IODP Proceedings    Volume contents     Search
iodp logo



Core catcher samples from Holes U1314A, U1314B, and U1314C contain rich assemblages of calcareous nannofossils, foraminifers, diatoms, and radiolarians that are well to moderately well preserved. A succession of biostratigraphic events provides a reliable chronological framework spanning the late Pliocene (~3 Ma) to the Holocene (Table T3).

Polar and subpolar calcareous and siliceous plankton assemblages are dominant throughout the Pliocene and Pleistocene, with a lower proportion of North Atlantic transitional species. Because this site is near the distribution limit of some temperate to subtropical species, some events that were very useful at the previous sites seem to be diachronous here, as the first and last occurrences of these species are probably not coincident with those in the mid-latitude North Atlantic. This may explain why some nannofossil events are somewhat depressed relative to foraminifer events in the late Pliocene (Figs. F15, F16, F17). Many of the samples analyzed at this site contain rare to abundant lithic grains of quartz, volcanic glass, basalts, and other rock fragments, including some gravel-sized rocks.

Sedimentation rates estimated using the approximate depth of biostratigraphic events defined in this study are relatively constant throughout the Pleistocene (7.0–7.5 cm/k.y.), whereas late Pliocene sedimentation rates are closer to 11 cm/k.y. (Figs. F15, F16, F17).

Calcareous nannofossils

We examined all core catcher samples from Holes U1314A through U1314C for calcareous nannofossils. All samples yielded abundant to very abundant nannofossil assemblages. Preservation was good to moderately good throughout the section. Gephyrocapsids, small reticulofenestrids, and Pseudoemiliania species dominate most Pleistocene assemblages, although several core catcher samples yielded very high percentages of Coccolithus spp. These samples are typically coarser grained, with lower overall abundance. Reworked Cretaceous and Paleogene nannofossils are present throughout the section, although Cretaceous species are more predominant within the Pleistocene, whereas Paleogene species are more common in the upper Pliocene and lowermost Pleistocene sediments.

The sections recovered at Site U1314 yielded Pleistocene and upper Pliocene assemblages (Tables T4, T5, T6, T7). We identified 11 Pleistocene nannofossil datums defined by Sato et al. (1999) in most holes at Site U1314. We were unable to identify the last occurrence of Helicosphaera inversa (0.16 Ma) at this site. In fact, no occurrences of this species are found within Martini’s (1971) nannofossil Zone NN21, although it does occur in other parts of the section. The first occurrence (FO) of Emiliania huxleyi (0.25 Ma), which marks the base of Zone NN21, is present in Samples 306-U1314A-3H-CC, 306-U1314B-3H-CC, and 306-U1314C-2H-CC. This event is problematic because E. huxleyi is very rare in the early stage of its evolution and difficult to recognize under the light microscope. Zone NN20, which spans the time period between the last occurrence (LO) of Pseudoemiliania lacunosa and the FO of E. huxleyi, occurs in Samples 306-U1314A-4H-CC, 306-U1314B-4H-CC, and 306-U1314C-3H-CC.

The LO of P. lacunosa (0.41 Ma), which defines the top of Zone NN19, occurs in Samples 306-U1314A-5H-CC, 306-U1314B-5H-CC, and 306-U1314C-4H-CC. The FO of H. inversa (0.51 Ma) is only identifiable in Samples 306-U1314A-5H-CC and 306-U1314C-6H-CC, as H. inversa is only found in one sample in Hole U1314B. The LO of Reticulofenestra asanoi (0.85 Ma) occurs in Samples 306-U1314A-8H-CC, 306-U1314B-8H-CC, and 306-U1314C-8H-CC. In Holes U1314A and U1314B, this event occurs in conjunction with the FO of Gephyrocapsa parallela (0.95 Ma). In Hole U1314C, the FO of G. parallela occurs in Sample 306-U1314C-8H-CC. The FO of R. asanoi (1.16 Ma) occurs in Samples 306-U1314A-10H-CC, 306-U1314B-9H-CC, and 306-U1314C-9H-CC.

Large specimens of Gephyrocapsa oceanica and Gephyrocapsa caribbeanica are quite prominent at this site. Specimens of these species range in size from small (<4 µm) to >6 µm. Careful measurements were required to identify the FO and LO of large Gephyrocapsa spp., which Sato et al. (1999) defines as those specimens >5 µm. Based on this size definition, the LO of large Gephyrocapsa spp. (1.21 Ma) occurs in Samples 306-U1314A-11H-CC, 306-U1314B-10H-CC, and 306-U1314C-10H-CC. The LO of Helicosphaera sellii (1.27 Ma) occurs with the LO of large Gephyrocapsa spp. in Sample 306-U1314C-10H-CC and in Samples 306-U1314A-12H-CC and 306-U1314B-12H-CC. The FO of large Gephyrocapsa spp. (1.45 Ma) is significantly more difficult to detect than the LO of this species, since larger specimens of this species grade from smaller specimens near its FO. Careful examination yielded the FO of this species in Samples 306-U1314A-13H-CC, 306-U1314B-12H-CC, and 306-U1314C-11H-CC.

The FOs of medium-sized (>4 µm) G. oceanica (1.65 Ma) and G. caribbeanica (1.73 Ma) occur together in Holes U1314A and U1314B. We use the latter species to approximate the Pliocene/Pleistocene boundary. Rare occurrences of these species are present below their FO datum. Specimens of these older occurrences are usually <4 µm, although rare occurrences of specimens >4 µm can occur in the upper Pliocene. We placed the FOs of G. oceanica and G. caribbeanica at the base of consistent occurrences of specimens >4 µm (Tables T4, T5, T6). The FOs of both species occur together in Samples 306-U1314A-14H-CC and 306-U1314B-15H-CC. In Hole U1314C, the FO of G. oceanica occurs in Sample 306-U1314-14H-CC, whereas the FO of G. caribbeanica occurs in Sample 306-U1314-15H-CC.

Pleistocene sediments at Site U1314 contain scattered occurrences of reworked Cretaceous and Paleogene material throughout much of the interval. The number of reworked specimens present is generally very rare, although some samples contain few reworked specimens. These samples are typically coarser grained and likely contain higher amounts of IRD. Reworked specimens are typically more poorly preserved than the in situ assemblage at this site.

Four Pliocene events dated by Sato et al. (1999) that should occur within the sedimentary section at Site U1314 are difficult to identify. These events are based on the LOs of discoasters, which are considered warm-water species. Discoasters are present at higher latitudes during the Neogene, although in reduced numbers. Therefore, the latitude of Site U1314 made it challenging to identify these Pliocene datums, particularly within the time constraints of drilling operations. Additional study of samples following completion of drilling allowed us to more accurately identify the LOs of these events, although some are still depressed relative to foraminifer and paleomagnetic datums (Figs. F15, F16).

The LO of Discoaster brouweri (1.97 Ma), which marks the top of Zone NN18, occurs in Samples 306-U1314A-18H-CC, 306-U1314B-18H-CC, and 306-U1314C-18H-CC. This datum is in close agreement with the FO of the planktonic foraminifer Globorotalia inflata at 2.08 Ma (Table T3). The LO of Discoaster pentaradiatus (2.38 Ma), which marks the top of Zone NN17, occurs in Samples 306-U1314A-24H-CC and 306-U1314B-24H-CC. Hole U1314C terminated above the LO of this species. D. pentaradiatus is particularly rare within this section, and as a result, the position of its LO is somewhat deeper than datums of similar age. The LO of Discoaster surculus (2.54 Ma), which marks the top of Zone NN16, occurs in Samples 306-U1314A-26H-CC and 306-U1314B-26H-CC. The LO of this species also occurs slightly deeper than correlative events. The LO of Discoaster tamalis (2.74 Ma) occurs in Samples 306-U1314A-28H-CC and 306-U1314B-28H-CC, indicating that the total depth of Hole U1314A just reached 2.74 Ma, whereas Hole U1314B penetrated 19 m beyond this datum.

Planktonic foraminifers

The planktonic foraminifer assemblages were studied in all core catchers from Holes U1314A to U1314C (Tables T8, T9, T10). In addition, the washout from the top of Cores 306-U1314A-1H, 306-U1314B-1H, and 306-U1314C-1H, for which only the >150 µm fraction is available, was examined. Even after intensive washing of the samples, many clay aggregates remained in the residues of some samples, making observation of the foraminifers more difficult. Planktonic foraminifers are the dominant component of the >63 µm fraction in nearly all samples, with lower proportions of benthic foraminifers, ostracodes, radiolarians, sponge spicules, diatoms, and ice-rafted grains of different mineralogical composition. Foraminifers are rare and the residue is completely dominated by IRD in Sample 306-U1314C-15H-CC. In addition, diatoms and radiolarians are more abundant than foraminifers in the washout surface sample of Section 306-U1314B-1H-1.

Site U1314 is located within the subpolar plankton province, and the foraminifer assemblages are dominated by Globigerina bulloides along with other subpolar to transitional species, such as Neogloboquadrina pachyderma (dextral), Turborotalia quinqueloba, Globigerinita glutinata, and G. inflata. These species, together with N. pachyderma (sinistral), Neogloboquadrina atlantica (sinistral), and Globorotalia puncticulata are the main components of the assemblage throughout the upper Pliocene and Pleistocene.

As most subtropical and temperate species are absent in this region, only subpolar to transitional species are used for definition of biostratigraphic events. The first event observed at this site is the first abundant occurrence (FaO) of N. pachyderma (sinistral), which is recorded in Samples 306-U1314A-16H-CC, 306-U1314B-15H-CC, and 306-U1314C-15H-CC. This species is common to abundant in most of the samples from the middle and upper Pleistocene, but is absent or very rare in the lowermost Pleistocene, where N. pachyderma (dextral) becomes the dominant component of the assemblage. The FaO of this species has been dated at 1.78 Ma by Weaver and Clement (1987) and 1.8 Ma by Lourens et al. (1996).

We observed the FO of G. inflata within the interval of dominant N. pachyderma (dextral) and only two cores below the previous event, (Samples 306-U1314A-18H-CC, 306-U1314B-18H-CC, and 306-U1314C-18H-CC). This species, which is absent in the upper Pliocene, becomes a significant component of the assemblage near the Pliocene/Pleistocene boundary at ~2.09 Ma (Weaver and Clement, 1987; Lourens et al., 1996). Globorotalia truncatulinoides is rarely present at this site in the upper Pleistocene, and therefore its FO was not used to build the biostratigraphic framework.

An interval with no globorotalids of the Globorotalia (Globoconella) lineage is observed between Samples 306-U1314A-18H-CC and 306-U1314A-23H-CC, 306-U1314B-18H-CC and 306-U1314B-23H-CC, and 306-U1314C-18H-CC and the bottom of Hole U1314C. The base of this interval is defined by the LO of G. puncticulata, dated at 2.41 Ma in the Mediterranean (Lourens et al., 1996). This event is nearly isochronous with the LO of N. atlantica (sinistral), which is identified in Samples 306-U1314A-23H-CC, 306-U1314B-23H-CC, and 306-U1314C-22H-CC. This is a cold-water species and therefore we assume this event, dated at 2.41 Ma by Weaver and Clement (1987), is synchronous across the northern latitudes of the North Atlantic.

Sand- to small gravel-sized lithic grains, most of which are likely to be IRD, are present in numerous core catchers in all three holes, with higher abundances in the Pleistocene samples (Tables T8, T9, T10). Several of the core catcher samples, especially in Holes U1314B and U1314C, contain basaltic tephra and hematite-stained quartz grains, two of the tracers used by Bond and Lotti (1995) to study phasing in the surges from circum-North Atlantic ice sheets. The presence of these marker grains in samples at Site U1314 will allow us to extend the detailed study of Bond et al. (1999) back to at least the early Pleistocene.

Benthic foraminifers

Benthic foraminifer assemblages from Site U1314 were only studied in Hole U1314A. Few moderately well preserved benthic foraminifers occur throughout the Neogene sequence, except in eight samples (306-U1314A-4H-CC, 6H-CC, 14H-CC, 16H-CC, 18H-CC, 20H-CC, 26H-CC, and 27H-CC), which contain very rare, poorly preserved specimens (Table T11). Three assemblages are determined in Hole U1314A.

Assemblage I (Epistominella exigua-Melonis pompilioides)

E. exigua and M. pompilioides are predominant and Cassidulina carinata is subordinate between Samples 306-U1314A-1H-CC and 3H-CC, in 5H-CC, between 7H-CC and 13H-CC, and in 23H-CC, 24H-CC, and 28H-CC.

Assemblage II (Nuttallides umboniferus)

This assemblage, represented by abundant occurrences of N. umboniferus, is recognized in Samples 306-U1314A-19H-CC and 21H-CC.

Assemblage III (Oridorsalis umbonatus)

This assemblage is characterized by the abundant occurrence of O. umbonatus and co-occurrence of M. pompilioides and Uvigerina peregrina in Samples 306-U1314A-17H-CC and 22H-CC.

According to Murray (1991), the association of abundant of E. exigua and O. umbonatus suggests the influence of NADW in the depositional environment. In contrast, high frequencies of N. umboniferus in Samples 306-U1314A-19H-CC and 21H-CC suggest an increased influence of Antarctic Bottom Water (AABW) between 2.09 and 2.38 Ma.

The only benthic foraminifer biostratigraphic event recognized in Hole U1314A is the LO of Stilostomella spp., which occurs in Sample 306-U1314A-10H-CC. Most species of Stilostomella disappear between 1.0 and 0.6 Ma (Hayward, 2001). Thus, the age indicated by the benthic foraminifer coincides with the nannofossil results.


Diatoms were investigated in smear slides from 80 core catcher samples of Holes U1314A to U1314C (Tables T12, T13, T14). Diatoms are generally present within all holes, although in lower abundances in the lower one-third of each hole. Also, abundances are highest in Hole U1314A and lowest in Hole U1314C.

The overall diatom assemblage is characterized by boreal and subarctic diatoms, such as Actinocyclus curvatulus, Thalassionema nitzschioides, and Rhizosolenia hebetata (Andersen et al., 2004). As a result, the relatively detailed diatom biostratigraphy of ODP Site 983 (Koç et al., 1999) must be used. Warm-water species like Fragilariopsis doliolus, Fragilariopsis reinholdii, and Hemidiscus cuneiformis are often present in lower numbers, so the low- to mid-latitude stratigraphy of Baldauf (1987) can be used as secondary datums when possible. This stratigraphy can also be applied to lower parts of the sedimentary successions.

The LO of Proboscia curvirostris (0.3 Ma), which marks the base of the Thalassiosira oestrupii Zone, is found in Samples 306-U1314A-4H-CC, 306-U1314B-3H-CC, and 306-U1314C-4H-CC. Koç et al. (1999) correlate this event to marine isotope Stage (MIS) 9.

The LO of F. reinholdii coincides with the LO of Fragilariopsis fossilis in Hole U1314A. In Hole U1314C, the same event coincides with the LO of Neodenticula seminae. In Hole U1314C, the LO of F. reinholdii occurs in Sample 306-U1314C-5H-CC, giving a secondary datum within the P. curvirostris Zone of Koç et al. (1999). The LO of F. fossilis is only present in Samples 306-U1314A-7H-CC and 306-U1314B-7H-CC. This event coincides with the FO of N. seminae in Hole U1314C.

The base of the P. curvirostris Zone is defined by the LO of N. seminae. This event occurs in Samples 306-U1314A-8H-CC, 306-U1314B-8H-CC, and 306-U1314C-7H-CC and marks the top of the N. seminae Zone. It occurs in MIS 21 at Site 983 (Koç et al., 1999).

The base of the N. seminae Zone, marked by the FO of N. seminae, is found in Samples 306-U1314A-11H-CC, 306-U1314B-10H-CC, and 306-U1314C-9H-CC. This suggests progressively lower sedimentation rates from Holes U1314A to U1314C in this interval. Koç et al. (1999) place this event somewhere between MIS 35 and 37 at Site 983.

Below the FO of N. seminae, the Koç et al. (1999) high-latitude stratigraphy merges with the lower-latitude stratigraphy of Baldauf (1987). The base of the F. reinholdii Zone is defined by the FO of F. doliolus. However, as this species is only sporadically present in trace numbers, it is not possible to constrain the base of the F. reinholdii Zone in any of the holes at Site U1314.

Thalassiosira convexa is utilized as a secondary datum in the Alveus marinus Zone of Baldauf (1987). It occurs in Sample 306-U1314A-25H-CC and below, as well as in Sample 306-U1314B-26H-CC and below, suggesting that these holes reached an age of at least 2.4 Ma. The sporadic occurrences of this species in Samples 306-U1314C-3H-CC and 19H-CC are the result of reworking in either the sediment column or during coring. The shorter Hole U1314C does not reach this datum and therefore the age of the bottom of the hole is not much older than 2 Ma, as F. doliolus fragments seem to be present at least as far down as Sample 306-U1314C-21H-CC.

Fragilariopsis jouseae occurs in Sample 306-U1314B-27H-CC and below. This suggests an age of at least 2.7 Ma for Sample 306-U1314B-27H-CC, as the LO of F. jouseae indicates the top of the F. jouseae Zone (Baldauf, 1987).

Characteristic for this site is the high abundance of fragments of the long pennate diatom Thalassiothrix longissima. Other diatoms that occur frequently in large numbers are Thalassionema nitzschioides and Thalassionema lineatum. The Thalassionema species seems to be as abundant as Thalassiothrix longissima in the lower half of each hole (Tables T12, T13, T14). In addition, remains of large specimens of Coscinodiscus spp., resting spores of Chaetoceros spp. and Rhizosolenia spp., are often very abundant, especially in Hole U1314C.

The presence of large numbers of long pennate diatoms has previously been ascribed to intensification of oceanic circulation and the development of oceanic fronts (Kemp and Baldauf, 1993). The dominant diatom assemblage at this site is similar to that at Expedition 303, Site U1304, although so far only one interval of actual diatom ooze and possible mat-forming has been found at Site U1314 (see “Lithostratigraphy”).

Frequent low-abundance occurrences of benthic diatoms, neritic species such as Paralia sulcata, as well as ice-edge diatoms such as Bacteriastrums spp. suggest southward transport of sediment, possibly delivered to these latitudes from the Irminger Basin region by the East Greenland Current (Koç and Flower, 1998).


We examined radiolarians in all core catcher samples from Holes U1314A, U1314B, and U1314C. In Hole U1314A, 4 of 28 core catcher samples (306-U1314A-4H-CC, 6H-CC, 20H-CC, and 21H-CC) contain rare to trace occurrences of poorly preserved radiolarians, whereas Sample 306-U1314A-16H-CC is barren (Table T15). Radiolarians are abundant to common and well preserved in the remaining samples from Hole U1314A. In Hole U1314B, radiolarians are generally abundant to common with good preservation throughout the hole. Only 3 of 30 core catcher samples (306-U1314B-16H-CC, 19H-CC, and 20H-CC) include rare specimens or traces of radiolarians that are of moderate to poor preservation (Table T16). Finally, abundant to common well-preserved radiolarians are generally observed throughout Hole U1314C. In 5 of 22 core catcher samples (306-U1314C-6H-CC, 13H-CC, 15H-CC, 20H-CC, and 21H-CC), moderate to poorly preserved radiolarians are rare or occur only as traces (Table T17).

We focused on the occurrence of Cycladophora davisiana at the previous sites and also found new, interesting information at this site. C. davisana can be traced to the bottom of all three holes (Samples 306-U1314A-28H-CC, 306-U1314B-30H-CC, and 306-U1314C-22H-CC). Holes U1314A and U1314B date to 2.74–3.54 Ma, which results in an earlier than expected FO for C. davisiana (2.6 Ma), although this is synchronous with Site U1313. In addition, high abundances of Cycladophora sakaii, the ancestor of C. davisiana (Motoyama, 1997), are observed in Samples 306-U1314A-25H-CC and 306-U1314B-25H-CC. These samples are dated to 2.4 Ma by the LO of the diatom T. convexa, 2.41–2.54 Ma by the LOs of the planktonic foraminifers N. atlantica (sinistral) and G. puncticulata, and the LO of the nannofossil D. surculus, respectively. Motoyama (1997) demonstrated that the evolutionary transition from C. sakaii to C. davisiana occurs between 2.7 and 2.4 Ma, when the percentage of C. davisiana exceeds the percent of C. sakaii at Deep Sea Drilling Project (DSDP) Site 192 in the North Pacific. This may indicate that C. davisiana also evolved from C. sakaii in the North Atlantic and the evolutionary transition is synchronous with the North Pacific, although we have not encountered C. sakaii in any remaining samples, except for 306-U1314A-28H-CC.

The preliminary data obtained from core catcher studies show the radiolarian fauna depict different assemblages between the younger 100 k.y. cycle world and the 41 k.y. cycle world. The stronger and more prominent 100 k.y. cycle glacial periods have a stronger, more intense southern penetration of subpolar waters, as indicated by the presence of Amphimelissa setosa. This species is found in Samples 306-U1314B-3H-CC and 306-U1314C-1H-CC and 3H-CC. Today it inhabits cold arctic/subarctic water masses, so when present at Site U1314, it indicates a more intensive southward penetration of these water masses. Additionally, the numbers of different Spongodiscidae species, Pseudodictyophimus gracilipes (at least four varieties), and the percent of C. davisiana may help us gain new insight into how water masses changed through time over this site.

Discussion on radiolarian stratigraphy in the North Atlantic

The only stratigraphic scheme for radiolarians in the North Atlantic is proposed by Haslett (1994) based on samples from ODP Site 609. He suggests that the standard tropical radiolarian zones applied in the Indian, Atlantic, and Pacific Oceans can also be applied to the mid-latitude North Atlantic. “It is significant that this same zonation is also applicable to the Atlantic, thus offering a standard cosmopolitan Neogene zonation enabling correlation throughout the world’s ocean” (Haslett, 1994). We are of the opinion that the low-latitude radiolarian zonation cannot be applied to the high northern latitudes, due to missing key boundary markers, as discussed below. Haslett (1994) recognizes the following low-latitude zones mainly based on the biostratigraphic assemblages that occur within each zone and not on the presence of species that define the zonal boundaries.

RN17: Buccinosphaera invaginata Taxon Range Zone (Nigrini, 1971)
  • Base: FO B. invaginata (0.18 Ma)

  • Remarks: B. invaginata is very rare in the equatorial Atlantic and has so far not been observed in the North Atlantic sites studied.

RN16: Collosphaera tuberosa Interval Zone (Nigrini, 1971)
  • Top: FO B. invaginata (0.18 Ma)

  • Base: LO Stylatractus universus (0.42 Ma)

  • Remarks: C. tuberosa is present in the equatorial Atlantic but rare in the North Atlantic, as documented with our observation that only a few individuals of this species have been identified in the Expedition 306 material.

Haslett (1994) was not able to separate these two zones, as the species defining them were not present, which is in accordance to our Expedition 306 observations.

RN15: S. universus Concurrent Range Zone (Caulet, 1979)
  • Top: LO S. universus (0.42 Ma)

  • Base: FO C. tuberosa (0.47–0.61 Ma)

  • Remarks: Hays (1965) described S. universus and used it as a stratigraphic marker in the Antarctic. It has since been applied as a stratigraphic marker in most ocean basins due to its cosmopolitan distribution. S. universus was listed by Westberg-Smith and Riedel (1984) in their range chart (table 1) in the North Atlantic DSDP Site 552A (pl. 1, fig. 7). They did not suggest any radiolarian biostratigraphy for this site, but their table 1 shows its LO in Sample 81-552A-2R-2, 122–123 cm. Similarly, Westberg-Smith et al. (1987) showed that the LO of S. universus was between Samples 94-609-3R-3, 40–42 cm, and 4H-2, 40–42 cm (0.25–0.56 Ma), thus marking the base of the RN16 Zone.

In the nine holes drilled during Expedition 306, we frequently observed “Stylatractus universus” as illustrated in Westberg-Smith and Riedel (1984). This is not S. universus as originally described by Hays (1965) and later used in North Pacific stratigraphy (Hays, 1970; Kling, 1973). The specimen illustrated by Westberg-Smith and Riedel (1984) is actually a Druppatractus species, probably D. irregularis (Popofsky).

RN14: Amphirhopalum ypsilon Interval Zone (Nigrini, 1971)
  • Top: LO C. tuberosa (0.47–0.61 Ma)

  • Base: LO Anthocyrtidium angulare (1.10–1.12 Ma)

  • Remarks: The species that defines the RN14 Zone are both absent in the Site 609 material, and we have not yet found them in our Expedition 306 material. Haslett (1994) states that the LO of Anthocyrtidium nosicaae and the FO of Lamprocyrtis hertwigii occur in this zone, the latter close to the base of the A. ypsilon Zone. We therefore also question the application of this zone.

RN13: A. angulare Interval Zone (Nigrini, 1971)
  • Top: LO of A. angulare (1.10–1.12 Ma)

  • Base: LO of Pterocanium prismaticum (1.65–1.74 Ma)

  • Remarks: Again Haslett (1994) states that the key markers are missing in the Site 609 material, but the LO of Lamprocyrtis neoheteroporus was encountered, an event that occurs in this zone. This is also reflected in our preliminary Expedition 306 results, and we have no reason to believe that this zone can be defined, especially since its application seems vague.

RN12b: P. prismaticum Interval Subzone (Riedel and Sanfilippo, 1970)
  • Top: LO of P. prismaticum (1.65–1.74 Ma)

  • Base: LO of Stichocorys peregrina (2.74–2.78 Ma)

  • Remarks: Neither of these two zonal markers were found by Haslett (1994), so neither the top nor the base of the zone was defined. We could not document the presence of the key marker species of this zone in the Expedition 306 results.

Haslett (1994) applied the low-latitude radiolarian biostratigraphic scheme to the mid-latitude North Atlantic. His zonal scheme, however, has two problems. The first is most of the species used to define zonal boundaries are not present. The second is the occurrence of species living in low latitudes is strongly affected by paleoceanographic conditions in the mid-latitudes, so we cannot detect the real FO or LO of those species. At the three Expedition 306 sites, we encountered some age-diagnostic species in the lower-latitude sites (Collosphaera tuberossa, P. hertwigii, and S. pregrina); however, their occurrences are sparse and the FO or LO of these species is not reliable compared to other microfossils. The radiolarian stratigraphy proposed by Haslett (1994) has to be more carefully tested. The Expedition 306 results show that only trace and spotty occurrences of several key marker species for the Pliocene–Pleistocene radiolarian zones could be identified. We think that a local radiolarian zonation based on species that really occur in the sediments will give the best and most reliable results. During our shore-based study we will develop a biostratigraphy that is based on existing species, which will be directly calibrated to the geomagnetic polarity timescale (Cande and Kent, 1995).

Radiolarian zones, a comparison: North Pacific–North Atlantic

The standard low-latitude radiolarian biostratigraphy cannot be applied in the North Pacific and therefore Hays (1970) used the Eucyrtidium tumidulum Zone for the uppermost radiolarian zone, whereas Kling (1973) decided to use the Artostrobium miralestensis Zone and Foreman (1975) devised a third alternative, the Artostrobium tumidulum Zone. Each zone utilizes the same marker species. Nigrini (1977) ended the taxonomic discussion by introducing the Botryostrobus aquilonaris Zone (also using the same species). She stated that the base of this zone is defined by the morphotypic last appearance of S. universus and is coincident with the upper limit of the S. universus Zone. Robertson (1975) stated that at present B. aquilonaris is most abundant north of 40°N in the Pacific, whereas its abundance during the Last Glacial Maximum was significantly reduced. This species is also present in the North Atlantic and has been recovered in all of our Expedition 306 holes. The species is easy to recognize but probably has no stratigraphic application in the North Atlantic. It is present throughout the entire Pliocene–Pleistocene section recovered, and therefore it has no biostratigraphic potential. Its abundance through time may fluctuate, however, probably increasing in glacial times due to its cold-water affinity, in contrast to the Pacific, where it decreases in number during glacials. This has to be tested during our shore-based study.

We found it opportune to comment on the radiolarian stratigraphy that at present is available from the North Atlantic and stress that we had difficulties identifying the RN zones as suggested by Haslett (1994). These zones where redefined by Sanfilippo and Nigrini (1998). We agree with Sanfilippo and Nigrini (1998) that the North Atlantic requires a regional radiolarian zonation. For consistency we will follow the recommendation by Sanfilippo and Nigrini (1998) to use the following notation in our stratigraphic scheme for the North Atlantic: RN (N. Atl)–Genus species zone type. As we are only working with the uppermost part of the Neogene, it is not possible for us to allocate a zone number at this stage.

Above, we outlined the problems we are facing, but during shore-based studies we hope to provide a better understanding of radiolarian associations and how they reflect different paleoecological (paleoceanographical) conditions, as well as a stratigraphic scheme for the Pliocene–Pleistocene sediments recovered during Expedition 306.