The Mechanisms of Sea-Level Change

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Rising sea levels as a result of global warming is now a widely publicised concern as strategies are currently being implemented to respond to them. However, global sea-levels have fluctuated many hundreds of metres either way of the present day sea-level and this essay proposes to examine mechanisms of sea-level change past and present and the its geomorphological impacts on the coasts.

Before one can explain the causes of sea-level change, it is important to define the different types of sea-level change. Eustatic sea-level, relative sea-level and water depth all have specific meanings (see figure 1 below)

Eustatic sea-level is the global sea-level and is measured between the sea-surface and a fixed datum, which has always been the centre of the Earth. Relative sea-level is the distance between the sea-surface and a local datum, such as the top of the basement rocks in a sedimentary basin. Water depth is the distance between the sea bed and the sea-surface (Coe at al 2005).

The principle causes of eustatic sea-level change are due to the variations of ocean-water volume and by the changes in the volume of the ocean basins. Variations in the volume of seawater are controlled by the formation and melting of the ice caps and glaciers. When water accumulates as ice on the land surface it causes sea-level to fall, whereas during inter-glacial periods the reverse happens and sea-levels begins to rise (Pethick 1984). This known as glacio-eustasy. In terms of the volume of water involved in this process, Pethick (1984) believes that if all the ice in the world were to melt, present day sea-level rise would rise by 40-60 metres, with Rothery (1998) and Coe et al (2005) suggesting this could be as high as 80 metres. Changes in the v...

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...ts and bedforms and thirdly it relies on outdated concepts (Cooper et al 2004).

Global and local sea-level has varied dramatically throughout the Earths history and are principally controlled by glacio-eustasy, thermal expansion and tectonic activity. Oxygen isotope records have help establish sea-level changes in recent geological history and it is now known that the sea-level in the British Isles has been rising for the last 18,000 years creating the beaches and coastline we see today. Present day sea-level rise is being exacerbated by anthropological causes, which is of concern due to the many millions who live by the coast. Predicting the effects of geomorphological effects of sea-level transgression is difficult, although Bruuns rule on sea-level change offer some, albeit, limited insight into predicting the geomorphological changes in the coastline.

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