2015
1. 地球史のなかの海
• なぜ地球には海があるか：他の惑星にはないのか
• いつどのように海ができたのか
• 海が地球の歴史の中で果たした役割は何か
• 海洋底を研究する意義
1
私たちは特異な存在か？
D’oú venons nous? Que sommes nous? Oú allons nous?
Blue Planet Earth: from Apollo 17 (NASA,1972)
2
3
他の地球型惑星との比較
（宮本ほか、惑星地質学、2008）
4
海が存在できる理由
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
5
他の惑星・衛星に海がある可能性：火星
（宮本ほか、惑星地質学、2008）
6
NASA Sept 28, 2015
7
他の惑星・衛星に海がある可能性：エウロパ
（Images: NASA）
8
地球の特徴
• 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
9
バイモーダルな標高分布
海：
重い、玄武岩地殻、若い
陸：
軽い、花崗岩地殻、大半は安定
10
90°
°
12
0°
elevation (km)
60°
90°
Downloaded from www.sciencemag.org on May 27, 2012
60°
30
Downloaded from www.sciencemag.org on May 27, 2012
elevation (km)
火星の場合
-60°
-60°
12
0°
°
30
1498
28 MAY 1999 VOL 284 SCIENCE www.sciencemag.org
Fig. 5. Histograms of (A) topography, (B)
-3
°
30
0°
-
0°
33
-60°
0°
heights with respect to an ellipsoid shifted by
"2.986 km along the z axis, and (C) 100-km
baseline slopes. The histogram in (A) shows
the distinct bimodal signature that represents the elevation difference between the
northern and southern hemispheres. That in
(B) shows elevations plotted with respect to
an ellipsoid whose center is shifted so as to
remove the effect of the COM-COF offset
along the polar axis. The effect of the shift is
to produce a unimodal distribution of elevations; that is, the hemispheric difference in
elevation largely disappears.
H2O
15
-60°
498
Downloaded from www.sciencemag.org on May 27, 2012
agnetometer
exface geology
regionalconvection
topography ap-sphere.
topography
the 6
south
polar re- that
ern hemisphere
crust byand
mantle
Together,
Figs.
5inand
demonstrate
resurfaced comparable
northernspatial
hemisphere.
the
hemispheric
elevation difference
is due is
to highest
long-wavelength
effect.
scale occur in Large
association
omalies
high anpears
to period
contain three
dominantplate
contribu-the gion
within the residual
ice deposits
(87°S,of the
(31,of 32),
early
tectonic
accounts
forofmost
(though smaller
than hemispheric-scale)
imtheoflong-wavelength
planetary COM-COF
shape rather offset A
projection
the northern hemisphere
with regional
topographic lows in the
northterpreted as evitions: (i) volcanic construction associated
!10°E), where a broad dome is present with
than
the
boundary
scarp (23,
37 ); thisdifference
issue
topography
from
the pole toand
the equator
(Fig.
ern
hemisphere,
except
for
themarked
previously
recycling
(33),
and
one
or
more
large
impacts
elevation
between
the
northern
pacts
such
as
Utopia
and
Hellas
are
s hydrodynamic
with Tharsis, (ii) major excavated deposits
more than 3 km of relief at one end of the cap
7
can(34
be–36).
explored from the global
distribution
7) further
illustrates
the nature
of the by
northern
known
Isidis impact
structure that
sits on the
in the
northern
hemisphere
and
indicate
distinctive
positive
gravitational
anoma43). The
largest
approximately
circumferential
to Hellas, withsouthern
(Fig. 8).hemispheres
The relief of the
southern
polar that
cap the
of elevations
and slopes
(Fig.
5). As shownelevation
in
hemisphere
depression.
Note athe distinctive
thus suggest
is to contributions
determine
how
much
ofprobhemispheric
difference
is primarily
onfined A
to key
the issue
lies (40). dichotomy
However, boundary.
no otherWeanomalies
of that
additional
from Isidis
and
is comparable to that of
the northern
cap.
Fig.and
5, which
plots histograms
of The
elevations
circular signature
the buried Utopia basin,
the long-wavelength topographic expression
because
ably elevation
Utopia ejecta,
(iii) modification
area ofeffect.
probable
ice-rich of
material
theanomahemispheric
difference
is due
to ofthe
long-wavelength
comparable
scale
occur in association
海洋底ダイナミクス2013-01
before
and
after
removing
offset
between
originally
proposed
asthat
an impact feature
on
ofspatial
the northern
hemisphere
depression was
r large
southern
the
intervening
region
by
fluvial
processes
greatly
exceeds
the
region
of
residual
ice
the long-wavelength planetary
shape
rather
A projection
the
northern
hemisphere
regional
topographic
lowsprocess
in theornorthand channels
COF
along
the polar
axis, the from
z ofthe
basis
of
geological
evidence (35),with
but not
shaped
by an internal
processes.
rapid decay in
associated with the
the COM
outflow
that
is apparent
images.
Support
for this
the boundary
scarp
(23,
37); In
this
issue
the
pole
equator
ern The
hemisphere, except for the previously
component
ofaddition,
the
COM-COF
differencefrom
is comes
observed
in the
earlier
topographic
and than
the global
empty into
Chryse
Planitia.
pre-topography
interpretation
from to
the
existence
of(Fig. studies.
to the
hemispheric
elevaexpression
Utopia is apparent
de-Isidis impact structure that sits on the
can be
explored
from
the largely
global
distribution
7) further
illustrates
the nature
of theofwith
northern
known
00 million
years
viously
documented
[for equivalent
example,
(35)]
condistinctive
plateau circular
regions
that correlate
tion
difference.
A
histogram
oflayered
the distribuspite
location
the area of dichotomy
northern
d (43).ofEither
an
tributions
from(Fig.
fracturing
resurfacing
terrain
units
(19),itsas
wouldthe
bewithin
expectelevations
and slopes
5).
As and
shown
in
hemisphere
depression.
Note
distinctive
boundary. We thus suggest that
tion of 100-km
baseline
slopes ed
(38)
hemisphere
resurfacing
(5, 39). However,
ecycling
(33,5,43)
dictate
thathistograms
the boundary
formed
in response
if calculatthe
layers were
deposited
on
cratered
Fig.
which
plots
ofthe
elevations
circular
signature
of
the
buried
Utopia
basin,
theoflong-wavelength
topographic expression
ed
from
global
topographic
grid
(Fig.
5C)
even
after
accounting
for
the
formation
the
n would be conto multiple, complex mechanisms.
terrain. In addition, impact craters within the
before
and
after
removing
the
offset
between
originally
proposed
as
an
impact
feature
on
of
the
northern
hemisphere
depression was
peaks
at
!0.3°
and
is
long-tailed,
the
latter
Tharsis
rise, presumably
namo.
Polar caps and present surface volatile
plateaus share unusual
geometric
properties subsequent to the
COF The
along
the
polar
axis, the
z lowthe with
basis
of geological
(35),
buthemispheric
not
shaped
feature
result
ofthe
topographic
excursions
process
that produced
the
eleva- by an internal process or processes.
ts onthe
the COM
litho- and
budget.
process
thata produced
counterparts
in theevidence
north
polar
region
associated
withearly
volcanoes,
and
tion
difference,
the
boundary
of the northern
of the COM-COF
difference
observed
earlier
topographic
studies.
heat component
flow in the
northern
hemisphere
occurred
inis mar-impact
(46basins,
) thatinare
observed
to have formed
in an The
features.
Thedifferslopecircular
corresponding
to
hemisphere
depression
is clearly
s come
from the
tian history.
Thetectonic
resulting
elevation
ice-rich
substrate.
This
similarity
suggests
largely
equivalent
to the
hemispheric
elevaexpression
of Utopia
is apparent
de- noncircular,
peak inthe
thetransport
distribution
isthat
close
to the portions
and we
see south
no compelling
y signature
(40)
ence A
must
have the
dominated
ofspite
ofthe
the
polar
ice topographic evition difference.
histogram
of the distribuitssignificant
location
within
area
of northern
average longitudinal
dence
for a mantling
hemispheric-scale
single impact
mplitudes of the
water on Mars throughout
its history.slope
The associated
cap may with
be buried
beneath
dust
100-km baseline slopes
(38) calculat(5, 39). proposed
However,
the COM-COF
offset alonghemisphere
the z axis andresurfacing
is
such as previously
(34 ). Formation
maliestion
(43).ofThe
from
(Fig.
5C)
even
forelevation
the formation
playededby
Hel-the global topographic
partly agrid
result
of the
offset. But
theafter
peak accounting
slope
of the
differenceofbythe
multiple smallal lithosphere
also containsthe
contributions
to Tharsis
er impactssubsequent
has also beento
suggested
peaks atof !0.3° and is long-tailed,
latter due
Tharsis
rise,andpresumably
the (36 ). Be-10
-2
-1
-0.5
0
0.5
1
2
10km
ormation
was ata result of topographic
regional-scale
features.
yondthe
Utopia,
however, no
circular structures
feature
excursions
process that produced
hemispheric
elevathickness and
Subtracting
the and
COM-COF
offset
along the theofboundary
comparableofscale
are
apparent in the toassociated
with
volcanoes,
impact
basins,
tion
difference,
the
northern
90°N
ied stress differpolar axis from the global topography model
pography of the northern plains, although
tectonic features. The slope
corresponding
to ofhemisphere
depression
is clearly
noncircular,
an lithospheric
(Fig.
6) eliminates most
the hemispheric
arguably
such structures
could have been
the peak
the distribution is close to the
and we see no compelling topographic evihe South
Pole– in 60°N
0) with
its lower longitudinal slope associated with
average
dence for a hemispheric-scale single impact
Fig. 4. Regional topowouldthe
be COM-COF
larger
the model
z axisofand
is
such0 as previously proposed (34). Formation
graphic
the Hel30°Noffset along
km thickness of
basin.
a result of the offset.lasBut
the(Top)
peakAzimuthslope
of the elevation difference by multiple smalln unitspartly
that blanally averaged radial toalso contains
contributions
due to Tharsis
andcal- er impacts has also been suggested (36). Bewere removed.
If
0°
pography
used in the
-5
ly compensated,
regional-scale features. culation of infilling the yond
Utopia, however, no circular structures
basinoffset
with along
surrounding
ata (40), Subtracting
then a
the
COM-COF
the
of
comparable
scale are apparent
in the to-3000
0
3000
30°S
material
postulated
to
ndicates that the
distance (km)
polar
axis
from
the
global
topography
model
pography
of
the
northern
plains,
although
have
been
excavated
at the time of
it. (Bottom)
Color- arguably such structures could have been
6) eliminates
most from
of the
hemispheric
km. (Fig.
The thick60°S
coded topography plotted
gnetized crustal
in an equal-area projecor the observed
with the same scale
90°S
Fig. 4.no Regional
topo- tion
agnetizations
as240°
that in Fig. 2.300°
The black
0°
60°
120°
180°
model of180°
the Hel- lines
0
correspond
to zero0°
estrialgraphic
rocks also
Fig. 6.
Map of Mars’
shape contours.
with zonal spherical harmonic degree 1 (COM-COF offset along the
basin.
Azimuthelevation
were las
below
the (Top)
polar
z axis) toremoved. The projection is rectangular to show topography from pole to pole. Note
averaged
radial
minantally
magnetic
the general similarity in elevation between the northern and southern hemispheres. The figure
pography
in the cals before
the Hel- used
highlights the two other-5significant components of martian topography: the Tharsis province and
culation
infilling
the basin. Here we have not removed shorter wavelength topographic features,
mplitudes
of the of the
Hellas impact
Fig. 5. Histograms of (A) topography, (B)
basin elaswith including
surrounding
those composing the dichotomy
boundary
scarp.
omparable
(Smith et
al., 1998）
heights with respect to an ellipsoid shifted by
-3000
0
3000
material postulated to
distance (km)
"2.986 km along the z axis, and (C) 100-km
-3
0°
0°
have www.sciencemag.org
been excavated SCIENCE VOL 284 28 MAY 1999
baseline slopes. The histogram in (A) shows
1499
-3
the distinct bimodal signature that repre- 11
from it. (Bottom) Color°
0
5
1
sents the elevation difference between the
coded topography plotted
northern and southern hemispheres. That in
in an equal-area projec(B) shows elevations plotted with respect to
tion with the same scale
8
33
an ellipsoid whose center is shifted so as to
0°
as that in Fig. 2. The black
remove the effect of the COM-COF offset
lines correspond to zero0°
along the polar axis. The effect of the shift is
to produce a unimodal distribution of elevaelevation contours.
tions; that is, the hemispheric difference in
elevation largely disappears.
28 MAY 1999 VOL 284 SCIENCE www.sciencemag.org
12
地球表層における水の分布
1.4×109 km3
2.5×105 km3
2.5×105 km3
2.3×107 km3
1.3×104 km3
1.5×105 km3/year
(日本海洋学会編、海と地球環境、1991）
13
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
(Press, Understanding Earth, 2003）
14
New!
Holocene開始
＝2.58
顕生代
この図は古い定義
原生代
New!
Quaternary開始
＝2.58
始生代（太古代）
冥王代
(Press, Understanding Earth, 2003）
15
16
初期地球の形成
EA40CH21-Arndt
ARI
23 March 2012
15:30
• 太陽系の惑星がほぼ一斉にできる∼およそ46億年前
INTRODUCTION
• 最初の一億年：微惑星衝突の時代
Our image of the early Earth has changed greatly over the past two decades, with important
consequences for models of the origin and early evolution of life. Figure 1 shows two contrasting
• 微惑星の衝突速度が増大（地球半径~2000km)
Ga: billion
years
views of the surface of earliest Earth. In the 20-year-old image in panel a, we see a Hell-like,
• 脱ガスによる原始大気（CO
Hadean landscape of fiery volcanoes
beneath a menacing red sky; the more recent image in panel
2,N2,水蒸気）の形成
b shows luxuriant microbial colonies on a tranquil beach. What caused this dramatic change in
• 衝突エネルギーの解放と大気の保温による地表の温度上昇
our vision of the young Earth?
The discovery of >4-Ga-old (>4-billion-year-old) zircons (Cavosie et al. 2005, 2007; Froude
マグマオーシャンの形成（地球半径~3000km, 表層100気圧）
et al. 1983; Wilde et al. 2001) radically changed our interpretation of the first part of Earth’s
history. The zircons, found first in Mount Narryer and then in Jack Hills, both in Western
微惑星の衝突頻度の減少
Australia, have ages from approximately 3.1 Ga (that of the quartzitic metasediment from which
they were extracted; Kinny et al. 1990) to just over 4.4 Ga (Wilde et al. 2001). The latter age
• 大気温度低下
is some 150 million years less than accretion at 4.56 billion years ago (4.56 Gya), and merely
100 million years after the impact of the planetesimal Thea that led to the formation of the
• 雲の形成→降雨
modern 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 compositions
• 原始海洋の形成
of 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 granitoids
マグマオーシャンの固化：（海洋性）地殻の形成
define the continents, it follows that some felsic crust had already formed at this early stage of
Earth’s history. Campbell & Taylor (1983) noted that, on the modern Earth, granite forms in
17
abundance 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 isotopes
in the minerals dating from >4 Gya—the oldest zircon they measured has a δ18 O of +4!, a value
Gya: billion years ago
•
Annu. Rev. Earth Planet. Sci. 2012.40:521-549. Downloaded from www.annualreviews.org
by University of Tokyo on 09/29/13. For personal use only.
•
•
Old concept of the Hadean Earth
a
Updated reconstruction
b
Figure 1
Two views of the young Earth. (a) Image reflects the old idea that the Hadean was Hell-like, covered by lava and impacted by
meteorites. (b) A more recent image shows oceans and primitive life (from http://archenv.geo.uu.nl/).
522
Arndt
·
Nisbet
18
海洋底ダイナミクス2013-01
%"
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(Rogers, Our Dynamic Planet, 2007）
8Rogers, Our Dynamic Planet, 2007）
19
11
海はいつできたのだろうか？
海洋底ダイナミクス2013-01
• 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
• 大陸があった証拠でもある
20
OLDEST PIECES OF THE PLANET
THE TALES THEY TELL
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 identified formed more than
COOL OCEANS
The oldest age for a Jack Hills zircon— 4.4 billion years (red) — is an
Acasta
exact match of
twoGneiss
geologic “clocks.” Two pairs of isotopes —
(oldest intact rock,
uranium 235
– lead
207
4 billion
years
old)(vertical axis) and uranium 238–lead 206
(horizontal axis) — form two radioactive timekeepers that start
ticking when a zircon forms. If they are well preserved, their final
Rocks older than
ratios plot
a single line (yellow). Dates from other parts of
2.5along
billion years
the zircon (pink)
fall off this line because some lead was lost
Jackfrom
Hills zircons
Inferred
(oldest earth material,
these areas,Exposed
but scientists can correct for this damage.
Oxygen isotope ratios in Jack Hills zircon samples (blue),
Fossilized
bed in theonly
Jack Hills
which range up
to 7.5, gravel
are possible
if their source rock
(above) contained
the world’s oldest
zirformed in a relatively
cool, water-rich
environment
near the
cons yet discovered. Geologists crushed
earth’s surface.
Had
magma
oceans
covered
and sorted hundreds of kilograms of thisthe planet when
these zirconsrock
formed,
values
would have
(below)their
to find
the 20 crystals
that clustered near
bearofsigns
of cool conditions
more
thanthat originates in
5.3, as do those
all crystals
from hot
rock
four billion
years
ago.
the planet’s deep
interior
(red).
1.20
20 Dates in billions
of years ago 4.2
Ancient rocks older than 2.5 billion years crop out or lie just underneath the soil in many spots
15regions
around the globe (red) and are probably hidden below younger rocks across even broader
0.80
(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 zircon
date from earth
Expected value for
the earth’s hot mantle
10 -
port the surviving grains great distances The Jack Hills conglomerate was deposbefore
0.40 they become incorporated into ited three billion years ago and marks
All may
dates inthe
thisnorthwestern
region
edge of a widespread
deposits of sand and gravel that
5fall between
4.2 billion
of rock formations that are all
later solidify into sedimentary rock.
In- assembly
and 4.4 billion years ago
deed, the Jack Hills zircons — separated older than 2.6 billion years. To recover
by perhaps thousands of kilometers from less than a thimbleful of zircons, my col0.00source— were found embedded in a leagues and I collected hundreds of0kilotheir
4.5
10
20
30
40
50
60
70
80
fossilized gravel bar called the Jack Hills grams of rock from these remote outRatio of Lead 206 to Uranium
238hauled them back to the labocrops and
conglomerate.
(Valley, 2005)
So, despite the excitement of finding ratory for crushing and sorting, similar
such primeval pieces of the earth, most to searching for a few special grains of
scientists,
including me, continued to sand on a beach.
FIRST
CONTINENTS?
Once extracted from their source
accept
the
view
that
the climate
of zircons
our
Rounded surfaces of
some
Jack Hills
under
rock,and
individual crystals could be dated
young planet
wasmicroscope
Hadean. It show
was not
a scanning
electron
that wind
because
until 1999
thatwater
technological
possibly
running
buffetedadvances
these crystals
overzircons make ideal timekeepers. In addition to their longevity, they
further
study ofacross
the ancient
zir-continental
— possibly
a large
longallowed
distances
contain
trace amounts of radioactive
con crystals
from
Western
— before
they
were fiAustralia
nally laid—to rest
(right).
landmass
Zircons
found near conventional
their place of origin
retain
their which decays at a known rate
uranium,
and challenged
wisdom
original
edgesearliest
( far right).
The large number of
aboutsharp
the earth’s
history.
Typical
mantle zircons
Jack Hills zircons
(includes all
billion-year-old
to lead. When a zircon forms from a>4
sozircons analyzed
lidifying magma, atoms of zirconium,
so far)
silicon and oxygen combine in exact
proportions (ZrSiO4) to create a crystal
structure unique to zircon; uranium occasionally substitutes as a trace impurity. Atoms of lead, on the other hand,
6.0 replace
6.5any
are5.0
too large 5.5
to comfortably
of the elements
in theIsotope
lattice, so
zircons
Oxygen
Ratio
start out virtually lead-free. The uranium-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 determine the age of an undamaged zircon
within 1 percent accuracy, which for the
early earth is about plus or minus 40
million years.
7.0
7.5
21
THE AUTHOR
ancient, rounded Jack Hills zircons suggests theirJOHN W. VALLEY received his Ph.D. in 1980 from the University of Michigan at Ann Arbor,
original
sourceDeep
rocks were widespread.
where he first became interested in the early earth. He and his students have since exDigging
t h e aus t r a l i a n z i rc ons did not
give up their secrets easily. For one thing,
the Jack
Hills andimplications
their surroundings
As the
possible
of are
the
dusty barrens at the edge of vast sheep
zircon discoveries
spread
through
the
scistations, called Berringarra and Mileuentific community,
was
ra, situated somethe
800excitement
kilometers north
of
Perth,
Australia’s
most
isolated
city.
palpable. In the superheated violence of
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
Van Hise
Professor of Geology
the University
of Wisconsin–
rock,
that evidence would supother
materials
thatR. were
encapsulated
as atgranitic
Madison, where he founded a multimillion-dollar laboratory called WiscSIMS. The cutport
the
hypothesis
that they are samtheting-edge
zircon capabilities
grew around
them.
Such
zirof the lab’s new CAMECA IMS 1280 ion microprobe will enable a
ples
of
the
world’s
fi
condiverse
inclusions
can
reveal
much
about
range of research; besides zircons, Valley and his colleagues will probe many rst continent. But
rare
or
extremely
small
materials
ranging
from stardust
cancer cells.
caution
is warranted. Quartz can form
where the crystal
came
from,
as can
the to
a Hadean world, no samples would have crystal’s growth patterns and the compow wfor
w. sgeologists
c ia m . c o m to study. But these
sition of trace elements. When Peck and
survived
COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
zircons pointed to a more clement and I studied the 4.4-billion-year-old zircon,
familiar world and provided a means to for instance, we found that it contained
unravel its secrets. If the earth’s climate pieces of other minerals, including
was cool enough for oceans of water ear- quartz. That was surprising because
ly on, then maybe zircons could tell us if quartz is rare in primitive rocks and was
continents and other features of modern probably absent from the very first crust
earth also existed. To find out, we had to on the earth. Most quartz comes from
look more closely into the interiors of granitic rocks, which are common in
more evolved continental crust.
single crystals.
If the Jack Hills zircons came from a
Even the smallest zircon contains
64
Expected value for cool,
wet environments
4.4
Number of Samples
Ratio of Lead 207 to Uranium 235
L U C Y R E A D I N G - I K K A N D A ; S O U R C E : W I L L I A M H . P E C K C o l g a t e U n i v e r s i t y ( m a p) ; J O H N W. V A L L E Y ( p h o t o g r a p h s)
4.4 billion years old)
SCIENTIFIC A MERIC A N
in the last stages of magma crystallizaS C I E Neven
T I F I C Aif
M Ethe
R I C Aparent
N 61
tion
rock is not granitic, 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 granitic, continental-style crust. Some scientists 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-
COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
OC TOBER 2005
22
L U C Y R E A D I N G - I K K A N D A ; S O U R C E : S I M O N A . W I L D E C u r t i n U n i v e r s i t y o f Te c h n o l o g y ( u r a n i u m - l e a d ) ;
ANCIENT AGE
Isua sediments
(oldest evidence
for life, 3.8 billion
years old)
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 fiery.
大気と海の進化 stage 1
• 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
23
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
(富山科学文化センター)
• 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
24
大気と海の進化 stage 2
• 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
• 22-19億年前か？
25
(ニュートン)
現世ストロマトライト（オーストラリア） (Wikipedia)
シアノバクテリア（藍藻類）
(富山科学文化センター)
ストロマトライト化石
(Wikipedia)
Banded Iron Formation (BIF) 縞状鉄鉱床
26
大気と海の進化 stage 3
• 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
27
28
海あってこそプレートテクトニクス？
• 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？
29
大陸と海：海あってこその陸
• 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
30
初期生命と海：海あってこその命
• ミラーの実験（1953）
• 水素・メタン・アンモニアからアミノ酸を合成
• 実際の原始大気は水素濃度が低いことが問題
• 化学進化による生命の進化
• 単純な有機物から複雑な高分子・タンパク質・核酸の生成、そして細胞膜
内システムの完成
• 化学進化を起こしたのはいつ？どこ？
• 炭素同位体比（生命活動が関与すると軽い）が示す38.5億年前
• 暖かい浅い海？深海の熱水噴出域？
31
数値は最適生息温度
32
(JAMSTEC, courtesy of Dr. Takai
33
地球の生命史は海・地球の歴史と結びつく
大量絶滅と海洋環境
34
現在の海洋底
35
海洋底を研究する意義
• 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
36
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