Dr Katie Szkornik, Keele University, [email protected] C-Change in GEES: Changing Coastal...

Dr Katie Szkornik, Keele University, [email protected]

C-Change in GEES: Changing Coastal Environments - Post-Glacial Sea Level Change

C-Change in GEES

Changing Coastal Environments

Session 1Session 1: Postglacial

Sea-Level Change



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These slides were last updated in December 2009



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• Key definitions

• Postglacial sea-level change

• Meltwater pulses

• Sea-level fingerprinting

• Holocene sea level change

• Reconstructing Holocene sea-level change

What you need to know about:

The history of sea-level change The causes of sea-level change The effects of sea-level change

Session Outline

Photo: wonderlane (flickr.com)



It is an important driving mechanism for (rapid?) coastal change, especially over glacial/interglacial cycles

Why is Sea-Level Change

Important?

Photo: skagman (flickr.com)

Sea-level change has always been a contributing driver of geomorphic change

within coastal environments



Understanding past sea-level change

Monitoring present day sea-level change

Predicting future sea-level rise

Coastal management, hazard mitigation

Need to fully understand

this before we can predict the future!



Key Definitions

Eustasy + Isostasy = Relative Sea-level Change

EUSTASY• Glacio-eustasy (hydro-eustasy)

The exchange of water between the World’s oceans and ice sheets and glaciers

• Tectono-eustasy. Change in ocean volumes due to tectonic activity

(orogeny, sea-floor subsidence, mid-ocean ridge growth), driven by geological processes

• Geoidal-eustacy. Ocean surface responds to changes in the Earth’s gravitational

field – increasing understanding of the importance in recent years



The Geoid

Source: NASA Image Exchange(nix.nasa.gov)

• The ocean surface responds to changes in the gravitational field of the Earth.

• Measured relative to a reference ellipsoid on the basis of gravitational measurements

• The ocean geoid is also known as geodetic sea level.

• There is a 180 m difference between the rise at New Guinea and the depression centred on the Maldives

Map showing spatial variation in gravitational field at the Earth’s surface



Dr Katie Szkornik, Kele University, [email protected]

C-Change in GEES Themes 2 and 3: Changing Coastal Environments - Post-Glacial Sea Level Change

Key Definitions

Eustasy + Isostasy = Relative Sea-level Change

ISOSTASYThe process by which the Earth’s crust seeks to reach equilibrium following loading or unloading by ice (glacio-isostasy) or water (hydro-isostasy)

Also need to consider:

•Sediment loading and unloading

•Volcanic loading/unloading



Glacial Forebulge

Image adapted from: Peltier and Andrews (1976)

Where this occurs in coastal locations it has important implications for relative sea level history



Water flows from the equator towards the collapsing forebulges

of the mid and high latitudes

Water loading of the continental shelf causes rebound at the coast

and a lowering sea bed

Images adapted from Gehrels & Long (2008)

Many of these processes are not fully considered by the IPCC & other agencies involved in predicting future sea-level rise…….

Ocean Siphoning Continental Levering



What else can affect sea level?

Influence of local and regional processes:

Storms, storm surges Tsunamis Earthquakes Plate tectonics Thermal expansion (steric

effects) Ocean currents Tides

Think about amplitudes and time scales over which these processes operate

Photo: ((brian)) (flickr.com)

There is no single location in the world where you can obtain a ‘global

sea-level curve’



Abundant, accessible(?) evidence of changes

Changes resulted in the coastlines that we see in existence today – rapid coastal change

Eustatic changes – 50 million cubic km of ice melted from the land-based ice sheets & glaciers

Dominance of glacio-isostasy in regions once covered by ice sheets lead to dramatic falls in RSL of hundreds of metres

Postglacial sea-level changes

Eustatic changes: global and rapidIsostatic changes: regional and slow(er)



Complexity of Postglacial Sea-Level Change

Differential melting of ice sheets produced a highly complex

pattern of sea-level & associated coastal change

• Ice sheets did not all melt at the same time• Antarctica and Greenland remain to this day• Phases of ‘rapid’ ice melt

*Meltwater pulses*

Existence and magnitude has implications for understanding rapid climate and coastal

change



Rates of glacio-eustatic sea-level change

• A high degree of geographic variability

• Evidence of meltwater pulses across locations

• Shennan (1999) constrains the magnitude and timing of two meltwater pulse events (ca.14,000 yr and 11,300 yr BP)

• Observable in the records of ‘far field’ sites and identified in NW Scotland from reconstructed records of RSL (diatoms)

Dataset evidence of meltwater pulses

Barbados Bard et al. (1990); Fairbanks (1989)

Tahiti Bard et al. (1996)

Huon Chappell and Polach (1991); Cutler et al. (2003)

Bonaparte Gulf Lambeck et al. (2002);

Yokoyama et al. (2000)

Sunda Shelf Hanebuth et al. (2000)



Where is the source location for these events?

SUGGESTION ONE Peltier (2005):

N. Hemisphere (Laurentide?)

• Source area is highly debated

• Current area of intense research (& funding!)

SUGGESTION TWO Bassett et al. (2005) & Clark et al.

(2002):Antarctic

Photos: cloudzilla (above); 23am.com (below) (flickr.com)



Sea-level fingerprinting – the

solution?

Influence of an ice mass on the vertical displacement of the ocean surface

Adapted from:Tamisiea et al. (2003)Diagram shows the sea level fingerprint of

a melting event – sea -evel fall in the near field of the ice and a gradual rise in sea level away from it (Tamisiea et al., 2003)



Global meltwater pulse 1A

Evidence thus far suggests that the Laurentide Ice Sheet was not the sole source of this meltwater pulse event and that the West Antarctic Ice Sheet was partially responsible

Key areas for additional research: S America (especially Argentine Shelf) and

Antarctica.

Source of Meltwater Pulse Events

Normalised sea-level change associated with melting from the southern Laurentide Ice Sheet

Normalised sea-level change associated with melting from the West Antarctic Ice Sheet

IMAGE

IMAGE

From: Clark, P.U., Mitrovica, J.X., Milne, G.A., Tamisiea, M.E. (2002) ‘Sea-level fingerprinting as a direct test for the source of global meltwater Pulse IA’ Science 295: 2438–2441. Reprinted with permission from AAAS. This figure may be used for non-commercial, classroom purposes only. Any other uses require prior written permission from AAAS.



Holocene Sea Level Changes

• The Holocene Transgression (period of ice melt/ice sheet collapse in the early Holocene)

• Younger Dryas ice had melted by about 6000 years BP

Changes in land above sea level during the sea level rise of the Holocene



Holocene sea-level curves

Clark, J.A. et al. (1978) ‘Global Changes in Post Glacial Sea Level: A Numerical Calculation’Quaternary

Research v.9



Reconstructing Holocene sea levels

Case study examples and key researchers:

•Isolation basins (microfossils, stratigraphy)

Shennan, Lloyd

•Salt marshes (microsfossils, stratigraphy)

Gehrels, Horton, Edwards, Haslett, Charman

Dee Estuary salt marshes

Raised beach, Isle of Skye



Reconstructing Holocene sea levels

What do we need to know?

- Age

- Present day altitude

- Indicative meaning (& range)

- tendency of sea-level (?)

Sea-level index point (SLIP)

Indicative meaningThe elevation relative to a reference tide level at which the sea-level indicators are found in the present environment. Commonly used RTLs: MSL, MHW, OD etc.).

Relative sea level (RSL) = H – I Where H is the present day altitude of the sample and I is the

indicative meaning.



Session Summary

Sea-level change is an important driving mechanism for coastal change, especially over G-IG cycles

Complex interplay between glacio-eustasy, tectono-eustasy, geoidal-eustasy, glacio-isostasy, hydro-isostasy results in highly complex patterns of RSL change

The concept of a ‘global’ sea-level curve does not exist, BUT regional trends can be identified

Postglacial (Holocene) trends are the most extensively studied – due to the abundant evidence

RSL change can be reconstructed from a number of different archives, BUT potential inaccuracies in the resulting RSL curves need to be acknowledged



References

Bassett, S.E., Milne, G.A., Mitrovica, J.X., Clark, P.U. (2005) ‘Ice sheet and solid earth influences on far-field sea-level histories’ Science 309: 925–928

Clark, J.A., Farrell , W.E. and Peltier, W.R. (1978) ‘Global Changes in Post Glacial Sea Level: A Numerical Calculation’ Quaternary Research v.9 : 265-287

Clark, P.U., Mitrovica, J.X., Milne, G.A., Tamisiea, M.E. (2002) ‘Sea-level fingerprinting as a direct test for the source of global meltwater Pulse IA’ Science 295: 2438–2441

Gehrels, W.R. and Long, A.J., (2008) Sea level is not level. The case for a new approach to predicting UK sea-level rise. Geography 93, 11-16.

Masselink, G. and Hughes, M. (2003) Introduction to Coastal Processes and Geomorphology Edward Arnold Publishers

Peltier, W.R. and Andrews, J.T.(1976) 'Glacial isostatic adjustment I: the forward problem’ Geophys. J. Roy. Astron. Soc. 46, 669–706

Peltier (2005) ‘On the hemispheric origins of meltwater pulse’ Quaternary Science Reviews 24: 1655–1671

Shennan, I. (1999) ‘Global meltwater discharge and the deglacial sea-level record from northwest Scotland’. Journal of Quaternary Science 14(7): 715-719

Tamisiea, M.E., Mitrovica, J.X., Davis, J.L. and Milne, G.A. (2003) ‘Long wavelength sea level and solid surface perturbations driven by polar ice mass variations: fingerprinting Greenland and Antarctic Ice Sheet flux’. Space Science Reviews 108(1): 81-93



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Author Dr Katie Szkornik

Stephen Whitfield

Institute – Owner Keele University, School of Physical and Geographical Sciences

Title Postglacial Sea-Level Change Powerpoint Presentation

Date Created December 2009

Description Postglacial Sea-Level Change - Powerpoint Presentation – Part One of Changing Coastal Environments

Educational Level 3

Keywords (Primary keywords – UKOER & GEESOER)

UKOER, GEESOER, Eustasy, Isostasy, Meltwater pulses

Creative Commons License Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales

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