The continuous efforts of the sea waves to scale as much land as possible before receding, provides the inspiration to many, to keep trying despite failures. It is not only the charm of watching this age old tug of war between land and sea, that has lead to man residing in its vicinity. The livelihood that man gains through the vast resources of the sea and the sea's great capacity of sustaining life has lead to the development of civilizations during ancient times and some of the most densely populated regions of the present day world, along the coast.

In this battle between land and sea, there are times when for brief and prolonged periods the sea or land has had an advantage over the other resulting in either sea conquering and occupying otherwise exposed land or land forcing the sea to recede thus extending its rule to a greater area. Brief periods of this alternating advantage are seen as diurnal high and low tides, regulated by changing positions of the earth, moon and sun. But it is the prolonged periods of ingress or regress of sea governed by changing global temperatures in response to changes in incident solar radiation, that are important for coastal population. Even a slight prolonged increase in sea level may result in submergence of kilometers of low-lying coastal regions and thus destruction of both life and property worth billions of Rupees. In view of enormous number of lives and property worth billions of Rupees at stake, it is important to predict future sea level variations. Taking clue from fortunetellers predicting future based on birth and subsequent past behavior of person concerned, understanding past sea level variations can help peep into future sea level variations.

Finding the extent of the sea during the past is possible as the sea, especially along the coastline, leaves its signatures in morphological features like wave cut terraces, notches, sea caves etc, as well as sedimentological characteristics, that can be seen in the light of time. Radioisotope dating of material ranging from sediments to shells and different types of calcareous and carbonized wood, making use of continuously decreasing number of radioactive isotopes of certain elements at known rates, after initial incorporation, provides chronological framework to these signatures. Thus finding signatures of sea stand during different times in past leads to the generation of continuous sea level variation history.

Generating sea-level variation history for different regions is necessary as sea level at any place is the cumulative result of both global and local factors. The global variation in sea level occurs due to changes in global temperature leading to variation in sea water volume due to thermal expansion during warmer conditions and vice versa. Fresh water from ice sheets due to temperature dependent melting also results in global sea-level variations. The localized variation in sea level is the result of site-specific tectonic uplift or submergence.

In one such study, the scientists at National Institute of Oceanography have developed sea level variation history of the last ~14,500 years B.P., for the western Indian continental margin. To generate the sea level variation curve for the west coast of India, they compiled all the dates of past shore line indicator features available between 21°N to 14°N latitude (till south of Saurashtra Peninsula). These dates ranging from 1,500 to 14,500 yr B.P. were then plotted against height/depth from which the dated material was recovered. The sea level curve was then drawn based on geological reasoning and supporting evidences such as presence of terraces, nature of samples dated and inferences of sea level rise from similar stable areas elsewhere.

The curve shows that the sea level along the west coast of India was about 100m lower as compared to present, around 14,500 years before and rose to 80m depth around 12,500 years before with a rate of ~10m/1,000 years. It was followed by a quiet period when the sea level remained unchanged for about 2,500 years. From 10,000 to 7,000 years before, sea level rose at a very high rate (~20m/1000 years). After 7,000 years B.P. it fluctuated to more or less the present level.

The potential application as well as validity of this sea level variation curve was established when ~7500 BC old Neolithic settlements were discovered from Gulf of Khambat from a seawater depth of 30-40m. Plotting the age of discovered settlement over the sea level variation curve generated by NIO scientists gives a water depth of 30-40m, exactly matching with the depth zone from which the new findings of ancient civilization were reported. Further observation of the curve indicates that the sea came to a stand still at that level for some time, thus providing time for civilization to flourish before being engulfed by the sea again, thus providing scientific proof for the possible ancient Neolithic settlements off Gujarat.

In view of past evidences of alarmingly high rates of sea level rise and this abrupt sea-level rise as possible cause for demise of civilizations, one needs to be cautious while planning high cost long term investments in the coastal regions, especially low lying areas along the sea. Study shows greater role played by the oceans in regulating the rise and demise of civilizations and emphasizes the need for continuous efforts to better understand past sea-level variations.

To consult original article, refer :

Hashimi, N.H.; Nigam, R.; Nair, R.R.; Rajagopalan, G. Holocene sea level fluctuations on western Indian continental margin: An update. J. Geol. Soc. India. 46; 1995; 157-162.

Nigam, R.; Hashimi, N.H. Has sea level fluctuations modulated human settlements in Gulf of Khambhat (Cambay)? J. Geol. Soc. India. 59; 2002; 583-584.