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A Paleogene deep-sea methane-seep community from Honshu, Japan
Palaeogeography, Palaeoclimatology, Palaeoecology 387 (2013) 126–133
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A Paleogene deep-sea methane-seep community from Honshu, Japan Kazutaka Amano a,⁎, Robert G. Jenkins b, Yukio Sako c, Masaaki Ohara d, Steffen Kiel e a
Department of Geoscience, Joetsu University of Education, 1 Yamayashiki, Joetsu, Niigata 943-8512, Japan School of Natural System, College of Science and Engineering, Kanazawa University, Kanazawa City, Ishikawa 920-1192, Japan c Kushimoto-cho, Wakayama 649-3503, Japan d Wakayama Prefectural Museum of Natural History, Kainan City, Wakayama 642-0001, Japan e Georg-August-Universität Göttingen, Geoscience Center, Geobiology Group, Goldschmidtstr. 3, 37077 Göttingen, Germany b
a r t i c l e
i n f o
Article history: Received 23 April 2013 Received in revised form 12 July 2013 Accepted 15 July 2013 Available online 24 July 2013 Keywords: Paleogene Seep Carbonate Honshu Chemosynthetic fauna
a b s t r a c t An isolated limestone deposit occurs within late Eocene to early Oligocene submarine fan deposits of the Tanamigawa Formation in the outskirts of Kushimoto Town in southern Honshu, Japan. The petrography, the very negative carbon isotope signature of early diagenetic cements, and the abundance of chemosymbiotic bivalves, namely thyasirids, vesicomyids and bathymodiolins, clearly identify this carbonate block as the first Paleogene methane seep deposit in Honshu. From a biogeographical point of view, distinctive features of Paleogene seep faunas in Japan are the apparently endemic vesicomyid Hubertschenckia ezoensis and the scarcity of lucinid bivalves, whereas the bivalves Conchocele bisecta and Bathymodiolus spp., and the gastropod Cryptonatica were widespread in North Pacific seep communities of this age. Although Cenozoic seep communities generally consists of members of the same families that inhabit seeps today, marked differences in the genus-level composition of the major chemosymbiotic bivalve families such as Vesicomyidae and Lucinidae, and the subfamily Bathymodiolinae are noted when Paleogene seep communities in Japan are compared to those of early Neogene age on the one hand, and to Paleogene seep communities elsewhere on the other. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrothermal vents and methane seeps in the deep sea are inhabited by a distinctive fauna that is largely restricted to these ecosystems (Van Dover et al., 2002). Despite the patchy distribution of these habitats many of the inhabiting taxa have a remarkably wide geographic distribution (Olu et al., 2010; Audzijonyte et al., 2012). The present-day biogeographic patterns have a historical basis (Tunnicliffe and Fowler, 1996; Bachraty et al., 2009; Baker et al., 2010) for which the fossil record can provide direct evidence. For example, while the Mediterranean seep fauna today consists of a mixture of local and northeast Atlantic species (Olu et al., 2004; Ritt et al., 2010), during the Miocene it still had close affinities to the seep fauna in the Gulf of Mexico (Taviani, 1994). Bathymodiolin mussels were abundant in Eocene and Oligocene seeps along the Pacific coast of North America, while they are nearly absent from this region today (Goedert and Squires, 1990; Duperron, 2010; Kiel and Amano, 2013). Lastly, a late Pliocene vent deposit on the Mid-Atlantic Ridge hosted the large provannid gastropod Kaneconcha, whose closest relative, Ifremeria, is restricted to vents in the Indian and western Pacific Oceans (Kaim et al., 2012). Fossil seep deposits are unevenly distributed in space and ⁎ Corresponding author. E-mail addresses: [email protected] (K. Amano), [email protected] (R.G. Jenkins), [email protected] (M. Ohara), [email protected] (S. Kiel). 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.07.015
time (Kiel, 2009) which hampers broader paleobiogeographic analyses. In the Cenozoic, Paleogene examples are particularly uncommon; most examples are from western North America (Goedert and Campbell, 1995; Squires and Gring, 1996; Peckmann et al., 2002; Burns et al., 2005; Kiel, 2010a) and a few other occurrences are from Peru and the Caribbean region, and from Hokkaido, northern Japan (Olsson, 1931; Majima et al., 2005; Kiel and Peckmann, 2007). The Japanese Islands have a rich fossil record of Cretaceous to Holocene seep deposits and also of other chemosynthetic habitats such as whale-falls, plesiosaur-falls and wood-falls (Amano and Little, 2005; Majima et al., 2005; Amano et al., 2007a,b; Jenkins et al., 2007; Kaim et al., 2008; Kiel et al., 2008; Nobuhara et al., 2008; Kiel et al., 2009; Amano and Kiel, 2010, 2011, 2012; Amano and Ando, 2011; Amano and Jenkins, 2011a,b). But occurrences of Paleogene age are restricted to a few examples on the island of Hokkaido (Majima et al., 2005; Amano and Jenkins, 2011a). An alleged Oligocene example from the Muroto Formation on Shikoku, southwestern Japan (cf. Matsumoto and Hirata, 1972; Majima et al., 2005) is in fact of early Miocene age (Iijima et al., 1981; Okamura and Taira, 1984; Suyari et al., 1989). Here we report a late Eocene–early Oligocene seep deposit and its fauna from southern Honshu, and discuss its biogeographic implications. 2. Geologic setting The studied seep deposit is situated near Tanami on the outskirts of Kushimoto Town in Wakayama Prefecture, southern Honshu (Fig. 1),
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Fig. 1. Map showing the fossil locality.
where Paleogene turbidites of the lower part of the Tanamigawa Formation (Suzuki et al., 2012) form the beach platform. These sediments belong to the Muro Group and represent an uplifted accretionary prism resulting from the subduction of the Pacific and Kula plates under the Eurasian plate during the Paleogene (e.g. Mizuno and Imai, 1964; Tateishi et al., 1979; Suzuki et al., 2012). Radiolarian fossils from the Tanamigawa Formation enclosing the seep deposit are indicative mainly of a late Eocene age, possibly ranging into the early Oligocene (Suzuki and Fukuda, 2012; Suzuki et al., 2012). The deposit was first mentioned by Katto and Masuda (1978) who reported a dark gray limestone with the bivalves Modiolus sp., Conchocele cf. nipponica, and Callista cf. hanzawai, and the gastropods Ancistrolepis sp. and Ancila? sp. Among these taxa, the thyasirid Conchocele is a common member of many fossil chemosynthetic communities in the North Pacific realm (Majima et al., 2005; Kiel, 2010a). The carbonate block itself is trapezoid in shape, 3.5–4.5 m × 1.5–2.5 m × 0.2–0.6 m (L × W × H) in size, and surrounded by black mudstones (Fig. 2A). Mollusk fossils are found mainly at the landward side of the deposit (0.5–1.1 m in width; Fig. 2A(a), B, C), while most of its seaward side consists of laminated ‘stromatolitic’ bands devoid of fossils (1.0–1.4 m in width; Fig. 2A(b), D). The Tanamigawa Formation was considered to be a submarine fan deposit but a more precise paleobathymetry is unknown (Nakaya and Sakamoto, 2012).
3. Materials and methods Carbonate samples and fossil specimens were collected in the field. Fossils were further prepared with a pneumatic vibrating tool and carbonate samples were used to prepare petrographic thin sections of ca. 60 μm thickness for standard observations with plane- and crosspolarized, reflected light, and cathodoluminescence microscopy. Samples for carbon and oxygen isotope analyses were extracted from
the polished surfaces of the counterparts of the thin sections using a microdrill. The analyses were performed on a Finnigan MAT252 (Thermo Fisher Scientific Inc., Wattham, MA, USA) isotope ratio mass spectrometer, equipped with an automated carbonate reaction device (Kiel III). The automatic reaction to generate carbon dioxide was performed at 90 °C with 100% phosphoric acid. All isotope values are reported with respect to the Pee Dee Belemnite standard (PDB). The external precision of samples of the powdered carbonate standard is 0.03% for both δ18O and δ13C. All fossil specimens are stored at the Wakayama Prefectural Museum of Natural History (WMNH).
4. Carbonate microfacies, stable isotopes, and the paleoenvironment The matrix consists of dark to light micrite and microspar with abundant angular clasts and numerous shell fragments (Fig. 3A). Voids are common and often lined by fibrous rim cements made of either banded or botryoidal crystal aggregates; the interior of the voids is filled by sparry calcite occasionally replaced by silica (Fig. 3B). The rim cements show no cathodoluminescence (CL) behavior but are sometimes partially recrystallized to micrite with strong CL behavior (Fig. 4). Burrows are common and often filled by fecal pellets and micrite with clotted texture (Fig. 3C). Stylolitic dissolution fronts lined with pyrite occur throughout the carbonate (Fig. 3D); otherwise pyrite is rare. Another feature is large stromatolitic bands or layers, reaching several decimeters in length. They are composed of alternating layers of fibrous, banded or botryoidal cements, seam micrite, and detritus-rich micrite, and occasionally small aggregates of pyrite (Fig. 3E, F). The light and dark micrites and microspar have δ13C signatures ranging from −35.3 to −16.2‰ with respective δ18O values ranging from −20.1 to −14.7‰; a sample from a stromatolitic band has a δ13C value of −29.9‰ and a δ18O value of −16.2‰, the micrite with clotted fabric and peloids had δ13C values of −31.2 to −23.5‰ and
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Fig. 2. Outcrop of the fossil locality. A. (a) limestone yielding molluscan fossils, (b) limestone with laminated ‘stromatolitic’ bands. B, C. Occurrence of molluscan fossils; white arrow in B indicates a geopetal structure in a bivalve. D. Enlargement of limestone with laminated ‘stromatolitic’ bands.
Fig. 3. Thin section images from Tanami. A. Micrite matrix with abundant angular clasts. B. Void with rim cements and sparry calcite. C. Burrow filled with fecal pellets. D. Example of a pyrite-lined dissolution front. E. Stromatolitic layer in upper part of image. F. Detail of the stromatolitic layer.
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Fig. 4. Cathodoluminescence images. A, B: Banded fibrous rim cement with dull CL behavior (center) surrounded by micrite and sparry calcite with strong CL behavior. C, D: Burrow filled with fecal pellets. E, F: Area replaced by silica (yellow in E) and small void with botryoidal crystal aggregates (lower right).
δ18O values of −20.1 to −18.8‰, and a sample from a rim cement showed the most negative δ13C value (−36.7‰) with the corresponding δ18O value of −8.7‰. The least negative δ13C values (−14.7‰ and −12.6‰) were measured in void-filling sparry calcite; the respective δ18O values (−19.5‰ and −17.8‰) were not different from those of the other carbonate phases. The stable isotope results are summarized in Fig. 5. The very negative δ13C values indicate that these carbonates were precipitated under the influence of anaerobic methane oxidation. Seep carbonates typically inherit the carbon isotope signature of the oxidized organic matter but with a certain amount of marine bicarbonate mixed in, resulting in δ13Ccarbonate values that are more positive those of the oxidized organic matter (Peckmann and Thiel, 2004). For example, oxidation of crude oil with a δ13C signature of −35‰ resulted in carbonate with δ13C values reaching only −25‰ (Joye et al., 2004). Biogenic methane exhibits the most negative isotopic signatures ranging from −110 to −50‰ (Whiticar et al., 1986; Whiticar, 1999), while thermogenic methane is somewhat heavier, with values ranging from −50 to −30‰ (Sackett, 1978). Thus in the case of the Tanami seep deposit, the δ13C signature does not allow distinguishing between a biogenic and a thermogenic methane source. The very negative δ18O values in virtually all carbonate phases indicates strong late diagenetic alteration under the influence of meteoric water and/or thermal stress during burial.
5. Molluscan fossils and paleoecology We recovered ten mollusk species from the carbonates and the vast majority of them are chemosymbiotic bivalves (Table 1, Fig. 6). The
Fig. 5. Cross plot of the carbon and oxygen isotope signature of carbonate phases.
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most common species with 66 specimens is the thyasirid bivalve Conchocele bisecta (Conrad) (Fig. 6E, I), which reaches up to 59.9 mm in length. Katto and Masuda (1978) reported Conchocele cf. nipponica (Yabe and Nomura) from Tanami, a taxon that was originally proposed as a variety of C. bisecta by Yabe and Nomura (1925). Due to the wide range of morphological variation in C. bisecta as pointed out by Makiyama (1934), Oyama et al. (1960) synonymized C. nipponica with C. bisecta. We follow their taxonomic treatment here. Extant thyasirids of this size typically harbor chemosymbiotic bacteria (Dufour, 2005) and this is assumed also for the specimens from the Tanami seep deposit. Next in abundance with 14 specimens is the bathymodiolin bivalve Bathymodiolus aff. inouei Amano and Jenkins (Fig. 6O, P) that reaches 29.2 mm in length. This is probably the same species that Katto and Masuda (1978) referred to as ‘Modiolus sp.’ The Tanami specimens differ slightly from B. inouei from Hokkaido by having the beak in a more anterior position: it is situated in the anterior 2−7% of the total shell length, whereas in the type specimens of B. inouei it is in the anterior 7−11%. We found 11 specimens of the vesicomyid bivalve Hubertschenckia ezoensis (Yokoyama) (Fig. 6K, L, M). The ‘Callista cf. hanzawai (Nagao)’ reported by Katto and Masuda (1978) from the Tanami seep deposit is most likely identical with H. ezoensis. The veneroid Callista is similar in shell outline to Hubertschenckia, but typically inhabits shallow water (e.g. Higo et al., 1999) and is thus unlikely to have inhabited the deep-water environment of the Tanamigawa Formation. All extant bathymodiolins and vesicomyids examined to date live in symbiosis with sulfur-oxidizing bacteria, or in the case of bathymodiolins, also with methane-oxidizing bacteria (Duperron, 2010; Taylor and Glover, 2010). This mode of life is thus also assumed for the specimens from Tanami. The same applies to the single specimen of the solemyid bivalve Acharax? sp. (Fig. 6J) found at the Tanami seep deposit (cf. Taylor and Glover, 2010). A single specimen of the nucinellid bivalve Nucinella sp., 22.5 mm long, was found (Fig. 6A, H). With its steeply sloping postero-dorsal margin and low umbo, this specimen is more similar to Nucinella sp. from uppermost Jurassic seep deposits in Svalbard Islands (Hammer et al., 2011) than to N. gigantea from Late Cretaceous seep deposits on Hokkaido (Amano et al., 2007a,b; Kiel et al., 2008). A chemosymbiotic mode of life was suggested for Nucinella based on the finding of a large species at a Cretaceous seep deposit in northern Japan (Amano
et al., 2007a,b). Recent anatomical investigations of a large Nucinella from the Arabian Sea confirmed the presence of symbiotic bacteria in its gills (Oliver and Taylor, 2012). A few heterotrophic mollusks were also found: the taxodont, infaunal deposit-feeding bivalve Neilonella cf. alferovi (Slodkewitsch) (Fig. 6B, C) (cf. Dame, 1996), the grazing trochid gastropod Margarites? sp. (Fig. 6F, G) (cf. Hickman and McLean, 1990), the turritellid gastropod Orectospira wadana Yokoyama (Fig. 6R) whose feeding type is unknown, and two predatory gastropods, the naticid Cryptonatica sp. (Fig. 6D, N) and the muricid Abyssotrophon? sp. (Fig. 6Q) (cf. Kohn, 1983). 6. Biogeographic relationships The late Eocene seep communities from the Tappu and Poronai Formations in Hokkaido (Amano and Jenkins, 2007; Amano and Kiel, 2007; and subsequent collections by KA and RGJ) share Conchocele bisecta and Hubertschenckia ezoensis with the Tanami seep fauna, but differ from it by lacking Acharax and Bathymodiolus (Table 1). Among the ‘vagrants’ (taxa from the background fauna that take advantage of the abundance of food at methane seeps) the Eocene and Oligocene seeps share the deposit feeding taxa Neilonella (Protobranchia) and the unknown feeder Orectospira (Turritellidae). The Tanami seep fauna is remarkably similar to an early Oligocene seep fauna from the Nuibetsu Formation in eastern Hokkaido (Amano and Jenkins, 2011a; Table 1), which is the only Oligocene seep deposit in Japan. They share the dominant chemosymbiotic bivalves Acharax spp., Bathymodiolus (s.l.) spp., Conchocele bisecta, and Hubertschenckia ezoensis. The differences are mainly among the vagrants; predatory gastropods such as Euspira meisensis, Colus spp. and Trominina japonica prevail in the Nuibetsu fauna. When the Tanami seep community is compared to early Miocene seep faunas in Japan, major differences especially among the dominant chemosymbiotic bivalve families become apparent. Among the vesicomyids, the moderately sized, compact genus Hubertschenckia is found only in the Eocene–Oligocene seep deposits and was replaced by the slender, elongate vesicomyids Adulomya chitanii and A. uchimuraensis, and the small Pliocardia kawadai in the early Miocene (Amano and Kiel, 2007, 2011, 2012). Lucinids are absent from the Tanami seep deposit and are generally extremely rare in Eocene and Oligocene seep deposits (KA, pers. obs.). In the early Miocene, Lucinoma became abundant with no less than five reported species (acutilineata, annulata, gracilistriata,
Table 1 Mollusk species at Paleogene seep deposits in Japan; those of the Nuibetsu, Poronai and Tappu Formations in Hokkaido were collected and identified by two of the authors (KA and RGJ). For locality information see Ohara (1966) and Amano and Jenkins (2007, 2011a). District Age Species
Formation
Bivalvia Acharax spp. Nucinella sp. Malletia poronaica Neilonella spp. Yoldia sobrina Takeda Portlandia watasei (Kanehara) Bathymodiolus inouei Amano and Jenkins B. aff. inouei Amano and Jenkins Conchocele bisecta (Conrad) Cyclocardia spp. Hubertschenckia ezoensis (Yokoyama) Gastropoda Orectospira wadana (Yokoyama) Cryptonatica sp. Euspira meisensis (Makiyama) Colus spp. Trominina japonica (Takeda) Ancistrolepis modestoideus (Takeda)
Wakayama
E. Hokkaido
C. Hokkaido
NW. Hokkaido
L. Eocene–? Early Oligocene
Early Oligocene
Late Eocene
Late Eocene
Tanamigawa F.
Nuibetsu F.
Poronai F.
Tappu F.
+ +
+ +
+
+ + + +
+ + +
+ + +
+
+ + + +
+ + +
+ +
+ + + +
+ +
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Fig. 6. Mollusk fossils from the Tanami seep deposit. All specimens are stored at the Wakayama Prefectural Museum of Natural History (WMNH) and were coated with ammonium chloride prior to photography. (A, H) Nucinella sp., same specimen, length = 22.5 mm, WMNH-Ge-1130110027. (B–C) Neilonella cf. alferovi (Slodkewitsch); (B) length = 7.8 mm, WMNH-Ge-1130110028; (C) length = 8.8 mm, WMNH-Ge-1130110029. (D, N) Cryptonatica sp., same specimen, height = 7.2 mm, WMNH-Ge-1130110030. (E, I) Conchocele bisecta (Conrad); (E) length = 59.9 mm, WMNH-Ge-1130110031; (I) length = 29.5 mm, WMNH-Ge-1130110032. (F, G.) Margarites? sp., same specimen, height = 3.4 mm, WMNH-Ge-1130110033. (J) Acharax? sp., length = 52.8 mm +, WMNH-Ge-1130110034. (K–M) Hubertschenckia ezoensis (Yokoyama); (K, L) same specimen, length = 49.5 mm, WMNH-Ge-1130110035; (M) length = 35.2 mm, white arrow shows pallial sinus, WMNH-Ge-1130110036. (O, P) Bathymodiolus aff. inouei (Amano and Jenkins); (O) length = 29.1 mm, WMNH-Ge-1130110037; (P) length = 29.1 mm, WMNH-Ge-1130110038. (Q) Abyssotrophon? sp., height = 11.6 mm, WMNH-Ge-1130110039. (R) Orectospira wadana (Yokoyama), height = 12.1 mm, WMNH-Ge-1130110040.
hannibali, otukai) (Majima et al., 2005) which, however, are in dire need of taxonomic revision, and also Nipponothracia appears (Amano and Ando, 2011). Bathymodiolins were common in the early Oligocene seep deposits, but have not yet been found at lower Miocene deposits in Japan (Majima et al., 2005). In sum, the late Eocene to early Oligocene Tanami seep fauna is most similar to the early Oligocene seep fauna in Hokkaido and is also similar to the late Eocene seep faunas in Hokkaido, which differ mainly by lacking bathymodiolins and solemyids. In contrast, the slightly younger, early Miocene seep faunas have a very different faunal composition, mainly due to the abundance of lucinids, and the presence of a different set of vesicomyid genera. Late Oligocene seep faunas are as-yet unknown from Japan, thus it remains unclear whether the faunal break between the Eocene/Oligocene seep communities on the one hand, and the early Miocene ones on the other, was as sharp as it currently appears, or if the transition was more gradual. There are three other areas worldwide with late Eocene to early Oligocene seep faunas: western North America, the Caribbean region, and northern Peru. The diverse seep faunas in western Washington and the Tanami seep fauna share the chemosymbiotic bivalves Bathymodiolus
inouei, Conchocele bisecta, and similar Acharax species; among the vagrants they share the gastropods Margarites and Cryptonatica (Goedert and Squires, 1990; Goedert and Campbell, 1995; Kiel, 2010a,b; Kiel and Amano, 2013). Remarkable differences include the lack of lucinid bivalves in the Tanami fauna, while Lucinoma and Epilucina are known from several sites in California and Washington (Goedert and Campbell, 1995; Squires and Gring, 1996; Peckmann et al., 2002), a different set of vesicomyid genera lacking Hubertschenckia in California and Washington (Goedert and Campbell, 1995; Peckmann et al., 2002; Amano and Kiel, 2007), and the lack of Nucinella from western North America (Kiel, 2010b). Oligocene seeps of the Caribbean region (although their stratigraphic age is not well constrained) and those of northern Peru are characterized by large lucinid bivalves, which are lacking at Tanami (Olsson, 1931; Kiel and Peckmann, 2007; Kiel et al., 2010; Kiel, in press) and by vesicomyid-like bivalves called ‘Pleurophopsis’ whose taxonomic affinities are still unclear (Kiel, 2007). The Caribbean seeps share with Tanami the chemosymbiotic bivalves Acharax and Bathymodiolus, but the accompanying gastropod fauna is quite different (Olsson, 1931; Kiel, 2007; Kiel and Peckmann, 2007; Kiel et al., 2010).
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7. Conclusions The petrography of the limestone deposit at Tanami in southern Honshu includes carbonate phases characteristic for seep carbonates, such as banded and botryoidal rim cements. The very negative δ13C signature of these cements, as low as −36.7‰, indicates that the oxidation of biogenic or thermogenic methane played a major role in their formation. These features, together with the abundance of chemosymbiotic bivalves, namely thyasirids, vesicomyids and bathymodiolins, clearly identify this carbonate block as the first Paleogene methane seep deposit in Honshu. From a biogeographical point of view, distinctive features of the Paleogene Japanese seep faunas are the apparently endemic vesicomyid genus Hubertschenckia and the scarcity of lucinid bivalves, whereas the bivalves Conchocele bisecta and Bathymodiolus inouei, and the predatory gastropod Cryptonatica appear to be widespread in North Pacific seep communities. Although virtually all Cenozoic seep faunas consist of members of the same mollusk families that inhabit these ecosystems today (see also Vrijenhoek, 2013), the Paleogene seep communities in Japan differ markedly from those of early Neogene age in generic composition and abundance of the chemosymbiotic bivalve families Vesicomyidae and Lucinidae, and the subfamily Bathymodiolinae. The same applies when Paleogene seep communities in Japan are compared to Paleogene seep communities in other parts of the world. Acknowledgements We thank J. L. Goedert and an anonymous reviewer for their constructive criticism. Part of this study was financially supported by a Grant-in-Aid for Science Research of the Japan Society for the Promotion of Science (C, 23540546, 2011–2013) to KA, Grant-in-Aid for JSPS Fellows and Grant for Program to Disseminate Tenure Tracking System (JST) to RGJ, and by the Deutsche Forschungsgemeinschaft through grant Ki802/6-1 and the Joetsu University of Education through a visiting scientist grant to SK. References Amano, K., Ando, H., 2011. Giant fossil Acharax (Bivalvia: Solemyidae) from the Miocene of Japan. Nautilus 125, 207–212. Amano, K., Jenkins, R.G., 2007. Eocene drill holes in cold-seep bivalves of Hokkaido, northern Japan. Marine Ecology 28, 108–114. Amano, K., Jenkins, R.G., 2011a. New fossil Bathymodiolus (sensu lato) (Bivalvia: Mytilidae) from Oligocene seep-carbonates in eastern Hokkaido, Japan, with remarks on the evolution of the genus. Nautilus 125, 29–35. Amano, K., Jenkins, R.G., 2011b. Fossil records of extant vesicomyid species from Japan. Venus (Japan Journal of Malacology) 69, 163–176. Amano, K., Kiel, S., 2007. Fossil vesicomyid bivalves from the North Pacific region. Veliger 49, 270–293. Amano, K., Kiel, S., 2010. Taxonomy and distribution of Archivesica (Vesicomyidae, Bivalvia) in Japan. Nautilus 124, 155–165. Amano, K., Kiel, S., 2011. Fossil Adulomya (Vesicomyidae, Bivalvia) from Japan. Veliger 51, 76–90. Amano, K., Kiel, S., 2012. Two Neogene vesicomyid species (Bivalvia) from Japan and their biogeographic implications. Nautilus 126, 79–85. Amano, K., Little, C.T.S., 2005. Miocene whale-fall community from Hokkaido, northern Japan. Palaeogeography Palaeoclimatology Palaeoecology 215, 345–356. Amano, K., Jenkins, R.G., Hikida, Y., 2007a. A new gigantic Nucinella (Bivalvia: Solemyoida) from the Cretaceous cold-seep deposit in Hokkaido, northern Japan. Veliger 49, 84–90. Amano, K., Little, C.T.S., Inoue, K., 2007b. A new Miocene fossil whale-fall community from Japan. Palaeogeography Palaeoclimatology Palaeoecology 247, 236–242. Audzijonyte, A., Krylova, E.M., Sahling, H., Vrijenhoek, R.C., 2012. Molecular taxonomy reveals broad trans-oceanic distributions and high species diversity of deep-sea clams (Bivalvia: Vesicomyidae: Pliocardiinae) in chemosynthetic environments. Systematic Biology 10, 403–415. Bachraty, C., Legendre, P., Desbruyères, D., 2009. Biogeographic relationships among deep-sea hydrothermal vent faunas at global scale. Deep-Sea Research Part I 56, 1371–1378. Baker, M.C., Ramirez-Llodra, E., Tyler, P.A., German, C.R., Boetius, A., Cordes, E.E., Dubilier, N., Fisher, C.R., Levin, L.A., Metaxas, A., Rowden, A.A., Santos, R.S., Shank, T.M., Van Dover, C.L., Young, C.M., Warén, A., 2010. Biogeography, ecology, and vulnerability of chemosynthetic ecosystems in the deep sea. In: McIntyre, A. (Ed.), Life in the World's Oceans: Diversity, Distribution, and Abundance. Wiley-Blackwell, pp. 161–182.
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A Paleogene deep-sea methane-seep community from Honshu, Japan Kazutaka Amano a,⁎, Robert G. Jenkins b, Yukio Sako c, Masaaki Ohara d, Steffen Kiel e a
Department of Geoscience, Joetsu University of Education, 1 Yamayashiki, Joetsu, Niigata 943-8512, Japan School of Natural System, College of Science and Engineering, Kanazawa University, Kanazawa City, Ishikawa 920-1192, Japan c Kushimoto-cho, Wakayama 649-3503, Japan d Wakayama Prefectural Museum of Natural History, Kainan City, Wakayama 642-0001, Japan e Georg-August-Universität Göttingen, Geoscience Center, Geobiology Group, Goldschmidtstr. 3, 37077 Göttingen, Germany b
a r t i c l e
i n f o
Article history: Received 23 April 2013 Received in revised form 12 July 2013 Accepted 15 July 2013 Available online 24 July 2013 Keywords: Paleogene Seep Carbonate Honshu Chemosynthetic fauna
a b s t r a c t An isolated limestone deposit occurs within late Eocene to early Oligocene submarine fan deposits of the Tanamigawa Formation in the outskirts of Kushimoto Town in southern Honshu, Japan. The petrography, the very negative carbon isotope signature of early diagenetic cements, and the abundance of chemosymbiotic bivalves, namely thyasirids, vesicomyids and bathymodiolins, clearly identify this carbonate block as the first Paleogene methane seep deposit in Honshu. From a biogeographical point of view, distinctive features of Paleogene seep faunas in Japan are the apparently endemic vesicomyid Hubertschenckia ezoensis and the scarcity of lucinid bivalves, whereas the bivalves Conchocele bisecta and Bathymodiolus spp., and the gastropod Cryptonatica were widespread in North Pacific seep communities of this age. Although Cenozoic seep communities generally consists of members of the same families that inhabit seeps today, marked differences in the genus-level composition of the major chemosymbiotic bivalve families such as Vesicomyidae and Lucinidae, and the subfamily Bathymodiolinae are noted when Paleogene seep communities in Japan are compared to those of early Neogene age on the one hand, and to Paleogene seep communities elsewhere on the other. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrothermal vents and methane seeps in the deep sea are inhabited by a distinctive fauna that is largely restricted to these ecosystems (Van Dover et al., 2002). Despite the patchy distribution of these habitats many of the inhabiting taxa have a remarkably wide geographic distribution (Olu et al., 2010; Audzijonyte et al., 2012). The present-day biogeographic patterns have a historical basis (Tunnicliffe and Fowler, 1996; Bachraty et al., 2009; Baker et al., 2010) for which the fossil record can provide direct evidence. For example, while the Mediterranean seep fauna today consists of a mixture of local and northeast Atlantic species (Olu et al., 2004; Ritt et al., 2010), during the Miocene it still had close affinities to the seep fauna in the Gulf of Mexico (Taviani, 1994). Bathymodiolin mussels were abundant in Eocene and Oligocene seeps along the Pacific coast of North America, while they are nearly absent from this region today (Goedert and Squires, 1990; Duperron, 2010; Kiel and Amano, 2013). Lastly, a late Pliocene vent deposit on the Mid-Atlantic Ridge hosted the large provannid gastropod Kaneconcha, whose closest relative, Ifremeria, is restricted to vents in the Indian and western Pacific Oceans (Kaim et al., 2012). Fossil seep deposits are unevenly distributed in space and ⁎ Corresponding author. E-mail addresses: [email protected] (K. Amano), [email protected] (R.G. Jenkins), [email protected] (M. Ohara), [email protected] (S. Kiel). 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.07.015
time (Kiel, 2009) which hampers broader paleobiogeographic analyses. In the Cenozoic, Paleogene examples are particularly uncommon; most examples are from western North America (Goedert and Campbell, 1995; Squires and Gring, 1996; Peckmann et al., 2002; Burns et al., 2005; Kiel, 2010a) and a few other occurrences are from Peru and the Caribbean region, and from Hokkaido, northern Japan (Olsson, 1931; Majima et al., 2005; Kiel and Peckmann, 2007). The Japanese Islands have a rich fossil record of Cretaceous to Holocene seep deposits and also of other chemosynthetic habitats such as whale-falls, plesiosaur-falls and wood-falls (Amano and Little, 2005; Majima et al., 2005; Amano et al., 2007a,b; Jenkins et al., 2007; Kaim et al., 2008; Kiel et al., 2008; Nobuhara et al., 2008; Kiel et al., 2009; Amano and Kiel, 2010, 2011, 2012; Amano and Ando, 2011; Amano and Jenkins, 2011a,b). But occurrences of Paleogene age are restricted to a few examples on the island of Hokkaido (Majima et al., 2005; Amano and Jenkins, 2011a). An alleged Oligocene example from the Muroto Formation on Shikoku, southwestern Japan (cf. Matsumoto and Hirata, 1972; Majima et al., 2005) is in fact of early Miocene age (Iijima et al., 1981; Okamura and Taira, 1984; Suyari et al., 1989). Here we report a late Eocene–early Oligocene seep deposit and its fauna from southern Honshu, and discuss its biogeographic implications. 2. Geologic setting The studied seep deposit is situated near Tanami on the outskirts of Kushimoto Town in Wakayama Prefecture, southern Honshu (Fig. 1),
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Fig. 1. Map showing the fossil locality.
where Paleogene turbidites of the lower part of the Tanamigawa Formation (Suzuki et al., 2012) form the beach platform. These sediments belong to the Muro Group and represent an uplifted accretionary prism resulting from the subduction of the Pacific and Kula plates under the Eurasian plate during the Paleogene (e.g. Mizuno and Imai, 1964; Tateishi et al., 1979; Suzuki et al., 2012). Radiolarian fossils from the Tanamigawa Formation enclosing the seep deposit are indicative mainly of a late Eocene age, possibly ranging into the early Oligocene (Suzuki and Fukuda, 2012; Suzuki et al., 2012). The deposit was first mentioned by Katto and Masuda (1978) who reported a dark gray limestone with the bivalves Modiolus sp., Conchocele cf. nipponica, and Callista cf. hanzawai, and the gastropods Ancistrolepis sp. and Ancila? sp. Among these taxa, the thyasirid Conchocele is a common member of many fossil chemosynthetic communities in the North Pacific realm (Majima et al., 2005; Kiel, 2010a). The carbonate block itself is trapezoid in shape, 3.5–4.5 m × 1.5–2.5 m × 0.2–0.6 m (L × W × H) in size, and surrounded by black mudstones (Fig. 2A). Mollusk fossils are found mainly at the landward side of the deposit (0.5–1.1 m in width; Fig. 2A(a), B, C), while most of its seaward side consists of laminated ‘stromatolitic’ bands devoid of fossils (1.0–1.4 m in width; Fig. 2A(b), D). The Tanamigawa Formation was considered to be a submarine fan deposit but a more precise paleobathymetry is unknown (Nakaya and Sakamoto, 2012).
3. Materials and methods Carbonate samples and fossil specimens were collected in the field. Fossils were further prepared with a pneumatic vibrating tool and carbonate samples were used to prepare petrographic thin sections of ca. 60 μm thickness for standard observations with plane- and crosspolarized, reflected light, and cathodoluminescence microscopy. Samples for carbon and oxygen isotope analyses were extracted from
the polished surfaces of the counterparts of the thin sections using a microdrill. The analyses were performed on a Finnigan MAT252 (Thermo Fisher Scientific Inc., Wattham, MA, USA) isotope ratio mass spectrometer, equipped with an automated carbonate reaction device (Kiel III). The automatic reaction to generate carbon dioxide was performed at 90 °C with 100% phosphoric acid. All isotope values are reported with respect to the Pee Dee Belemnite standard (PDB). The external precision of samples of the powdered carbonate standard is 0.03% for both δ18O and δ13C. All fossil specimens are stored at the Wakayama Prefectural Museum of Natural History (WMNH).
4. Carbonate microfacies, stable isotopes, and the paleoenvironment The matrix consists of dark to light micrite and microspar with abundant angular clasts and numerous shell fragments (Fig. 3A). Voids are common and often lined by fibrous rim cements made of either banded or botryoidal crystal aggregates; the interior of the voids is filled by sparry calcite occasionally replaced by silica (Fig. 3B). The rim cements show no cathodoluminescence (CL) behavior but are sometimes partially recrystallized to micrite with strong CL behavior (Fig. 4). Burrows are common and often filled by fecal pellets and micrite with clotted texture (Fig. 3C). Stylolitic dissolution fronts lined with pyrite occur throughout the carbonate (Fig. 3D); otherwise pyrite is rare. Another feature is large stromatolitic bands or layers, reaching several decimeters in length. They are composed of alternating layers of fibrous, banded or botryoidal cements, seam micrite, and detritus-rich micrite, and occasionally small aggregates of pyrite (Fig. 3E, F). The light and dark micrites and microspar have δ13C signatures ranging from −35.3 to −16.2‰ with respective δ18O values ranging from −20.1 to −14.7‰; a sample from a stromatolitic band has a δ13C value of −29.9‰ and a δ18O value of −16.2‰, the micrite with clotted fabric and peloids had δ13C values of −31.2 to −23.5‰ and
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Fig. 2. Outcrop of the fossil locality. A. (a) limestone yielding molluscan fossils, (b) limestone with laminated ‘stromatolitic’ bands. B, C. Occurrence of molluscan fossils; white arrow in B indicates a geopetal structure in a bivalve. D. Enlargement of limestone with laminated ‘stromatolitic’ bands.
Fig. 3. Thin section images from Tanami. A. Micrite matrix with abundant angular clasts. B. Void with rim cements and sparry calcite. C. Burrow filled with fecal pellets. D. Example of a pyrite-lined dissolution front. E. Stromatolitic layer in upper part of image. F. Detail of the stromatolitic layer.
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Fig. 4. Cathodoluminescence images. A, B: Banded fibrous rim cement with dull CL behavior (center) surrounded by micrite and sparry calcite with strong CL behavior. C, D: Burrow filled with fecal pellets. E, F: Area replaced by silica (yellow in E) and small void with botryoidal crystal aggregates (lower right).
δ18O values of −20.1 to −18.8‰, and a sample from a rim cement showed the most negative δ13C value (−36.7‰) with the corresponding δ18O value of −8.7‰. The least negative δ13C values (−14.7‰ and −12.6‰) were measured in void-filling sparry calcite; the respective δ18O values (−19.5‰ and −17.8‰) were not different from those of the other carbonate phases. The stable isotope results are summarized in Fig. 5. The very negative δ13C values indicate that these carbonates were precipitated under the influence of anaerobic methane oxidation. Seep carbonates typically inherit the carbon isotope signature of the oxidized organic matter but with a certain amount of marine bicarbonate mixed in, resulting in δ13Ccarbonate values that are more positive those of the oxidized organic matter (Peckmann and Thiel, 2004). For example, oxidation of crude oil with a δ13C signature of −35‰ resulted in carbonate with δ13C values reaching only −25‰ (Joye et al., 2004). Biogenic methane exhibits the most negative isotopic signatures ranging from −110 to −50‰ (Whiticar et al., 1986; Whiticar, 1999), while thermogenic methane is somewhat heavier, with values ranging from −50 to −30‰ (Sackett, 1978). Thus in the case of the Tanami seep deposit, the δ13C signature does not allow distinguishing between a biogenic and a thermogenic methane source. The very negative δ18O values in virtually all carbonate phases indicates strong late diagenetic alteration under the influence of meteoric water and/or thermal stress during burial.
5. Molluscan fossils and paleoecology We recovered ten mollusk species from the carbonates and the vast majority of them are chemosymbiotic bivalves (Table 1, Fig. 6). The
Fig. 5. Cross plot of the carbon and oxygen isotope signature of carbonate phases.
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most common species with 66 specimens is the thyasirid bivalve Conchocele bisecta (Conrad) (Fig. 6E, I), which reaches up to 59.9 mm in length. Katto and Masuda (1978) reported Conchocele cf. nipponica (Yabe and Nomura) from Tanami, a taxon that was originally proposed as a variety of C. bisecta by Yabe and Nomura (1925). Due to the wide range of morphological variation in C. bisecta as pointed out by Makiyama (1934), Oyama et al. (1960) synonymized C. nipponica with C. bisecta. We follow their taxonomic treatment here. Extant thyasirids of this size typically harbor chemosymbiotic bacteria (Dufour, 2005) and this is assumed also for the specimens from the Tanami seep deposit. Next in abundance with 14 specimens is the bathymodiolin bivalve Bathymodiolus aff. inouei Amano and Jenkins (Fig. 6O, P) that reaches 29.2 mm in length. This is probably the same species that Katto and Masuda (1978) referred to as ‘Modiolus sp.’ The Tanami specimens differ slightly from B. inouei from Hokkaido by having the beak in a more anterior position: it is situated in the anterior 2−7% of the total shell length, whereas in the type specimens of B. inouei it is in the anterior 7−11%. We found 11 specimens of the vesicomyid bivalve Hubertschenckia ezoensis (Yokoyama) (Fig. 6K, L, M). The ‘Callista cf. hanzawai (Nagao)’ reported by Katto and Masuda (1978) from the Tanami seep deposit is most likely identical with H. ezoensis. The veneroid Callista is similar in shell outline to Hubertschenckia, but typically inhabits shallow water (e.g. Higo et al., 1999) and is thus unlikely to have inhabited the deep-water environment of the Tanamigawa Formation. All extant bathymodiolins and vesicomyids examined to date live in symbiosis with sulfur-oxidizing bacteria, or in the case of bathymodiolins, also with methane-oxidizing bacteria (Duperron, 2010; Taylor and Glover, 2010). This mode of life is thus also assumed for the specimens from Tanami. The same applies to the single specimen of the solemyid bivalve Acharax? sp. (Fig. 6J) found at the Tanami seep deposit (cf. Taylor and Glover, 2010). A single specimen of the nucinellid bivalve Nucinella sp., 22.5 mm long, was found (Fig. 6A, H). With its steeply sloping postero-dorsal margin and low umbo, this specimen is more similar to Nucinella sp. from uppermost Jurassic seep deposits in Svalbard Islands (Hammer et al., 2011) than to N. gigantea from Late Cretaceous seep deposits on Hokkaido (Amano et al., 2007a,b; Kiel et al., 2008). A chemosymbiotic mode of life was suggested for Nucinella based on the finding of a large species at a Cretaceous seep deposit in northern Japan (Amano
et al., 2007a,b). Recent anatomical investigations of a large Nucinella from the Arabian Sea confirmed the presence of symbiotic bacteria in its gills (Oliver and Taylor, 2012). A few heterotrophic mollusks were also found: the taxodont, infaunal deposit-feeding bivalve Neilonella cf. alferovi (Slodkewitsch) (Fig. 6B, C) (cf. Dame, 1996), the grazing trochid gastropod Margarites? sp. (Fig. 6F, G) (cf. Hickman and McLean, 1990), the turritellid gastropod Orectospira wadana Yokoyama (Fig. 6R) whose feeding type is unknown, and two predatory gastropods, the naticid Cryptonatica sp. (Fig. 6D, N) and the muricid Abyssotrophon? sp. (Fig. 6Q) (cf. Kohn, 1983). 6. Biogeographic relationships The late Eocene seep communities from the Tappu and Poronai Formations in Hokkaido (Amano and Jenkins, 2007; Amano and Kiel, 2007; and subsequent collections by KA and RGJ) share Conchocele bisecta and Hubertschenckia ezoensis with the Tanami seep fauna, but differ from it by lacking Acharax and Bathymodiolus (Table 1). Among the ‘vagrants’ (taxa from the background fauna that take advantage of the abundance of food at methane seeps) the Eocene and Oligocene seeps share the deposit feeding taxa Neilonella (Protobranchia) and the unknown feeder Orectospira (Turritellidae). The Tanami seep fauna is remarkably similar to an early Oligocene seep fauna from the Nuibetsu Formation in eastern Hokkaido (Amano and Jenkins, 2011a; Table 1), which is the only Oligocene seep deposit in Japan. They share the dominant chemosymbiotic bivalves Acharax spp., Bathymodiolus (s.l.) spp., Conchocele bisecta, and Hubertschenckia ezoensis. The differences are mainly among the vagrants; predatory gastropods such as Euspira meisensis, Colus spp. and Trominina japonica prevail in the Nuibetsu fauna. When the Tanami seep community is compared to early Miocene seep faunas in Japan, major differences especially among the dominant chemosymbiotic bivalve families become apparent. Among the vesicomyids, the moderately sized, compact genus Hubertschenckia is found only in the Eocene–Oligocene seep deposits and was replaced by the slender, elongate vesicomyids Adulomya chitanii and A. uchimuraensis, and the small Pliocardia kawadai in the early Miocene (Amano and Kiel, 2007, 2011, 2012). Lucinids are absent from the Tanami seep deposit and are generally extremely rare in Eocene and Oligocene seep deposits (KA, pers. obs.). In the early Miocene, Lucinoma became abundant with no less than five reported species (acutilineata, annulata, gracilistriata,
Table 1 Mollusk species at Paleogene seep deposits in Japan; those of the Nuibetsu, Poronai and Tappu Formations in Hokkaido were collected and identified by two of the authors (KA and RGJ). For locality information see Ohara (1966) and Amano and Jenkins (2007, 2011a). District Age Species
Formation
Bivalvia Acharax spp. Nucinella sp. Malletia poronaica Neilonella spp. Yoldia sobrina Takeda Portlandia watasei (Kanehara) Bathymodiolus inouei Amano and Jenkins B. aff. inouei Amano and Jenkins Conchocele bisecta (Conrad) Cyclocardia spp. Hubertschenckia ezoensis (Yokoyama) Gastropoda Orectospira wadana (Yokoyama) Cryptonatica sp. Euspira meisensis (Makiyama) Colus spp. Trominina japonica (Takeda) Ancistrolepis modestoideus (Takeda)
Wakayama
E. Hokkaido
C. Hokkaido
NW. Hokkaido
L. Eocene–? Early Oligocene
Early Oligocene
Late Eocene
Late Eocene
Tanamigawa F.
Nuibetsu F.
Poronai F.
Tappu F.
+ +
+ +
+
+ + + +
+ + +
+ + +
+
+ + + +
+ + +
+ +
+ + + +
+ +
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Fig. 6. Mollusk fossils from the Tanami seep deposit. All specimens are stored at the Wakayama Prefectural Museum of Natural History (WMNH) and were coated with ammonium chloride prior to photography. (A, H) Nucinella sp., same specimen, length = 22.5 mm, WMNH-Ge-1130110027. (B–C) Neilonella cf. alferovi (Slodkewitsch); (B) length = 7.8 mm, WMNH-Ge-1130110028; (C) length = 8.8 mm, WMNH-Ge-1130110029. (D, N) Cryptonatica sp., same specimen, height = 7.2 mm, WMNH-Ge-1130110030. (E, I) Conchocele bisecta (Conrad); (E) length = 59.9 mm, WMNH-Ge-1130110031; (I) length = 29.5 mm, WMNH-Ge-1130110032. (F, G.) Margarites? sp., same specimen, height = 3.4 mm, WMNH-Ge-1130110033. (J) Acharax? sp., length = 52.8 mm +, WMNH-Ge-1130110034. (K–M) Hubertschenckia ezoensis (Yokoyama); (K, L) same specimen, length = 49.5 mm, WMNH-Ge-1130110035; (M) length = 35.2 mm, white arrow shows pallial sinus, WMNH-Ge-1130110036. (O, P) Bathymodiolus aff. inouei (Amano and Jenkins); (O) length = 29.1 mm, WMNH-Ge-1130110037; (P) length = 29.1 mm, WMNH-Ge-1130110038. (Q) Abyssotrophon? sp., height = 11.6 mm, WMNH-Ge-1130110039. (R) Orectospira wadana (Yokoyama), height = 12.1 mm, WMNH-Ge-1130110040.
hannibali, otukai) (Majima et al., 2005) which, however, are in dire need of taxonomic revision, and also Nipponothracia appears (Amano and Ando, 2011). Bathymodiolins were common in the early Oligocene seep deposits, but have not yet been found at lower Miocene deposits in Japan (Majima et al., 2005). In sum, the late Eocene to early Oligocene Tanami seep fauna is most similar to the early Oligocene seep fauna in Hokkaido and is also similar to the late Eocene seep faunas in Hokkaido, which differ mainly by lacking bathymodiolins and solemyids. In contrast, the slightly younger, early Miocene seep faunas have a very different faunal composition, mainly due to the abundance of lucinids, and the presence of a different set of vesicomyid genera. Late Oligocene seep faunas are as-yet unknown from Japan, thus it remains unclear whether the faunal break between the Eocene/Oligocene seep communities on the one hand, and the early Miocene ones on the other, was as sharp as it currently appears, or if the transition was more gradual. There are three other areas worldwide with late Eocene to early Oligocene seep faunas: western North America, the Caribbean region, and northern Peru. The diverse seep faunas in western Washington and the Tanami seep fauna share the chemosymbiotic bivalves Bathymodiolus
inouei, Conchocele bisecta, and similar Acharax species; among the vagrants they share the gastropods Margarites and Cryptonatica (Goedert and Squires, 1990; Goedert and Campbell, 1995; Kiel, 2010a,b; Kiel and Amano, 2013). Remarkable differences include the lack of lucinid bivalves in the Tanami fauna, while Lucinoma and Epilucina are known from several sites in California and Washington (Goedert and Campbell, 1995; Squires and Gring, 1996; Peckmann et al., 2002), a different set of vesicomyid genera lacking Hubertschenckia in California and Washington (Goedert and Campbell, 1995; Peckmann et al., 2002; Amano and Kiel, 2007), and the lack of Nucinella from western North America (Kiel, 2010b). Oligocene seeps of the Caribbean region (although their stratigraphic age is not well constrained) and those of northern Peru are characterized by large lucinid bivalves, which are lacking at Tanami (Olsson, 1931; Kiel and Peckmann, 2007; Kiel et al., 2010; Kiel, in press) and by vesicomyid-like bivalves called ‘Pleurophopsis’ whose taxonomic affinities are still unclear (Kiel, 2007). The Caribbean seeps share with Tanami the chemosymbiotic bivalves Acharax and Bathymodiolus, but the accompanying gastropod fauna is quite different (Olsson, 1931; Kiel, 2007; Kiel and Peckmann, 2007; Kiel et al., 2010).
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7. Conclusions The petrography of the limestone deposit at Tanami in southern Honshu includes carbonate phases characteristic for seep carbonates, such as banded and botryoidal rim cements. The very negative δ13C signature of these cements, as low as −36.7‰, indicates that the oxidation of biogenic or thermogenic methane played a major role in their formation. These features, together with the abundance of chemosymbiotic bivalves, namely thyasirids, vesicomyids and bathymodiolins, clearly identify this carbonate block as the first Paleogene methane seep deposit in Honshu. From a biogeographical point of view, distinctive features of the Paleogene Japanese seep faunas are the apparently endemic vesicomyid genus Hubertschenckia and the scarcity of lucinid bivalves, whereas the bivalves Conchocele bisecta and Bathymodiolus inouei, and the predatory gastropod Cryptonatica appear to be widespread in North Pacific seep communities. Although virtually all Cenozoic seep faunas consist of members of the same mollusk families that inhabit these ecosystems today (see also Vrijenhoek, 2013), the Paleogene seep communities in Japan differ markedly from those of early Neogene age in generic composition and abundance of the chemosymbiotic bivalve families Vesicomyidae and Lucinidae, and the subfamily Bathymodiolinae. The same applies when Paleogene seep communities in Japan are compared to Paleogene seep communities in other parts of the world. Acknowledgements We thank J. L. Goedert and an anonymous reviewer for their constructive criticism. Part of this study was financially supported by a Grant-in-Aid for Science Research of the Japan Society for the Promotion of Science (C, 23540546, 2011–2013) to KA, Grant-in-Aid for JSPS Fellows and Grant for Program to Disseminate Tenure Tracking System (JST) to RGJ, and by the Deutsche Forschungsgemeinschaft through grant Ki802/6-1 and the Joetsu University of Education through a visiting scientist grant to SK. References Amano, K., Ando, H., 2011. Giant fossil Acharax (Bivalvia: Solemyidae) from the Miocene of Japan. Nautilus 125, 207–212. Amano, K., Jenkins, R.G., 2007. Eocene drill holes in cold-seep bivalves of Hokkaido, northern Japan. Marine Ecology 28, 108–114. Amano, K., Jenkins, R.G., 2011a. New fossil Bathymodiolus (sensu lato) (Bivalvia: Mytilidae) from Oligocene seep-carbonates in eastern Hokkaido, Japan, with remarks on the evolution of the genus. Nautilus 125, 29–35. Amano, K., Jenkins, R.G., 2011b. Fossil records of extant vesicomyid species from Japan. Venus (Japan Journal of Malacology) 69, 163–176. Amano, K., Kiel, S., 2007. Fossil vesicomyid bivalves from the North Pacific region. Veliger 49, 270–293. Amano, K., Kiel, S., 2010. Taxonomy and distribution of Archivesica (Vesicomyidae, Bivalvia) in Japan. Nautilus 124, 155–165. Amano, K., Kiel, S., 2011. Fossil Adulomya (Vesicomyidae, Bivalvia) from Japan. Veliger 51, 76–90. Amano, K., Kiel, S., 2012. Two Neogene vesicomyid species (Bivalvia) from Japan and their biogeographic implications. Nautilus 126, 79–85. Amano, K., Little, C.T.S., 2005. Miocene whale-fall community from Hokkaido, northern Japan. Palaeogeography Palaeoclimatology Palaeoecology 215, 345–356. Amano, K., Jenkins, R.G., Hikida, Y., 2007a. A new gigantic Nucinella (Bivalvia: Solemyoida) from the Cretaceous cold-seep deposit in Hokkaido, northern Japan. Veliger 49, 84–90. Amano, K., Little, C.T.S., Inoue, K., 2007b. A new Miocene fossil whale-fall community from Japan. Palaeogeography Palaeoclimatology Palaeoecology 247, 236–242. Audzijonyte, A., Krylova, E.M., Sahling, H., Vrijenhoek, R.C., 2012. Molecular taxonomy reveals broad trans-oceanic distributions and high species diversity of deep-sea clams (Bivalvia: Vesicomyidae: Pliocardiinae) in chemosynthetic environments. Systematic Biology 10, 403–415. Bachraty, C., Legendre, P., Desbruyères, D., 2009. Biogeographic relationships among deep-sea hydrothermal vent faunas at global scale. Deep-Sea Research Part I 56, 1371–1378. Baker, M.C., Ramirez-Llodra, E., Tyler, P.A., German, C.R., Boetius, A., Cordes, E.E., Dubilier, N., Fisher, C.R., Levin, L.A., Metaxas, A., Rowden, A.A., Santos, R.S., Shank, T.M., Van Dover, C.L., Young, C.M., Warén, A., 2010. Biogeography, ecology, and vulnerability of chemosynthetic ecosystems in the deep sea. In: McIntyre, A. (Ed.), Life in the World's Oceans: Diversity, Distribution, and Abundance. Wiley-Blackwell, pp. 161–182.
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