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Sea-Level changes

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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?

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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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The height of the Earth surface

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by definition:Mean Sea Level = 0 m = equilibrium level

Changes in volume of water Changes in shape and volume of ocean basins

Changes are measured as relative changes to a reference datum

This reference datum can be a fixed one (e.g. distance from the center of the earth) or local (coastline).

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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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Sea Level Change

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Other effects of plate tectonicse.g. Upper Cretaceous (90 Ma) MSL > 300 m

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Summary of spatial-temporal scale of processes contributing to Mean Sea Level

TIME (years)

MSL (meters)

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 (PGR) !!!

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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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U-shaped valley

Aerial view of glaciated Bylot Island, Canada

Glacial Striations

Glacial Flow

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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?

Corals for paleo-sea level reconstruction

From corals we know thatLGM sea level was -125m

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The world looked different during the LGM

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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