Friday, February 10, 2012

Is the sea level rise is recent phenomenon or it has happened in earlier geological ages too?

Over the past 20,000 years the oceans have risen by more than 110 meters.

by

Dr. Nitish Priyadarshi


The U.S. is shrinking –physically. It has lost nearly 20 meters of beach from its East coast during the 20th century. The oceans have risen by roughly 17 centimeters since 1900 through expansion (warmer water taking up more space) and the ongoing melt down of polar ice. That increase, however, is a small fraction compared with what’s to come. Sea level is of great concern to human beings especially to those living in coastal regions. India has the sixth longest coastline in the world and the fate of the occupants of the coastal areas critically depends, among other things, on the abrupt rise and fall of the sea level.

Is the sea level rise is recent phenomenon or it has happened in earlier geological ages too? Different researches have shown that our planet earth has seen drastic sea level changes in past geological times.

Before going in detail first we should understand the causes for relative sea level changes. This is a topic of utmost importance in sea level measurements, be they present day or paleo type. Many processes can bring about an apparent sea level changes at any particular location. These processes are of two types, (a) which are caused by land movements and (b) caused by changes in sea surface level. Local or regional uplift or subsidence (like near Dwarka, India) of land, changes of atmospheric pressure, winds, ocean currents, changes in the volume of ocean basins due to volcanic activity, marine sediment deposition, isostatic adjustments of the earth’s crust under the sea or sea floor spreading, changes in the mass of water brought about by melting or accumulation of ice sheets and alpine glaciers ( manifests in the form of salinity lowering) and thermal expansion or concentration of ocean waters when they become warmer or colder. The last two processes are the most important ones that bring about global changes which can result from climate change, e.g. warming as a result of increasing greenhouse gases in the atmosphere.

The subject of sea-level variations over the last 18,000 years is treated in detail by different geo scientists. They emphasized two sharp rises, to about +30 m at about 15,000 years ago and to about +20m near 8500 years ago. Since 7000 years ago the amplitudes of sea- level change have been increased.

A theory proposed by different researchers dates the flooding of the Black Sea basin to an earlier time and from a different cause. According to research, global warming beginning from about 16,000 years ago caused the melting of the Scandinavia Ice Sheet, resulting in massive river discharge that flowed into the Caspian Sea, raising it to as much as 50 metres (160 ft) above normal present-day levels. The rise was extremely rapid and the Caspian basin could not contain all the floodwater, which flowed from the northwest coastline of the Caspian Sea, through the Kuma-Manych Depression and Kerch Strait, over the current eastern coastline of the Sea of Azov into the ancient Black Sea basin. By the end of the Pleistocene this would have raised the level of the Black Sea by some 60 to 70 metres (200 to 230 ft) 20 metres (66 ft) below its present-day level, and flooding large areas that were formerly available for settlement or hunting.

A catastrophic flood refilled the Mediterranean Sea 5.3 million years ago, at the beginning of the Zanclean age that ended the Messinian salinity crisis.The flood occurred when Atlantic waters found their way through the Strait of Gibraltar into the desiccated Mediterranean basin, following the Messinian salinity crisis during which it repeatedly became dry and re-flooded, dated by general consensus to before the emergence of modern humans.

Global sea level rose by about 120 m during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilized between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century. The instrumental record of modern sea level change shows evidence for onset of sea level rise during the 19th century. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7mm yr–1.


During much of the Paleozoic, the rock record shows, sea level stood quite high relative to the continents. But there were also substantial fluctuations. Continental flooding was pervasive, but it was intermittent.

The key to understanding how sea level changes are recorded in rocks is very simple: Most sedimentary rocks were originally deposited in water, and all bodies of water accumulate sediments at their bottoms. Rainfall and erosion ensure that the rocks of the continents will be worn down, and the detritus of that process is carried by rivers to lakes and the ocean, where it settles out in layers of sediment. Furthermore, the coarse material drops out first, close to the shore line, while the finer grains remain suspended and are carried further out to sea. Thus the type of sediment that is formed depends on the water depth. Even by using just these fairly obvious principles, a great deal can be learned about the sea level changes that occurred in the Paleozoic.

The sea level lowering was not an isolated event, but part of several cycles of rising and lowering seas near the end of the Permian period that must have played havoc with life in the shallow waters around the continental margins. When sea level fell, not only were vast shallow-water habitats removed from existence, but also great quantities of organic matter that has been deposited in the sediments, the remains of shallow-water-dwelling organisms were exposed to the atmosphere.

Coral reefs grow very close to the sea level. Raise sea level by a few meters, and the corals die- but new ones grow on top of them, closer to the new ocean surface. By continuously growing upward, the reefs keep pace as water levels rise, and they are thus very good indicators of past sea level. In places like the Caribbean such reefs have been cored and studied, and their ages determined using the Carbon 14 method and other techniques. Corals that lived near the sea surface thousands of years ago are now found at depths of tens of meters, buried in the reef by their progeny. By measuring their ages, and the depth at which they now reside, a record of past ocean levels has been constructed. It illustrates that the most recent low stand of sea-water occurred at the same time the oxygen isotope data indicate a glacial maximum, about 20,000 years ago. It also indicates that there have been two or three times over the past 20,000 years when sea level rose more rapidly, almost instantaneously in geologic terms, presumably in response to especially rapid melting of the ice sheets. Over the past 20,000 years the oceans have risen by more than 110 meters, covering vary large areas that were dry land at the peak of the glacial episode.

Reference:

Frakes, L.A. 1979. Climates throughout geologic time. Elsevier.

Macdougall, J.D. 1996. A short history of planet earth. Mountains, mammals, fire and ice. John Wiley & Sons, New York.

Narayana, A.C. (Ed) 2002. Late Quaternary Geology of India and Sea level changes. Memoir, 49. Geological soc. of Ind., Bangalore.


http://en.wikipedia.org/wiki/Outburst_flood
http://www.sciencedaily.com/releases/2009/02/090209205314.htm
http://judithcurry.com/2011/07/12/historic-variations-in-sea-levels-part-1-from-the-holocene-to-romans/
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