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Learning Objectives
The shape of the planet: difference between Geoid and Ellipsoid
The concept of Mean Sea LevelArticle: 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?
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 world-wide 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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Summary of spatial-temporal scale of processes contributing to Mean Sea Level
TIME (years)
MS
L (
mete
rs)
100 1000 100 Ka 10 Ma 100 Ma
1 cm
1 m
10 m
100 m
1 day
(A) Exchange of water with continents (Groundwater, Lakes, etc.)(B) Temperature expansion
(D) Plate Tectonics
(C) Melting of ICE Load from ice sheets deforms crust
• Thickness and area of continental crust• Thermal state (age) of crust• sediment loading
A,B,C change in volume of waterD change in shape of container
NOTE:
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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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Date coral samples from various paleo-sea levels.
Barbados is the “dipstick” for eustatic sea level reconstruction
Now what?
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The subsidence of the Northern Sea (associated with relaxation from glacial loading)
Northern Sea
Great Britain
Rate of change in Sea Levelmm/year
Scandinavia
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Geological proxy for sea level change:18O/16O in foraminifera
Oxygen has two stable isotopes: 16O (99.8%) and 18O (0.2%)
Rainfall and Ice are very depleted in 18O (lots more 16O)
So when you build ice sheets, ocean loses 16O, becomes 18O-rich
Forams record ocean 18O/16O 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 underand 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