2B.7A - Long-Term Sea Level Change

Longer-term sea level changes result from a complex interplay of factors eustatic (ice formation/melting, thermal changes) and isostatic (post glacial adjustment, subsistence, accretion) and tec​tonics. ​

Sea level change in general
  • Sea level changes constantly
    • Due to: tides, variations in surface air pressure, winds pushing on the water surface, creating temporary bulges of higher sea level. 
  • Long term sea level changes occur over thousands of years. 
  • Long term sea level change is due to eustatic and isostatic factors, and tectonics.

Eustatic
  • Eustatic change is a change in global sea level, usually due to a change in the volume of water in the oceans.  ​
 
  • Climate change occurs cyclically, in Milankovitch Cycles, due to changes in the Earth's orbit around the Sun. 
  • Glacials are 90,000 year colder phases, which lead to the formation of ice sheets. 
  • Water evaporated from the oceans falls as snow on the land and compresses to form ice. 
  • Over the course of the cycle, the distribution of water within the hydrological cycle changes, with transfers from the ocean store to sheets on land.
  • A decrease in the volume of water in the oceans produces a global fall in sea level. 
  • The sea bed is exposed as land - marine regression. 
  • The most recent glacial was the Devensian, where global sea levels were 120 m lower than they are today. The English Channel, Irish Sea and most of the North Sea was dry land. 
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  • Interglacials are 10,000 year warmer phases that shrink ice sheets. 
  • Water is transferred from the land store back to the oceans. increasing the volume of water in the oceans and raising global sea levels. 
  • (Melting of sea ice has no effect on global sea levels as the floating ice mass already displaces its volume.)
  • Rising water temperature leads to the thermal expansion of water, increasing its volume even further. 
  • This leads to marine transgression - rising sea levels flood low land areas. 
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  • Since 1750, humans are thought to be accelerating natural interglacial warming through greenhouse gas emissions. 
    • Anthropogenic forcing
  • Sea levels rose by 21 cm from 1870 - 2010. 
  • The melting of Antarctic ice sheets are predicted to raise sea levels by 50 m

Summary: 
  1. Eustatic fall in sea level
    1. During glacial periods, when ice sheets form on land in high latitudes, water evaporated from the sea is locked up on land as ice, leading to global fall in sea level. 
  2. Eustatic rise in sea level
    1. At the end of a glacial period, melting ice sheets return water to the sea and sea level rises globally. Global temperature increases and causes the volume of ocean water to increase (thermal expansion) leading to sea level rise. 

Isostatic Changes
An isostatic change is a change in local land level. 
  • Rises in local land level causes a fall in local sea level. This may be due to:
    • post-glacial adjustment
    • accretion
      • sink regions in the sediment cell are experiencing net deposition, land is built up, leading to a fall in sea level (in delta regions accretion -> subsidence -> accretion and so on)
    • tectonics
  • A fall in local land level produces a rise in local sea level. This may be due to:
    • post-glacial adjustment 
    • subsidence
      • subsidence of land produces a rise in sea level
      • the deposition of sediment, especially fluvial deposits in large river deltas, the weight of sediment deposition overcomes the threshold and leads to very slow 'crustal sag' and delta subsidence, e.g. Nile, Mississippi, Amazon
      • can also be caused by the lowering of the water table (from increased evaporation from climate change or human abstraction) can lead to settling of overlying sediment and land subsidence as pore water pressure is removed
      • or by heavy buildings 
    • tectonics

Post-Glacial Adjustment
During glacial periods, the weight of the ice depresses the crust in areas below the ice sheets. The solid lithosphere is forced down into the plastic asthenosphere. The rigid nature of the solid crust means that when sections of the crust are depressed by ice and forced down, adjacent areas are uplifted in a see-saw effect. 
The melting of ice causes previously ice-covered crust to slowly rebound upwards whilst adjacent areas subside. 

At the end of the last ice age 12,000 years ago, the UK was covered in ice as far down as Birmingham. Northern Britain is experiencing a isostatic fall in sea level as land is uplifted by 1.5 mm per annum. 
Southern Britain is experiencing an isostatic rise in sea level as land is lowered by 1 mm per annum. 
The UK is pivoting, with the south sinking and the north rising. 

In northern Britain, sea levels are falling as isostatic rebound exceeds eustatic rise in global sea levels. However, in the south isostatic subsidence is accelerating a rise in sea level produced by global warming (eustatic). Land's End in Cornwall is sinking isostatically by 1.1 mm p.a., and there is a 2.8 mm eustatic rise in sea level due to climate change, producing a sea level rise of 3.9 mm per annum.

Tectonics
Eustatic
  1. Rising magma at a constructive plate margin/hot spots lifts the overlying crust, reducing the capacity of the ocean and producing eustatic sea level rise. 
  2. Uplift of crustal plate reduced Indian Ocean capacity
    causing 0.1 mm eustatic rise in global sea levels.
Isostatic
  1. Folding of sedimentary rock by compressive forces at a destructive plate margin produces an isostatic fall in sea level for anticlines and a fall for synclines. E.g. the Alpine folding at the Eurasian-African destructive plate boundary produced an isostatic fall of 60 cm in the Bakar-Vindol area of Croatia. 
  2. Lava or ash from volcanic activity produces an isostatic fall, e.g. Hawaiian hot spot island chain or Caribbean island arc. 
  3. Sea floor spreading - carries volcanic islands away from the uplifted crust at mid-ocean ridge. Colder, more dense crust subsides and sea levels rise, e.g. Tonga, Fiji, Kiribati.
  4. FAULTING can uplift HORST blocks of crust producing
    isostatic rise in land & fall in local sea level.
  5. Subsidence of crust blocks by faulting form GRABEN
    experiencing isostatic fall in land level & rise in local sea
    level.
  6. During 2004 Boxing Day Tsunami in Indian Ocean
    extension of crustal plate caused isostatic fall in land on
    island of Sumatra by 20 cm in Banda Aceh region.