Hostname: page-component-848d4c4894-m9kch Total loading time: 0 Render date: 2024-05-20T12:47:48.329Z Has data issue: false hasContentIssue false
  • English
  • Français

Paleobiology of a Neoproterozoic tidal flat/lagoonal complex: the Draken Conglomerate Formation, Spitsbergen

Published online by Cambridge University Press:  20 May 2016

A. H. Knoll ,
K. Swett  and
J. Mark
A. H. Knoll
Affiliation:
Botanical Museum, Harvard University, Cambridge, Massachusetts 02138
K. Swett
Affiliation:
Department of Geology, University of Iowa, Iowa City 52242
J. Mark
Affiliation:
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138
Rights & Permissions [Opens in a new window]

Abstract

Core share and HTML view are not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Carbonates and rare shales of the ca 700–800 Ma old Draken Conglomerate Formation, northeastern Spitsbergen, preserve a record of environmental variation within a Neoproterozoic tidal flat/lagoon complex. Forty-two microfossil taxa have been recognized in Draken rocks, and of these, 39 can be characterized in terms of their paleoenvironmental distributions along a gradient from the supratidal zone to permanently submerged lagoons. Supratidal to subtidal trends include: increasing microbenthic diversity, increasing abundance and diversity of included allochthonous (presumably planktonic) elements, decreasing sheath thickness of mat-building organisms (with significant taphonomic consequences), and an increasing sediment/fossil ratio in fossiliferous rocks. Five principal and several minor biofacies can be distinguished. The paleoecological resolution obtainable in the Draken Conglomerate Formation rivals that achieved for most Phanerozoic fossil deposits. It documents the complexity and diversity of Proterozoic coastal ecosystems and indicates that both environment and taphonomy need to be taken into explicit consideration in attempts to understand evolutionary trends in the early fossil record. Three species, Coniunctiophycus majorinum, Myxococcoides distola, and M. chlorelloidea, are described as new; Siphonophycus robustum, Siphonophycus septatum, and Gorgonisphaeridium maximum are proposed as new combinations.

Type
Research Article
Information
Journal of Paleontology , Volume 65 , Issue 4 , July 1991 , pp. 531 - 570
Copyright
Copyright © The Paleontological Society 

References

Aitken, J. D. 1989. Giant “algal” reefs, Middle/Upper Proterozoic Little Dal Group (> 770, < 1200 Ma), Mackenzie Mountains, N.W.T., Canada, p. 13–23. In Geldsetzer, H. H. J., James, N. P., and Tebbutt, G. E. (eds.), Reefs: Canada and Adjacent Areas. Canadian Society of Petroleum Geologists Memoir 13.Google Scholar
Aizenshtat, Z., Lipiner, G., and Cohen, Y. 1984. Biogeochemistry of carbon and sulfur in the microbial mats of the Solar Lake (Sinai), p. 281–312. In Cohen, Y., Castenholz, R. W., and Halvorson, H. O. (eds.), Microbial Mats: Stromatolites. Alan R. Liss, New York.Google Scholar
Allison, C. W., and Awramik, S. M. 1989. Organic-walled microfossils from earliest Cambrian or latest Proterozoic Tindir Group rocks, northwest Canada. Precambrian Research, 43:253–294.CrossRefGoogle Scholar
Atkinson, A. W., Gunning, B. E. S., and John, P. C. L. 1972. Sporopollenin in the cell wall of Chlorella and other algae: ultrastructure, chemistry, and incorporation of 14C-acetate. Planta, 107:1–32.Google Scholar
Barghoorn, E. S., and Tyler, S. A. 1965. Microorganisms from the Gunflint chert. Science, 147:563–577.CrossRefGoogle ScholarPubMed
Bauld, J. 1981. Geobiological role of cyanobacterial mats in sedimentary environments: production and preservation of organic matter. BMR Journal of Australian Geology and Geophysics, 6:307–317.Google Scholar
Bauld, J. 1986. Benthic microbial communities of Australian saline lakes, p. 95–111. In de Deckker, P. and Williams, W. D. (eds.), Limnology in Australia. Dr. J. Junk, The Hague.CrossRefGoogle Scholar
Behrensmeyer, A. K., and Kidwell, S. M. 1985. Taphonomy's contributions to paleobiology. Paleobiology, 11:105–119.Google Scholar
Black, M. 1933. The algal sediments of Andros Island, Bahamas. Philosophical Transactions, Royal Society, London, B222:165–192.Google Scholar
Butterfield, N. J. 1990. Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale. Paleobiology, 16:272–286.Google Scholar
Butterfield, N. J., and Knoll, A. H. 1989. Large, complex acritarchs in Upper Proterozoic rocks. Abstracts, Geological Association of Canada–Mineralogical Association of Canada, Annual Meeting, Montreal, p. A100.Google Scholar
Butterfield, N. J., and Swett, K. 1988. Exceptional preservation of fossils in Upper Proterozoic shale. Nature, 334:424–427.Google Scholar
Derry, L. A., Keto, L. S., Jacobsen, S. B., Knoll, A. H., and Swett, K. 1989. Sr isotopic variations in Upper Proterozoic carbonates from Svalbard and East Greenland. Geochimica et Cosmochimica Acta, 53:2331–2339.Google Scholar
Dill, R. F., Shinn, E. A., Jones, A. T., Kelly, K., and Steinen, R. P. 1986. Giant subtidal stromatolites forming in normal salinity waters. Nature, 324:55–58.Google Scholar
Downie, C., and Sarjeant, W. A. S. 1963. On the interpretation and status of some Hystrichosphere genera. Palaeontology, 6:83–96.Google Scholar
Eisenack, A. 1958. Microfossilien aus dem Ordovizium des Baltikums. 1. Markasitschicht, Dictyonema-Schiefer, Glaukonitsand, Glaukonitkalk. Senckenbergiana Lethaea, 39:389–404.Google Scholar
Ercegovic, A. 1932. Etudes écologiques et sociologiques des Cyanophycées lithophytes de la côte Yougoslave de l'Adriatique. Bulletin Internationale, Academie Yougoslave Sciences, 26:33–56.Google Scholar
Fairchild, I. J., Knoll, A. H., and Swett, K.In press. A coastal lithofacies-biofacies continuum with syndepositional dolomitization and silicification (Draken Conglomerate Formation, Upper Riphean, Spitsbergen). Precambrian Research.Google Scholar
Fairchild, T. 1975. The geologic setting and paleobiology of a Late Precambrian stromatolitic microflora from South Australia. Unpubl. Ph.D. dissertation, University of California, Los Angeles, 272 p.Google Scholar
Francis, S., Margulis, L., and Barghoorn, E. S. 1978. On the experimental silicification of microorganisms. II. On the time of appearance of eukaryotic organisms in the fossil record. Precambrian Research, 6:65–100.Google Scholar
Geitler, L. 1925. Cyanophyceae. A. Pascher's Die Süsswasserflora Deutschlands, Osterreichs, und der Schweiz, 12:1–450.Google Scholar
Geitler, L. 1930–1932. Cyanophyceae. Band 14, Dr. L. Rabenhorst's Kryptogamen-Flora. Leipzig, 1197 p.Google Scholar
German, T. 1974. Nakhodi massevikh skoplyennii trikhomov v Rifeyi. Microphytofossilii Proterozoya i Rannyogo Paleozoya SSSR, p. 6–10. [The discovery of massive accumulations of trichomes in the Riphean. Microphytofossils of the Proterozoic and Lower Paleozoic, USSR, p. 6–10.]Google Scholar
German, T. 1981. Nitchatye mikroorganizmy lakhandinskoj svity reki Mai [Filamentous microorganisms from the Lakhanda Suite along the River Maya]. Paleontologicheskiy Zhurnal, 1981:126–131.Google Scholar
Golubic, S. 1976. Organisms that build stromatolites, p. 113–126. In Walter, M. R. (ed.), Stromatolites. Elsevier, Amsterdam.CrossRefGoogle Scholar
Golubic, S., and Barghoorn, E. S. 1977. Interpretation of microfossils with special reference to the Precambrian, p. 1–14. In Flügel, E. (ed.), Fossil Algae. Springer Verlag, Heidelberg.Google Scholar
Golubic, S., and Hofmann, H. J. 1976. Comparison of Holocene and mid-Precambrian Entophysalidaceae (Cyanophyta) in stromatolitic algal mats: cells division and degradation. Journal of Paleontology, 50:1074–1082.Google Scholar
Green, J. W., Knoll, A. H., Golubic, S., and Swett, K. 1987. Paleobiology of distinctive benthic microfossils from the Upper Proterozoic Limestone–Dolomite ‘Series’, central East Greenland. American Journal of Botany, 62:835–852.Google Scholar
Green, J. W., and Swett, K. 1988. Microfossils in oolites and pisolites from the Upper Proterozoic Eleonore Bay Group, central East Greenland. Journal of Paleontology, 62:835–852.CrossRefGoogle ScholarPubMed
Green, J. W., and Swett, K. 1989. Microfossils from silicified stromatolitic carbonates of the Upper Proterozoic Limestone-Dolomite ‘Series’, central East Greenland. Geological Magazine, 126:567–585.Google Scholar
Hambrey, M. J. 1982. Late Precambrian diamictites of northeastern Svalbard. Geological Magazine, 119:527–551.Google Scholar
Hardie, L. A. (ed.). 1977. Sedimentation on the Modern Carbonate Tidal Flats of Northwest Andros Island, Bahamas. The Johns Hopkins University Studies in Geology 22, 202 p.Google Scholar
Harland, W. B., and Wright, N. J. R. 1979. Alternative hypothesis for the pre-Carboniferous evolution of Svalbard. Norsk Polar Institutt Skrifter, 167:89–117.Google Scholar
Hofmann, H. J. 1976. Precambrian microflora, Belcher Islands, Canada: significance and systematics. Journal of Paleontology, 50:1040–1073.Google Scholar
Horodyski, R. J., and Donaldson, J. A. 1980. Microfossils from the Middle Proterozoic Dismal Lakes Group, arctic Canada. Precambrian Research, 11:125–159.Google Scholar
Horodyski, R. J., and vonder Haar, S. 1975. Recent calcareous stromatolites from Laguna Mormona (Baja California) Mexico. Journal of Sedimentary Petrology, 45:894–906.Google Scholar
Jankauskas, T. V. 1979. Nizhnyerifeyiskii mikrobioti Yzhnogo Urala [Lower Riphean microfossils of the Southern Urals]. Doklady, Akademii Nauk SSSR, 247:1465–1467.Google Scholar
Jankauskas, T. V. 1982. Mikrofossilii Rifeya Yzhnogo Urala [Riphean microfossils from the Southern Urals], p. 84–120. In Keller, B. M. (ed.), Stratotyp Rifeya: Paleontologiya ee Paleomagnetizm [Stratotype of the Riphean: Paleontology and Paleomagnetics]. Trudy Geologicheskii Institut, SSSR Akademyii Nauk, Moscow.Google Scholar
Jankauskas, T. V. 1989. Mikrofossilii Dokembriya SSSR [Precambrian Microfossils of the USSR]. Trudy Institut Geologii ee Geokhronologii SSSR Akademii Nauk, Leningrad, 188 p.Google Scholar
Johnson, J. L. 1961. Limestone-building Algae and Algal Limestones. Colorado School of Mines, Golden, 297 p.Google Scholar
Knoll, A. H. 1981. Paleoecology of late Precambrian microbial assemblages, p. 17–54. In Niklas, N. J. (ed.), Paleobotany, Paleoecology, and Evolution, Volume 1. New York, Praeger.Google Scholar
Knoll, A. H. 1982. Microorganisms from the late Precambrian Draken Conglomerate Formation, Ny Friesland, Spitsbergen. Journal of Paleontology, 56:755–790.Google Scholar
Knoll, A. H. 1984. Microbiotas of the late Precambrian Hunnberg Formation, Nordaustlandet, Svalbard. Journal of Paleontology, 58:131–162.Google Scholar
Knoll, A. H., and Bauld, J. 1989. The evolution of ecological tolerance in prokaryotes. Transactions, Royal Society of Edinburgh, Earth Sciences, 80:209–223.Google Scholar
Knoll, A. H., and Butterfield, N. J. 1989. New window on Proterozoic life. Nature, 337:602–603.Google Scholar
Knoll, A. H., and Calder, S. 1983. Microbiotas of the late Proterozoic Ryssö Formation, Nordaustlandet, Svalbard. Palaeontology, 26:467–496.Google Scholar
Knoll, A. H., and Golubic, S. 1979. Anatomy and taphonomy of a Precambrian algal stromatolite. Precambrian Research, 10:110–151.CrossRefGoogle Scholar
Knoll, A. H., Hayes, J. M., Kaufman, J., Swett, K., and Lambert, I. 1986. Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland. Nature, 321:832–838.Google Scholar
Knoll, A. H., and Swett, K. 1985. Micropaleontology of the Late Proterozoic Veteranen Group, Spitsbergen. Palaeontology, 28:451–473.Google Scholar
Knoll, A. H., and Swett, K. 1990. Carbonate deposition during the Late Proterozoic Era: an example from Spitsbergen. American Journal of Science, 290A:104–132.Google Scholar
Knoll, A. H., and Burkhardt, E. 1989. Paleoenvironmental distribution of microfossils and stromatolites in the Upper Proterozoic Back-lundtoppen Formation, Spitsbergen. Journal of Paleontology, 63:129–145.Google Scholar
Komar, V. 1979. Klassifikatsia stromatolitov po mikrostrukturam [The classification of stromatolites by microstructure], p. 42–45. In Sokolov, B. V. (ed.), Paleontologia Dokembriya i Rannyevo Kembriya [Paleontology of the Precambrian and Lower Cambrian]. Trudy, Institut Geologii i Geokhronologii Dokembriya, Akademii Nauk SSSR, Leningrad.Google Scholar
Komarek, J., and Anagnostidis, K. 1986. Modern approach to the classification system of cyanophytes. Archiv für Hydrobiologie, Supplement to 73(2):157–226.Google Scholar
Licari, G. R. 1978. Biogeology of the late pre-Phanerozoic Beck Spring Dolomite of eastern California. Journal of Paleontology, 52:767–792.Google Scholar
Lo, S.-C. C. 1980. Microbial fossils from the lower Yudoma Suite, earliest Phanerozoic, eastern Siberia. Precambrian Research, 13:109–166.CrossRefGoogle Scholar
Maliva, R. G., Knoll, A. H., and Siever, R. 1989. Secular change in chert distribution: a reflection of evolving biological participation in the silica cycle. Palaios, 4:519–532.CrossRefGoogle ScholarPubMed
Mendelson, C. V., and Schopf, J. W. 1982. Proterozoic microfossils from the Sukhaya Tunguska, Shorikha, and Yudoma Formations of the Siberian Platform, USSR. Journal of Paleontology, 56:42–83.Google Scholar
Mikhailova, N. S. 1986. Noviye nakhodki mikrophytofossilii iz otlozhyenii vekhogo rifeya Krasnoyarskogo kraya [New discoveries of microphytofossils from Upper Riphean sediments, Krasnoyarskog region]. Aktualniye voprosi sovryemennoi paleoalgologii [Topical Problems in Contemporary Paleoalgology], Kiev, p. 31–37.Google Scholar
Monty, C. L. V. 1967. Distribution and structure of Recent stromatolitic algal mats, eastern Andros Island, Bahamas. Annales de la Société Géologique de Belgique, 90:55–100.Google Scholar
Muir, M. D., and Sarjeant, W. A. S. 1971. Bibliographic commentée des Tasmanacées et de formes vivantes apparentés (Algues: Prasinophyceae), p. 51–117. In Jardine, S. (ed.), Microfossiles organiques du Paleozoique, 3, les Acritarches. Editions du CNRS, Paris.Google Scholar
Nägeli, C. 1849. Gattungen einzellinger Algen, physiologisch und systematisch bearbeitet. Neue Denkschriften der Allgemeinen schweizerischen Gesellschaft für die gesamten Naturwissenschaften, 8:44–60.Google Scholar
Nyberg, A. V., and Schopf, J. W. 1984. Microfossils in stromatolitic cherts from the Upper Proterozoic Min'yar Formation, southern Ural Mountains, USSR. Journal of Paleontology, 58:738–772.Google Scholar
Pia, J. 1927. Thallophyta, p. 31–136. In Hirmer, M. (ed.), Handbuch der Paläobotanik. Oldenbourg, Munich.Google Scholar
Pickett-Heaps, J. D. 1975. Green Algae: Structure, Reproduction, and Evolution in Selected Genera. Sinauer, Sunderland, Massachusetts, 606 p.Google Scholar
Pjatiletov, V. G. 1988. Mikrophytofossilii pozdnyeto dokembriya Uchuro-Maiskogo raiona [Later Precambrian microphytofossils from the Uchuro-Maya Uplift], p. 47–104. In Khomentovsky, V. V. (ed.), Pozdnii dokembrii i rannii paleozoi Sibiri [Later Precambrian and Early Paleozoic of Siberia: Riphean and Vendian]. Trudy, Sibirskoye Otdelyeniye Institut Geologii i Geofysiki, Akademii Nauk SSSR, Novosibirsk.Google Scholar
Raaben, M. Ye. 1969. Stromatoliti verkhego rifeya (Gymnosolenidi) [Upper Riphean Stromatolites (Gymnosolenida)]. Trudy, Institut Geologii, Akademii Nauk SSSR, Moscow, 100 p.Google Scholar
Riding, R. 1979. Origin and diagenesis of lacustrine algal bioherms at the margin of the Ries crater, Upper Miocene, southern Germany. Sedimentology, 26:645–680.Google Scholar
Riding, R. 1982. Cyanophyte calcification and changes in ocean chemistry. Nature, 299:814–815.Google Scholar
Rippka, R., Deuelles, J., Waterbury, J. B., Herdman, M., and Stanier, R. Y. 1979. Generic assignments, strain histories, and properties of pure cultures of cyanobacteria. Journal of General Microbiology, 111:1–61.Google Scholar
Round, F. E. 1981. The Ecology of Algae. Cambridge University Press, Cambridge, 653 p.Google Scholar
Schieber, J. 1989. Facies and origin of shales from the mid-Proterozoic Newland Formation, Belt Supergroup, Montana, USA. Sedimentology, 36:203–219.Google Scholar
Schopf, J. W. 1968. Microflora of the Bitter Springs Formation, Late Precambrian, Central Australia. Journal of Paleontology, 42:651–688.Google Scholar
Schopf, J. W., and Blacic, J. M. 1971. New microorganisms from the Bitter Springs Formation (Late Precambrian) of the north-central Amadeus Basin, central Australia. Journal of Paleontology, 45:925–960.Google Scholar
Sergeev, V. N. 1984. Mikrofossilii v okryemnistikh stolochatikh stromatolitakh verkhego rifeya Turukhanskogo raiona [Microfossils in Upper Riphean silicified columnar stromatolites from the Turukhansk area]. Doklady Akademii Nauk SSSR, 278:436–439.Google Scholar
Sergeev, V. N. 1988. Okryemnenniye mikrofossili v stratotipe sryednego rifeya na yuzhom Urale [Silicified microfossils from the stratotype of the Middle Riphean, southern Urals]. Doklady Akademii Nauk SSSR, 303:708–710.Google Scholar
Sergeev, V. N., and Krylov, I. N. 1986. Mikrofossilii minyarskoi sviti Urala [Microfossils of the Min'yar Suite, Urals]. Paleontologichesky Zhurnal, 1986:84–95.Google Scholar
Southgate, P. N. 1986. Despositional environment and mechanism of preservation of microfossils, upper Proterozoic Bitter Springs Formation, Australia. Geology, 14:683–686.Google Scholar
Stanier, R. Y., Sistrom, W. R., Hansen, T. A., Whitton, B. A., Castenholz, R. W., Pfennig, N., Gorlenko, V. N., Kondratieva, E. N., Eimhjellen, K. E., Whittenbury, R., Gherna, R. L., and Trüper, H. G. 1978. Proposal to place nomenclature of the Cyanobacteria (blue-green algae) under the rules of the International Code of Nomenclature of Bacteria. International Journal of Systematic Bacteriology, 28:335–336.Google Scholar
Staplin, F. L., Jansonius, J., and Pocock, S. A. 1965. Evaluation of some acritarchous hystrichosphere genera. Neues Jahrbuch für Geologie und Paläontologie, Abhandlung 123:167–201.Google Scholar
Strother, P. K., Knoll, A. H., and Barghoorn, E. S. 1983. Microorganisms from the late Precambrian Narssârssuk Formation, northwestern Greenland. Palaeontology, 26:1–32.Google Scholar
Swett, K., and Knoll, A. H. 1985. Stromatolitic bioherms and microphytolites from the Late Proterozoic Draken Conglomerate Formation, Spitsbergen. Precambrian Research, 28:327–347.Google Scholar
Tappan, H. 1980. Paleobiology of Plant Protists. Wiley, San Francisco, 1028 p.Google Scholar
Thuret, G. 1875. Essai de classification des Nostochinées. Annales Sciences Naturelle (Botanie), 6:372–382.Google Scholar
Timofeev, B. V. 1959. Drevejshava flora Pribaltiki i ee stratigrafi-cheskoeznachenie [Ancient flora of the Baltic area and its stratigraphic significance]. Trudy, Vsesoyuznogo Neftyanogo Nauchno-Issledo-vatel'skogo Geologorazvedochnogo Instituta, Leningrad, 129:1–320.Google Scholar
Timofeev, B. V. 1966. Mikropaleofitologichneskoe issledovanie drevnikh svit [Microphytological Investigations of Ancient Formations]. Nauka, Moscow, 145 p.Google Scholar
Timofeev, B. V., German, T. N., and Mikhailova, N. S. 1976. Mikrofitofossilii dokembiya, kembriya, i ordovika [Microfossils from the Precambrian, Cambrian, and Ordovician]. Trudy, Institut Geologii i Geochronologii Dokembri, Leningrad, 106 p.Google Scholar
Tyler, S. A., and Barghoorn, E. S. 1954. Occurrence of structurally preserved plants in Precambrian rocks of the Canadian Shield. Science, 119:606–608.Google Scholar
Vidal, G. 1976. Late Precambrian microfossils from the Visingsö beds in southern Sweden. Fossils and Strata, 9:1–57.Google Scholar
Vidal, G., and Knoll, A. H. 1983. Proterozoic plankton. Geological Society of America, Memoir 161:265–277.CrossRefGoogle Scholar
Vidal, G., and Nystuen, J. P. 1990. Stratigraphy and micropaleontology of the late Precambrian Hedmark Group, Norway. American Journal of Science, 290A:170–211.Google Scholar
Waterbury, J. B., and Stanier, R. Y. 1978. Patterns of growth and development in pleurocapsalean cyanobacteria. Microbiological Reviews, 42:2–44.Google Scholar
Wettstein, R. 1924. Handbuch der Systematischen Botanik, Band 1. Franz Deuticke, Leipzig, 1017 p.Google Scholar
Whitton, B. A., and Potts, M. 1982. Marine littoral, p. 515–542. In Carr, N. G. and Whitton, B. A. (eds.), The Biology of Cyanobacteria. Blackwell, Oxford.Google Scholar
Wilson, C. B. 1958. The Lower Middle Hecla Hoek rocks of Ny Friesland, Spitsbergen. Geological Magazine, 94:305–327.Google Scholar
Wilson, C. B. 1961. The Upper Middle Hecla Hoek rocks of Ny Friesland, Spitsbergen. Geological Magazine, 98:89–116.Google Scholar
Woese, C., and Fox, G. 1977. Phylogenetic structure of the prokaryotic domain. Proceedings, National Academy of Science, U.S.A., 74:5088–5090.CrossRefGoogle ScholarPubMed
Wray, J. L. 1977. Calcareous Algae. Elsevier, Amsterdam, 185 p.Google Scholar
Yin, L. 1987. Microbiotas of latest Precambrian sequences in China. Stratigraphy and Paleontology of Systemic Boundaries in China, Precambrian–Cambrian Boundary, 1:415–494.Google Scholar
Zang, W. 1989. Late Proterozoic–Early Cambrian microfossils and biostratigraphy in China and Australia. Unpubl. Ph.D. thesis, Australian National University, Canberra, Australia, 432 p.Google Scholar
Zang, W., and Walter, M. R. 1989. Latest Proterozoic plankton from the Amadeus Basin in central Australia. Nature, 337:642–645.Google Scholar
Zhang, Y. 1981. Proterozoic stromatolite microfloras of the Gaoyuzhuang Formation (Early Sinian: Riphean), Hebei, China. Journal of Paleontology, 55:485–506.Google Scholar