4 Geoid and sea level Geoid = shape of the mean sea level - Planetary rotation (ellipsoid) - Seafloor topography (irregularity) Geoid changes very slowly via plate techtonics (>10 million years) Sea level = volume of seawater - How much ocean basins are filled with seawater - Sea level can change relatively rapidly by (1)Changing the volume of sea water (2)Changing the shape of ocean basin
5 1. Changing ocean volume Eustatic sea level change - Changing the total mass of the ocean - Ex. Melting of ice on land, and the melt water runs off to the ocean Steric sea level change - Changing the density of the ocean - Ex. Heating up the seawater, and warm water takes up more volume
6 Eustatic sea level change Global change - More ice on land = Lower sea level - Global warming and loss of ice sheets and land glaciers (decades to centuries) - Glacial cycles (100k years)
7 Can melting Arctic sea ice cause global sea level rise? Why or why not?
8 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?
9 U-shaped valley Aerial view of glaciated Bylot Island, Canada Glacial Striations Glacial Flow
10 OK, so we’ve mapped the extent of glaciation. Now what?
11 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 that LGM sea level was -125m
14 Steric sea level change Global/regional change - Warmer ocean = Higher sea level - Heating up the ocean makes the seawater less dense, and it expands to take up more volume - A few centimeters to meters variation
15 2. Changing the shape of ocean basins Relative sea level change - A change in local sea level with respect to a land reference point Ex. Land uplift Changes in plate techtonics - Changing thickness of ocean crust and sediments - Changing land crust and distribution Up to a few hundred meters of sea level change (>10 million years)
16 Relative sea level change Local techtonic effect - Land uplift Lower relative sea level - Land sinking Higher relative sea level
17 Sea Level Changes over the timescale of plate techtonics
22 Summary of spatial-temporal scale of processes contributing to Mean Sea Level TIME (years) MSL (meters) 100 1000100 Ka10 Ma100 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 water D change in shape of container NOTE:
23 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 !!!
24 The subsidence of the Northern Sea (associated with relaxation from glacial loading) Northern Sea Great Britain Rate of change in Sea Level mm/year Scandinavia
25 Geological proxy for sea level change: 18 O/ 16 O in foraminifera Oxygen has two stable isotopes: 16 O (99.8%) and 18 O (0.2%) Rainfall and Ice are very depleted in 18 O (lots more 16 O) So when you build ice sheets, ocean loses 16 O, becomes 18 O-rich Forams record ocean 18 O/ 16 O ratio in shells 21,000 ybp
27 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 18 O record