The causes of sea level change vary considerably:
- Tectonic movement, changing the shape of ocean basins
- Climatic variations (changes in temperature, precipitation rates etc)
- Isostatic land movements
- Back ground noise in records (volcanic eruptions, anthropogenic influences etc.) [1]
The various causes of change have caused much debate over the best way to modelling past sea level rise, a debate that has not yet and may never be satisfied.
This post will document the various techniques of past sea level modelling and illustrate the weaknesses with each, an important point as it has been noted that no single technique should be relied upon to produce a history of sea level change [2].
Tide Gauges
Whilst the first tide gauge was set up some 300 years ago [1], reliable sea level records are only available for the past 150 years [3]. Such records began after the use of tide gauges in ports, harbours, rivers and estuaries became the most popular way of recording monthly and annual means of sea level change.
Tide gauges record the sea level, in relation to a set norm. water enters the gauge through the bottom and the height reached is recorded by electronic sensors, allowing the production of raw tide data to an accuracy level of 1mm (figure 1). The data is then used to produce month and yearly averages of sea levels.
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| Figure 1: An example of raw tide gauge data, taken from Ponta da Armação, Brazill [4] |
Tide gauge networks have now been set up across the globe, greatly increasing the reliability of the measurements. Examples of such tide gauge organisations include the Global Sea Level Observing System and the Permanent Service for Mean Sea Level, both of which aim to observe sea level changes and use the collected data to warn of dangerous events, such as storm surges and tsunamis.
For more information, please see the websites of the aforementioned organisations:
Whilst tide gauges allow the monitoring of current and recently past sea level changes, the study of paleo-sea levels requires the use of other techniques.
Sea Level Reconstruction Using Paleotemperatures
Temperature has a leading roll in the story of sea level rise, a point illustrated in modern day sea level rise, as rising temperatures cause thermal expansion of the oceans and ice sheet melting. It is this modern day occurrence that allows past temperatures to be related to sea level rise: the study of glacier and ocean expansion reaction to modern temperature changes can be used to predict past changes, once temperature is know [3].
The study of oxygen isotopes in proxy records, such as ice cores, coral reefs, foraminfera and fossilised diatoms allows the reconstruction of past temperatures, as the different weights of oxygen isotopes cause varying levels of evaporation [3].
To find the temperature of past climates, the ratio between Oxygen-16 and the 12% heavier Oxygen-18 [5] isotope is considered:
Colder Climates
Ice Cores: Higher content of Oxygen-16 in the ice core, as these are lifted by weak evaporative forces, to form precipitation and extend glaciers.
Corals, Foraminfera and Diatoms: Higher content of Oxygen-18, as this heavier isotope is left in the ocean waters by the weak forces of evaporation. (figure 2)
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| Figure 2: Variations in the content of Oxygen-18 isotope of fossilised oceanic calcitic and phosphatic shells, illustrating the increase in Oxygen-18, as temperatures fall towards glacial level [6] |
Such a technique allows the reconstruction of past temperatures and the relation of such temperatures to sea levels, as modern day reactions to change are able to be used as a point of reference. This technique has however been accused of being too simplistic, with little appreciation of other possible differences in the past, such as the temperature of ocean waters and varying isotopic content of paleo-ice sheets [2&6]. It is for this reason that other methods must be researched.
Seismic Stratigraphy
The study of seismic stratigraphy allows the reconstruction of sea levels from some 250 million years ago [1]. The technique uses coastal sediments to identify periods of maritime erosion and deposition [7], a variation that indicates the rise and fall of sea levels over the coast.
The maritime sediment is studied for duration and magnitude of the change, a perk that allows the identification of the speed and length of sea level change and can allow the construction of charts that demonstrate the observations [7]. Whilst such an approach can supply a good representation of sea level change in an area, it can be easily affected by isostatic sea level change and the tectonic alteration of ocean basins, it must therefore be cross referenced against other techniques.
Whilst the tectonic effects on sea level may seem whimsical, the following video illustrates the evolution of the earth, aptly demonstrating tectonic movement's ability to alter ocean basin size.
Percentage Flooding of Continents
This less documented technique uses paleogeographic maps to calculate areas of the land previously covered by ocean (figure 3). Such maps are created through the study of sediment cores, enabling the identification of areas that were once connected land masses. The extent of sea water coverage is also assessed through sediment observation, as maritime sediment in terrestrial cores indicates that the land was once covered.
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| Figure 3: Paleogeographic map illustrating the hypothesised positions of the continents 245 to 228 willion years ago [8] |
Such a method falls to the same shortcomings as Seismic Stratigraphy; local changes could easily affect the sediment core and cause errors in any conclusions made. The techniques should not however be dismissed, as the study of other techniques should help to avoid mistakes.
Raised Marine Terraces
The study of uplifted marine terraces and coral reefs is one that can prove very useful, as long as all of the necessary information is available; the researcher must be aware of:
- Date of uplift
- Rate of uplift
- Extent of uplift
- Height of original position, with relation to the seafloor [1]
With such information, the research is in a good position to estimate the previous sea levels, as this is simply as far as the terrace extends. The use of uplifted coral reefs (figure 5) makes the study particularly easy, as the extent of the previous sea levels is demonstrated by the extent of the reef.
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| Figure 5: An example of an uplifted coral reef in Moria [9] |
The technique does however, fall when isostatic sea levels are considered, as uplift in one area may have no influence on sea levels across the globe. Again, the study of past sea levels must combine the findings of different techniques, in an effort to identify suggestions of sea level rise, that are in fact caused by local changes.
The techniques documented above have all provided sea levels estimations for varying periods of time, the next post will document the different findings, illustrating the need to combine the techniques, as the strengths and weaknesses of each become clear.
To help you relax after reading this information-heavy post, please watch the following video of my main man David Attenborough talking about fish and shrimps.
References
[1] Pirazzoli P.A. (1992) Global Sea-level changes and their meaurement. Global and Planetary Change, 8, 135
[2] Rabineau, M. S. Berné, J-L. Olivet, D. Aslanian, F. Guillocheau, P. Joseph (2006) Paleo sea levels reconsidered from direct observation of paleoshoreline position during Glacial Maxima (for the last 500,000 yr). Earth and Planetary Science letters, 252, 119
[3] Grinsted, A., J.C. Moore, S. Jevrejeva (2009) Reconstructing sea level from paleo and projected temperatures 200 to 2100AD. Clim. Dyn.
[4] Lieutenant Commander M.F. Cavalcante (2003) Brazilian Navy Hydrographical Centre of the Navy the Digilevel [online]. Available at http://www.mares.io.usp.br/aagn/7/dhn/presentation-brasil-digilevel.htm [4.3.2011]
[5] Emsley, John (2001). "Oxygen". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press.
[6] Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O. and Strauss, H. (1999) 87Sr/86Sr, d13C and d18O evolution of Phanerozoic seawater. Chemical Geology 161, 59-88.
[7] Vail, P.R., R.M. Mitchum Jr., Thompson, S. (1977) Seismis Stratigraphy and Global Changes of Sea level
[8] Malaysian Triassic Blog [online]. Available at: http://malaysiantriassic.blogspot.com/2009_03_01_archive.html [4.3.2011]
[9] cruising newcaledonia (2010) Coastal Uplift [online]. Available at: http://www.cruising-newcaledonia.com/ncal80/3UPLIFT.HTM
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