Transcript 07-Paleo-Sealevel

Sea-Level changes
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Questions:
•What causes the sea level to change
over time?
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Geoid and sea level
• Geoid = shape of the mean sea level
- Planetary rotation (ellipsoid)
- Seafloor topography (irregularity)
 Geoid changes very slowly via plate
techtonics (>10 million years)
• Sea level = volume of seawater
- How much ocean basins are filled with seawater
- Sea level can change relatively rapidly by
(1) Changing the volume of sea water
(2) Changing the shape of ocean basin
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1. Changing ocean volume
• Eustatic sea level change
- Changing the total mass of the ocean
- Ex. Melting of ice on land, and the melt water
runs off to the ocean
• Steric sea level change
- Changing the density of the ocean
- Ex. Heating up the seawater, and warm water
takes up more volume
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Eustatic sea level change
• Global change
- More ice on land = Lower sea level
- Global warming and loss of ice sheets and land
glaciers (decades to centuries)
- Glacial cycles (100k years)
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Can melting Arctic sea ice cause
global sea level rise?
Why or why not?
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Last Glacial Maximum: 20 thousand years ago
Laurentide Ice Sheet, 3-4km thick
All this ice caused a EUSTATIC sea level drop of 125m
How do we know this?
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Aerial view of glaciated Bylot Island, Canada
U-shaped valley
Glacial Striations
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OK, so we’ve mapped the extent of glaciation.
Now what?
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Date coral samples from various paleo-sea levels.
Barbados is the “dipstick” for eustatic sea level reconstruction
Now what?
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Corals for paleo-sea
level reconstruction
From corals we know that
LGM sea level was -125m
The world looked different during the LGM
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Steric sea level change
• Global/regional change
- Warmer ocean = Higher sea level
- Heating up the ocean makes the seawater less
dense, and it expands to take up more volume
- A few centimeters to meters variation
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2. Changing the shape of ocean basins
• Relative sea level change
- A change in local sea level with respect to a land
reference point
 Ex. Land uplift
• Changes in plate techtonics
- Changing thickness of ocean crust and
sediments
- Changing land crust and distribution
 Up to a few hundred meters of sea level
change (>10 million years)
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Relative sea level change
• Local techtonic effect
- Land uplift  Lower relative sea level
- Land sinking  Higher relative sea level
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Sea Level Changes over the timescale of
plate techtonics
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Effects of plate tectonics
e.g. Upper Cretaceous (90 Ma) MSL > 300 m
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Summary of spatial-temporal scale of
processes contributing to Mean Sea Level
MSL (meters)
(D) Plate Tectonics
100 m
(C) Melting of ICE
Load from ice sheets
deforms crust
• Thickness and area of
continental crust
• Thermal state (age) of crust
• sediment loading
10 m
1m
(A) Exchange of water with continents (Groundwater, Lakes, etc.)
(B) Temperature expansion
NOTE:
A,B,C  change in volume of water
D  change in shape of container
1 cm
1 day 100
1000
100 Ka
TIME (years)
10 Ma
100 Ma
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Other processes complicating the study of
mean sea level (ice or sediment loads)
The concept of Post Glacial Rebound:
Scandinavia is STILL bouncing back up from
glaciers that melted 10 thousand years ago !!!
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The subsidence of the Northern Sea
(associated with relaxation from glacial loading)
Rate of change in Sea Level
mm/year
Scandinavia
Northern Sea
Great Britain
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Geological proxy for sea level change:
18O/16O in foraminifera
Oxygen has two stable isotopes:
16O
Rainfall and Ice are very depleted in
(99.8%) and
18O
(lots more
So when you build ice sheets, ocean loses
Forams record ocean
18O/16O
16O,
18O
(0.2%)
16O)
becomes
18O-rich
ratio in shells
21,000 ybp
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Take-home points:
-eustatic vs. local sea level
-lots of new, young, hot crust means higher sea level;
tectonic changes on 10-100Ma timescales  Wilson cycle
-glacial cycles have several impacts on sea level:
1) ice sheets remove water  lower sea level
2) glacial loading/unloading reshapes crust under
and surrounding ice sheets
- changes occur on 10-100ky timescales
-tools for studying sea level change through geologic time:
1) radiocarbon-date marine shells & corals found at
known elevation (above MSL) and depth (below MSL)
2) deep-sea sediment 18O record
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