Sea Level Change

• • Eustatic Change - actual change in level of the sea relative to the land; global change.
  • Isostatic Change - change in level of the land relative to the sea; local change.
• Fall in Sea Level:

  • sea level falls and land rises/stays still/subsides at slower pace
  • sea level remains fixed whilst land rises
  • sea level rises but land rises at greater pace

  Rise in Sea Level:

  • sea level rises and land subsides/stays still/rises at slower pace
  • sea level remains fixed whilst land subsides
  • sea level falls and land subsides at greater pace
• • Marine Transgression - sea level rises relative to land and shoreline moves towards higher ground resulting in flooding.
  • Marine Regression - areas of submerged seafloor are exposed during a drop in sea level (coastlines advance/sea retreat).
• Sea Level Change from Glacial Advance/Retreat:

  • in glacials, temperatures fall and ice sheets/glaciers form on land (global eustatic fall in sea level)
  • continental ice increases in size causing localised isostatic depression of land; sea level rises locally
  • temperatures rise in interglacials, ice begins to melt leading to global eustatic rise in sea level
  • continental ice melts; as weight of ice on land decreases, isostatic rebound takes place resulting in localised fall in sea level
• Amount of ice on land:

  • influence on sea level - changes when ice forms/melts as continental ice caps/sheets formed from water taken from (and released back into) oceans; accumulation/melting affects level
  • influence on land - accumulation of ice on land depresses the crust + melting of ice in interglacials leads to isostatic rebound as land uplifts
• Steric Changes (change in atom arrangement):

  • influence on sea level; thermal contraction/expansion (as oceans cool they reduce in size/expand when they warm) only affects global sea level by around 10m
• Epeirogenic Movements (tectonic movements of crust involving depression/uplift of ocean floor and continents):

  • influence on sea level - if ocean basin reduces in size, the water holding capacity decreases so global sea levels rise (and if increases, a fall in global sea level)
  • influence on land - changes land level so sea level is reduced relative to land when continents rise periodically
• Orogenic Movements (tectonic mountain building):

  • influence on land - any gradual displacement of height of land from mountain building process can lead to uplift of land (lowering of sea level relative to land) or lowering of land (relative rise in sea level)
• Volcanic Activity:

  • influence on sea level - creation of volcanic ocean islands displaces water and increases sea level + weight of volcanic islands can depress crust leading to epeirogenic changes and lowering of sea level
  • influence on land - as magma chambers below surface fill/empty resulting uplift/subsidence of land causes relative rises/falls in sea level
• Sediment Accumulation:

  • influence on sea level - large accumulation of sediments can reduce ocean basin's holding capacity leading to sea level rise + sediment mass can depress ocean floor, increasing ocean's volume leading to sea level fall
  • influence on land - where sediments build up in coastal regions, weight causes land to subside resulting in relative sea level rise (unless sediment accumulation equals rate of subsidence)
• Ice History and Global Sea Levels:

  • 2.6Ma, Quaternary began with pleistocene ice ages; changes in global temperatures lead to colder glacial/warmer interglacials
  • last ice age (devensian) began 70,000 years ago/ended 10,000 years ago; 18,000 years ago, temperatures 5$^o$C lower and continental ice at maximum meaning sea level 100-150m lower
  • ice melted in interglacials; glacio-eustatic rise in sea level caused coastline inundation during Holocene (flandrian transgression; oceans reached current levels 6,000 years ago)
• Estuaries (Submergent): eg. Humber Estuary

  • tidal mouths of rivers resulting from lower part of valleys in lowland areas being drowned by post-glacial rise of sea level; subject to tidal patterns + widen towards sea/narrow inland
  • estuaries are affected by residual currents (mixing of fresh/saline water), tidal currents and variations in discharge/velocity from tidal cycle
• Rias (Submergent): eg. Kingsbridge Estuary, Devon

  • flooded mouth of river valley in an upland area; formed when valleys in a dissected upland area are submerged by rising sea levels
  • rias are shelted, winding inlets with gentle sides where lower parts of rivers/tributaries were drowned; deepest water at mouth, with depth decreasing inland + occur on irregular shorelines
• Fjords (Submergent): eg. Loch Torridon, Scotland

  • flooded mouth of glaciated valley in an upland area (flooded glacial U-shaped valleys) - glacier erodes V-shaped valley to create steep-sided, flat bottomed valley, which are flooded when sea levels rise
  • Fiards - drowned, glaciated lowland areas; eg. Strongford Lough, Northern Ireland.
• Drowned Forests (Submergent): eg. Bexhill, East Sussex

  • vegetation/forests established on glacial outwash/boulder clay once glaciers retreated; as sea levels rose they were drowned by the sea
• Shingle Ridges (Submergent): eg. Chesil Beach, Dorset (forms tombolo with Isle of Portland)

  • after glacier retreat, outwash plains (of reworked glacial material) were deposited; as sea levels rose these deposits were transported and sorted by the sea then pushed landwards by waves
• Dalmation Coasts (Submergent): eg. Croatia

  • valleys in a dissected upland area are submerged but with rivers following parallel to the coast + form in landscape of ridges and valleys running parallel to the coast
• Raised Beaches/Wave Cut Platforms (Emergent): eg. Isle of Arran, Western Scotland

  • as land rose, former wave-cut platforms/beaches were raised above wave level + recognised by line of degraded cliffs fronted by a wave-cut platform (may be relict landforms; wave-cut notch, caves, arches and stacks) + deposits of sand/pebbles backed by an old cliff line
• River Rejuventation (Emergent):

  • rivers produce graded long profile from source-mouth; any protrusions are eroded/depressions infilled (produce smooth profile)
  • when sea levels drop (rise in land), rivers are rejuventated startig to incise more deeply into their course (establish new graded profile due to change in sea level)
  • happens first at coasts where knickpoint develops; knickpoint will migrate upstream by headward erosion until new graded profile created
• River Rejuventation Features (Emergent):

  • River Terraces; former floodplains left high/dry above present day flooding following vertical erosion from rejuventation (pair of terraces will flank floodplain if river cuts in rapidly).
  • Entrenched Meander; from rejuventation, rivers dig themselves in so deep into bedrock they are unable to migrate.
  • if land over river flows rebounds, river's erosive processes are rejuvenated as it tries to erode down to new base level, eg. River Wear, Durham