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 fi

ltered 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

Oxygen isotope record for the past 2.6Ma from benthonic foraminifera of ODB core 677 (Shackleton et al., 1990)

Warm climate

cold climate

Causes of sea-level change• Isostatic imbalance Glacio-isostasy

Volcano-isostasy Sediment-isostasy Hydro-isostasy

Causes of sea-level change

• Neotectonic deformation

Coseismic uplift

Gradual sea-level fall

A sea-level flucutation superimposed on a gradual uplifting trend

Causes of sea-level change

• Compaction of sediments• Human-induced subsiden

ce• 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, b

each-rock, peat layers……• Archaeological and historical sea-level indicators: two ma

in 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. The 14C activity is defined internationally and is assumed to have

always been the same;2. There has been no inclusion of new 14C in the sample after its

death; 3. The 14C half-life used is 5568 years;4. 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.

To construct a sea-level curve

C14 time

Sea

-leve

l (m

)

The ice age Earth• Glacial-interglacial sequences, constructed by oxyge

n 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 ice-

core:

• 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 cha

nge 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

Emerged land

Paleoriver valley

submerged terrace

paleolagoon

Sea floor

Deglacial sea-level changes• The last deglaciation seems to have started earlier in Antarctica

(17ka BP) than in Greenland (15 ka BP).

• The coldest periods of the last glacial were probably 21 oC colde

r than at present over the Greenland ice sheet.• Increasing evidence supports the idea that the deglaciation occu

rred 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 f

orced by deglaciation.• The models are usually based on the mathematical analysis of t

he 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 sea-level 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 sea-level fall to the present position.

Intermediate-field sites: the relative sea-level 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 sea-level histories in the east Asia

A gradually rising or a fluctuating sea-level• Major climatic changes which lead to substantial variations in

the world balance. A long-debated point is whether minor sea-level fluctuations may also have occurred during the last glacio-eustatic sea-level rise……

• In fact, the major problem is related to different scales:

1. Global scale—a eustatic oscillation occurred in the past, it has produced the same sea-level fluctuation everywhere around the world.

2. 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.

3. 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 hydro-isostatic 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!