Sea level Flashcards
Ways to reconstruct sea level changes e.g. ancient coastlines
Corals - utilises the fact that coral reefs adjust due to changing sea level
Factors controlling sea level change
- Tectonic movements ( local scale)
- Temperature cycles
- Global warming
- Glacial melt
Consequences on ocean of waxing and waning of the continental ice masses
A lot of O16 was extracted from the ocean and stored in ice leading to higher oxygen isotopes in ocean waters and lower sea level
What processes impact global sea level?
- Amount of ice on the continent
- Temperature of seawater
- Seafloor spreading rates (shape of the ocean basins )
- Amount of continental margin (mega-continent or lots of continental fragments
- On a local level additional effects like regional tectonic movement may play a role
Impact of sea level on climate
Change of albedo when sea level covers or exposes continental shelves
- With LESS continental shelf exposed, MORE sunlight is absorbed- by the surface of the earth. E.g. HIGH sea level absorbs more sun light (warmer)
Sea floor spreading rate
Faster sea floor spreading rate= a hotter crust and a more elevates mid ocean ridge- continues elevated even on the ridge flanks.
Slower sea floor spreading=
Less elevated crust and a lower sea level
Continental margins
Have a lot of volume and displace large amounts of seawater
Most promising result for reconstructing sea level changes
Corals
Why can we use corals to reconstruct sea level change?
Some coral species only occur in a very narrow depth interval in the ocean- coral reefs adjust to changing sea levels
Acropora Palmata
Only survived within top 5m of ocean
Isostatic Pressure
The Earth’s crust responds to the weight above it by exerting an equalizing pressure (isostatic pressure) in all directions, helping to maintain balance between continents and the ocean floor
Hudson Bay- isostatic rebound
Needs to rebound 150m to compensate for pressure applied by ice sheets
Weight applied to the crust:
Dispersed throughout the lithosphere
Consequence of the lithosphere being ridgid
Weight is transferred across the crust resulting in a peripheral depression and a forebulge
Extent of depression beyond ice margin 150-180km
Can record relative sea level change
Lateral displacement of mantle material from below the center of the ice sheet loading:
Results in the formation of an area of slight uplift (10-20m) beyond the peripheral depression (forebulge)
Isostatic rebound in Scotland
- Emergence at a modest rate
- Outpaced by sea level rise
Processes impacting global sea level:
- Amount of ice on the continent
- Temp of seawater
- Sea floor spreading rate (shape of basin)
- Amount of continental margin (mega-continent or lots of continental fragments)
- Regional tectonic movement may play a role
Estimates of sea level further back in time
Suffer from a growing uncertainty
Sea water albedo
0.06
Soil Albedo
0.25
Sand albedo
0.40
Less continental shelf exposed=
MORE sunlight absorbed- by the surface of the earth
Faster sea floor spreading
Produces a hotter crust and a more elevates mid ocean ridge
Slower sea floor spreading
Produces less elevates crust and a lower sea level
Continental margins …
have a large volume and displace large amounts of seawater
Lots of continental fragments
= lots of margins therefore sea level=high
Effect of the break up of Pangea on global sea level?
High sea level flooded MOST continental interiors
Oceanic plateaus (large igneous provinces)
Can be very big and displace large volumes of water
Factors contributing to sea level fall in the last 80 million years
- Decrease in ocean ridge volume
- Collision of India and Asia
- Decrease in volcanic plateau volume
- Water stored in ice sheets
-Thermal contraction of seawater
The Messinian Event
The Mediterranean Sea dried up due to continental movement
Why did the Messinian event occur?
5.9 mya Water exchange with the Atlantic Ocean via the Strait of Gibraltar was interrupted
Evidence for the Messinian event
Evaporite minerals, soils and fossil plants
2 scenarios used to explain the salinity crisis
- Uplift Model
- Drying- out model
Uplift Model
- Large-scale regional uplift
- Draining of the basin into the Atlantic Ocean
- Large-scale regional sinking of crust
Drying Out Model
- Small-scale local uplift and evaporation
- Drying out of basin
- Small-scale local sinking of sill
