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Taxonomy of Middle Eocene Diatom Resting Spores and Their Allied Taxa from the Central Arctic Basin

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Taxonomy of Middle Eocene Diatom Resting Spores and Their Allied Taxa from the Central Arctic Basin Taxonomy of middle Eocene diatom resting spores and their allied taxa from the central Arctic Basin Itsuki Suto,1 Richard W. Jordan2 and Mahito Watanabe3 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa, Nagoya 464-8601, Japan 2Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, Kojirakawa-machi 1-4-12, Yamagata 990-8560, Japan 3Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan email: [email protected] email: [email protected] email: [email protected] ABSTRACT: In the late summer of 2004, Integrated Ocean Drilling Program (IODP) Expedition 302, also called the Arctic Coring Ex- pedition (ACEX), successfully drilled the first deep boreholes on the Lomonosov Ridge in the central Arctic Ocean. The well preserved fossil diatoms used here are from biosiliceous Unit 2 in Holes 2A and 4A of middle Eocene age. In the lower part of Unit 2, resting spores occurred abundantly with other fossil diatoms. 25 diatom resting spore taxa and five allied vegetative cell taxa are described in this study of ACEX samples. Moreover 11 diatom taxa which did not occur in these sediments are also described for comparison with the Eocene Arctic resting spores. Their biostratigraphic ranges are also indicated. 10 of the resting spore species which occur in the ACEX samples had already appeared during the late Cretaceous while the rest of them appeared in Eocene. 21 of 25 (84%) resting spore taxa became ex- tinct during the middle Eocene to early Oligocene. Most resting spore taxa described in this study do not belong to Chaetoceros resting spores because they lack a single ring of puncta on the hypovalve mantle that characterizes the resting spores of Chaetoceros and became extinct before Oligocene, therefore it is clear that Chaetoceros did not flourish in the middle Eocene in the Arctic Ocean. Other diatom genera that produced resting spores such as Pterotheca and Pseudopyxilla, might have prospered before the Eocene/Oligocene bound- ary, although their vegetative cells are unknown so far. Since some Chaetoceros resting spore taxa are reported in this study, most coastal regions experienced regular seasonal environmental change, which benefitted genera such as Pterotheca, Pseudopyxilla and Odontotropis, but also there might have been some patchy coastal upwelling regions with nutrient depletion and sporadic supplies where Chaetoceros may have survived. The abundant dinoflagellate cysts preserved in middle Eocene ACEX cores provide evidence of stable conditions before the Eocene/Oligocene boundary. The resting spore ecology of most resting spore taxa before the Eocene may have been similar to that of dinoflagellate cysts rather than that of Chaetoceros resting spores after the Oligocene. INTRODUCTION ate and boreal waters, but are also found in polar and tropical re- gions (e.g. Schrader 1978, Leventer 1991), especially in Fossil diatoms have been reported in many oceanic sediment upwelling areas (Hargraves 1984) and reported from ancient cores, especially those of the DSDP (Deep-Sea Drilling Project) sediments, extending back to the Cretaceous (Hanna 1927b, and ODP (Ocean Drilling Program), as biostratigraphic mark- Ross and Sims 1974). Fossil diatom resting spores have been ers in various geological epochs, particularly Miocene (e.g. used as paleoclimatic, especially upwelling, indicators. Suto Yanagisawa and Akiba 1998). Although extensive studies of (2006a) proposed that the increase in diversity and abundance fossil Arctic diatoms in diatomites (e.g. Strelnikova 1974, of Chaetoceros resting spores from the late Eocene to early Barron 1985, Medlin and Priddle 1990, Tapia and Harwood Oligocene in the Norwegian Sea indicated a change from a sta- 2002) have been reported, these studies only documented the ble environment with regular seasonal supply of nutrients to an late Cretaceous or Holocene and Pleistocene diatoms, and since unstable one with depletion and sporadic supply. He also men- then there have been few papers on Eocene Arctic diatoms. tioned that Chaetoceros might have established itself as the Moreover, the taxonomy of fossil diatom resting spores has main primary producer in the Oligocene Norwegian Sea, re- been neglected. placing dinoflagellates and/or nannoplankton which had been the main producers till the late Eocene because their diversities Some coastal planktonic diatoms survive unfavorable environ- decreased across the boundary (Falkowski et al. 2004). mental conditions as resting spores. The model of Gran (1912) proposed that resting spores were benthic resting stages, and subsequent studies showed that they are formed in response to In the late summer of 2004, Integrated Ocean Drilling Program nutrient depletion, darkness and low temperature (e.g. Kuwata (IODP) Expedition 302, also known as the Arctic Coring Expe- et al. 1993, Oku and Kamatani 1995, 1997, 1999, McQuoid and dition (ACEX), successfully drilled the first deep boreholes in Hobson 1996). Resting spores having thick silicified valves and the central Arctic Ocean, penetrating a ~430m-thick package of lacking areolae are preserved frequently as fossils in nearshore sediment on the Lomonosov Ridge (Backman et al. 2005a, b, sediments. The occurrences of fossil diatom resting spores are Moran et al. 2006) (Text-figure 1). The well preserved Eocene concentrated in coastal waters and are most common in temper- fossil diatoms used here are from biosiliceous Unit 2 in Holes micropaleontology, vol. 55, nos. 2-3, pp. 259-312, text-figures 1-9, plates 1-13, 2009 259 Itsuki Suto et al.: Taxonomy of middle Eocene diatom resting spores and their allied taxa from the central Arctic Basin cies is separated from Peripteropsis norwegica Suto (2005b) by Pterotheca cf. aculeifera Grunow sensu HAJÓS and STRADNER 1975, lacking branched thin and wide processes. p. 933, pl. 28, figs. 1, 2 nec pl. 12, fig. 6. Pterotheca carinifera Grunow in VAN HEURCK sensu MCCOLLUM Stratigraphic and geographic distributions: 1975, p. 535, pl. 10, fig. 4 nec pl. 16, figs. 6, 7. This species oc- Pterotheca danica (Grunow) FORTI 1909, p. 13. – GOMBOS 1983, p. curred in middle Eocene sediments from IODP Leg 302 Sites 570, pl. 3, fig. 9. – GOMBOS and CIESIELSKI 1983, p. 603, pl. 13, 2A and 4A in the central Arctic Ocean. figs. 1-3, 9. – BARRON et al. 1984, p. 156, pl. 8, fig. 10. – BALDAUF 1985, p. 464, pl. 12, figs. 8, 9. – HARWOOD 1988, p. 86, fig. 18.12. – Remarks: This species does not appear to belong to the fossil DESIKACHARY and SREELATHA 1989, p. 218, pl. 100, figs. 1, 2, resting spore morpho-genus Peripteropsis of extant 5. Chaetoceros because of the absence of a ring of puncta on the Pterotheca major JOUSÉ 1955, p. 101, text-fig. 1; pl. 6, fig. 2. – GOMBOS 1983, p. 570. – GOMBOS and CIESIELSKI 1983, p. 603, hypovalve margin. pl. 13, figs. 6-8. – HARWOOD 1988, p. 86, fig. 18.16. Pterotheca spada TEMPÈRE et BRUN sensu GOMBOS and Porotheca danica (Grunow) Fenner 1994 CIESIELSKI 1983, p. 603, pl. 13, figs. 4, 5. Plate 7, figures 1-28 Pterotheca (Grunow) FORTI sensu HARGRAVES 1984, p. 71, figs. 14-16. Porotheca danica (Grunow) FENNER 1994, p. 114, pl. 4, figs. 16, 17; Pterotheca carinifera (Grunow in Van Heurck) FORTI sensu HAR- pl. 15, figs. 1-6. WOOD 1988, p. 86, fig. 18.6. Basionym: Stephanogonia (Pterotheca?) danica GRUNOW in VAN Stephanogonia novazelandica Grunow sensu DESIKACHARY and HEURCK 1880-1885, pl. 83 bis., figs. 7, 8. SREELATHA 1989, p. 228, pl. 100, figs. 3, 4. Pyxilla? carinifera Grunow sensu HOMANN 1991, p. 139, pl. 55, fig. 6 References: Stephanogonia danica GRUNOW 1866, p. 146. – CLEVE- nec figs. 1-5, 8. EULER 1951, Handl. 2: 1, p. 110, figs. 232a, b. – HOMANN 1991, p. Pterotheca carinifera Grunow sensu HARWOOD and BOHATY 2000, 141, pl. 55, figs. 7, 9-16. p. 93, pl. 3, fig. t; pl. 9, fig. o. Synonymy: Pyxilla carinifera var. russica PANTOCSEK 1905, Bd. 3, pl. 35, fig. 491; Bd. 3, pl. 29, fig. 423. Pterotheca danica GRUNOW, HANNA 1927a, p. 119, pl. 20, fig. 11. – Emended description: Epivalve convex, cylindrical with a high PROSCHKINA-LAVRENKO 1949, p. 203, pl. 75, fig. 9. – HAJÓS mantle, diameter 13-45µm, transapical axis 30-65µm. The cen- 1976, p. 829, pl. 16, figs. 12-15. – GOMBOS 1977, p. 596, pl. 23, fig. tral part of epivalve face protracted forming a hollow tube with 5. – LEE 1993, p. 42, pl. 3, fig. 4. – DELL’AGNESE and CLARK a flat top. Epivalve surface generally structured by seven to 1994, fig. 9.11. TEXT-FIGURE 8 Geographic and stratigraphic distribution of Trochosira spinosa Kitton. 1-25. Trochosira spinosa 15 Fur Formation, Denmark (Fenner 1994); 1-17. Reported as Trochosira spinosa. 16 ODP Hole 908A (Scherer and Koç1996); 1 Mors, Denmark (Kitton 1871); 17 DSDP Site 338 (This study). 2 Mors Formation, Denmark (Van Heurck 1880-1885); 18, 19. Reported as Trochosira spinosus. 3 Lower course of the Anadyr River, Russia (Sheshu- 18 Jutland, Denmark (Sims 1988); kova-Poretskaya 1967); 19 Cape Roberts Project, Antarctica (Scherer et al. 4 DSDP Site 173 (Schrader 1973a); 2000). 5 west Kazakhstan (Glezer et al. 1974); 20. Reported as Trochosira spinosa? 20 McMurdo Sound, Antarctica (Harwood and Bohaty 6 DSDP Site 337 (Schrader and Fenner 1976); 2000). 7 DSDP Site 338 (Schrader and Fenner 1976); 21, 22. Reported as Trochosira ornata. 21 Jutland, Denmark (Van Heurck 1880-1885); 8 DSDP Site 339 (Schrader and Fenner 1976) 22 Fur Formation, Denmark (Fenner 1994). 9 DSDP Site 343 (Schrader and Fenner 1976); 23. Reported as Sceletonema ornatum. 10 DSDP Site 338 (Dzinoridze et al. 1978); 23 eastern slopes of Ural Mountains, USSR (Jousé 11 DSDP Site 339 (Dzinoridze et al. 1978); 1955).
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