When Did Life Begin? It is Older than 3.8 Ga: Evidence from Greenland

Transcription

1 地学雑誌 (Chigaku Zasshi) Journal of Geography 120(5) The 100s: Significant Exposures of the World No. 3 When Did Life Begin? It is Older than 3.8 Ga: Evidence from Greenland Yuichiro UENO Abstract The oldest record of life is a carbonaceous material preserved in Ga metamorphosed sedimentary rocks in western Greenland. The carbonaceous material is now graphitized due to extensive metamorphism, although it exhibits a low 13 C/ 12 C ratio. The 13 C-depleted isotopic composition is comparable to organics produced through biological carbon fixation, so it could support the biological origins of the graphite. This interpretation has been debated for 15 years. Here, the ongoing controversy is briefly reviewed. In summary, geologists have good reason to believe that life emerged on Earth at least in 3.8 billion years ago. Key words:graphite, carbon isotopes, Akilia island, Isua Greenstone Belt I Introduction The oldest fossil records of life date back to ca. 3.5 Ga microfossils from Pilbara craton, Western Australia (Ueno et al., 2001, 2006). The age is 1.1 Ga younger than the birth date of the Earth. We do not yet know when life first appeared on this planet. Older, but more metamorphosed rocks, occur in the Acasta gneiss complex, which dates back to 3.96 Ga (Bowring et al., 1989)or even older to 4.2 Ga (Iizuka et al., 2007). However, none of these rocks include the signature of life because the protoliths for their gneisses are derived from igneous rocks of TTG and related rocks. The Isua supracrustal belt (ISB hereafter)in western Greenland (Fig. 1)includes the oldest known sedimentary rocks ( Ga), which contain graphite of a possible biological origin. Fig. 1 Locality index of the Isua Belt, Greenland. For this reason, a number of researchers have investigated the origins of graphites in the ISB since the pioneering work of Schidlowski et al. (1979). The problem that makes matters difficult is the high-grade regional metamorphism in the ISB belt. Moreover, the geology is not yet detailed enough to identify the tectonic setting for the origins of the carbonaceous matter. A Japanese team GMT * Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, , Japan 877

2 Fig. 2 Locality map of the 3.8 Ga Isua metasediments (after Ueno et al., 2002). Metamorphic zonations from Zone A (greenschist facies)through B, C (amphibolite facies)to D (upper amphibolite facies)is Hayashi et al. (2000). Outcrop location of Fig. 3 is close to K244 sample locality of metasediments. visited the area to map it in great detail, introducing accretionary complex geology established in Japan prior to 1990 to identify the origins of rocks and graphite-bearing rocks (Maruyama, 1997). The results have been published in a series of papers, geology as a Pacific-type orogeny (Komiya et al., 1999), regional metamorphism (Hayashi et al., 2000), igneous petrology and source mantle composition and temperature in the Eoarchean (Komiya et al., 2002). The author has completed carbon isotope studies on the ISB using SIMS (Ueno et al., 2002). Since then, further studies have been completed on the ISB and western equivalent region (Akilia) on the origins of graphites (e.g., Mojzsis et al., 2003; Papineau et al., 2010b), I summarize the oldest chemical record of life in this region together with a representative outcrop. II Interpretation of Outcrop The graphite-bearing meta-sediments are exposed on the eastern side of the ISB (Fig. 2), as top-sitting meta-sedimentary rocks in an accretionary complex (Fig. 2). A general outline of the 878

3 accretionary complex is given in this issue by Maruyama and Komiya (2011). The location of the representative outcrop is the site of K244 at the center of the map, and it belongs to Zone D of the upper amphibolite facies of regional metamorphism (Ueno et al., 2002). Fig. 3 photo looks south of the accreted fragments of the Early Archean oceanic crust (right) overlain by white-colored bedded cherts (on which a man stands), which in turn are overlain by graphite-bearing meta-sediments (left), presumably derived from trench-turbidites in an intraoceanic environment (Komiya et al., 1999). Conglomerate layers of a few meters thick appear on top of the meta-sediments (Komiya et al., 1999). Fig. 4 photo shows an area further to the south of this zone with isoclinal folds (center)of carbonate-layers interlayered with graphite-bearing metasediments (cm- to a few tens of cm scale). The hill at the back is overlain with white-colored bedded cherts and meta-sediments. Graphites are derived from these meta-sediments resting on top of the accreted oceanic crust. III Akilia Controversy The oldest evidence of life on Earth comes from 13 C-depleted graphite preserved in the Isua-Akilia Eoarchean supracrustal sequence ( Ga)in western Greenland, 100 km west of the ISB, as an isolated enclave (Mojzsis et al., 1996). In the 3.9 Ga Akilia section, high-grade quartz-pyroxene rock hosts graphite with a 13 C-depleted isotopic composition as low as -44 (Mojzsis et al., 1996). The quartz-pyroxene rock may have originally been deposited as a banded iron formation (BIF)and subsequently suffered from upper amphibolite to granulite facies metamorphism (Mojzsis and Nutman, 1994; Nutman et al., 1997). Since the first discovery, many questions concerning the original interpretation have arisen. The sedimentary origin of the Akilia quartz- pyroxene rock was once questioned by trace element geochemistry, suggesting that the putative meta-bif is metasomatized komatiite and not sedimentary rock (Fedo and Whitehouse, 2002a, b; Whitehouse et al., 2009). However, a subsequent reanalysis revealed that all isotopic, trace element geochemical, and textual characters are not incompatible with the BIF-origin of the quartzpyroxene rock (Friend et al., 2002; Nutman et al., 2002; Mojzsis and Harrison, 2002a, b; Manning et al., 2003, 2006; Dauphas et al., 2004, 2007). In particular, the S-MIF signature of sulfides in the quartz-pyroxene rock (Mojzsis et al., 2003)clearly indicates the sedimentary origin of this graphitebearing BIF. Independent reanalysis by Lepland et al. (2005) and Nutman and Friend (2006)failed to recognize graphite within the Akilia BIF, casting doubt on the presence of graphite in this rock. However, these reports do not address the same hand specimen originally studied by Mojzsis et al. (1996). Recent studies of this sample by Papineau et al. (2010a, b)and McKeegan et al. (2007)clearly relocated the graphite within the Akilia BIF. A thorough petrographic study by Papineau et al. (2010a, b), however, showed that most of the graphites are attached to the surface of apatite grains and do not occur as inclusions within apatite crystals, suggesting the possibility that the graphite was deposited from hydrothermal fluid with co-occurring base-metal sulfides. This fluid deposited scenario does not discard biological origins, because carbon-bearing fluid is often derived from sedimentary organic matter during metamorphism of C- and S-bearing sedimentary rock. Papineau et al. (2010b)also revealed that the 13 C values of Akilia graphites range from -24 to -4, which are more 13 C-enriched than those reported originally by Mojzsis et al. (1996: -44 ~-28 ). The large 13 C-depletion under -30 has never been reproduced (McKeegan et 879

4 Fig. 3 The accreted Eoarchean oceanic crust pillow lava and hyaloclastite on the right and overlying pelagic sediments a man is standing at the middle, 2 3 m thick and trench turbidite to the left. Fig. 4 The highly folded trench turbidite interlayered with dolostones. The location is ca. 500 m south of Fig. 3. The same stratigraphical horizon as that shown in Fig. 3 appears again to the south. The outcrop shows a stratigraphically higher succession of ocean plate stratigraphy OPS. 880

5 al., 2007; Papineau et al., 2010b), despite using the same analytical technique (secondary ion mass spectrometry). Hence, the Akilia graphite is not so 13 C-depleted that the 13 C value may have also been explicable if the graphite had been formed by an abiological reaction during metamorphism. An alternative abiotic scenario proposed so far is that the graphite may have been formed through a disproportionation reaction of siderite (van Zuilen et al., 2002; Lepland et al., 2005). During prograde metamorphism, siderite may decompose into magnetite and graphite at about 450 (French, 1971): 6FeCO 3 2Fe 3O 4+5CO 2+C (1) In this case, the 13 C value of the deposited graphite can be depleted in 13 C by about 15 compared to the original carbonate phase. Hence, a large part of the Akilia graphites are isotopically indistinguishable from those produced abiotically through reaction (1), although some Akilia graphites are slightly more 13 C-depleted up to -24, compared to co-occurring carbonate (-3~-6 ; Papineau et al., 2010b). Sano et al. (1999)measured the age of the apatite including graphites in the same samples as described by Mojzsis et al. (1996), and obtained a U-Pb age of 1.5 Ga, casting doubts on the interpretation of the oldest signs of life. The 13 C-depleted Akilia graphite has no guarantees of the oldest known record of life ca Ga, but still remains a possibility (see also Eiler, 2007). IV Isua Greenstone Belt In contrast to the Akilia controversy, growing evidence supports the existence of life at 3.8 Ga, based on investigations of lower grade metasediments in the 3.8 Ga ISB. The ISB hosts greenschist to amphibolite facies metamorphosed chemical and clastic sedimentary rocks deposited on the 3.8 Ga seafloor (Nutman et al., 1984; Baadsgaard et al., 1984; Appel et al., 1998, 2001; Komiya et al., 1999, 2002; Hayashi et al., 2000; Myers, 2001; Friend et al., 2002; Nutman and Firend, 2009). Some of the ISB metasediments contain graphite, which show whole rock 13 C values of -5 to -29 (Oehler and Smith, 1977; Perry and Ahmad, 1977; Schidlowski et al., 1979; Hayes et al., 1983; Shimoyama and Matsubaya, 1992; Naraoka et al., 1996; Rosing, 1999; Ueno et al., 2002). In addition to metamorphism, some parts of the ISB are severely metasomatized (Rose et al., 1996; Fedo et al., 2001). Due to metasomatism, some mafic to ultramafic volcanics were severely carbonatized, which sometimes made identification of the protolith difficult. Nonetheless, less carbonatized unambiguous sedimentary rocks also host graphite with 13 C-depleted carbon isotopic compositions (Rosing, 1999). Also, the carbonate-metasomatism could possibly have deposited graphite abiologically, although the mechanism forming graphite from carbonate fluid is largely unknown. Van Zuilen et al. (2002) reported some of the ISB graphite co-occurring with siderite and magnetite, thus they interpreted that the graphite may have been deposited by a siderite decomposition mechanism through reaction (1). However, the presence of abundant siderite is by itself problematic for a fluid-deposited scenario, because siderite should not have survived if reaction (1)proceeded at an elevated temperature over 450. This abiotic mechanism cannot be applicable to sub-amphibolite grade ISB metasediments, which occur sporadically in West ISB (Rosing, 1999)and especially in the eastern margin of the ISB (Hayashi et al., 2000; Ueno et al., 2002). Hayashi et al. (2000)and Komiya et al. (2001)mapped the eastern part of ISB and separated the region into three mineral zones, from greenschist, through epidote-amphibolote, to amphibolite facies zones (Fig. 2). The graphite- 881

6 Fig. 5 Carbon isotopic composition of minute graphite globules. A to D show the metamorphic grade, based on the mineral assemblage of metabasites (Hayashi et al., 2000). Note that the well-buffered graphites through progressive metamorphism changed the carbon isotope from low to high with increasing metamorphism (above). Extraporating the temperature effect to surface temperature below 100, the original 13 C must be -25 to -30 which is similar to the value by Nishizawa et al. (2005)(below). bearing sedimentary rocks were systematically collected covering these three zones and carbon isotopes were measured at different localities (Ueno et al., 2002). The results showed a systematic decrease of graphite 13 C values from -5 to -17, along the low- to high-grade metamorphic profile. Note that carbonate rocks of putative metasomatized origin also host graphite, but the graphite within the carbonate rocks exceptionally shows heavier 13 C values compared to metasediment of the same mineral zone (Ueno et al., 2002). Moreover, graphites enclosed within premetamorphic minerals show a more 13 C-depleted isotopic composition than those outside these minerals (Ueno et al., 2002). Thus, apart from the effects of sporadic carbonatization, the original sedimentary organic matter and carbonate would have been re-equilibrated isotopically, as is commonly observed in younger regional and contact metamorphic profiles (Hoefs and Frey, 1976; Val- 882

7 ley and O Neil, 1981; Wada and Suzuki, 1982). This reconfirms the earlier suggestion from pioneering work of Schidlowski et al. (1979)and Hayes et al. (1983)pointing out that the observed wide range of graphite 13 C values and smaller isotopic fractionation between graphite and carbonate resulted from metamorphic re-equilibration, thus the original isotopic compositions of organic and carbonate carbon may have been similar to those of younger Archean sequences. If established, not only 13 C-depletion of organic carbon, but also 13 C-enrichment of carbonate carbon relative to mantle carbon suggests the biological primary productivity should have controlled the global carbon cycle. Considering the effects of metamorphic temperature of above 300 (Fig. 5 above), the original 13 C of organic carbon could be below -20 and around -25 to -30 (Fig. 5 below). Nishizawa et al. (2005)extract carbon by stepwise heating from the lowest grade BIF at the northeastern margin of the ISB, and demonstrated that 13 C-depleted carbon (-30 )is released at 1200, which may represent graphite enclosed within magnetite of the greenschist facies BIF. This value is sufficient to regard it to be of biotic origin (Fig. 5 below). Now, we have reason to believe that life had already appeared on the Earth at least about 3.8 Ga. The presence of 13 C-depleted carbon suggests the onset of biological carbon fixation. Moreover, the Eoarchean ecosystem may not have been local but global. It is still not known if the Eoarchean primary producers were phototrophic or chemotrophic. Physiological and taxonomic aspects of Eoarchean ecosytems await further detailed studies. References References referred in the text are not all listed below to minimize the total length of paper. Those are included in Ueno et al. (2002)and Website of the author. Appel, P.W.U., Fedo, C.M., Moorbath, S. and Myers, J.S. (1998): Recognizable primary volcanic and sedimentary features in a low-strain domain of the highly deformed, oldest known (~ Gyr)Greenstone Belt, Isua, West Greenland. Terra Nova, 10, Appel, P.W.U., Rollinson, H.R. and Touret JLR (2001): Remnants of an Early Archaean (>3.75 Ga)sea-floor, hydrothermal system in the Isua Greenstone Belt. Precambrian Research, 112, Baadsgaard, H., Nutman, A.P., Bridgwater, D., Rosing, M., McGregor, V.R. and Allaart, J.H. (1984): The zircon geochronology of the Akilia association and Isua supracrustal belt, West Greenland. Earth and Planetary Science Letters, 68, Dauphas, N., van Zuilen, M.A., Wadhwa, M., Davis, A.M., Marty, B. and Janney, P.E. (2004): Clues from Fe isotope variations on the origin of Early Archean BIFs from Greenland. Science, 306, Dauphas, N., van Zuilen, M.A., Busigny, V., Lepland, A., Wadhwa, M. and Janney, P.E. (2007): Iron isotope, major and trace element characterization of early Archean supracrustal rocks from SW Greenland: Protolith identification and metamorphic overprint. Geochimica et Cosmochimica Acta, 71, Eiler, J.M. (2007): The oldest fossil or just another rock?. Science, 317, Fedo, C.M. and Whitehouse, M.J. (2002a): Metasomatic origin of quartz-pyroxene rock, Akilia, Greenland, and implication for Earth s earliest life. Science, 296, Fedo, C.M. and Whitehouse, M.J. (2002b): Origin and significance of Archean quartzose rocks at Akilia, Greenland: Response. Science, 298, 917a. Fedo, C.M., Myers, J.S. and Appel, P.W.U. (2001): Depositional setting and paleogeographic implications of earth s oldest supracrustal rocks, the >3.7 Ga Isua Greenstone belt, West Greenland. Sedimentary Geology, 141/142, French, B.M. (1971): Stability relations of siderite (FeCO3)in the system Fe-C-O. American Journal of Science, 271, Friend, C.R.L., Nutman, A.P. and Bennett, V.C. (2002): Protoliths of the Ga Isua greenstone belt-comment. Precambrian Research, 117, Iizuka, T., Komiya, T., Ueno, Y., Katayama, I., Uehara, Y., Maruyama, S., Hirata, T., Johnson, S.P. and Dunkley, D. (2007): Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: new constraints on its tectonothermal history. Precambrian Research, 153, Komiya, T., Hayashi, M., Maruyama, S. and Yurimoto, H. (2002): Intermediate-P/T type Archean metamorphism of the Isua supracrustal belt: Implications for secular change of geothermal gradients at subduction zones and for Archean plate tectonics. American Journal of Science, 302, Lepland, A., van Zuilen, M.A., Arrhenius, G., Whitehouse, M.J. and Fedo, C.M. (2005): Questioning the evidence for Earth's earliest life - Akilia revisited. Geology, 33, 883

8 Manning, C.E., Mojzsis, S.J. and Harrison, T.M. (2003): Geology, age, and origin of Akilia supracrustal rocks, Greenland. Geochimica et Cosmochimica Acta, 67, A271. Manning, C.E., Mojzsis, S.J. and Harrison, T.M. (2006): Geology, age and origin of supracrustal rocks at Akilia, West Greenland. American Journal of Science, 306, Maruyama, S. (1997): Pacific-type orogeny revisited: Miyashiro-type orogeny proposed. The Island Arc, 6, Maruyama, S. and Komiya, T. (2011): The oldest pillow lavas, Ga from the Isua Supracrustal belt, SW Greenland: Plate Tectonics had already begun by 3.8 Ga. Journal of Geography (Chigaku Zasshi), 120, McKeegan, K.D., Kudryavtsev, A.B. and Schopf, J.W. (2007): Raman and ion microscopic imagery of graphitic inclusions in apatite from older than 3830 Ma Akilia supracrustal rocks, west Greenland. Geology, 35, Mojzsis, S.J. and Nutman, A.P. (1994): Evidence for a marine sedimentary system at > 3.87 Ga in southern West Greenland. EOS Transactions, 75, 690. Mojzsis, S.J. and Harrison, T.M. (2002a): Origin and significance of Archean quartzose rocks at Akilia, Greenland. Science, 298, 917a. Mojzsis, S.J. and Harrison, T.M. (2002b): Establishment of a 3.83-Ga magmatic age for the Akilia tonalite (southern West Greenland). Earth and Planetary Science Letters, 202, Mojzsis, S.J., Coath, C.D., Greenwood, J.P., McKeegan, K.D. and Harrison, T.M. (2003): Mass-independent isotope effects in Archean (2.5 to 3.8 Ga)sedimentary sulfides determined by ion microprobe analysis. Geochimica et Cosmochimica Acta, 67, Myers, J.S. (2001): Protoliths of the Ga Isua greenstone belt, West Greenland. Precambrian Research, 106, Nishizawa, M., Takahata, N., Terada, K., Komiya, T., Ueno, Y. and Sano, Y. (2005): Rare-earth element, lead, carbon, and nitrogen isotope geochemistry of apatite-bearing metasediments from the ~ 3.8 Ga Isua Supracrustal Belt, West Greenland. International Geology Review, 47, Nutman, A.P. and Friend, C.R.L. (2006): Petrography and geochemistry of apatites in banded iron formation, Akilia, W. Greenland: Consequences for oldest life evidence. Precambrian Research, 147, Nutman, A.P. and Friend, C.R.L. (2009): New 1:20,000 scale geological maps, synthesis and history of investigation of the Isua supracrustal belt and adjacent orthogneisses, southern West Greenland: a glimpse of Eoarchaean crust formation and orogeny. Precambrian Research, 172, Nutman, A.P., Friend, C.R.L. and Bennett, V.C. (2002): Evidence for Ma assembly of the northern end of the Itsaq Gneiss Complex, Greenland: Implication for early Archaean tectonics. Tectonics, 21, Papineau, D., De Gregorio, B.T., Cody, G.D., Fries, M.D., Mojzsis, S.J., Steele, A., Stroud, R.M. and Fogel, M.L. (2010a): Ancient graphite in the Eoarchean quartzpyroxene rocks from Akilia in southern West Greenland I: Petrographic and spectroscopic charactgerization. Geochimica et Cosmochimica Acta, 74, Papineau, D., De Gregorio, B.T., Stroud, R.M., Steele, A., Pecoits, E., Konhauser, K., Wang, J. and Fogel, M.L. (2010b): Ancient graphite in the Eoarchean quartzpyroxene rocks from Akilia in southern West Greenland II: Isotopic and chemical compositions and comparison with Paleoproterozoic banded iron formations. Geochimica et Cosmochimica Acta, 74, Sano, Y., Terada, K., Takahashi, Y. and Nutman, A.P. (1999): Origin of life from apatite dating?. Nature, 400, 127. Ueno, Y., Yurimoto, H., Yoshioka, H., Komiya, T. and Maruyama, S. (2002): Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition. Geochimica et Cosmochimica Acta, 66, Ueno, Y., Yamada, K., Yoshida, N., Maruyama, S. and Isozaki, Y. (2006): Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era. Nature, 440, Valley, J.W. and O Neil, J.R. (1981): 13 C/ 12 C exchange between calcite and graphite: a possible thermometer in Grenville marbles. Geochimica et Cosmochimica Acta, 45, van Zuilen, M.A., Lepland, A. and Arrhenius, G. (2002): Reassessing the evidence for the earliest traces of life. Nature, 418, Wada, H. and Suzuki, K. (1982): Carbon isotopic thermometry calibrated by dolomite-calcite solvus temperatures. Geochimica et Cosmochimica Acta, 47, Whitehouse, M.J., Myers, J.S. and Fedo, C.M. (2009): The Akilia controversy: Field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland. Journal of the Geological Society, London, 166,

9 地球上にいつ生命が誕生したのだろうか? この疑問に答えるために 世界最古の地層を探し その痕跡を突き止める努力がなされてきた 西グリーンランドイスア地域には 億年前の太平洋型造山帯が見られるが そのなかの付加体を構成する海溝堆積物中に石墨が産する この石墨の炭素同位体組成は起源推定に有用であるのだ が この堆積岩はのちに 程度の広域的な変成作用を被っているため同位体組成は改変されている そこで変成作用による影響を考慮すると もとの炭素同位体組成は-25 から-30 程度であったと予測される これらの同位体的特徴は 地球誕生後 8 億年時点ですでに生物は地球に出現していたことを示す : 石墨 炭素同位体 アキリア島 イスア緑色岩帯 * 東京工業大学大学院理工学研究科 885