Studium Generale

18 June, 2019

Fossil coral reefs and sea-level change:
diving into the past to predict the future
Marc Humblet
Department of Earth and Planetary Sciences, Nagoya University❖ What is a coral reef?
❖ What is sea-level change?

❖ Reconstructing sea-level history using
fossil coral reefs
Case studies: ⚫ Great Barrier Reef, Australia⚫ Seychelles Islands
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1. What is a coral reef?

REEF
Old Norse word (Vikings, 8th-11th century) → “RIF” = shallow ridge in the sea
Online Etymology Dictionary (Douglas Harper, 2010)“A shallow elevation composed of consolidated material that may constitutea hazard to surface navigation.”International Hydrographic Organization (2013)

Wikipedia

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1. What is a coral reef?

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1. What is a coral reef?

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1. What is a coral reef?
“Diversity of coral reef ecosystems represents 25% of known marine species”
from Ecological Society of America (ESA) website
Branching corals (Ishigaki-jima)

Branching corals (Ishigaki-jima)

Foliaceous coral (Ishigaki-jima)

Massive coral (Ishigaki-jima)

Flat worm on Halimeda sp. (Reunion) Nudibranch feeding on tunicate

Soft coral (Micronesia)

Sea star (GBR, Australia)

Saddled butterfly fish

Clown fish and anemone

Sea turtle (Madagascar)

Blenny fish (Solomon Isl.)

Image credits: reefbase.org, nationalgeographic.com, nature.org

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1. What is a coral reef?

MAIN REEF-BUILDING ORGANISMSCORALSCORALLINE ALGAE

Photosynthetic

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1. What is a coral reef?

MAIN REEF-BUILDING ORGANISMSCORALSCORALLINE ALGAE
tentaclecorallites

costaeoral disc
body cavity
oral cone
mouth
pharynx
gonadsmesenteries

septapaliformlobes
coenosteum

1 cm
columellawall

Photosynthetic symbionts
Veron (2000)

1.5 mmCusack et al. (2015)

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1. What is a coral reef?
FRINGING REEFS<1-2 km

Ishigaki Isl., Japan

Image credits (from left to right): Google Earth, NASA, Wikipedia

BARRIER REEFS

10-100s km

New Caledonia

ATOLLS

1m-10s km

Atafu Atoll

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1. What is a coral reef?

FACTORS INFLUENCING CORAL REEFS
Environ. factor

Optimum

Temperature

23-29ºC (mean winter >18ºC)
Prolonged heat wave leads to coral bleaching

Salinity

33-37‰ (normal marine)

Solar radiation

Within 30º Lat. N/S

Water motion

Some needed for metabolism, feeding

OxygenNear saturation

pH8.1-8.4 (related to pCO2)

Nutrients

Low concentration

Terrigenous sediments

Low input

Hallock, 2001. "Coral reefs, carbonate sedimentation, nutrients, and global change".
In: Stanley, G.D. (Ed.), " The History and Sedimentology of Ancient Reef Ecosystems ".

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1. What is a coral reef?

~18ºC

www.reefbase.org

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1. What is a coral reef?

Image credits (clockwise from top left): Wikipedia, Google Earth, Kathryn Berry/James Cook University, NOAA

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1. What is a coral reef?

A reef is a local accumulation of hard parts of marine organisms
living on the sea floor (sessile organisms) topographically higher than
the surrounding sediments.
After Reef Evolution (Wood, 1999)

SEA SURFACE

Topographic high
SEAFLOOR

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1. What is a coral reef?

REEF SLOPE
REEF CRESTLAGOON

CoralsImage credits: Left- Google Earth, Right- IODP Exp. 325

Coralline algae
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1. What is a coral reef?

380,000,000 years old reef (Belgium)

REEF1m15/63

1. What is a coral reef?

380,000,000 years old reef (Australia)

Today, the Great Barrier Reef (Australia)

4 km

REEF

2 km

Google Earth300 km

16/63❖ What is sea-level change?

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2. What is sea-level change?

⚫ Recent sea-level rise

1900–2010

1993–2012

1.65 +/- 0.2 mm.yr-1

3.2 +/- 0.4 mm.yr-1

Tide gauges

Satellite altimetry

Cazenave & Le Cozannet (2014)

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2. What is sea-level change?

Bertler & Barrett (2014)

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2. What is sea-level change?

⚫ Predictions
10 mm.yr -1

To predict the future, learn about the past.
Pessimistic
Models predicting
the future trend of sea level 0.2
or –climate
1 m / 100 yrs(RCP8.5) what wen know of the longneed to bescenario
validatedagainstterm sea-level and climate history.

Models need to be able to reproduce changes in sea level
mm.yr -1and climate that we identify when we study Earth2.5
history.
Optimistic
scenario (RCP 2.6)

IPCC (2013)

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2. What is sea-level change?

Image credit: Hironobu Kan

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2. What is sea-level change?

ABSOLUTE sea-level changeRELATIVE sea-level change

EarthEarth

Relative to the center of Earth

Relative to the land at a particularlocation

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2. What is sea-level change?

Absolute sea-level change:⚫ Increase/decrease of ocean volume⚫ Variations in polar ice volume⚫ Temperature-related changes in ocean volume

Relative sea-level change:⚫ Up/down movements of the land

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2. What is sea-level change?

Variations in polar ice volume
Glacial (cold, ice age)

Interglacial (warm)
Polar ice cap
LandOceanEarth

Earth25/63

2. What is sea-level change?

Warm periodIce age****mbsl = meter below sea level**ka = kilo-annum = 1 thousand years

https://www.dur.ac.uk/geography/, http://jan.ucc.nau.edu/, Lisiecky & Raimo (2005)

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2. What is sea-level change?

Temperature-related changes in ocean volume
Interglacial (warm)

Glacial (cold, ice age)

EarthEarth

DILATATION of ocean water

CONTRACTION of ocean water

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2. What is sea-level change?

Up/down movements of the land/seafloor
1. Isostatic deformation

ice«Forebulge»

2. Tectonic deformation«Ocean syphoning»

RelativeSeaLevelHigh latitudes

Low latitudes

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2. What is sea-level change?

Albert et al. (2007)

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2. What is sea-level change?

Objective:⚫ Reconstruct changes in absolute sea level related to glacialinterglacial cycles

Why:⚫ Learn how much sea-level rise/fall (and how fast) can result fromchanges in polar ice volume⚫ Learn about the dynamics of polar ice sheets (expansion/retreat)How:⚫ We need a good sea-level indicator⚫ We need regions far away from the poles and stable (limitedup/down movements of land)
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2. What is sea-level change?

Measure of the elevation (relative to present sea level) and age of geologicalfeature that can be linked to sea level.

0

Sea-level indicator (age: 3000 years)
Sea level 3000 years ago

221

Sea level today

LAND

1000 2000 3000 4000 5000 6000
3SEA

Sea-level drop
66.7 cm/1000 years

010-1-1-2-2-3-31000 2000 3000 4000 5000 6000

0

Elevation (in m)

2m

3Time (in years)

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2. What is sea-level change?

Examples of sea-level indicators:⚫ Marine notches⚫ Fossilized beach sediments⚫ Coastal marshes and wetlands⚫ Archeological indicators (harbor structures)⚫ Fossil coral reefs and reef organisms

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2. What is sea-level change?

Image credits (clockwise from top left): Boulton and Stewart (2015), Wikipedia, Gehrels and Anderson (2015), Benjamin et al. (2017)33/63

❖ Reconstructing sea-level history using
fossil coral reefs

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3. Sea-level history

Reef
crestReef
Reef slope flat0m

LagoonSea level
Coral reef

Activereef growth*

1-20 mm/yr

few mm to 10 cm/yr
-30 m

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3. Sea-level history
15a 5c 5e7Stable (e.g., Great Barrier Reef)

Sea 0Level(m)-50

5e5cSL-100

175a-150
050100150200250Time (ka)Subsiding (e.g., Hawaii)
Uplifting (e.g., Ryukyus, Papua New Guinea)SL715a5e5c75c5a5eSL

Adapted from Woodroffe & Webster (2014)

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3. Sea-level history

Image credits (clockwise from top left): Google map, Wikipedia, Webster et al. (2009)37/63

3. Sea-level history⚫ Coral communities as sea-level indicators

Coral community A

LAGOON

Coral community BREEF
CREST-5 mLightCoral community C
REEF
SLOPE
-20 m
Coral community D

Image credit: Australian Institute of Marine Sciences

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3. Sea-level history⚫ Algal communities as sea-level indicators

LAGOON

Algal community A

REEF
CREST-5 m

REEF
SLOPE

Light

Algal community B-20 m
Algal community C

Image credits: IRD, UC Santa Barbara, Kansas University (Derek Keats)

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3. Sea-level history

Microatolls

Image credit: Dr. Alton Meltzner (Earth Observatory of Singapore)

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3. Sea-level history

Mean low water spring tide

Smithers (2011)

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3. Sea-level history – study case 1: Great Barrier Reef

1650 km

AUSTRALIA

Google Earth42/63

3. Sea-level history – study case 1: Great Barrier Reef

Objective: Reconstruct history of Great Barrier Reef and sea level since thelast Glacial period
Today

20,000 years ago
-30 m

-60 m

-125 m

X
X
X
X
X
X

Hopley at al. (2007)

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3. Sea-level history – study case 1: Great Barrier Reef
abc

Hinestrosa et al. (2016)

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3. Sea-level history – study case 1: Great Barrier Reef
Continental Shelf

IODP Exp. 325Reefs(Highstand)Land SeaReefs(Lowstand)

Last Glacial
Niv. 0marin(m)-50

Highstand-100

Lowstand-150050100

150200250

Last Glacial
Maximum
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3. Sea-level history – study case 1: Great Barrier Reef

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3. Sea-level history – study case 1: Great Barrier Reef

3 sites
4 transects
34 boreholes
225 m of cores⚫

Hydrographers
Passage⚫
⚫

⚫

Noggin Pass⚫

⚫

HYD-01C
HYD-02A

NOG-01B

Ribbon Reef⚫

RIB-02A

Humblet et al. (2019)

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3. Sea-level history – study case 1: Great Barrier Reef
98 m

67 m

110 m

88 m

90 m47 m57 m

15 kyr

12.5 kyr

20 kyr

15.5 kyr24 kyr

10.5 kyr12.5 kyr
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Image credit: IODP

3. Sea-level history – study case 1: Great Barrier Reef
Acropora gr. humilis

Seriatopora hystrix

Favia sp.

Massive Porites sp.
Massive Isopora sp.

Montipora sp.

Stylophora pistillata

Image credit: Australian Institute of Marine Sciences

Tubipora musica

Pachyseris speciosa

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3. Sea-level history – study case 1: Great Barrier Reef
5,000

10,000 15,000 20,000 25,000 30,000 35,000-20

New data (IODP Exp. 325)-40

Old data-60-60-80-80-100-100-120

*-140
5,000

Global Mean Sea Level (in m)-40-20-120-140

10,000 15,000 20,000 25,000 30,000 35,000Time (in years)* Rapid glaciation → 20 m sea-level drop in 1400 years (1.4 cm/year)
Yokoyama et al. (2018)

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3. Sea-level history – study case 1: Great Barrier Reef
Water
Depth (m)0E

RD

Sea Level (m)0ERDRD

-60

E-120

0

70

102030Time (ka)100

RDE100

RD: reef drowning
E: Erosional surface

LGM

LGM: Last Glacial Maximum reef

130
Adapted from Webster et al. (2018)51/63

3. Sea-level history – study case 2: Seychelles Islands

MahePraslin
AldabraAldabraAssumption

10 km

Assumption
2.5 kmMahe10 km

Praslin

7 km
Google Earth52/63

3. Sea-level history – study case 2: Seychelles Islands

Objective: Reconstruct history of coral reef and sea level at Aldabra andAssumption during the Last Interglacial

AldabraSea 0Level(m)
-50
-100
-150
050
Time (ka)100150200250

Wikipedia

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3. Sea-level history – study case 2: Seychelles Islands✓ Geochemistry(datings & climate proxies)✓ Sedimentology✓ Paleobiology,✓ Diagenesis✓ Geophysical modeling✓ Climate modeling54/63

3. Sea-level history – study case 2: Seychelles Islands
4 to 5-day trip

MaheAntsiva
AldabraImage credits: Nicolas Tisne

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3. Sea-level history – study case 2: Seychelles Islands

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3. Sea-level history – study case 2: Seychelles Islands

METHODS⚫

3D mapping and photo coverage⚫ Mapping of the LIG reef unit
⚫ Aerial photography of the studied outcrops⚫

著作権等の都合により、ここに挿入されていた画像を削除しました

Mavic Pro mini-droneの画像

High-precision GPS measurements⚫ Hundreds of GPS measurements on the 4 study sites⚫ 40 topographic transects⚫

Field descriptions⚫ Description/interpretation of depositional environments⚫ Description/interpretation of reef communities⚫

Sampling⚫ Coring/sampling in situ fossil coral colonies for radiometric dating

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3. Sea-level history – study case 2: Seychelles Islands©︎いらすとや

BaseMobile unit

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3. Sea-level history – study case 2: Seychelles Islands

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3. Sea-level history – study case 2: Seychelles Islands

One of 150,000 giant turtles of Aldabra

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3. Sea-level history – study case 2: Seychelles Islands

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THANK YOU!

ANYQUESTIONS?