Transcript Document
Sea-Level changes 1 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 2 What does it mean to be at an altitude of 4000 m? 3 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) 4 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 5 The height of the Earth surface 6 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). 7 8 9 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 10 11 12 13 Sea Level Change ? 14 15 16 Other effects of plate tectonics e.g. Upper Cretaceous (90 Ma) MSL > 300 m 17 18 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 19 Other processes complicating the study of mean sea level (ice or sediment loads) The concept of Post Glacial Rebound (PGR) !!! 20 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? 21 Aerial view of glaciated Bylot Island, Canada U-shaped valley Glacial Striations 22 OK, so we’ve mapped the extent of glaciation. Now what? 23 Date coral samples from various paleo-sea levels. Barbados is the “dipstick” for eustatic sea level reconstruction Now what? 24 Corals for paleo-sea level reconstruction From corals we know that LGM sea level was -125m The world looked different during the LGM 26 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 27 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 28 29 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 30