Surface of Equal Density
(Pacific example)
Diapycnal Diffusivity
High Resolution Profiler
Sea-Level changes
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Learning Objectives
The shape of the planet: difference between Geoid and Ellipsoid
The concept of Mean Sea Level
Article: http://www.esri.com/news/arcuser/0703/geoid1of3.html
Processes that control the Mean Sea Level and its changes
Sea level changes over millions of year
Sea level changes over the recent geologic past
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What does it mean to be at an altitude
of 4000 m?
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What does it mean to be at an altitude
of 4000 m?
It means that I am 4000 m above the
Mean Sea Level (MSL)
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Model of the shape of the Earth
geoid: The equipotential surface of the Earth's gravity field which best fits, in a
least squares sense, global mean sea level (MSL)
http://www.ngs.noaa.gov/GEOID/geoid_def.html
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by definition:
Mean Sea Level = 0 m = equilibrium level
Changes in sea level can be caused by:
 Changes in volume of water
 Changes in shape and volume of ocean basins
Changes are measured as relative changes to a
reference level
This reference level can be a fixed one (e.g.
distance from the center of the earth = eustatic)
or local (coastline = relative).
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A change in volume of seawater in one ocean
will affect the level in all others. Any such worldwide change in sea-level is called EUSTATIC
SEA-LEVEL change
A change in local sea level measured with
respect to a land reference point is referred to
as a RELATIVE SEA-LEVEL change
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Sea Level Change
?
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Other 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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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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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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