How do coastal landforms evolve over time as climate changes

Difference between a glacial and an interglacial period

Glacial - a period in the earths history when polar and mountain ice sheets were unusually extensive across the earths surface

• lasts c. 100,000 years

• the last glacial period (Devensian) ended 11,700 years ago

Interglacial - a geological interval of warmer global average temp. lasting thousands of years that separates consecutive glacial periods

• lasts c. 10,000-15,000 years

• the current interglacial period (Holocene) started 11,700 years ago

Impact of temperature on sea level

Warm - Holocene:

• Thermal expansion of water due to warming temps

• land ice melts and discharges into oceans

• result is eustatic sea level rise

Cool - Devensian:

• thermal contraction of particles in the water meaning sea level falls

• land ice expands as more precipitation falls as snow, leading to ice formation

• result is eustatic sea level fall

• snow = albedo 90% = Short wave radiation not absorbed + temps drop, positive feedback cycle

Emergence coastline features - distinctive landforms as a result of sea level fall

• the landforms created on emergent coastlines are created by a eustatic fall in SL resulting in sea level being at a lower level relative to the land than it has been previously

• as a result, landforms shaped by wave processes during higher SL are left exposed when SL falls e.g. raised beach, abandoned cliff, marine terrace

• some of these landforms may be found some distance inland from the modern coastline

• they are left high and dry

Raised beaches and abandoned cliffs

• raised beaches are former shore/WCP that are left exposed at a higher level than the present SL and therefore, are removed from current marine processes

• the raised shore platform, often with beach deposits creates a terrace, an area of flat land, which is backed by the abandoned cliffs

• the presence of the raise platform indicates that before SL fell, the sea must have been at a relatively fixed level for a considerable period of time to have allowed that platform to develop

• abandoned cliffs can often be identified due to their steep slope angle, but may not be as steep as they were at the time of their formation due to modification via sub-aerial processes

• Example of raised beach - Isle of Portland near Weymouth, 15m above present day SL, formed around 125,000 years ago during the Tyrrhenian IG when SL were 20m higher than today → Portland limestone was eroded by hydraulic action, erosion rates ~1m/yr

Marine terraces

• SL can fall in a series of stages

• when that happens, then marine erosion is able to shape a number of raised beaches, which then appear as steps in the landscapes

• each one represents a period of stable temps. and SL during which wave action had sufficient time to act significantly on the landscape to produce flat, eroded WCP

Modification of landforms pt. 1

• following their emergence, these landforms were no longer affected by wave processes due to being high + dry, but they continue to be affected by sub-aerial processes

1. on top of abandoned cliff on the Isle of Portland there is a 1.5m layer of frost-shattered limestone debris deposited when the area experienced periglacial conditions during the end of the last glacial period

2. the cliff face was gradually degraded by frost weathering processes leading to rock fall from the cliff face

Modifications of landforms pt. 2

3. the periglacial process of cryoturbation created distortions in the bedding planes of limestone on the isle of portland. the freezing and thawing of the water in the cracks created mix ups in the bedding planes

4. post-glacial, warmer and wetter conditions have led to the development of vegetation on exposed surfaces making them harder to recognise

5. warmer climates under a climate change will lead to more chemical weathering, especially carbonation of limestone cliffs and platforms. Bio weathering on the raised beach may also become more sig. with the colonisation of the surface by an increasing no. of marine organisms e.g. limpets and whelks

Modification of landforms during the periglacial period

• periglacial suggests an environment on the periphery of a glacier

• these areas would have been periglacial in nature i.e. with long, cold winters and short mild summers, when partial melting of frozen ground would occur

• mean summer temps would be c. 10 degrees C

• during spring and autumn, critically temps would fluctuate around 0, enabling freeze thaw weathering to occur

• some modification of emergent features would occur during the periglacial phase

Submergent coastline features

• these form due to eustatic SL rise from a warming climate e.g. following the end of a glacial (e.g. the end of the Devensial glacial 11,700 years ago)

• temperatures then warmed and sea levels rose too

• continental ice melts and water discharges in oceans

Rias e.g. Kingsbridge Estuary

• Rias are sheltered, winding inlets formed as the lower parts of rivers and their tributaries were drowned as SL rose in a warming cliamte

• rias are tidal at their lower reaches revealing extensive mudflats

• an increase in global temps lead to -

  • higher rates of melting ice stored on land

  • thermal expansion of the water in the ocean

  • therefore, a global increase in the vol of water in the ocean

• the lowest part of the rivers course and its floodplains may now be completely drowned

• the higher land forming tops of the valley sides and the middle and upper part of the rivers course remain exposed

• they were formed during the post-glacial SL rise of the Flandrian Transgression

• prior to its submergence, geomorphic processes of fluvial erosion caused the river channel to be carved out during a cooler phase

Ria from different view sections

Cross-section:

• gently sloping sides

• variable depth due to deposition of sediment as rivers had less energy for erosion once the flooding had occurred

• wider and deeper than a normal river

Long-section:

• deepest water is at the mouth with depth decreasing inland with increasing distance up the river’s course

Birds eye view:

• winding plan form reflecting the original route of the river and its valley, formed by fluvial erosion within the channel and subaerial processes on valley sides

Fjords e.g. Sogne Fjord in Norway

• = a long, narrow, straight and deep inlet of the sea between high cliffs formed by submergence of a glaciated trough as SL rose in a warming climate

• they have steep, almost cliff-like sides and the water is uniformly deep, often reaching 1000m in depth

• the sogne fjord in Norway is nearly 200km long

• they were originally formed as glacial troughs

• glacial troughs were formed by glaciers eroding their path through former river valleys

• when the ice melted, the glacial troughs were flood by a eustatic SL rise to form long, deep, narrow inlets with a U-shaped cross section

• prior to its submergence, geomorphic erosion processes of plucking and abrasion caused the glacial trough to be shaped, with the glacial acting like a bulldozer

Fjord section view

Cross-section:

• straight, deep, U-shaped valley sides due to the critical erosive power of the glacial that carved out the trough

Long section:

• shallower area at the mouth (the rock threshold) where glacial ice thinned as it reached the sea and hence lost its erosional power

• Due to the depth of water that occupied fjords during the marine transgression, marine erosion rates were high and in some cases the fjords were further deepened

• in others there has been some infilling with sediments, deposited by meltwater from the glaciers

• this modification has happened increasingly due to the melting of glaciers associated with climate change and warming temperatures

Shingle beaches

• shingle beach - a depositional landform shapes as SL rose during a marine transgression pushing sediment landward following SL rise in a warming climate

• prior to marine transgression, during a cooler climate, lower SL existed due to an accumulation of water on land as ice

• at this point, large areas of new land emerged from the sea

• sediment accumulated on this land, deposited by rivers, meltwater streams and low energy waves

• as SL rose during a marine transgression at the end of the ice age, this sediment was transported onshore by wave action

• in some places, they beached at the base of former cliff lines and elsewhere they formed tombolos and bars

Modification of submergent landforms

Fjords becoming less steep:

• valley sides are modified by sub-aerial processes in todays warmer, wetter, climate conditions

• e.g. chemical, mechanical weathering and biological weathering rates will be enhanced, weakening the cliffs and causing rock falls

Water depths in Rias and Fjords increase:

• SL predicted to rise by a further 0.6m in the next 100 years, so water depth in Rias and Fjords will increase

Marine erosion of F, R and S will increase:

• CC causing stormier conditions and therefore higher energy waves

• increase in KE increase the rates of marine erosion due to higher inputs of energy

Sediment on shingle beach becoming increasingly sorted:

• increased marine erosion by increase stormy conditions and larger waves cause shingle to be more eroded and sorting will become increasingly common

• sediment is likely to be removed to the offshore zone

Waves during storm events at Chesil beach overshooting the beach:

• high energy waves increasingly found at Chesil beach means sediment moves even further NE and beach of Tombolo can occur in future, esp. with large storm surge

Cooling climates

• landforms of emergence - shaped by wave processes during times of higher SL are left exposed creating distinctive landforms when SLF e.g. Raised beach at Isle Portland

• due to fluctuation of temps around 0, freeze thaw, rock fall due to weathering and cryoturbation occurred as a result of cooling climates

A02:

• not geographically widespread → not as sig. as headland/bay landscape, they are distinct and unique

• But on IOP, cooling climate has been critical sig. in formation of landforms

• but there is not large spatial evidence of cooling in landscape systems

Warming climates

• landforms shaped by geomorphic processes during times of low SL are submerged when SL rises, creating distinctive landforms e.g. Sogne Fjord in Norway

• rates of chemical weathering increase resulting in landforms being degraded, more biological weathering

• steep U shaped valley sides associated with fjords will become headlands + V-shaped valley sides associated with rias, these will be eroded due to conc. wave energy from refraction, modifying these landforms, leading to formation of caves, arches, stacks + stumps in future

• highly sig. as landscape is changing sig. - brand new landforms and even processes

A02:

• Fjord landscapes geographically widespread in high latitudes but are distinct and unique so not fully globally geographically widespread

• Rias more dispersed between mid to low latitudes so in these locations, these landforms are highly sig.

• current CC is brining more thermal and KE into coastal systems e.g. via storms and storm surges, modifying through geomorph. processes e.g. CW and erosional processes