Climate and Sea-level ChangesClimate and Sea-level Changesin Hong Kongin Hong Kong

During the Past 500,000 YearsDuring the Past 500,000 Years

過去過去 5050萬年香港氣候和海平面變化萬年香港氣候和海平面變化 W.W.-S. YimW.W.-S. Yim DSc (London) PhD (Tasmania) DSc (London) PhD (Tasmania)

Department of Earth Sciences, The University of Hong KongDepartment of Earth Sciences, The University of Hong Kong

Order of presentationOrder of presentation

(1) Background

(2) Evidence for climate and sea-level changes in Hong Kong

(3) Applications -

Engineering properties of seafloor deposits

Global carbon cycle

Evolution of coastal landforms

What is the evidence for climate and sea-level What is the evidence for climate and sea-level changes in Hong Kong?changes in Hong Kong?

Based on the recognition of 2 types of deposits on the seafloor in excavations and boreholes –

(1) Marine deposits

Formed during ‘warm’ interglacial periods with ‘high’ sea levels with elevations similar to the present day.

(2) Terrestrial deposits

Formed during ‘cold’ glacial periods with ‘low’ sea levels about 120 m below the present day. Land bridges existed e.g. between Asia and north America and between the Kowloon Peninsula and Hong Kong Island.

Satellite photo of Hong Kong Sea-level datumsSatellite photo of Hong Kong Sea-level datumsand the Pearl River Estuaryand the Pearl River Estuary(band 5)(band 5)

Examples of excavations and drilling of Examples of excavations and drilling of the seafloor resultingthe seafloor resulting

in important discoveriesin important discoveries

(1) Sheung Wan Station excavation, Island Line, MTR

(2) Western Harbour Crossing excavation

(3) Drilling of the seafloor to search of fill resources

Excavation of the Sheung Wan Station,Excavation of the Sheung Wan Station,Island Line, MTR Island Line, MTR

Radiocarbon age 30 kaUranium-series age 130 kaMTR construction

Sea-floor excavation of Victoria HarbourSea-floor excavation of Victoria Harbourfor the Western Harbour Crossing for the Western Harbour Crossing

7,000-year old oyster shells

Corals

Evidence from offshore drillingEvidence from offshore drilling

A drill barge

Drilling in action A continuous 60-m core drilled for sand search

Comparison between marine & terrestrial depositsComparison between marine & terrestrial deposits_______________________________________________________________________

Feature Marine deposits Terrestrial deposits_______________________________________________________________________

Fossils Marine fossils may be Terrestrial fossils may bepresent e.g. corals, present e.g. plant remains,clams, snails, etc. pollen, etc.

Colour Usually darker Usually paler

Grain size Usually finer Usually coarser

Compaction Usually softer Usually firmer

Water content Usually higher Usually lower

Iron content Usually lower Usually higher_______________________________________________________________________

Two examplesTwo examples

(1) High Island Reservoir

(2) Drillhole made in the West Lamma Channel during 1992 for the Western Harbour Development Study

Example 1 High Island ReservoirExample 1 High Island Reservoir

Before

AfterWest Dam

East Dam

M1

T1

?T2

M2

T1

Palaeo-desiccated crust

Example 3 Drillhole in the West Lamma ChannelExample 3 Drillhole in the West Lamma Channelfor the Western Harbour Development Studyfor the Western Harbour Development Study

+ WB7

Samples at about 0.5 mIntervals in Borehole WB7

Simplified logsheet

M1

M2

T2

M3T3

M4

M5

T5

Residual soil

+ WB7

M1 M2T2

M3T3 M4

M5T5

Offshore geological modelOffshore geological model_______________________________________________________________

Unit Age Estimated age (ka) Maximumthickness

(m)_______________________________________________________________

M1 Postglacial < 8.2 21.5T1 Last glacial 8.2 – 70 6.5M2 Last interglacial 90 – 140 15.7T2 2nd last glacial 150 – 180 9.5M3 2nd last interglacial 190 – 240 12T3 3rd last glacial 250 – 300 7.3M4 3rd last interglacial 310 – 340 14.1T4 4th last glacial 350 – 370 6M5 4th last interglacial 380 – 420 3.5T5 5th last glacial > 440 7______________________________________________________________

How is age determined?How is age determined?

(1) Order of marine and terrestrial deposits(2) Dating of suitable samples

Methods used -Radiocarbon (reliable when younger than 8.2

ka)Uranium-series (up to 500 ka)Luminescence (up to 1000 ka)Cosmogenic nuclides (up to 5000 ka)

(3) Correlation with other parts of the world e.g. ice cores

(4) Other indirect methods e.g. fossil evidence, engineering properties

Palaeotemperature reconstructionPalaeotemperature reconstructionwith oxygen isotopeswith oxygen isotopes

Oxygen isotopes 16O 17O 18O

% distribution 99.763 0.033 0.204

Samples - foraminifers (marine) seawater

corals (marine) seawater

snow and ice (terrestrial) air

Palaeotemperature estimation –18O : 16O ratios is measured by a mass spectrometer

Vostok ice core in AntarcticaVostok ice core in Antarctica

Vostok Station

Annual layers

Present day conditions (interglacial periods)Present day conditions (interglacial periods)Mean temperature – 24oC

Temperature range – 16 to 29oC

Sea level – within 3 m from present

Frost – uncommon

Rainfall – ca. 2225 mm/annum

Wind – southwest monsoon in summer and northeast monsoon in winter

Flooding of the Tin Shui Wai areaduring Typhoon Brenda in May 1989

Conditions during ice ages (glacial periods)Conditions during ice ages (glacial periods)

Mean temperature – 15oC

Temperature range – 8 to 21oC

Sea level – ca. 120 m below

present

Frost – common on high ground during winter

Rainfall – drier than present

Wind – stronger during winter

Tafonis a feature common on rocky granite coasts were probably formed by wind erosion during ice ages

Cheung Chau

What happened during ice ages?What happened during ice ages?

+

Hong Kong

Evidence from vegetation in the pastEvidence from vegetation in the past

A mangrove pollen found in the M2 unit of the New Hong Kong Airport site

_________0.1 mm

ApplicationsApplications

(1) Improvement in understanding in the engineering properties of seafloor deposits.

(2) Termination of ice ages and the global carbon cycle.

(3) Evolution of coastal landforms.

Foundation studies of the New Hong Kong AirportFoundation studies of the New Hong Kong Airport

Impact rollers used in ground compaction

Evidence from engineering properties after Choy (2004)Evidence from engineering properties after Choy (2004)

M1

M2

M3

T3

M4

Both COBoth CO22 and CH and CH44 show changes resembling temperature in the show changes resembling temperature in the

Vostok ice core of AntarcticaVostok ice core of AntarcticaProblem –Problem –Source of COSource of CO22 and CH and CH44 following the glacial maxima following the glacial maxima

Chemical reactionsChemical reactionsAbove water table :

FeS2 → Fe+++ + SO4--

[O]

2 Fe+++ + 4 H2O → Fe2O3 . H2O + 6 H+

2 H+ + SO4-- → H2SO4

CaCO3 + H2SO4 → CaSO4 + H2CO3

H2CO3 → H2O + CO2

Below water table :

H2CO3 → 2 H+ + CO3--

Fe+++ → Fe++

[R]

Fe++ + CO3-- → FeCO3

Evolution of coastal features –Evolution of coastal features –How long does it takes to form such features?How long does it takes to form such features?

Sea cave in Ap Chau

East Ping Chau

48-m high sea cliff

3-m high wave-cut notch

Wave-cut platform

Main conclusionsMain conclusions(1)Study of the Hong Kong sea floor has provided a record of 0.5

million years of sea-level changes.

(2) Production of greenhouse gases from aub-aerially exposed continental shelves may be responsible for the termination of ice ages.

(3)Earth history provides explanation of -

engineering properties of sea-floor sediments

changes in the global carbon cycle

coastal features found in Hong Kong

“… and we can save 2,000 liraby not carrying out a geological investigation of the foundations.”