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 3.9–3.8 Ga metamorphosed sedimentary rocks in western Greenland. The carbonaceous material is now graphitized due to extensive metamorphism, although it exhibits a low 13C/12C ratio. The 13C-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 on-going 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, oc-

cur in the Acasta gneiss complex, which dates

back to 3.96 Ga （Bowring et al., 1989） or even old-

er 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 （3.9–3.8 Ga）, which

contain graphite of a possible biological origin.

For this reason, a number of researchers have in-

vestigated 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 ori-

gins of the carbonaceous matter. A Japanese team

＊ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551, Japan

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地学雑誌（Chigaku Zasshi）Journal of Geography120（5）877–885 2011

The 100s: Significant Exposures of the World （No. 3）

Fig. 1　Locality index of the Isua Belt, Greenland.

GMT

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visited the area to map it in great detail, introduc-

ing 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 pa-

pers, 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 stud-

ies 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 togeth-

er with a representative outcrop.

II．Interpretation of Outcrop

　The graphite-bearing meta-sediments are ex-

posed on the eastern side of the ISB （Fig. 2）, as

top-sitting meta-sedimentary rocks in an accre-

tionary complex （Fig. 2）. A general outline of the

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.

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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 metamor-

phism （Ueno et al., 2002）.　Fig. 3 photo looks south of the accreted frag-

ments 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）, presum-

ably derived from trench-turbidites in an intra-

oceanic environment （Komiya et al., 1999）. Con-

glomerate 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 carbon-

ate-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 13C-depleted graphite preserved in the Isua-Akilia

Eoarchean supracrustal sequence （3.9–3.6 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 13C-depleted isotopic

composition as low as －44‰ （Mojzsis et al.,

1996）. The quartz-pyroxene rock may have origi-

nally been deposited as a banded iron formation

（BIF） and subsequently suffered from upper am-

phibolite to granulite facies metamorphism （Mojz-

sis and Nutman, 1994; Nutman et al., 1997）. Since the first discovery, many questions concern-

ing the original interpretation have arisen.

　The sedimentary origin of the Akilia quartz-

pyroxene rock was once questioned by trace ele-

ment 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 in-

compatible with the BIF-origin of the quartz-

pyroxene 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 graphite-

bearing BIF.

　Independent reanalysis by Lepland et al. （2005） and Nutman and Friend （2006） failed to recog-

nize graphite within the Akilia BIF, casting doubt

on the presence of graphite in this rock. However,

these reports do not address the same hand speci-

men originally studied by Mojzsis et al. （1996）. Recent studies of this sample by Papineau et al.

（2010a, b） and McKeegan et al. （2007） clearly re-

located the graphite within the Akilia BIF. A thor-

ough 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-occur-

ring base-metal sulfides. This fluid deposited sce-

nario does not discard biological origins, because

carbon-bearing fluid is often derived from sedi-

mentary organic matter during metamorphism of

C- and S-bearing sedimentary rock.

　Papineau et al. （2010b） also revealed that the

δ13C values of Akilia graphites range from

－24‰ to －4‰, which are more 13C-enriched than

those reported originally by Mojzsis et al. （1996:

－44 ～－28‰）. The large 13C-depletion under

－30‰ has never been reproduced （McKeegan et

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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）.

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al., 2007; Papineau et al., 2010b）, despite using

the same analytical technique （secondary ion

mass spectrometry）. Hence, the Akilia graphite is

not so 13C-depleted that the δ13C value may have

also been explicable if the graphite had been

formed by an abiological reaction during meta-

morphism. An alternative abiotic scenario pro-

posed 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）:

　6FeCO3→2Fe3O4＋5CO2＋C （1）

　In this case, the δ13C value of the deposited

graphite can be depleted in 13C 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 graph-

ites are slightly more 13C-depleted up to －24‰,

compared to co-occurring carbonate （－3～－6‰;

Papineau et al., 2010b）.　Sano et al. （1999） measured the age of the apa-

tite 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 interpre-

tation of the oldest signs of life. The 13C-depleted

Akilia graphite has no guarantees of the oldest

known record of life ca. 3.9–3.8 Ga, but still re-

mains 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 metasedi-

ments in the 3.8 Ga ISB. The ISB hosts green-

schist to amphibolite facies metamorphosed chem-

ical and clastic sedimentary rocks deposited on

the 3.8 Ga seafloor （Nutman et al., 1984; Baads-

gaard 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 δ13C 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; Na-

raoka 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 car-

bonatized, which sometimes made identification of

the protolith difficult. Nonetheless, less carbon-

atized unambiguous sedimentary rocks also host

graphite with 13C-depleted carbon isotopic compo-

sitions （Rosing, 1999）.　Also, the carbonate-metasomatism could possi-

bly 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 reac-

tion （1）. However, the presence of abundant sid-

erite is by itself problematic for a fluid-deposited

scenario, because siderite should not have sur-

vived if reaction （1） proceeded at an elevated tem-

perature over 450℃. This abiotic mechanism can-

not 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-

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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 system-

atic decrease of graphite δ13C values from －5‰

to －17‰, along the low- to high-grade metamor-

phic profile. Note that carbonate rocks of putative

metasomatized origin also host graphite, but the

graphite within the carbonate rocks exceptionally

shows heavier δ13C values compared to metasedi-

ment of the same mineral zone （Ueno et al.,

2002）. Moreover, graphites enclosed within pre-

metamorphic minerals show a more 13C-depleted

isotopic composition than those outside these min-

erals （Ueno et al., 2002）. Thus, apart from the ef-

fects of sporadic carbonatization, the original sedi-

mentary organic matter and carbonate would

have been re-equilibrated isotopically, as is com-

monly observed in younger regional and contact

metamorphic profiles （Hoefs and Frey, 1976; Val-

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 δ13C must be －25 to －30‰ which is similar to the value by Nishizawa et al. （2005） （below）.

 883—　　—

ley and O’Neil, 1981; Wada and Suzuki, 1982）. This reconfirms the earlier suggestion from pio-

neering work of Schidlowski et al. （1979） and

Hayes et al. （1983） pointing out that the observed

wide range of graphite δ13C values and smaller

isotopic fractionation between graphite and car-

bonate resulted from metamorphic re-equilibra-

tion, thus the original isotopic compositions of or-

ganic and carbonate carbon may have been

similar to those of younger Archean sequences. If

established, not only 13C-depletion of organic car-

bon, but also 13C-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 13C of organic carbon could be

below －20 and around －25 to －30‰ （Fig. 5 be-

low）. Nishizawa et al. （2005） extract carbon by

stepwise heating from the lowest grade BIF at the

northeastern margin of the ISB, and demonstrat-

ed that 13C-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 al-

ready appeared on the Earth at least about 3.8

Ga. The presence of 13C-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 chemotro-

phic. Physiological and taxonomic aspects of Eo-

archean ecosytems await further detailed studies.

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地球生命はいつ誕生したのか？世界最古の生命の痕跡―西グリーンランドの高変成泥質岩中の石墨の炭素同位体組成―

上 野 雄 一 郎＊

　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 38–37億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 300–600℃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。

キーワード： 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