Sea level changes

Sea level changes

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  • INTRODUCTION • EVIDENCESFOR SEA LEVEL CHANGES • MECHANISMS OF SEA LEVEL CHANGES • IMPACT OF SEA LEVEL CHANGES • IMPACT OF PROJECTED SEA LEVEL RISE • COCLUSIONS • REFERENCES
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  Sea level changeis a central concern of the geomorphologists. Sea level determines the base level for erosion . Besides , relative vertical movements of land and sea can greatly alter the area of land exposed by exogenic subaerial geomorphic processes.  The study of sea level is important because it can provide key evidences of climatic change and can also provide a benchmark of estimating rates of tectonic.
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  A wide rangeof techniques can be used to unravel the complex history of Sea level changes. For different time scales, there are different methods. As the time period grows older , the task of reconstructing the record of sea level changes, either locally or globally becomes difficult. In contrast to the direct information from tide gauge records, sea level changes, over longer periods of time, are based on indirect information. Relative falls in sea level are demonstrated by elevated shoreline features such as raised beaches, which provide deposits yielding material suitable for dating by radiometric techniques. A relative rise in sea level is indicated by submerged coastal topography and by drowned river valleys or glacial troughs which, in some instances, can be traced seawards as submarine canyons and channel networks. Changes in global sea level during the quaternary have been reflected in the oxygen isotope record preserved in the calcareous shells of microfossils, which have accumulated in the sediments deposited on the ocean floor. The recovery of cores drilled at various locations across the world’s oceans have thus
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   Average sealevel over all the changes of the last few glaciations and interglacials has been about 50-60 m below it’s present level. During the height of the last glaciation (about 18,000 years ago) it was between 110 m and 140 m lower than it is today; and the large areas of the continental shelves were dry land. Sea level has risen through the post-glacial period(this is called the flandrian transgression). The post-glacial sea level rise was initially fast , but it leveled off about 5,000 to 6,000 years ago.
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  Sea level hasremained relatively close to it’s present position over the last 5,000 to 6,000 years. Evidences of marine animals in the rocks of highest mountains are found and deep drilling has recorded shallow marine sediments hundreds and even thousands of meter below the modern sea level.
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  The sea levelfluctuations over the past 100 M.Y or so indicates that they have involved total movement in excess of 200 M. and have occurred over a wide range of time scales. These fluctuations have been caused by several possible mechanism. However the long-term changes can be grouped into three basic categories:- 1. Those that alter the volume of the world’s ocean. 2. Those that change the volume of world’s oceans. 3. Those that produce change in the geoid. The various geological evidences suggest that the mass of water at the surface of earth has not changed significantly since precambrian time. The volume of seawater does vary both in the short-term of a few years, or even during a year, and to a much greater extent over the tens of thousands of year.
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  The Antarctic Icesheet developed during the middle and late Tertiary and added it’s effect to that of ocean-basin volume change. About 3 to 4 million years ago, extensive ice sheets developed for first time on the northern hemisphere continents and since then ice volume and ice extent have fluctuated widely . At it’s most extreme, sea level may have fallen 200 m below that of the present , and risen 5-8 m above present levels. If ice volume along controlled sea levels uniformly around the coasts , then the sea level evidences would uniformly follow a pattern imposed on it by climate. The sea level curves for the last 15,000 years would show a progressive rise from about 100-130 m below present levels to about the present level 6,000 to 40,000 years ago.
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  A relative risein sea level reduces the drainage basin area , the sea level fluctuations of the late Cenozoic caused major shifts in drainage area and the periodic sub mergence of extensive drainage systems in the relatively brief interludes of high interglacial sea level such as the one being experienced in the present day.
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  • A fallin the level of the sea especially where the continental shelf is shallow will lead to greater aridity in the continental hinterland and a consequential reduction in runoff. • A drop in the sea level causes ice caps and glacial tongues to be extended onto the shelf proper . In temperate and high latitude, flowing ice scored and plucked the shelf bottom. In some case, it smoothed out the tops of hills; In others, it gouged out irregular topography, including fjords, and created considerable local relief. Also, glaciers dumped debris on the shelf floor, forming blankets of till (unsorted deposits of boulders, sand, and mud) and hummocky hill deposits at their terminal.
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  It is projectedthat the sea level will rise in the near future. There are many possible impacts of such rise , these include- 1. Damage to many important coastal ecosystem , including deltas , coral atolls and reefs. 2. Flooding of many densely populated areas. 3. Damage to port facilities and coastal structures. 4. Severe coastal erosion in many countries, including loss of beaches and dunes. 5. Decline of loss of production in up to one-third of the world’s croplands .
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  CONCLUSIONS  A widerange of techniques can be used to unravel the complex history of sea level changes. For different time scales, there are different methods. As the time period grows older, the task of reconstructing the record of the sea level changes, either locally or globally becomes difficult.  Evidences of marine animal in the rock of highest mountains are found and deep drilling has record shallow marine sediments even hundred to thousands of meter below the modern sea level.  The various geological evidence suggest that the water at the surface of the earth has not changed significant since Precambrian time.
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  REFERENCES 1. K. Siddhartha(2005), Oceanography Kisalaya publications Pvt Ltd,(3rd Edition 1999), P to p- 263-279 2. A.C.Narayana (2002), Late Quaternary geology of India, Published by Geological society of India, Pp(2-40) 3. www.sealevelchanges.com 4. www.wikipedia.com