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doi:10.2204/iodp.proc.303306.110.2006

Biostratigraphy

Preliminary ages were assigned primarily based on core catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. Ages for calcareous nannofossil, foraminifer, diatom, and radiolarian events from the late Miocene–Quaternary were estimated by correlation to the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995). The biostratigraphic events, zones, and subzones for nannofossils, planktonic foraminifers, diatoms, and radiolarians are summarized in Figure F8. The Pliocene/Pleistocene boundary has been formally located just above the top of the Olduvai (C2n) magnetic polarity subchronozone (Aguirre and Pasini, 1985) and just below the lowest occurrence of Gephyrocapsa caribbeanica at 1.73 Ma (Takayama and Sato, 1993–1995). Gephyrocapsa species occur in upper Pliocene sediments; however, medium-sized forms (>4 µm) first appear just above the Pliocene/Pleistocene boundary (de Kaenel et al., 1999), coincident with the lowest occurrence of G. caribbeanica (Takayama and Sato, 1993–1995; Sato et al., 1999). Thus, we define G. caribbeanica and Gephyrocapsa oceanica as specimens >4 µm to distinguish between Pliocene and Pleistocene forms. In addition, the first occurrences (FOs) of Globorotalia truncatulinoides and Globorotalia inflata at 2.09 Ma immediately below the Olduvai subchronozone is very useful to approximate the Neogene/Quaternary boundary. Thus for this study, we locate the Pliocene/Pleistocene boundary between the FO of G. caribbeanica (>4 µm) and the FOs of G. truncatulinoides and G. inflata.

The Miocene/Pliocene boundary has not yet been formally defined. Its location is not well constrained because no biostratigraphic event has been identified in this interval of time; however, the last occurrence (LO) of Discoaster quinqueramus is at 5.5 Ma and the FO of Thalassiosira convexa is between 6.1 and 6.4 Ma.

Details of the shipboard methods are described below for each microfossil group.

Calcareous nannofossils

The zonal scheme established by Martini (1971) was used for Miocene–Quaternary sequences during Expedition 306. In addition, the late Pliocene–Quaternary biostratigraphic horizons defined by Sato et al. (1999) were used for more detailed correlations. Correlation of the zonal scheme and biostratigraphic horizons to the GPTS of Cande and Kent (1995) is shown in Figure F8. Age estimates for datums tied to the geomagnetic timescale are also shown in Figure F8 and listed in Table T1.

Methods

Samples were prepared utilizing standard smear slide methods with Norland optical adhesive as a mounting medium. Calcareous nannofossils were examined with a Zeiss light microscope at 1250× magnification using cross-polarized light, transmitted light, and phase contrast light.

We followed the taxonomic concepts summarized in Takayama and Sato (1987; Deep Sea Drilling Project [DSDP] Leg 94). Calcareous nannofossil preservation was assessed as follows:

• G = good (little or no evidence of dissolution or overgrowth, little or no alteration of primary morphological features, and specimens are identifiable to the species level).

• M = moderate (minor dissolution or crystal overgrowth observed, some alteration of primary morphological features, but most specimens are identifiable to the species level).

• P = poor (strong dissolution or crystal overgrowth, significant alteration of primary morphological features, and many specimens are unidentifiable at the species and/or generic level).

The total abundance of calcareous nannofossils for each sample was estimated as follows:

• V = very abundant (>100 specimens per field of view [FOV]).

• A = abundant (11–100 specimens per FOV).

• C = common (1–10 specimens per FOV).

• F = few (1 specimen per 2–10 FOVs).

• R = rare (1 specimen per 11 or more FOVs).

• B = barren.

Nannofossil abundances of individual species were recorded as follows:

• A = abundant (11 or more specimens per FOV).

• C = common (1–10 specimens per FOV).

• F = few (1 specimen per 2–10 FOVs).

• R = rare (1 specimen per 11 or more FOVs).

• P = present, but abundance was not quantitatively determined.

• ? = questionable occurrence.

• * = reworked occurrence.

Foraminifers

Preliminary ages were assigned based on the occurrence of planktonic foraminifers from core catcher samples. Stratigraphic events of Weaver and Clement (1987) for the North Atlantic and Lourens et al. (1996) for the Mediterranean were applied to the Pliocene and Pleistocene samples of Expedition 306 (Fig. F8). For the late Miocene, however, planktonic foraminiferal events as defined by Sierro et al. (1993, 2001), Krijgsman et al. (1995), and Hilgen et al. (2000) for the northeast Atlantic and Mediterranean Sea were used (Table T2). Middle Miocene stratigraphy is based on Spezzaferri (1998). The species identified as Globorotalia cf. crassula by Weaver and Clement (1987) has a highly convex dorsal side, which led us to relate this species to Globorotalia hirsuta during Expedition 306. Consequently, only G. hirsuta is listed in the tables and the LO of G. cf. crassula, dated at 3.18 Ma by Weaver and Clement (1987), is renamed to “disappearance of G. hirsuta.” Globorotalia conomiozea and Globorotalia miotumida have been grouped together as the G. miotumida group. For the abundance estimates, Neogloboquadrina acostaensis is combined with Neogloboquadrina pachyderma, and in the Miocene the Globorotalia menardii group also includes Globorotalia plesiotumida and Globorotalia merotumida following Sierro et al. (1993). Otherwise, taxonomic concepts for Neogene taxa are adopted from Kennett and Srinivasan (1983). The magnetic stratigraphy for the Cenozoic is derived from Cande and Kent (1995).

The benthic foraminifers are defined by assemblage zones. Boundaries are placed at major shifts in the fauna. Each zone is named after its dominant species. Benthic foraminifers provide limited biostratigraphic age control as currently applied to Expedition 306 samples. Whenever possible, the genus Stilostomella was used as a marker because most of its species disappeared from the global ocean at different latitudes during the interval of 1.0–0.6 Ma (Hayward, 2001). Taxonomic assignments on the generic level follow Loeblich and Tappan (1988). Benthic foraminifers were identified mainly to determine past changes in oceanographic conditions and surface water productivity. Oxygenation and carbon flux are the main factors controlling abundance and species composition in deep-sea assemblages (Jorissen et al., 1995; Altenbach et al., 2003).

During the benthic assemblage analysis, the presence or absence of ostracodes was noted, as they will be used for postcruise paleoceanographic studies.

Methods

From each core catcher, 20 cm³ of sediment was analyzed for planktonic and benthic foraminifers. Unlithified sediment samples were soaked in tap water and then washed over a 63 µm sieve. Semilithified material was soaked in a 3% H₂O₂ solution before washing. Washed samples were dried at 60°C and analyzed under a ZEISS Stemi SV11 or ZEISS DR binocular microscope. The residue was split, with three-quarters of the sample being used for the benthic and one-quarter for the planktonic foraminifers analysis. Sieves for wet sieving were cleaned in a sonicator for several minutes and for dry sieving were blown out with compressed air to avoid contamination between successive samples.

Planktonic foraminifer abundance in the fraction >125 µm in relation to the total residue was categorized as follows:

• D = dominant (>30%).

• A = abundant (10%–30%).

• F = few (5%–10%).

• R = rare (1%–5%).

• P = present (<1%).

• B = barren.

Benthic foraminifer abundance in a sample of the >63 µm size fraction is registered as follows:

• D = dominant (>30%).

• A = abundant (10%–30%).

• F = few (5%–10%).

• R = rare (1%–5%).

• P = present (<1%).

• B = barren.

Preservation includes the effects of diagenesis, epigenesis, abrasion, encrustation, and/or dissolution. Preservation of planktonic and benthic species was categorized as follows:

• VG = very good (no evidence of breakage or dissolution).

• G = good (dissolution effects are rare; >90% of specimens unbroken).

• M = moderate (dissolution damages, such as etched and partially broken tests, occur frequently; 30%–90% of specimens unbroken).

• P = poor (strongly recrystallized or dominated by fragments or corroded specimens).

Diatoms

Except for previous Leg 94 and ODP Leg 162, diatom data from the mid-latitude North Atlantic Ocean are sparse, and only a few diatom biostratigraphic studies have been completed. The Neogene and Quaternary diatom zonation used for the high-latitude sites of Expedition 306 is that proposed by Baldauf (1984) and Koç et al. (1999). The late Miocene–Holocene portion of this zonation is based on the occurrence in the North Atlantic of a warm-temperature diatom assemblage similar to that recorded from the eastern equatorial Pacific (Burckle, 1972, 1977; Baldauf, 1984; Barron, 1985). In addition, diatom stratigraphies from the Japan Trench (Akiba and Yanagisawa, 1986), as well as from the North Pacific–Bering Sea region (Koizumi, 1971) were adopted in an attempt to cover the gap in the late Miocene–late Pliocene high-latitude North Atlantic diatom stratigraphy (Koç et al., 1999). This is partly justified by the rare possible finding of Neodenticula kamtschatica during Expedition 303 (see “Biostratigraphy” in “Site U1302–U1308 methods”). Figure F8 lists diatom biostratigraphic events, paleomagnetic calibration, and age estimates used during this expedition.

Typical diatom assemblages preserved in the sedimentary record can be used as tracers for the corresponding hydrographic conditions in the surface waters. Diatoms characterize fertile surface waters of high latitudes and coastal upwelling areas. In the North Atlantic north of ~50°N, they are diverse and are the main contributors to the biogenic silica preserved in the sediment (Baldauf, 1984; Koç et al., 1999).

Methods

Two types of slides were prepared for diatom analysis depending on overall abundance. For intervals of high abundance, smear slides were prepared from a small amount of raw material from the core catcher. When dictated by a low concentration of diatom valves and/or abundant clay, selected core catcher samples were processed by boiling in a solution of H₂O₂ and sodium pyrophosphate to remove organic matter and to disperse the clay-sized material followed by treatment with HCl to remove CaCO₃. The treated samples were then washed several times with distilled water. In each case, aliquots of raw and cleaned samples were mounted on microscope slides using Norland optical adhesive. All slides were examined with illumination at 1000× magnification for stratigraphic markers and paleoenvironmentally sensitive taxa. The counting convention of Schrader and Gersonde (1978) was adopted. Overall diatom abundance and species relative abundances were determined based on smear slide evaluation using the following conventions:

• A = abundant (>100 valves per traverse).

• C = common (40–100 valves per traverse).

• F = few (20–40 valves per traverse).

• R = rare (10–20 valves per traverse).

• T = trace (2–9 valves per traverse).

• T = single occurrence or fragment per tranverse.

• B = barren.

For computing purposes, a number was assigned to each abundance category (0 = B, 1 = T, 2 = R, 3 = F, 4 = C, and 5 = A).

Preservation of diatoms was determined qualitatively as follows:

• G = good (weakly silicified forms present and no alteration of frustules observed).

• M = moderate (weakly silicified forms present, but with some alteration).

• P = poor (weakly silicified forms absent or rare and fragmented, and the assemblage is dominated by robust forms).

A number was assigned to each category (1 = P, 2 = M, and 3 = G).

Radiolarians

There is currently no working high-resolution radiolarian biostratigraphic framework applicable to the North Atlantic, but several existing sites could potentially be used for this matter. The stratigraphic framework established by Goll and Bjørklund (1989) is only applicable for the Norwegian Sea. Goll and Bjørklund (1989) found that most radiolarians encountered in the Norwegian Sea are endemic and thus proposed a unique biostratigraphic scheme for that area. Lazarus and Pallant (1989) did not establish a biostratigraphic zonal scheme for the Labrador Sea, but as the species are clearly different from those in the Norwegian Sea, the biostratigraphic scheme of the Norwegian Sea cannot be used. Thus, there is no available radiolarian biostratigraphic framework for the Labrador Sea at present. On the other hand, Westberg-Smith and Riedel (1984) recognized two radiolarian zones, including the Stichocorys peregrina and Sphaeropyle langii Zones, for the late Miocene and early Pliocene in deep-sea cores from the Rockall Plateau, high-latitude North Atlantic. Haslett (1994, 2004) reported several radiolarian biostratigraphic events throughout the Pliocene and Pleistocene at the mid-latitude North Atlantic DSDP Site 609, which is directly comparable to this expedition.

For the above reason, we will try to apply the scheme suggested by Haslett (1994, 2004), especially for the Pliocene–Pleistocene section, and Westberg-Smith and Riedel (1984) for the late Miocene and early Pliocene. Numerical age datums were tied to the GPTS of Cande and Kent (1995). Primary datums from late Miocene to Pleistocene are listed in Figure F8.

As mentioned above, radiolarians in the North Atlantic show a different faunal distribution than in the Norwegian Sea, the Labrador Sea, and the mid-latitude North Atlantic, making it difficult to establish a biostratigraphic framework in this region. However, radiolarian biostratigraphy of the North Atlantic will play an important role not only in a local biostratigraphic zonation, but also in the paleoceanographic reconstruction of this area. During preglacial times, oceanic conditions should have been more homogeneous in the North Atlantic and radiolarian associations have a better chance for a more uniform distribution. The hydrographical and ecological situations are more provincial in the postglacial period, so radiolarian associations should have been endemic to specific regions. Therefore, most likely, local biostratigraphic schemes have to be established in the Labrador Sea, the North Atlantic, and the Norwegian Sea, respectively.

Methods

Core catcher samples were treated with a 5%–8% solution of HCl to dissolve all calcareous components. The solution was sieved through a 45 µm mesh screen. The residue was disaggregated by gentle boiling in 5%–8% H₂O₂ with a few grams of Calgon and sieved again. After settling in the beaker, the residue was picked up and dropped on a slide using an eyedropper. The sample was spread with a toothpick and dried on a hot plate. A few drops of Norland optical adhesive were used to apply a 22 mm × 50 mm glass coverslip.

Overall radiolarian abundances were determined based on slide evaluation with a 20× objective lens using the following convention:

• A = abundant (>100 specimens per traverse).

• C = common (51–100 specimens per traverse).

• F = few (11–50 specimens per traverse).

• R = rare (1–10 specimens per traverse).

• T = trace (<1 specimens per traverse).

• B = barren.

The abundance of individual species was recorded relative to the fraction of the total assemblages as follows:

• A = abundant (>10% of the total assemblage).

• C = common (5%–10% of the total assemblage).

• F = few (<5% of the total assemblage).

• R = rare (a few or more specimens per slide).

• T = trace (present in slide).

• B = barren.

Preservation was recorded as follows:

• G = good (majority of specimens complete, with minor dissolution, recrystallization, and/or breakage).

• M = moderate (minor but common dissolution, with a small amount of breakage of specimens).

• P = poor (strong dissolution, recrystallization, or breakage, many specimens unidentifiable).

The abundances of lithic grains, including tephra and organic compounds in the slides, were also evaluated.

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