全球海準面變遷
--兩萬年來的記錄
黃文樹
Introducation
• The importance of sea level
change……
• Paleoclimatic,
paleoenvironmental change
could be recognized by the
identification of sea level
change.
• Hence, it is important to
many scientist of varied
fields, such as geologist,
geomorphologist,
geographer, archeologist,
oceanographer,
paleobiologist… and so on.
Mean sea level (MSL)
• When periodical and random sea movements are
filtered out, a stable sea surface can be obtained.
• Altimetric measurements are related to a reference
level which corresponds to MSL calculated at a
selected tide-gauge station over a specified period.
• However, accurate levellings have shown that along
the coasts of a continent MSL varies from place to
place, and at each place it also varies over time.
• As a generalization, if tidal forcing, differences in
water density, currents and atmospheric forcing are
left aside, MSL can be defined as an equipotential
surface of the gravity field.
Causes of sea-level change
• Climate change: Glacial time and deglacial
Causes of sea-level change
• Climate change: Glacial time and deglacial
Warm climate
cold climate
Oxygen isotope record for the past 2.6Ma from benthonic foraminifera of
ODB core 677 (Shackleton et al., 1990)
Causes of sea-level change
• Isostatic imbalance
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Glacio-isostasy
Volcano-isostasy
Sediment-isostasy
Hydro-isostasy
Causes of sea-level change
Coseismic uplift
• Neotectonic
deformation
Gradual sea-level fall
A sea-level flucutation superimposed on a gradual
uplifting trend
Causes of sea-level change
• Compaction of
sediments
• Human-induced
subsidence
• Steric changes
• Tidal changes
Evidence of former sea levels
• Biological features, fossils, residuals in littoral (marine)
environment.
• Erosional features: geomorphologic features.
• Depositional features: coral reef, reef plat …and so on.
• Sedimentary shores: mangrove, grain-size distribution,
beach-rock, peat layers……
• Archaeological and historical sea-level indicators: two
main categories—(1)located near shoreline;
(2)depending for their use on tidal fluctuations and
marine conditions.
Dating a sea-level indicator
•
The predominant dating methods is: Carbon 14. The convention of
C14 dating:
1.
2.
3.
4.
The 14C activity is defined internationally and is assumed to have always
been the same;
There has been no inclusion of new 14C in the sample after its death;
The 14C half-life used is 5568 years;
14C ages are given in years before present (BP), i.e. before AD 1950 and
have to be corrected for isotope fractionation, based on the measure
13C/12C ratio.
•
Uranium/thorium dating.
Sea-level (m)
To construct a sea-level curve
C14 time
The ice age Earth
• Glacial-interglacial sequences, constructed by
oxygen isotope analyses, show an approx. 100 ka
periodicity.
• The last interglacial: 140-110 ka BP; (up to 4 oC
warmer than the Holocene)
• The last glacial period: approx. 18 ka BP
The ice age Earth
• But, under high-resolution isotopic analysis of icecore:
•
The duration
of these
interstadials
ranges from
about 500 to
2000 years,
and their
irregular
occurrence
suggests
complexity in
the behavior
of the North
Atlantic
Ocean
circulation.
The sea-level positions during the last ice age
• Shackleton and Opdyke (1973) suggested that a
change of about 10m in the global sea level
corresponded with 0.1 0/00 change in isotope record.
• The global sea-level rise since the last glacial
maximum can be estimated to be about 130m
(Chappell and Shackleton, 1986)
• However, inferences to the end of the last glacial was
different according to varied investigations. Moreover,
the rise volume of sea-level since last glacial was
estimated discrepantly.
Low sea-level land bridges
• Many
continental
shelf areas
which are
now
submerged
were
exposed
land at the
last glacial
maximum.
Low sea-level land bridges
Two migratory waves from
Asia to north America: one
was at least 25ka ago; one
is dated from the early
holocene.
Geat Britain was indeed
connected to the European
continent when the channel
and the southern part of
North Sea were emerged
land.
A land bridge connected
Asia, New Guinea and
Australia. Some evidences
pointed out two episodes
of invasion: the first is
more than 50ka ago; the
2nd is about 10 or 20 ka.
Low sea-level landscape-oceanic island case
Paleoriver valley
submerged terrace
Emerged land
Sea floor
paleolagoon
Deglacial sea-level changes
• The last deglaciation seems to have started earlier in Antarctica
(17ka BP) than in Greenland (15 ka BP).
o
• The coldest periods of the last glacial were probably 21 C
colder than at present over the Greenland ice sheet.
• Increasing evidence supports the idea that the deglaciation
occurred in two steps: a first warming period which peaked at
the Bolling (about 13-12 ka BP.); a second warming period after
about 10.3 ka BP, separated by a temporary cooling and
southward migration of the polar front, very marked in North
Atlantic areas.  the Younger Dryas
• Broecker et al. (1985, 1989)suggested that it may have been
caused by a rapid lowering of salinity in the surface water of the
whole northern part of the North Atlantic, following a diversion of
the Canadian ice-sheet meltwater flow from the Mississippi to
the St. Lawrence River.
Global isostatic model results
• Global isostatic models may predict relative sea-level changes
forced by deglaciation.
• The models are usually based on the mathematical analysis of
the deformation of a viscoelastic Earth produced by surface
mass loads.
• Isostatic effects of deglaciation have also been modelled on a
regional scale:
Global isostatic model results
Near-field sites
(zone I): the
dominant
contribution to
sea-level
change comes
from ice-load
effects, and the
characteristic
late-glacial and
postglacial
relative sea-leve
curve is one of
almost
exponential fall
up to the
present because
of rising land
Ice-margin sites
(zone I-II): the
relative sealevel change is
characterized by
and initially
rapid fall during
the late-glacial
stage, followed
by a period of
relative stabiligy,
then by a rise in
level, and finally
by a more less
uniform sealevel fall to the
present position.
Intermediatefield sites: the
relative sealevel continues
to rise even
when
deglaciation has
ceased, though
at gradually
decreasing rates.
Far-field sites:
relative sea-level rise
only predominates
during the
deglaciation period.
Case: far-field sites
from East Asia
However,
insufficient to
provide precise
local relative sealevel histories in the
east Asia
A gradually rising or a fluctuating sea-level
•
•
1.
2.
3.
Major climatic changes which lead to substantial variations in
the world balance. A long-debated point is whether minor sealevel fluctuations may also have occurred during the last
glacio-eustatic sea-level rise……
In fact, the major problem is related to different scales:
Global scale—a eustatic oscillation occurred in the past, it has
produced the same sea-level fluctuation everywhere around
the world.
Regional scale—relative sea-level oscillations may have
occurred during the same time interval throughout wide areas.
Many factors will effect the regional sea-level.
Local scale—possible causes of relative sea-level oscillation
are even more numerous.
Relative sea-level changes in the late Holocene
• Rates of relative sea-level change decelerated
considerably during the period from 10 to 6 ka BP.
• At about 6 ka BP, most of deglaciation had been
completed and most present-day deltas and coral
reefs were already in place.
• Any relative sea-level change after the period is a
consequence of: ice and water loading changes in
deglaciaion period; tectonic effects; climate and
oceanographic changes.
Continuance of
isostaty
The near-field
localities, the ice
load effects are
predominant!
Continuance of isostaty
The intermediated-field sites, a gradual rise, at rates varying
with the distance from the ice load.
Continuance of isostaty
The far-field sites, mild-Holocene emergence up to few
meters…….
Seismo-tectonic
displacements
Site to site differences
in height between late
Holocene shorelines
cannot always be
explained only by
glacio and hydroisostatic processes.
Present-day seal-level trends
• In some regions archaeological and historical data
may help to identify the present-day sea-level
changes.
• But poor supported by reliable sea-level indicators.
According to oxygen isotope……
• The
present
interglacial
period may
be nearing
an end and
another
cold glacial
phase is
likely to
ensue?
BUT, In the future……
• Intergovernmental Panel on Climate Change
assessment (IPCC, 1990): the “best estimate”
scenario for 2100: a sea-level rise of 66 cm
(high 110 cm to low 31 cm)
Thank you for your attention!