1.海洋底序説...

1.海洋底序説 Introduction

なぜ地球には海があるか：他の惑星にはないのか Why do we have “ocean” on the Earth?

いつどのように海ができたのか When and How was the Earth’s ocean born?

海が地球の歴史の中で果たした役割は何か What is the role of “ocean” in Earth’s history?

海洋底を研究する意義 Why do we study “ocean floor”?

海洋底ダイナミクス 2018Ocean Floor Geodynamics 2018

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The Blue Marble Apollo 17, NASA, 1972

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他の地球型惑星との比較 comparison with other terrestrial planets

（宮本ほか、惑星地質学、2008）

Mercury

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• 表面が水（Ｈ２Ｏの水と氷）に満ちた唯一の惑星

• 酸素の多い大気を持つ唯一の惑星

• 珪素に富む岩石（例えば花崗岩）を多量に持つおそらく唯一の惑星

• 表面の高度がバイモーダル（海と陸）なおそらく唯一の惑星

• 地球型惑星の中では唯一の強い固有磁場を持つ惑星

• 生命のいる唯一の惑星？

これらの特徴はみなリンクしている

地球の特徴 Uniqueness of the Earth

Full of water (liquid / ice H2O) on the surface

Oxygen-rich atmosphere

Large amount of granitic rocks

Bimodal distribution of hipsometry

Strong intrinsic magnetic field

Full of life !

these features are mutually linked4

��

��From the Sun [km] 108 ×106 150×106 228×106

Surface Temp [°C] 460 15 -60

Surface Press [atm] 100 1 0.01

Major Atmospheric comp

CO2,N2 N2,O2 CO2,N2,Ar

海がなぜ存在できるか existence of “ocean”

5

海洋 Ocean 1.4×109�km3

湖沼 Lake 2.5×105�km3

河川・地下水 River/ground water

2.5×105�km3

氷山・氷塊iceberg/ice sheet

2.3×107�km3

水蒸気 vapor 1.3×104�km3

蒸発（降水）rain fall

1.5×105�km3/year

(日本海洋学会編、海と地球環境、1991）

地球表層における水の分布 Distribution of water on the Earth’s surface

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(NASA)

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（宮本ほか、惑星地質学、2008）

他の惑星・衛星に海がある可能性：火星 Ocean on other planets/moons “Mars”

gully on carter’s wall

gully alcove

fan gul‧ly, gulley /ˈɡʌli/ noun (plural gullies)

[countable] 1 a small narrow valley, usually formed by a lot of rain flowing down the side of a hill

8

NASA Sept 28, 2015

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（Images: NASA）

他の惑星・衛星に海がある可能性：エウロパ Ocean on other planets/moons “Europa” Jupiter II

core (metal)

silicate (=rock)

ocean?

ice crust

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Full of life !


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Full of life !


陸：軽い、花崗岩地殻、大半は安定

海：重い、玄武岩地殻、若い

地球の標高分布：ヒプソメトリ Hypsometry: Earth

bimodal

ocean: heavy, basaltic, young

land (continent) light, granitic, old/stable

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火星

地球　

金星

(b)

(中西・沖野「海洋底地球科学」))

(Rogers, Our Dynamic Planet, 2007）

他の惑星のヒプソメトリ Hypsometry: Mars and Venus

Earth

Mars

Venus

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Full of life !


Magnetic filed of planets

rotation mag.field (Earth’s)Mercury 58.8d 0.006Venus 243.7d 0.00Earth 23.9h 1Mars 24.6h 0.00

Jupiter 9.9h 19,519

NASA Facts Sheet

up to spherical harmonic degree 13. Higher order harmonics of theinternal field are masked by crustal remnant magnetism. The dy-namic nature of the geodynamo is revealed through secular varia-tion, where changes in the strength and spatial structure of thefield occur over timescales of months (Olsen and Mandea, 2008)to centuries (Bloxham et al., 1989) to millenia (Cande and Kent,1995). The most notable aspects of secular variation are polarityreversals, westward drift of the non-axisymmetric field, anticy-clonic field motions at high latitudes, growth of the South Atlanticanomaly where the field is unusually weak, and geomagnetic jerks(Finlay et al., 2010). The physical mechanisms that lead to thesebehaviors, however, are not yet understood.

2.1.2. MercuryMercury has the weakest intrinsic magnetic field in the solar

system with a mean surface strength of only about 0.3 lT. Thisfield was first measured during the Mariner 10 flybys in 1974and 1975 (Ness et al., 1975, 1976) and again during the MESSEN-GER flybys in 2008 and 2009 (Anderson et al., 2008, 2010; Unoet al., 2009; McNutt et al., 2010; Solomon, 2011). These observa-tions suggest that the field is dipole-dominated and nearly alignedwith the rotation axis. The surface radial magnetic field plotted upto spherical harmonic degree three is shown in Fig. 2a. Comparedto the Earth, the field is about 100 times weaker at the surfaceand about 500 times weaker at the core-mantle boundary, assum-ing Mercury’s core has a radius of 1900 km. It is not yet understoodwhy Mercury has such a weak magnetic field.

The source of the field is not agreed upon in the literature (e.g.,Schubert et al., 1988; Stevenson, 2010), although dynamo action isthe general consensus because libration measurements by Margotet al. (2007), gravity measurements of MESSENGER (Smith et al.,2010), and thermodynamic phase equilibrium simulations ofMalavergne et al. (2010) support the presence of a liquid core. Otherpossibilities include remnant magnetism (Aharonson et al., 2005) orthermoelectric currents (Stevenson, 1987; Giampieri and Balogh,2002). Further, MESSENGER found that the internal field interactswith currents in the planet’s magnetosphere (McNutt et al., 2010),but the dynamical implications of this coupling are not well con-strained (Glassmeier et al., 2007; Kabin et al., 2008; Gomez-Perezand Solomon, 2010; Gomez-Perez and Wicht, 2010).

2.1.3. GanymedeThe Galileo spacecraft revealed that the Jovian satellite Gany-

mede also has an intrinsic magnetic field (Kivelson et al., 1996).

As shown in Fig. 2b, these measurements suggest that the field isnearly axially-aligned and dipole-dominated with a mean surfacestrength of about 1 lT. However, it might also be appropriate toconsider the value of Ganymede’s magnetic field at the ice-rockboundary since the body has a terrestrial-style internal structurebeneath the ice shell. Assuming an ice shell thickness of about900 km (Schubert et al., 2007), the mean magnetic field is 3 lT atthe ice-rock boundary. This is about an order of magnitude larger(smaller) than Mercury’s (Earth’s) surface field. The field resolu-tion, however, is limited to the dipole and quadrupole componentsand additional data are needed to further characterize the mag-netic field. Since Ganymede is thought to have a metallic core witha fluid component, the field is likely driven by dynamo action(Schubert et al., 1996; Sarson et al., 1997; Kivelson et al., 2002).The size of the core is poorly constrained, and the energy sourcefor maintaining a liquid or partially liquid core in such a small bodyis not fully understood.

2.1.4. JupiterThe magnetic field of Jupiter was discovered when Burke and

Franklin (1955) showed that the planet was a strong radio source.This was the first detection of a planetary magnetic field besidesthat of the Earth. The existence of the field was confirmed whenthe planet was visited by the Pioneer 10 flyby in 1973 (Smithet al., 1974). Additional measurements were obtained by the Pio-neer 11 flyby in 1974 (Smith et al., 1975), the Voyager 1 and 2 fly-bys in 1979 (Ness et al., 1979), the Ulysses flybys in 1992 and 2004(Balogh et al., 1992), the Galileo orbiter between 1995 and 2003(e.g., Connerney, 1993; Yu et al., 2010), and the New Horizons flybyin 2007 (e.g., McComas et al., 2007; McNutt et al., 2007). Thesemeasurements show that the magnetic field is dominated by theaxial dipole component and is the strongest field in the solar sys-tem with a mean surface strength of 550 lT. The surface of gaseousplanets is taken to be the 1 bar pressure level. The surface radialmagnetic field including up to the octupole component is shownin Fig. 2c. The spatial and temporal resolution of the field is a sig-nificant limitation. While Jupiter’s magnetic field has beenmeasured by several missions, the orbits do not extend inward of1.6 Jupiter radii and have longitudinal data gaps (Russell andDougherty, 2010). Consequently, the observations are only re-solved to spherical harmonic degree three. Jupiter has not exhib-ited unambiguous secular variations in field strength or dipoletilt in the 20 years between the Pioneer and Galileo missions (Yuet al., 2010).

-80 µT 80 µT0 -800 µT 800 µT0

(a) Earth surface (b) Earth CMB

Fig. 1. Radial magnetic field of Earth at (a) the surface and (b) the core-mantle boundary (CMB). The colors represent field intensity where purple (green) indicates outward(inward) directed field. A mollweide projection is used in which the horizontal lines indicate constant latitude. IGRF-11 coefficients are used and are available on the IAGADivision V-MOD geomagnetic field modeling website.

94 G. Schubert, K.M. Soderlund / Physics of the Earth and Planetary Interiors 187 (2011) 92–108

The magnetic field is produced by convectively-driven dynamoaction in the highly electrically conducting metallic hydrogenmantle and in the less electrically conducting region near the baseof the molecular envelope (Stanley and Glatzmaier, 2010). The ex-act location and extent of the dynamo region is poorly constrained.Nellis (2000) argues that the dynamo can extend up to 95% of theplanet radius. Consequently, the dynamo region is expected to ex-hibit significant changes in density and electrical conductivity, theeffects of which are relatively unknown. The upcoming Juno mis-sion will critically improve our understanding of Jupiter by resolv-ing the magnetic field up to spherical harmonic degree 14,

detecting short timescale secular variation, and constraining theplanet’s internal structure (Connerney, private communication).

2.1.5. SaturnSaturn was revealed to have a magnetic field during the Pioneer

11 flyby in 1979 (Smith et al., 1980), and subsequent observationswere made by Voyager 1 in 1980 (Ness et al., 1981) and Voyager 2in 1981 (Ness et al., 1982). The Cassini spacecraft arrived at Saturnin 2004 and is still collecting data (Dougherty et al., 2005; Burtonet al., 2009). These observations have mapped the large-scale radialmagnetic field, shown in Fig. 2d, and suggest that no resolved

-0.6 µT 0.6 µT0 -1.5 µT 1.5 µT0

(a) Mercury

-1200 µT 1200 µT0 -60 µT 60 µT0

-100 µT 100 µT0 -100 µT 100 µT0

(b) Ganymede

(c) Jupiter (d) Saturn

(e) Uranus (f) Neptune

Fig. 2. Radial magnetic field at the surfaces of (a) Mercury, (b) Ganymede, (c) Jupiter, (d) Saturn, (e) Uranus, and (f) Neptune. Data taken from Uno et al. (2009) for Mercury(with spectral resolution l, m 6 3), Kivelson et al. (2002) for Ganymede (l, m 6 2), Yu et al. (2010) for Jupiter (l, m 6 3), Burton et al. (2009) for Saturn (l, m 6 3), and Holme andBloxham (1996) for the ice giants (l, m 6 3).

G. Schubert, K.M. Soderlund / Physics of the Earth and Planetary Interiors 187 (2011) 92–108 95

(Schubert and Soderlund, 2011)

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Full of life !


(Press, Understanding Earth, 2003）

月の誕生

最初の海の証拠?

主な大陸の誕生

酸素大気の出現

殻を持つ生物の出現

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(Press, Understanding Earth, 2003）

顕生代

原生代

始生代（太古代）

冥王代

New! Quaternary start

＝2.58

New! Holocene start ＝0.011784

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初期地球の形成 Early history of the Earth

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planetesimal 微惑星太陽系の惑星がほぼ一斉にできる～およそ46億年前最初の1億年　微惑星衝突の時代4.6 Ga Birth of Solar System First 1 billion year:

formation of planetesimals and accretion

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衝突速度の増大脱ガスによる原始大気の形成（CO2,N2,水蒸気）衝突エネルギーの解放と大気の保温による地表温度の上昇 acceleration of accretion causes - primordial (early) atmosphere: CO2, N2, water vapor - increase of surface temperature (collision and green house)

protoplanet 原始惑星 planet 惑星giant impact　巨大衝突

(Rogers, Our Dynamic Planet, 2007）

planetary embryos 惑星胚子

マグマオーシャンの形成 formation of magma ocean

衝突頻度の減少 - 大気温度低下 - 雲の形成と降雨 - 原始海洋の形成 - 地殻の形成

Decrease of accretion (collision) causes - decrease of surface temperature - formation of cloud and rainfall - primordial (early) ocean - formation of primary (early) crust

22

EA40CH21-Arndt ARI 23 March 2012 15:30

Ga: billion years

Gya: billion years ago

INTRODUCTIONOur image of the early Earth has changed greatly over the past two decades, with importantconsequences for models of the origin and early evolution of life. Figure 1 shows two contrastingviews of the surface of earliest Earth. In the 20-year-old image in panel a, we see a Hell-like,Hadean landscape of fiery volcanoes beneath a menacing red sky; the more recent image in panelb shows luxuriant microbial colonies on a tranquil beach. What caused this dramatic change inour vision of the young Earth?

The discovery of >4-Ga-old (>4-billion-year-old) zircons (Cavosie et al. 2005, 2007; Froudeet al. 1983; Wilde et al. 2001) radically changed our interpretation of the first part of Earth’shistory. The zircons, found first in Mount Narryer and then in Jack Hills, both in WesternAustralia, have ages from approximately 3.1 Ga (that of the quartzitic metasediment from whichthey were extracted; Kinny et al. 1990) to just over 4.4 Ga (Wilde et al. 2001). The latter ageis some 150 million years less than accretion at 4.56 billion years ago (4.56 Gya), and merely100 million years after the impact of the planetesimal Thea that led to the formation of themodern Earth-Moon system and the start of the Hadean (Goldblatt et al. 2009, Halliday 2003).

Zircon is ubiquitous in granite but rare in rocks of more mafic compositions. The compositionsof the >4-Ga-old zircons are similar to those in modern granitoids (Maas et al. 1992, Mojzsis et al.2001). Thus, this mineral provides evidence for the existence of granite >4 Gya. Because granitoidsdefine the continents, it follows that some felsic crust had already formed at this early stage ofEarth’s history. Campbell & Taylor (1983) noted that, on the modern Earth, granite forms inabundance only when hydrated basaltic crust is subducted.

Oxygen isotopic analysis of the old zircons supports the inference by Campbell and Taylor.Cavosie et al. (2005) and Mojzsis et al. (2001) reported significant fractionation of oxygen isotopesin the minerals dating from >4 Gya—the oldest zircon they measured has a δ18O of +4!, a value

a b

Old concept of the Hadean Earth Updated reconstruction

Figure 1Two views of the young Earth. (a) Image reflects the old idea that the Hadean was Hell-like, covered by lava and impacted bymeteorites. (b) A more recent image shows oceans and primitive life (from http://archenv.geo.uu.nl/).

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• 過去に海が存在した証拠は何か？

• 堆積層の存在：陸では「堆積」は起こらない

• 海底下で噴出した溶岩(枕状溶岩）の存在：水による急冷の構造

• 最古の海の証拠があるのは？

• グリーンランド西部イスア地方：38億年前の堆積層＋枕状溶岩片

• オーストラリア西部ジャックヒル地域：44億年前のジルコン

• ZrSiO4:　風化に強い、水と反応する際に同位体分別

海の証拠 Early evidence for “ocean (liquid water)”

Geological evidence for ocean (water)

sedimentary sequence

pillow lava = eruption of underwater volcano

The “earliest” evidence?

Isua, western Greenland: sedimentary rocks and pillow lava @ 3.8Ga

Jack Hills, western Australia: zircon @ 4.4Ga

isotopic fractionation during reaction with water

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Isua

Greenland

oldest sedimentary rocks (3.8Ga)

oldest pillow lava

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(Valley, 2005)

w w w. s c i a m . c o m S C I E N T I F I C A M E R I C A N 61

port the surviving grains great distances before they become incorporated into deposits of sand and gravel that may later solidify into sedimentary rock. In-deed, the Jack Hills zircons—separated by perhaps thousands of kilometers from their source—were found embedded in a fossilized gravel bar called the Jack Hills conglomerate.

So, despite the excitement of fi nding such primeval pieces of the earth, most scientists, including me, continued to accept the view that the climate of our young planet was Hadean. It was not until 1999 that technological advances allowed further study of the ancient zir-con crystals from Western Australia—and challenged conventional wisdom about the earth’s earliest history.

Digging Deepthe austr alia n zircons did not give up their secrets easily. For one thing, the Jack Hills and their surroundings are dusty barrens at the edge of vast sheep stations, called Berringarra and Mileu-ra, situated some 800 kilometers north of Perth, Australia’s most isolated city.

The Jack Hills conglomerate was depos-ited three billion years ago and marks the northwestern edge of a widespread assembly of rock formations that are all older than 2.6 billion years. To recover less than a thimbleful of zircons, my col-leagues and I collected hundreds of kilo-grams of rock from these remote out-crops and hauled them back to the labo-ratory for crushing and sorting, similar to searching for a few special grains of sand on a beach.

Once extracted from their source rock, individual crystals could be dated because zircons make ideal timekeep-ers. In addition to their longevity, they contain trace amounts of radioactive uranium, which decays at a known rate

to lead. When a zircon forms from a so-lidifying magma, atoms of zirconium, silicon and oxygen combine in exact proportions (ZrSiO4) to create a crystal structure unique to zircon; uranium oc-casionally substitutes as a trace impu-rity. Atoms of lead, on the other hand, are too large to comfortably replace any of the elements in the lattice, so zircons start out virtually lead-free. The urani-um-lead clock starts ticking as soon as the zircon crystallizes. Thus, the ratio of lead to uranium increases with the age of the crystal. Scientists can reliably de-termine the age of an undamaged zircon within 1 percent accuracy, which for the early earth is about plus or minus 40 million years.

JOHN W. VALLEY received his Ph.D. in 1980 from the University of Michigan at Ann Arbor, where he fi rst became interested in the early earth. He and his students have since ex-plored the ancient rock record throughout North America and in Western Australia, Greenland and Scotland. Currently Valley is president of the Mineralogical Society of America and Charles R. Van Hise Professor of Geology at the University of Wisconsin–Madison, where he founded a multimillion-dollar laboratory called WiscSIMS. The cut-ting-edge capabilities of the lab’s new CAMECA IMS 1280 ion microprobe will enable a diverse range of research; besides zircons, Valley and his colleagues will probe many rare or extremely small materials ranging from stardust to cancer cells.

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Ancient rocks older than 2.5 billion years crop out or lie just underneath the soil in many spots around the globe (red) and are probably hidden below younger rocks across even broader regions (pink). Zircon crystals as old as those discovered in the Jack Hills of Western Australia may eventually be discovered at another of these locations.

OLDEST PIECES OF THE PL ANET

Fossilized gravel bed in the Jack Hills (above) contained the world’s oldest zir-cons yet discovered. Geologists crushed and sorted hundreds of kilograms of this rock (below) to fi nd the 20 crystals that bear signs of cool conditions more than four billion years ago.

InferredExposed

Jack Hills zircons(oldest earth material,

4.4 billion years old)

Acasta Gneiss(oldest intact rock, 4 billion years old)

Isua sediments (oldest evidencefor life, 3.8 billion

years old)

Rocks older than2.5 billion years

COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.

64 S C I E N T I F I C A M E R I C A N O C T O B E R 2 0 0 5

As the possible implications of the zircon discoveries spread through the sci-entifi c community, the excitement was palpable. In the superheated violence of a Hadean world, no samples would have survived for geologists to study. But these zircons pointed to a more clement and familiar world and provided a means to unravel its secrets. If the earth’s climate was cool enough for oceans of water ear-ly on, then maybe zircons could tell us if continents and other features of modern earth also existed. To fi nd out, we had to look more closely into the interiors of single crystals.

Even the smallest zircon contains

other materials that were encapsulated as the zircon grew around them. Such zir-con inclusions can reveal much about where the crystal came from, as can the crystal’s growth patterns and the compo-sition of trace elements. When Peck and I studied the 4.4-billion-year-old zircon, for instance, we found that it contained pieces of other minerals, including quartz. That was surprising because quartz is rare in primitive rocks and was probably absent from the very fi rst crust on the earth. Most quartz comes from granitic rocks, which are common in more evolved continental crust.

If the Jack Hills zircons came from a

granitic rock, that evidence would sup-port the hypothesis that they are sam-ples of the world’s fi rst continent. But caution is warranted. Quartz can form in the last stages of magma crystalliza-tion even if the parent rock is not gra-nitic, although such quartz is much less abundant. For instance, zircons and a few grains of quartz have been found on the moon, which never developed a gra-nitic, continental-style crust. Some sci-entists have also wondered if the earth’s earliest zircons formed an environment more like the early moon or by some other means that is no longer common, perhaps related to giant meteorite im-

THE TALES THEY TELL

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ANCIENT AGE

FIRST CONTINENTS? Rounded surfaces of some Jack Hills zircons under a scanning electron microscope show that wind and possibly running water buffeted these crystals over long distances—possibly across a large continental landmass—before they were fi nally laid to rest (right). Zircons found near their place of origin retain their original sharp edges ( far right). The large number of ancient, rounded Jack Hills zircons suggests their original source rocks were widespread.

COOL OCEANS

Zircons from the Jack Hills of Western Australia have changed the way scientists think about the early history of the earth. These crystals are the oldest terrestrial materials yet discovered—hundreds of those identifi ed formed more than

four billion years ago. Many of these tiny timekeepers also bear clear chemical signs that oceans of liquid water and possibly even continents existed on the earth’s surface at a time once thought to be molten and fi ery.

Ratio of Lead 206 to Uranium 238 70 80

0.80

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20 30 40 50 60

4.24.4Dates in billions

of years ago

Oldest zircondate from earth

All dates in this region fall between 4.2 billion and 4.4 billion years ago

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The oldest age for a Jack Hills zircon—4.4 billion years (red)—is an exact match of two geologic “clocks.” Two pairs of iso topes—uranium 235–lead 207 (vertical axis) and uranium 238–lead 206 (horizontal axis)—form two radioactive time keep ers that start ticking when a zircon forms. If they are well pre served, their fi nal ratios plot along a single line (yellow). Dates from other parts of the zircon (pink) fall off this line because some lead was lost from these areas, but scientists can correct for this damage.

Oxygen isotope ratios in Jack Hills zircon samples (blue), which range up to 7.5, are possible only if their source rock formed in a relatively cool, water-rich environment near the earth’s surface. Had magma oceans covered the planet when these zircons formed, their values would have clustered near 5.3, as do those of all crystals from hot rock that originates in the planet’s deep interior (red).

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Typical mantle zircons

Jack Hills zircons(includes all>4 billion-year-old zircons analyzed so far)

Expected value forthe earth’s hot mantle

Expected value for cool,wet environments

COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.

oldest zircons

26

• 大気も海洋も酸素に乏しい

• シアノバクテリアによって部分的に酸素濃度の高まった海洋表層部も存在

• 海洋で縞状鉄鉱床(BIF)が生成

• 陸上に赤色砂岩はない

大気（還元的）

海洋深層（還元的）

海洋表層（還元的）

大気と海の進化 Evolution of atmosphere and ocean stage 1

atmosphere: reductive

surface water: mostly reductive

deep water: reductive

Mostly reductive, less oxygen

Partly oxidative by cyanobacteria photosynthesis

Formation of Banded Iron Formation (BIF)

No red sandstone (redbed) on land

27

縞状鉄鉱床 Banded Iron Formation

• 鉄分に富む鉱物から成る層と珪酸からなる層の繰り返し

• 水に溶けた二価鉄イオンがシアノバクテリアの光合成で生み出された酸素と結びついて沈殿

• 鉄イオンが海中に溶解するためには、無酸素状態（還元的）な海が必要

• つまり、ほとんどが還元的な海洋環境で、酸化的な環境が一部にある、という状態が必要(富山科学文化センター)

Iron-rich layer + silicate layer

Fe2+ in water + oxygen created by photosynthesis

Fe2+ in ocean = reductive ocean

Mostly reductive, partly oxidative !!

28

• シアノバクテリアの光合成によって大気と海洋表層が酸化的に

• 海洋ではひきつづき縞状鉄鉱床が形成

• 陸上では赤色砂岩が形成

大気（酸化的）

海洋深層（還元的）

海洋表層（酸化的）

atmosphere: oxidative

surface water: oxidative

deep water: reductive


photosynthesis of cyanobacteria == oxidative Earth’s surface

Formation of Banded Iron Formation (BIF)

Formation of red sandstone

2.2-1.9 Ga?

29

現世ストロマトライト（オーストラリア）

ストロマトライト化石

シアノバクテリア（藍藻類）

Banded Iron Formation (BIF) 　縞状鉄鉱床

(Wikipedia)

(Wikipedia)

(ニュートン)

(富山科学文化センター)

cyanobacteria

modern stromatolite　（Australia)

fossil stromatolite

30

• 大気も海洋も酸化的になり現在に至る

• 海洋で縞状鉄鉱床(BIF)は生成されない

• 陸上では赤色砂岩が形成

大気（酸化的）

海洋深層（酸化的）

海洋表層（酸化的）

atmosphere: oxidative

surface water: oxidative

deep water: oxidative


oxidative Earth

No Banded Iron Formation (BIF) formation

red sandstone formation

31

32

• なぜ「適当に柔らかい」プレートが存在できるのか？

• 冷却により鉱物の粘性は急増するので、惑星表層は硬い殻に覆われて対流は深部でのみ起こるほうが自然＝沈み込みは起こらない

• 適度に粘性を下げる何か...海？

プレートテクトニクスと海 Role of ocean: plate tectonics

Why are “plates” moderately soft?

surface cooling of planets = rapid increase of viscosity = stagnant lid convection, no subduction

viscosity decrease by ….

33

少量の水の存在がマントルの粘性率を劇的に変えるsmall amount of water can drastically change mantle viscosity

upper mantle viscosity vs depth, based on laboratory experiments

34

• 大陸（花崗岩の火山活動）の証拠

• 38-37, 32-31, 27-26億年前に活動のピークがある

• 27-26億年前が最大の大陸形成期で現存の大陸の大半がこの時代に既にできていた

• 水が加わらないと花崗岩の火山活動は起こらない

大陸と海 Role of ocean: formation of continents

Formation of continental crust (granitic/rhyolitic magmatism)

Peaks of magmatism at 3.8-3.7Ga, 3.2-3.1Ga, and 2.7-2.6 Ga

Largest Peak at 2.7-2.6 Ga: most present-day continents were formed

“Water” is required to make granitic magma

35

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現在、大規模に大陸がつくられている場所はないToday, no large scale continental formation (c.f. arc magmatism)

36

(JAMSTEC, courtesy of Dr. Takai

生命と海 Role of ocean: Life

37

数値は最適生息温度

38

現在の海洋底 Features on ocean floor

39

average altitude of land ：840ｍ　average depth of ocean：3800ｍ　

「海」は水があるかないかの違いだけではない＝海底と陸は違う Ocean Floor and Land (continent) are fundamentally different!

もう一度標高分布 Hypsometry again…

40

物理的区分と化学的区分

物理的区分・リソスフェア・アセノスフェア・メソスフェア・外核・内核

化学区分・地殻・マントル・核

復習:地球内部の層構造 summary: Earth’s interior

Physical ViewChemical(compositional) View

lithosphereasthenosphere

mesosphereouter coreinner core

coremantlecrust

crust

mantle

lithosphere

asthenosphere

41

studies of rock velocities and densities and nonseismic geo-physical studies using gravity, magnetic, and geoelectricalmethods. Geophysical studies of the crust and subcrustal lith-osphere have become so numerous that it is difficult for anyindividual or research group to keep abreast of all recentresults. This suggests a need for a searchable data bank ofresults, where it would be possible to sort by technique,location, depth of penetration, or geologic setting.

The process of synthesizing global studies of crustal struc-ture is more than 30 years old (Soller et al., 1982), and numer-ous regional and global models have become available(Tables 16 and 17). Until recently, these models have beenlargely based on seismic measurements of compressional-wavespeeds (Vp), with the shear-wave speed (Vs) and density esti-mated using empirical relations. Future models should be ableto rely more heavily on measured shear-wave velocities. Esti-mating deep crustal density will continue to be a challenge butis greatly aided by borehole and laboratory measurements. Theuse of multiple seismic data sets in constructing regional andglobal crustal models will be aided by the use of inverse tech-niques (e.g., Pasyanos et al., 2004). As high-resolution surface-wave models become available, particularly from ambientnoise data (Campillo and Paul, 2003; Shapiro and Campillo,2004), it will be possible to directly compare regional modelsderived from body-wave studies with models derived fromsurface waves. Likewise, nonseismic methods are rapidly devel-oping, including satellite observations of long-wavelengthgravity and magnetic fields. The synthesis of all of thesemethods promises to provide ever-increasing resolution onthe global structure of the Earth’s crust and subcrustallithosphere.

Acknowledgments

Discussions with my scientific colleagues at the USGS andelsewhere over the past decade have aided greatly in preparingthis chapter. Thanks to Claus Prodehl, Nina Pavelnkova, ShaneDetweiler, Gary Chulick, Rolf Meissner, Gary Fuis, Gabi Laske,and Guy Masters for sharing their thoughts and insights. SusanMcDonald, Martina Strakova, Honglei Wong, and ShaneDetweiler provided valuable assistance in preparing this report.

References

Abbott DH (1996) Plumes and hotspots as sources of greenstone belts. Lithos37: 113–127.

Acton CE, Priestley K, and Mitra S (2011) Crustal structure of the Darjeeling–SikkimHimalaya and southern Tibet. Geophysical Journal International 184: 829–852.

Agostinetti NP, Lucente FP, Selvaggi G, et al. (2002) Crustal structure and Mohogeometry beneath the Northern Apennines (Italy). Geophysical Research Letters29: 1999. http://dx.doi.org/10.1029/2002GL015109.

Agudelo W, Ribodetti A, and Collot J-Y (2009) Joint inversion of multichannel seismicreflection and wide-angle seismic data: Improved imaging and refined velocitymodel of the crustal structure of the north Ecuador-south Colombia convergentmargin. Journal of Geophysical Research 114: B02306. http://dx.doi.org/10.1029/2008JB005690.

Aki K (1982) Three-dimensional seismic inhomogeneities in the lithosphere andasthenosphere. Reviews of Geophysics and Space Physics 20: 161–170.

Aki K, Christoffersson A, and Husebye ES (1976) Three-dimensional seismic structureof the lithosphere under Montana LASA. Bulletin of the Seismological Society ofAmerica 66: 501–524.

Aki K and Lee WHK (1976) Determination of three-dimensional velocity anomaliesunder a seismic array using first P-arrival time from local earthquakes, 1.A homogeneous initial model. Journal of Geophysical Research 81: 4381–4399.

Aki K and Richards P (1980) Quantitative Seismology. Theory and Methods. SanFrancisco: Freeman.

–180–90

–60

–30

0

30

60

90Crustal thickness at 2×2 degrees

–120 –60 0 60 120 1800

10

20

30

40

50

60

70

(Km)

Figure 24 Crustal thickness provided by the 2! " 2! CRUST2.0 model (Bassin et al., 2000). The locations of seismic profiles used to make this modelare shown in Figure 11. Typical crustal velocity models are summarized in Figure 21. The crustal structure is estimated in regions with no seismicmeasurements using the statistic average illustrated in Figure 23. This crustal model is useful to make crustal velocity (P- and S-wave) or densitycorrection in many types of geophysical models.

372 Crust and Lithospheric Structure - Global Crustal Structure

全球の地殻の厚さ Global crust thickness model

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• 地殻の古典的な認識：地殻はマントルから分化

• モホロビチッチによる地震学的不連続面の発見

• 地殻・マントル境界の地震学的な定義

• 大陸と海のモホ面の深さの違い

• 大陸 39.17±8.52km (Christensen and Mooney, 1995)

• 海　7.0±0.8km (White et al., 1992)

モホ面がいつも観測できるとは限らないモホ面と岩石境界が一致するとは限らない

地殻の厚さ average thickness of crust

Classic petrological view: crust is differentiated from mantle

Discovery of Mohorovičić discontinuity (seismic discontinuity)

“seismically” definition of crust/mantle boundary

Depths of Moho discontinuity

continent : thick and large regional variation

ocean floor: thin and homogeneous

attention! Moho can NOT ALWAYS clearly observed.

Moho does NOT coincides with petrological boundary. 43

海洋底の年齢 age of oceanic crust

young! no old seafloor remains…

44

• 海底は現在の地球上で最も活動的な場所

• ほとんどすべてのプレート境界は海底にある

• 海底には過去2億年の地球の歴史が刻まれている

• 連続的な地球環境変動を追えるのは海底堆積物だけ

• 風化浸食の影響が少ない

海洋底を研究する意義 Why do we learn “ocean floor”?

Ocean floor is most active part of the present-day Earth’s surface.

Most plate boundaries locate in deep ocean.

Ocean floor records the Earth’s history since 200 Ma.

Marine sediments are unique, continuous record of environmental change.

Little disturbance by erosion/weathering

Main Themes of “Ocean Floor Geodynamics”

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