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doi:10.2204/iodp.proc.302.103.2006

Micropaleontology

Calcareous nannofossils

The zonal scheme of Martini (1971) was used for Cenozoic calcareous nannofossil biostratigraphy. This zonation represents a general framework for biostratigraphic classification of calcareous nannofossil assemblages. Many additional nannofossil biohorizons are also used for dating and correlation. Nannofossil taxonomy follows that of Perch-Nielsen (1985a).

Calcareous nannofossils were examined on smear slides using standard light microscope techniques under cross-polarized and transmitted light at 1000× magnification. The following abbreviations were used to describe nannofossil preservation:

• G = good (little or no evidence of dissolution and/or recrystallization; diagnostic characters fully preserved).

• M = moderate (dissolution and/or secondary overgrowth; partially altered primary morphological characteristics; most specimens were identifiable to the species level).

• P = poor (severe dissolution, fragmentation, and/or overgrowth; primary morphological characteristics largely destroyed; specimens often could not be identified at the species and/or generic level).

Five calcareous nannofossil abundance levels are recorded as follows:

• A = abundant (>10 specimens observed/field of view [FOV]).

• C = common (5–10 specimens observed/FOV).

• S = scarce (1–5 specimens observed/FOV).

• R = rare (<1 specimen observed/FOV).

• B = barren.

Diatoms

The Neogene Nordic Seas diatom zonation of Koç and Scherer (1996), the North Pacific diatom zonation of Akiba (1986), and the Neogene North Pacific diatom (NPD) zone code system of Yanagisawa and Akiba (1998) are used for the Neogene intervals. The Oligocene and Eocene zonation is modified from the Norwegian Sea diatom zonation of Schrader and Fenner (1976), Dzinoridze et al. (1978), Fenner (1985), and Scherer and Koç (1996). Absolute ages for diatom biostratigraphic horizons were updated from the Cande and Kent (1995) geomagnetic polarity timescale (GPTS).

Most slides were prepared as smear slides. However, strewn slides were prepared from the core catcher samples using the method of Akiba (1986) by placing a small amount of material in a snap-cap vial and removing part of the upper suspension with a pipette. The sample (~1 g) was placed in a 250 mL beaker, heated at 100°C for 1–2 h, and broken into pieces, after which 100 mL of boiling distilled water was added. After the distilled sample soaked for 6 h, the supernatant fluid was skimmed. Additional distilled water was then added to obtain a solution of suitable density. The solution was left for 30 s to let grains denser than diatoms, such as grains of quartz, settle to the bottom of the beaker. Strewn slides were prepared by spreading the pipette suspension on a coverslip (22 mm × 30 mm), drying on a hot plate (50°–60°C), and mounting in Pleurax.

Slides were examined under an Olympus BX41 microscope at 400× magnification, with identifications checked at 1000× magnification. On all slides, the abundance of total diatoms and other biosiliceous components, together with assemblage composition, were recorded. In core catcher samples, whenever possible, 200 specimens (other than Chaetoceros resting spores) were counted. After counting, the slides were scanned to record the presence of other species missed in the original tally. Between 100 and 1000 valves were observed for samples containing sufficient diatom remains. When fewer than 200 diatom valves were encountered on a slide, all taxa were enumerated in a single count.

Except for the core catcher samples, assessment of total diatom abundance was qualitative. Diatoms were recorded as follows:

• A = abundant (≥6 specimens/FOV.

• C = common (1–5 specimens/FOV).

• F = few (1–4 specimens/5 FOV).

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

Diatom preservation categories reported in the range charts are described according to Koç and Scherer (1996) as follows:

• G = good (finely silicified and robust forms present; no significant alternation of the frustules other than moderate fragmentation).

• M = moderate (concentration of more heavily silicified forms and/or a high degree of fragmentation of finely silicified forms).

• P = poor (finely silicified forms virtually absent; heavily silicified forms fragmented and/or corroded).

Silicoflagellates

Prior to Expedition 302, silicoflagellates had not been observed in any sediment from the Lomonosov Ridge. Previous silicoflagellate studies from neighboring regions, however, including the Alpha Ridge in the Arctic Ocean, were used for reference. Based on the sediments recovered during Leg 151 from the Fram Strait and the Norwegian-Greenland Sea, a biostratigraphic zonal scheme has been established for the early Eocene through the Quaternary (Locker, 1996). Amigo (1999) worked on ODP Leg 162 materials from the Iceland and Rockall plateaus, and proposed new Miocene silicoflagellate zones. Perch-Nielsen (1985b) compiled comprehensive references on silicoflagellate biostratigraphy from the World Ocean. Silicoflagellates and ebridians were reported from the Alpha Ridge in Core FL-422 (Bukry, 1984; Ling, 1985; Dell’Agnese and Clark, 1994). Estimated ages were all adjusted to the GPTS for ACEX.

Core catcher samples were disaggregated and decalcified by gentle boiling in a solution of 10% H₂O₂ and 10% HCl for ~2 h. A solution of Calgon was added to the sample solution and thoroughly stirred to further disaggregate the sediments and raise the pH level. Distilled water was repeatedly added to neutralize the sample solution before sieving through a 45 µm stainless steel screen. Smear slides of both the coarse fraction (>45 µm) and the fine fraction (<45 µm) were prepared by pipetting the sediments onto glass slides. The water was allowed to evaporate, and a drop of xylene was added to purge the sediments of remaining water. Finally, Canada balsam was added to the slide, and a 22 mm × 50 mm coverslip was placed on top. The coarse fraction was primarily used for the study of silicoflagellates, although the fine fraction was also investigated to account for smaller-sized specimens (<45 µm).

Total silicoflagellate and ebridian abundances were determined along eight traverses (perpendicular to the length of slide) at 400× magnification, using the following convention:

• A = abundant (>50 specimens).

• C = common (16–50 specimens).

• F = few (3–15 specimens).

• R = rare (2–3 specimens).

• T = trace (1 specimen).

• B = barren (none on a slide).

Preservation of silicoflagellates and ebridians was recorded as follows:

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

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

• P = poor (strong dissolution and/or breakage; many specimens unidentifiable).

Radiolarians

No radiolarian biostratigraphic zonation has been developed for the Arctic Ocean. However, a biostratigraphic scheme for the Norwegian Sea (e.g., ODP Leg 104, Goll and Bjørklund, 1989) extends back to near the base of the Miocene and contains 30 zones. It builds upon the study carried out during DSDP Leg 38 by Bjørklund (1976), which includes eight zones covering the Oligocene and Eocene. The radiolarian biostratigraphy used for Expedition 302 is based largely on these two radiolarian zonations. Other high-latitude zonations have been developed for the North Pacific Ocean (e.g., Kamikuri et al., 2004), but it is uncertain how applicable these zonations will be in the Arctic Ocean.

Samples were disaggregated by heating in a solution of 10% H₂O₂. Calcareous components were then dissolved by adding a 10% solution of HCl and sieved through a 45 µm screen. When clumps of clay and radiolarians remained on the 45 µm sieve after initial treatment, the coarse residue was returned to the beaker and soaked in a concentrated solution of NaOH for 1–5 min on a warm hotplate, immersed in an ultrasonic bath for no more than 5 s, and then resieved. A strewn slide was prepared by pipetting the microfossils onto a glass slide, allowing the water to evaporate, adding a drop or two of xylene and some Canada balsam, and covering with a 22 mm × 40 mm glass coverslip. Alternatively, after drying, the glass coverslips were mounted using 8–12 drops of Norland optical adhesive 61.

Total radiolarian abundances were determined based on strewn slide evaluation at 100× magnification, using the following convention:

• A = abundant (>100 specimens/slide traverse).

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

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

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

• T = trace (<1 specimen/slide traverse).

• B = barren (no radiolarians in sample).

Individual species abundances were recorded relative to the fraction of the total assemblages as follows:

• C = common (>2000 specimens/slide).

• F = few (200–2000 specimens/slide).

• R = rare (20–200 specimens/slide).

• VR = very rare (2–20 specimens/slide).

• + = trace (<2 specimens/slide).

• B = barren (no radiolarians in sample).

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 specimen breakage).

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

Foraminifers

Preparation methods used to obtain calcareous, agglutinated, and planktonic foraminifers include disaggregation and wet-sieving over a 63 µm sieve. The sieved residues were dried, and the foraminifers were separated from the sand fraction under a binocular microscope. Several methods were used to disaggregate the sediment sample, including boiling in hot Calgon solution. The volume of original core catcher sample was variable, but ~10–20 cm³ was processed whenever possible. Abundances of the foraminifers per ~10–20 cm³ sample are characterized as follows:

• A = abundant (>250 specimens).

• C = common (25–250 specimens).

• F = few (5–25 specimens).

• R = rare (1–5 specimens).

• B = barren (no foraminifers in sample).

Calcareous benthic foraminifers

Calcareous benthic foraminifers have been studied from Legs 104 and 151 (Osterman and Qvale, 1989; Osterman and Spiegler, 1996; Osterman, 1996). Shallow-water, high-latitude benthic foraminifers from Baffin Island, Greenland, and the North Sea have been extensively studied by Feyling-Hanssen and colleagues (e.g., Feyling-Hanssen et al., 1983; Feyling-Hanssen, 1990; Knudsen and Asbjørnsdottir, 1991). Quaternary calcareous foraminifers from central Arctic Ocean deep-sea cores have been studied by Scott et al. (1989), Ishman et al. (1996), and Polyak et al. (2004). Taken together, calcareous benthic foraminifers provide a means to correlate Pleistocene sediments from the central Arctic Ocean with those from adjacent regions.

Agglutinated benthic foraminifers

No formal zonal scheme for agglutinated foraminifers yet exists for the Cenozoic of the central Arctic Ocean, as these microfossils have only been studied from comparatively short piston cores that recovered the last few glacial cycles (Evans and Kaminski, 1998). Fortunately, however, regional zonations have been set up for several areas along the circum-Arctic shelves, including the Cenozoic of the Beaufort Sea–MacKenzie Delta region (Schröder-Adams and McNeil, 1994; McNeil, 1997), the Paleogene of the Western Barents Sea (Nagy et al., 2000, 2004), and the Paleogene of the western Siberian lowlands (Podobina, 1998). Studies of ODP sites in the Norwegian-Greenland Sea provide a basis for establishing the taxonomical affinities of Arctic foraminifers with localities farther south. Miocene biostratigraphy at ODP Site 909 in Fram Strait was established by Osterman and Spiegler (1996); Kaminski and Austin (1999) studied the Oligocene at ODP Site 985 on the Iceland Plateau. The Eocene foraminiferal record is known from ODP Site 643 on the Vøring slope (Kaminski et al., 1990). The Cenozoic biostratigraphic record (mostly Paleocene to Miocene) of the North Sea and Labrador Sea was established by Gradstein et al. (1994). The stratigraphic record of agglutinated foraminifers in the Norwegian Sea area is continuous from the early Paleocene to the late Miocene, and with well over a hundred taxa reported from the area, this group provides good potential for stratigraphic correlation.

Planktonic foraminifers

The standard low-latitude biostratigraphic zonation of Berggren et al. (1995) is not applicable to the Arctic region; therefore, direct correlations of planktonic foraminifers to the GPTS is not possible. Instead, comparisons were made with the regional biostratigraphy established at ODP sites in the Norwegian-Greenland Sea (Spiegler and Jansen, 1989; Spiegler, 1996) that make use of local planktonic foraminiferal first and last occurrences, coiling changes in Neogloboquadrina, and the biostratigraphic zonation of Weaver and Clement (1986). Unfortunately, the pre-Pliocene record of planktonic foraminifers in the Norwegian-Greenland Sea area is patchy at best and no biostratigraphic zonation exists for the area.

Ostracodes

There is no formal biostratigraphic zonation for Cenozoic ostracodes from the Arctic Ocean; however, there are stratigraphic range data for many species found in widely scattered literature. Prior ODP legs have yielded preliminary information on Neogene ostracodes from the Vøring Plateau (Malz, 1989; Leg 104 Sites 642–644) and the Yermak Plateau (Cronin and Whatley, 1996; Leg 151 Sites 910 and 911). Both studies were of a preliminary nature, and no formal ostracode zonation was developed.

Outcrop exposures and shallow-water cores from the circum-Arctic and sub-Arctic regions in the North Atlantic and Pacific regions provide additional stratigraphic information on high-latitude northern hemisphere species (see Cronin et al., 1993, for a review). These studies include the Pliocene of the Alaska Coastal Plain (Repenning et al., 1987), the Pliocene–Pleistocene Iperk sequence in the eastern Beaufort Sea (Siddiqui, 1988), the Pliocene Kap Kobenhaven and Lodin Elv formations of Greenland (Brouwers et al., 1991; Penney, 1990, 1993), the Tjornes, Iceland, Pliocene–Pleistocene (Cronin, 1991), and Paleocene of West Greenland (Szczechura, 1971).

Shallow-water ostracode zonations for the Cenozoic of the British Isles (Keen, 1978), France (Oertli, 1985), and Germany and the Cenozoic of the deep North Atlantic (Coles and Whatley, 1989; Whatley and Coles, 1987) will also be useful in correlating Expedition 302 cores with other regions.

Previous coring in the deep Arctic Ocean has yielded abundant data on ostracodes from the last few glacial and interglacial cycles of the Quaternary (Cronin et al., 1994, 1995; Polyak et al., 2004). These studies show that ostracodes are highly sensitive to oceanographic changes related to Arctic water mass and sea-ice conditions.

Ostracodes were picked from the >150 µm size fraction of the micropaleontological residue used for foraminiferal studies.

The following categories were used to describe ostracode abundance:

• A = abundant (>250 specimens).

• C = common (25–250 specimens).

• F = few (5–25 specimens).

• R = rare (1–5 specimens).

• B = barren (no specimen present).

Palynology, organic-walled dinoflagellate cysts

In the absence of an established zonation scheme for the Arctic Ocean, the distribution pattern of index taxa is compared to the global Late Cretaceous–Cenozoic compilation of Williams et al. (2004). In addition, comparisons were made to relevant northern high-latitude studies, including those of Head and Norris (1989), Firth (1996), Williams and Manum (1999), and Eldrett et al. (2004).

Samples were processed on board using sodium hexametaphosphate, following the procedures described by Riding and Kyffin-Hughes (2004). “Heavy-liquid” (ZnCl₂) separation was applied. Following sieving over a 20 µm sieve, strewn mounts of the residue were made using glycerine jelly. Organic-walled dinoflagellate cysts (dinocysts) were counted to >100 specimens, whereas other major categories of palynomorphs (e.g., bisaccate pollen, spores, and foraminifer linings) were also taken into account. Dinocyst taxonomy is in accordance with that cited in Williams et al. (1998).

Fish teeth

Although fish teeth have been used for biostratigraphy in the Pacific Ocean (e.g., Doyle and Riedel, 1979), we extracted ichthyoliths from the sediment primarily for the purpose of making geochemical measurements (e.g., Gleason et al., 2004, and references therein). Samples (20 cm³) of raw sediment were disaggregated and sieved using only distilled water. Individual fish teeth were picked from the coarse residue and stored in a cardboard slide for each sample. These specimens were then further chemically cleaned prior to analysis according to the procedure of Gleason et al. (2004). The presence of other fossil fish material in Hole M0004A is shown in the foraminiferal occurrence table.

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