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doi:10.2204/iodp.proc.337.103.2013

Paleontology

During Expedition 337, a succession of different lithologies was penetrated, reflecting environments ranging from warm-temperate coastal backswamps to cool-water continental shelf. These different environments yielded fossil groups with varying degrees of utility for biostratigraphy. Considerable challenges were posed by contamination from drilling mud that affected core samples, core catcher samples, and especially cuttings samples. Large volumes of sand also led to no yield throughout some parts of the hole for palynological investigations. Despite these obstacles, micropaleontology was able to successfully secure an age of late Pliocene at the top of Hole C0020A at 636.5 m MSF and indicate a probable age of late Oligocene–early Miocene at the base of the hole (2466 m MSF).

Shipboard micropaleontology included diatoms, calcareous nannofossils, organic-walled dinoflagellate cysts (dinocysts), pollen, and spores. A range of different sample types was analyzed, including well cuttings, core catcher samples, and discrete samples taken from working halves. In documenting fossil distribution in Hole C0020A, discussion is split into four units: Unit I spans the top of the hole from 647 to 1256.5 m MSF, Unit II ranges from 1256.5 to 1826.5 m MSF, Unit III from 1826.5 to 2046.5 m MSF, and the final unit (Unit IV) ranges from 2046.5 to 2466 m MSF (see “Lithostratigraphy”). Calcareous nannofossils are rare and poorly preserved. Calcareous nannofossils do not inform the age model presented here. Preservation varied greatly throughout the hole for the other microfossil groups, but diatoms were best and most abundantly preserved in Unit I together with predominantly heterotrophic dinocysts. Diatom floras indicate a Pliocene cool-water continental shelf succession in Unit I. In contrast, Unit II yielded few identifiable diatoms and poor dinocysts. Pollen and spores are moderately well represented but are abundant near the base of Unit II in the more terrestrial to very shallow marine sediments. Unit III contains excellent pollen and spore assemblages in the coals and associated terrestrial to coastal shallow-marine sediments, but dinocysts are scarce and contain few useful biostratigraphic markers. The same remarks can also be made for Unit IV, which is marked by good pollen and spore assemblages from the cores but contains very few dinocysts. A feature of Units II–IV is that reworked Paleogene dinocysts are encountered that have ranges from early middle Eocene to late Oligocene. Therefore, the pollen floras provide some indication of the age in Units III and IV, indicating a maximum age of late Oligocene for the base of Unit IV and a likely age of early middle Miocene for Unit III.

Palynology

A total of 101 samples were analyzed for dinocysts, pollen, and spores. Preservation and yield of palynomorphs varied significantly throughout the different units with the greatest yield and abundance of dinocysts from Unit I and greatest abundance and best preservation of pollen and spores in Units III and IV. Clast grain size, as well as depositional environment, plays a major role in determining whether samples yield palynomorphs. Despite the predominance of spores and especially pollen in Units III and IV and a general lack of dinoflagellate cysts, stratigraphic information is provided by the palynomorphs. Reworking is noted throughout the core and mixes Eocene and Oligocene dinoflagellates into Neogene assemblages. Remobilization of palynomorphs in drilling fluid has a significant role in smearing stratigraphic ranges for those palynological data taken from cuttings samples. However, this is a minor issue for core and core catcher samples because they were selected away from the core edge and could be washed effectively. In particular, samples from terrestrial deposits such as lignites in Units III and IV contain pollen and spores in such high concentrations that palynomorph presence totally masks any potential contamination that is remnant even after washing the raw sample.

Unit I (647–1256.5 m MSF)

A total of 41 cuttings samples were analyzed from Unit I and captured a Pliocene sequence of dinocysts and gymnosperm pollen (Table T4). Palynomorph abundance was greatest from 636.5 to 846.5 m MSF, but the lower half of Unit I below 846.5 m MSF contained generally poor samples with sparse representation of any palynomorph group. Dinocyst assemblages are dominated by a restricted assemblage of primarily heterotrophic forms such as Brigantedinium spp., Selenopemphix spp., and Xandarodinium variabile. The composition of the assemblages is identical to that of Pliocene samples from ODP Leg 186 Hole 1151A located slightly further south off the Sanriku Coast of northern Japan (Kurita and Obuse, 2003), as well as those from the Bering Sea (Bujak, 1984; Bujak and Matsuoka, 1986). The only notable event is the first downhole occurrence (FDO) of Capillicysta fusca at 746.5–756.5 m MSF. The dinocyst flora contains occasional specimens of other dinocysts such as Operculodinium spp. together with some reworked Miocene and Paleogene dinoflagellates. The pollen flora is skewed toward large specimens that were trapped in the 20 µm mesh used during the processing of these samples in Unit I. Conifer pollen (including Tsuga and gymnosperms) is very common, indicating development of significant coniferous vegetation on nearby land areas. The paleoenvironment of deposition is interpreted as continental shelf with cool water and high productivity that allowed the development of restricted heterotrophic dinocyst communities feeding off diatom blooms.

Unit II (1256.5–1826.5 m MSF)

This unit is represented by 30 samples, of which 12 are barren, and the rest contain moderate to poor palynomorph abundance (Table T5). A total of 18 samples are from cuttings, 10 are from core catchers, and 2 are from working halves. The sandy nature of sediment in some parts of this unit results in the poor preservation and abundance of palynomorphs. All samples from Unit II were processed using a 10 µm mesh to enhance the capture of pollen and spores as well as dinoflagellate cysts. Pollen and spores are considerably more abundant than dinoflagellates in this unit, with particularly strong representation of Betulaceae (Alnus, Betula, and Carpinus). Pollen from other angiosperm trees is frequently encountered, including Juglandaceae (Juglans and Pterocarya), Fagus, deciduous Quercus (D. Quercus), and Ulmus/Zelcova. Gymnosperm pollen are represented by abundant Taxodiaceous pollen (a group that includes Glyptostrobus, Metasequoia, and Taxus here), bisaccate pollen (that includes undifferentiated Pinaceae grains), and Tsuga. Pollen of Larix is also encountered, although it is not abundant. The only notable FDOs are evergreen Quercus (E. Quercus) that first appears at 1546.5 m MSF and Platanus at 1816.5 m MSF. The flora is similar to late Neogene assemblages of Japan (Yamanoi, 1978, 1992; Wang et al., 2001) and northeast China (Wang, 2006) but lacks significant quantities of herbaceous taxa. Reduced herb presence may be an artifact of depositional environment that is interpreted as marine shelf environment. The pollen flora suggests the presence of a temperate paleovegetation type dominated by trees in northeast Honshu, Japan, during deposition of Unit II.

Dinocyst assemblages include the presence of reworked Trinovantedinium boreale and assemblages that offer limited biostratigraphic information. The last downhole occurrence (LDO) of Xandarodinium variable at 1378.83 m MSF may indicate the middle–late Miocene based on Leg 186 Hole 1151A (Kurita and Obuse, 2003) and age ranges from west Japan (Matsuoka et al., 1987). The dominant dinocyst remains Brigantedinium spp. in Unit II, with the continued presence of Lejeunecysta spp., Operculodinium spp., and Selenopemphix spp. The presence of Tuberculodinium vancampoae is also noted, which supports a middle Neogene age determination. The paleoenvironment of deposition is interpreted as shallow marine and more proximal to land than depositional environments in Unit I. The presence of pyrite damage to some organic matter throughout the unit confirms the likely deposition in or near seawater.

Unit III (1826.5–2046.5 m MSF)

A total of 20 samples from Unit III yield moderate to abundant palynomorphs (Table T6). Only two samples are barren. Five samples are from cuttings, five are from core catchers, and the remaining ten are taken from working halves. Lithologies include fine sand to coal, and the best pollen and spore assemblages are found associated with the coal facies. Dinoflagellates are present in the more clastic samples but are neither diverse nor abundant and capture a range of taxa from the late Eocene and Oligocene (Kurita and Matsuoka, 1994; Kurita, 2004) that are interpreted as reworked. This group includes Bellatudinium hokkaidoanum, Kallosphaeridium, Phthanoperidinium spp., Systematophora ancyrea, and Trinovantedinium boreale. These dinocysts are uniformly poorly preserved and associated with sediments that are not open shelf but rather lagoonal to very shallow marine or even terrestrial deposits. The presence of Evittosphaerula sp. A at 1846.5 m MSF may indicate that the sediments are early middle Miocene (Bujak and Matsuoka, 1986; Kurita and Obuse, 2003). This age estimate is further supported by the pollen and spore flora that contain a notable presence of evergreen taxa and thermophilic elements, such as Engelhardtia, Liquidamber, Malvaceae/Euphorbiaceae, Myrtaceae, Pasania, E. Quercus, and Reevsia. Pollen of Taxodiaceous plants is very common and so are Liquidamber, Nyssa, D. Quercus, E. Quercus, and Ulmus/Zelcova. Frequently encountered pollen includes Alnus, Carya, and Fagus. Conifers and Larix are not present in the coals but are present in clastic sediments. Therefore, conifers, Larix, and Tsuga are transported into the depositional environment from drier regions, presumably at the basin edge where active uplift and erosion of marine Paleogene strata facilitate the deposition of reworked dinocysts.

The composition of the flora in the terrestrial sediments suggests a warmer climate than that of Unit II, and the flora is comparable to the pollen Zone NP2 of Yamanoi (1978, 1992) that spans the early–middle Miocene (≈13–19 Ma) and to pollen floras from northeastern China that are early–middle Miocene (Wang, 2006). The relative abundance and composition of angiosperm pollen are very similar to those documented for the middle Miocene Sugata Formation of central Japan (Wang et al., 2001). The abundance of Liquidamber, Ulmaceae (Celtis and Ulmus/Zelcova), and also taxodiaceous pollen is a feature of other Miocene swamp deposits in Japan (Shimada, 1967). None of these coalbeds or terrestrial sediments are comparable with the early Oligocene and middle Eocene coal fields of Hokkaido reported by either Sato (1994) or Kurita and Obuse (1994). The presence of unusual Paleogene-type pollen, such as Taliisipites pulvifluminis, together with modern genera and families, is a feature shared with lignites within the Taneichi Formation in the Iwate Prefecture, northeast Japan. In the Taneichi Formation, Paleogene dinoflagellates are reworked into clastic sediments and even lignite that contains pollen floras assigned to the late Oligocene–early Miocene (Yagishita et al., 2003). The paleoenvironment of Unit III is interpreted as coastal swamps with associated mosaics of coastal to very shallow marine depositional environments. A notable feature is that pyrite damage to organic matter is observed in many clastic samples, suggesting some deposition in very shallow marine sediments.

Unit IV (2046.5–2446 m MSF)

Pollen and spores are the dominant palynomorphs throughout the nine samples analyzed in Unit IV. All samples are taken from working halves, and most yield abundant fossils (Table T7). Fern spores are present in abundance together with pollen from taxodiaceous trees, Alnus, and Betula. Throughout Unit IV, pollen from deciduous trees such as Nyssa, Platanus, Pterocarya, and Ulmus/Zelcova are consistently present. The LDO of Cathaya is noted at 2110.5 m MSF, and the LDO of Larix is at 2456.5 m MSF. Cathaya has been previously reported from the early Miocene of Japan (Wang et al., 2001), and Larix is most commonly found in the Neogene as well (Shimada, 1967). The first occurrence (FO) of Larix in northern Japan is probably in the late Oligocene based on correlation from eastern Hokkaido between pollen occurrences with dinocysts and diatom stratigraphy (Kurita et al., 1998). Unit IV is no older than late Oligocene based on the presence of these gymnosperms.

Dinoflagellates from Unit IV are poorly represented, although Batiacasphaera micropapillata is consistent with an age of late Oligocene–early Miocene (Schiøler, 2005). A similar species, Batiacasphaera hirsuta, occurs in the late Oligocene of Japan (Kurita, 2004). The occurrence of Diphyes latisculum may indicate early Miocene because this species has biostratigraphic significance (Matsuoka et al., 1987; Kurita, 2004). The presence of reworked Paleogene dinocysts are noted and is in accord with Units II and III, of which Phthanoperidinium spp. and Trivantodinium boreale are most persistent. Both these dinocyst groups are noted from Hokkaido and northeastern Honshu and are reworked into late Oligocene–early Miocene sediments (Kurita et al., 1998; Yagishita et al., 2003). The depositional environment for the majority of Unit IV is interpreted as a shallow- to very shallow marine environment, based on the palynomorph assemblages. The base of the unit contains coal. Therefore, coastal swamps are also encountered.

Diatoms

Diatoms are present and identifiable throughout the entirety of Unit I but are either too poorly preserved throughout Units II, III, and IV to be useful as geologic age indicators or are completely absent. Throughout Hole C0020A, the zonation of Yanagisawa and Akiba (1998) is followed.

Unit I (647–1256.5 m MSF)

In Unit I, diatom samples range from barren to abundant with poor to good preservation (Table T8). Samples can be dated and zoned with varying degrees of precision to 1076.5 m MSF. All samples are Pliocene in age. Samples 337-C0020A-24-SMW (636.5–646.5 m MSF) through 55-SMW (886.6–896.5 m MSF) are assigned to the upper Pliocene Neodenticula koizumii Zone (North Pacific diatom [NPD] 9), but Samples 25-SMW (646.5–656.5 m MSF) through 45-SMW (846.5–856.5 m MSF) belong to the upper part of this zone based on the FO of N. seminae. Samples 56-SMW (896.5–906.5 m MSF) through 61-SMW (946.5–956.5 m MSF) have been assigned to the underlying middle–upper Pliocene N. koizumii–Neodenticula kamtschatica Zone (NPD 8) based on an increase in N. koizumii abundance. Samples 66-SMW (996.5–1006.5 m MSF) through 75-SMW (1066.5–1076.5 m MSF) have been assigned to the lower Pliocene Thalassiosira oestrupii Subzone (NPD 7Bb). Diatoms are identifiable in Samples 79-SMW (1076.5–1086.5 m MSF) through 98-SMW (1246.5–1256.5 m MSF)—the base of Unit I at 1256.5 m MSF—and appear to be Miocene; however, marker species were not identified in any of these samples and the potential exists that specimens in these sediments have been reworked. The early Pliocene/late Miocene boundary is not observed.

Contamination became more apparent at ~1126.5 m MSF, just below the first lost interval near the base of Unit I, with persistent remobilization of Miocene–Pliocene diatoms down the hole. Microfossils polluted the drilling mud as it recirculated through the hole, causing contamination in nearly every sample to the base of the hole despite thorough washing of the sediment. Contamination proved problematic for core samples, core catcher samples, and cuttings samples of all fraction sizes. Samples of the smallest size fraction often contained no rock cuttings and were of a sandy or muddy consistency, therefore completely saturated with the contaminated drilling mud. Most samples taken from the center of the core were free of contamination but contained no diatoms in Units II, III, or IV. Diatoms were observed in deeper parts of the hole (see “Lithostratigraphy”) but are restricted to small, uninformative fragments or contaminants. The most prolific contaminants to 2119 m MSF were those of late Miocene and Pliocene age, including such marker species as N. kamtschatica and N. koizumii. Two Pleistocene diatoms, Actinocyclus oculatus and Proboscia curvirostris, were also identified as contaminants.

Calcareous nannofossils

Calcareous nannofossils were very low in diversity and rare to absent in all sediments recovered from Hole C0020A. Nannofossils were only observed in the shallower sediments and consisted of younger species of Reticulofenestra and Sphenolithus—species that tend to be too small for precise identification using a light microscope and, therefore, of little biostratigraphic significance when present.

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