Surface of Equal Density Pacific example Diapycnal Diffusivity

Surface of Equal Density (Pacific example) Diapycnal Diffusivity

High Resolution Profiler

Sea-Level changes 3

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/geoid 1 of 3. 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 4

What does it mean to be at an altitude of 4000 m? 5

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) 6

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 7

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). 8

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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 11

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Sea Level Change ? 15

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Other effects of plate tectonics e. g. Upper Cretaceous (90 Ma) MSL > 300 m 18

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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 1 m (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 1000 100 Ka TIME (years) 10 Ma 100 Ma 20

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 !!! 21

Last Glacial Maximum: 20 thousand years ago Laurentide Ice Sheet, 3 -4 km thick All this ice caused a EUSTATIC sea level drop of 125 m How do we know this? 22

Aerial view of glaciated Bylot Island, Canada U-shaped valley Glacial Striations w Gl Flo ial ac 23

OK, so we’ve mapped the extent of glaciation. Now what? 24

Date coral samples from various paleo-sea levels. Barbados is the “dipstick” for eustatic sea level reconstruction Now what? 25

Corals for paleo-sea level reconstruction From corals we know that LGM sea level was -125 m

The world looked different during the LGM 27

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 28

Geological proxy for sea level change: 18 O/16 O in foraminifera Oxygen has two stable isotopes: 16 O Rainfall and Ice are very depleted in (99. 8%) and 18 O (lots more So when you build ice sheets, ocean loses Forams record ocean 18 O/16 O 16 O, 18 O (0. 2%) 16 O) becomes 18 O-rich ratio in shells 21, 000 ybp 29

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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 -100 Ma 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 -100 ky 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 18 O record 31