987) to be diachronous between the high and low latitudes, is dated to 3.98 Ma in the Mediterranean (Lourens et al., 1996). As Site U1313 is at a similar latitude, we use the Mediterranean age for this marker event.

The previous event is directly preceded by the FOs of G. puncticulata and Globorotalia crassaformis, which occur together in Samples 306-U1313A-20H-CC, 306-U1313B-20H-CC, and 306-U1313C-21H-CC (Table T10). This event can be traced throughout the North Atlantic and Mediterranean. It is calibrated to the astronomical timescale and dated to 4.52 Ma (Lourens et al., 1996).

The oldest definitive age marker at Site U1313 is the LO of Globigerina nepenthes during the Sidufjall magnetic subchron, which lasted from 4.89 to 4.8 Ma. This event occurs in Samples 306-U1313A-22H-CC, 306-U1313B-22H-CC, and 306-U1313C-23H -CC.

For the lower parts of the three deeper holes, no stratigraphic event could be placed with great certainty. The next biostratigraphic event would be the FaO of G. margaritae at ~6.0 Ma near the top of the C3A.1n Chron (F.J. Sierro, unpubl. data). At Site U1313, however, G. margaritae is relatively common in most samples analyzed from the lowermost cores, with only Sample 306-U1313A-33H-CC containing rare specimens of this species. Based on these data, the FaO of G. margaritae can tentatively be placed in Sample 306-U1313A-33H-CC. Beginning at 6.3 Ma, N. pachyderma, which is preferentially sinistral in the late Miocene, underwent a series of changes in the coiling direction to become dominantly dextral during the Pliocene (Hilgen and Krijgsman, 1999; Sierro et al., 2001; Hodell et al., 2001). Analysis of coiling direction of this species indicates that from Samples 306-U1313A-33H-CC through 26H-CC, 306-U1313B-33H-CC through 26H-CC, and 306-U1313C-32H-CC through 27H-CC, N. pachyderma underwent some fluctuations in coiling direction, which are typical for the latest part of the Messinian. Even though the lowermost samples of Holes U1313A and U1313B contain abundant numbers of N. pachyderma (sinistral), the remaining occurrences of G. margaritae and the scarcity of specimens of the Globorotalia miotumida group indicate that the oldest sediments recovered at Site U1313 are younger than 6.3 Ma and somewhere closer to 6.0 Ma. Furthermore, the G. miotumida group, including Globorotalia conomiozea, was dominant in the North Atlantic and Mediterranean between 7.2 and 6.3 Ma (Sierro et al., 1993); however, only a few specimens of this group are observed in the bottommost samples of this site.

Benthic foraminifers

Benthic foraminifer assemblages from Site U1313 were only studied in Hole U1313A. Few moderately well preserved benthic foraminifers occur throughout the Neogene sequence, except in Sample 306-U1313A-28H-CC, which contains very rare, poorly preserved specimens (Table T15). Four assemblages are determined for this site.

Assemblage I (Epistominella exigua-Uvigerina proboscidea)

This assemblage occurs between Samples 306-U1313A-1H-CC and 3H-CC and is characterized by the highest abundance of E. exigua and U. proboscidea and common occurrences of Globocassidulina subglobosa.

Assemblage II (Oridorsalis umbonatus)

This assemblage occurs between Samples 306-U1313A-3H-CC and 11H-CC and is characterized by the abundant occurrence of O. umbonatus.

Assemblage III (Globocassidulina subglobosa)

This assemblage is found from Samples 306-U1313A-12H-CC through 15H-CC, and also from 20H-CC through 23H-CC. It is represented by the highest abundances of G. subglobosa.

Assemblage IV (Nuttallides umboniferus)

This assemblage is represented by abundant occurrences of N. umboniferus and is recognized in Samples 306-U1313A-16H-CC through 18H-CC and below Core 24H.

Studies of benthic foraminifers in the modern Atlantic Ocean have shown that foraminiferal biofacies are well correlated with individual water masses (e.g., Murray, 1991). Murray (1991) identified specific foraminiferal biofacies associated with NADW and AABW. Whereas G. subglobosa and O. umbonatus are present beneath NADW, N. umboniferus prefers deeper water depths under the influence of AABW. In addition, G. subglobosa, Melonis barleeanus, and Uvigerina peregrina are dependent on food availability (Murray, 1991) and E. exigua is an opportunistic species linked to seasonal deposition of phytodetritus on the seafloor (Thomas et al., 1995). Thus, their high abundances may indicate an increased flux of organic matter, even if only seasonally.

Benthic foraminifer assemblages from Hole U1313A suggest sediments above Sample 306-U1313A-3H-CC (0.41–0.85 Ma) (Table T4) were probably deposited under the influence of NADW. The relatively high abundances of E. exigua and G. subglobosa suggest an increased supply of phytodetritus to the seafloor and thereby at least seasonal high productivity in the surface waters at Site U1313. Sediments between Samples 306-U1313A-4H-CC and 15H-CC also show evidence of NADW influence, particularly from 0.85 to 2.74 Ma and also from 4.56(?) to 5.089 Ma. Prior to 2.38 Ma, the high abundance of G. subglobosa reveals either high organic carbon flux to the seafloor or sluggish deepwater circulation (Thomas et al., 1995). Although increased organic carbon is not preserved in the sediments (see “Geochemistry”), seasonally high phytodetritus fluxes could have caused the blooms of G. subglobosa, especially because E. exigua is also present in minor numbers. Between 3.85 and 4.56(?) Ma and before 5.089 Ma, N. umboniferus dominates the fauna in most core catchers, suggesting the influence of AABW at Site U1313 and thereby shoaling of the NADW/AABW interface, at least during the intervals recovered in the core catcher samples.

The only benthic foraminifer biostratigraphic event recognized at Site U1313 is the LO of Stilostomella lepidula, which occurs in Sample 306-U1313A-5H-CC. Most species of Stilostomella disappeared between 1.0 and 0.6 Ma (Hayward, 2001). Thus, the age indicated by the benthic foraminifers coincides with the nannofossil results.

Diatoms

Diatoms were investigated in smear slides from 113 core catcher samples from Holes U1313A through U1313D (Tables T16, T17, T18, T19). In addition, Core 306-U1313C-32H was investigated at 10 cm resolution in an attempt to constrain the age of the bottom of the hole. Diatoms are generally present within the Pliocene–Pleistocene intervals (from 0 to 40–70 m); however, they are usually only abundant in the uppermost two core catchers and occur only occasionally as traces below 50–60 m. Silicoflagellates are generally present with the diatoms. Placement of defined datums (Baldauf, 1987) was therefore difficult due to the generally low abundance of diatoms.

The base of the Fragilariopsis doliolus Zone, defined by the LO of Fragilariopsis reinholdii (0.48–0.45 Ma), occurs in Samples 306-U1313A-4H-CC, 306-U1313B-4H-CC, and 306-U1313D-4H-CC. In Hole U1313C, this event occurs in Sample 306-U1313C-5H-CC, but this is the only occurrence of that species in Hole U1313C. The LO of Proboscia curvirostris is observed in Samples 306-U1313A-2H-CC, 306-U1313B-2H-CC, and 306-U1313D-2H-CC, supporting an age of at least 0.3 Ma for those sediments.

The F. reinholdii Zone is represented by the co-occurrence of F. reinholdii and F. doliolus and is constrained at the base by the FO of F. doliolus. In Holes U1313A and U1313B, the FO of F. doliolus occurs in Samples 306-U1313A-4H-CC and 306-U1313B-4H-CC. In Hole U1313C, this event is found in Sample 306-U1313C-1H-CC. The LO of F. reinholdii should be stratigraphically above the FO of F. doliolus. These events are reversed in Hole U1313C, which is likely an artifact of very low concentrations of diatoms in the sediment, making them more susceptible to influence by contamination. The same reversal is present in Hole U1313D, where the FO of F. doliolus occurs in Sample 306-U1313D-3H-CC and the LO of F. reinholdii occurs in Sample 306-U1313D-4H-CC. Again, this is more likely a function of low diatom abundances than any disturbance in the cores.

Section 306-U1313C-32H-2 includes Thalassiosira convexa in several samples, suggesting a maximum age of 6 Ma for the upper part of this core (Baldauf, 1987). However, as the abundances of T. convexa are very low and the surrounding samples contain no diatoms, the occurrences are not suitable for biostratigraphic use. Unfortunately, it was not possible to establish a diatom-based biostratigraphy below the F. reinholdii Zone because of the low abundances of diatoms.

The diatom flora is diverse in all holes, especially within the upper 4–5 core catchers. A total of 74 categories were applied at this site, including 45 diatom species, several generic groups, identification of fragmented specimens, and several types of silicoflagellates. The assemblage is dominated by warm-water flora (Baldauf, 1987), including Coscinodiscus nodulifer, Thalassiora oestrupii group, Alveus marinus, Thalassionema nitzschioides group, F. doliolus, F. reinholdii, and F. fossilis. Chaetoceros resting spores are often present, indicating high productivity.

The upper core catchers of all holes contain plentiful Thalassiothrix fragments, as well as Actinocyclus curvatulus, indicating influence from Arctic and subarctic waters (Andersen et al., 2004). Neodenticula seminae in Samples 306-U1313A-1H-CC through 4H-CC, 306-U1313B-1H-CC through 4H-CC, and 306-U1313C-1H-CC through 4H-CC, as well as partly dissolved specimens of Triceratium and Trochosira in Sample 306-U1313C-1H-CC, suggest transport of diatoms from colder waters in the upper Pleistocene sections of Site U1313 (Koç et al., 1999; Schrader and Fenner, 1976).

Radiolarians

We examined radiolarians in all core catcher samples from Site U1313 (Tables T20, T21, T22, T23).

In Hole U1313A, 5 of 33 core catcher samples (306-U1313A-10H-CC, 14H-CC, 15H-CC, 16H-CC, and 19H-CC) include no radiolarians (Table T20). Radiolarians are abundant and well preserved in Samples 306-U1313A-1H-CC through 5H-CC and are common and moderately preserved in Samples 306-U1313A-9H-CC, 12H-CC, 25H-CC, 26H-CC, 30H-CC, and 33H-CC.

Some age-diagnostic species used in the mid-latitude North Atlantic by Haslett (1994, 2004) and Ciesielski and Bjørklund (1995) were encountered in this hole. The FO of Pterocorys hertwigii is dated to 0.75–0.87 Ma (Haslett, 1994; 2004) and is observed in Samples 306-U1313A-1H-CC and 2H-CC. However, Sample 306-U1313A-2H-CC, which contains the oldest occurrence of this species in Hole U1313A, is younger than 0.25 Ma based on the FO of the nannofossil E. huxleyi. Its absence below 306-U1313A-2H-CC indicates the occurrence of P. hertwigii depends on oceanographic conditions because the distribution of this species in surface sediments is restricted to low latitudes (Caulet and Nigrini, 1988). Thus, the FO datum of P. hertwigii in the North Atlantic is considered unreliable.

The FO of Cycladophora davisiana is dated to 2.6 Ma (Haslett, 1994, 2004; Ciesielski and Bjørklund, 1995), and this datum was compatible with other microfossil data at Site U1312. In Hole U1313A, however, the FO of C. davisiana occurs in Sample 306-U1313A-25H-CC, where the age is dated to 5.5 Ma by the LO of the nannofossil D. quinqueramus.

The LO of Stichocorys peregrina is dated to 2.65–2.8 Ma (Haslett, 1994, 2004). This species is found only in Samples 306-U1313A-25H-CC, 26H-CC, 30H-CC, and 33H-CC, where radiolarians are common and moderately preserved. The uppermost occurrence of this species in Sample 306-U1313A-25H-CC is dated to 5.5 Ma (LO of D. quinqueramus) in this hole. This discrepancy in the age of the LO of S. peregrina may result from one of two reasons. First, the preservation is poor and its occurrence might be hampered by dissolution. Second, as S. peregrina is a tropical/subtropical species, its absence might be due to ecological conditions in this area. Therefore, we regard the LO of S. peregrina in our samples as not reliable biostratigraphically.

In Hole U1313B, abundant to common radiolarians, with good to moderate preservation, are present in Samples 306-U1313B-3H-CC through 7H-CC, except in 6H-CC, which contains few, poorly preserved radiolarians (Table T21). Radiolarians are also common but poorly preserved in Sample 306-U1313B-32H-CC. All other core catcher samples yield only traces of opal with overall poor to moderate preservation. Two samples (306-U1313B-20H-CC and 21H-CC) are barren of radiolarians.

The FO of C. davisiana occurs in Sample 306-U1313B-29H-CC. This sample is dated to at least 5.5 Ma by the LO of D. quinqueramus. The C. davisiana datum again occurs much earlier than the expected 2.6 Ma in this hole but is synchronous with the event in Hole U1313A. In Samples 306-U1313B-30H-CC and 32H-CC, S. peregrina is present, but poor preservation does not allow for a firm determination of its LO (2.65–2.8 Ma).

Moderate to well preserved radiolarians are abundant to common in the uppermost four cores of Hole U1313C (Table T22). Common radiolarians are also present in Samples 306-U1313C-6H-CC, 9H-CC, 27H-CC, and 31H-CC. The preservation is good in the first two samples and moderate in the latter two. The remaining core catcher samples contain only traces of radiolarians and the preservation is generally poor to moderate.

The first occurrence of C. davisiana occurs in Sample 306-U1313C-27H-CC (5.5–6.0 Ma). This is synchronous with its FO in Holes U1313A and U1313B. S. peregrina occurs in Samples 306-U1313C-26H-CC, 27H-CC, 30H-CC, and 31H-CC, but poor preservation does not allow us to determine its LO, dated at 2.65–2.8 Ma (Haslett, 1994).

Abundant, well-preserved radiolarians are observed in the uppermost four cores of Hole U1313D, except for Sample 306-U1313D-1H-CC, where radiolarians are rare and poorly preserved (Table T23). Poorly preserved radiolarians generally occur as traces or are rare in the remainder of Hole U1313D (Samples 306-U1313D-5H-CC through 16H-CC); however, in Sample 306-U1313D-8H-CC, radiolarians are abundant and moderately preserved.

C. davisiana occurs in the following six samples: 306-U1313D-2H-CC, 3H-CC, 4H-CC, 8H-CC, 10H-CC, and 11H-CC. The lowermost sample is assigned an age of 2.38 Ma based on nannofossil and planktonic foraminifer events from this hole.

Low radiolarian abundance and the generally poor state of preservation at this site precludes recognition of any biostratigraphic marker species used in the low-latitude radiolarian biostratigraphic scheme. Offset cores drilled in the four holes indicate that within short distances there are great differences in opal content. We expect to recover opal-rich intervals containing well-preserved, rich radiolarian faunas between core catchers, similar to DSDP Site 609, where Haslett (1994) established a radiolarian biostratigraphy. Even though opal preservation was a problem and many marker species were missing, he was still able to recognize several low-latitude key marker species.

Interestingly, C. davisiana shows some peculiar occurrences. This species has an inconsistent distribution in Site U1313 holes. Its more or less consistent distribution in the upper part of the four holes is in fairly close agreement with observations by S. Funakawa at Site U1308, where he found its FO in Sample 303-U1308A-20H-CC (see “Biostratigraphy” in the “Site U1308” chapter). This datum is assigned an age of 2.6 Ma. We have found C. davisiana in 14 core catcher samples below its anticipated FO at Site U1313. As reported by Motoyama (1997), morphotypic individuals of C. davisiana have been found back to 4.2 Ma at DSDP Site 192 in the North Pacific.

The occurrence of C. davisiana at 2.6 Ma (Motoyama, 1997) represents its first common and continuous stratigraphic distribution. If our recording of C. davisiana in the deeper parts of the sections is real and not a result of downhole reworking, we conclude that its oldest occurrence is in Sample 306-U1313B-29H-CC, or at ~6 Ma (LO of A. amplificus). Even if Motoyama (1997) convincingly demonstrated that C. davisiana evolved from Cycladophora sakaii, our findings imply C. davisiana occurs ~2 m.y. earlier in the North Atlantic than the North Pacific. This discrepancy will be addressed during our shore-based studies.

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