Climate Change

Signatures of past climate change found on West coast

Researchers unravel imprints of sea-level fluctuations and climate change that may have occurred along coastal rivers in Gujarat's Saurashtra region over 1.5 lakh years

 
By Ravi Mishra
Last Updated: Wednesday 09 January 2019
Photo: India Science Wire

Researchers have unravelled imprints of sea-level fluctuations and climate change that may have occurred along coastal rivers in Gujarat's Saurashtra region over 1.5 lakh years. They analysed samples taken from more than six metres of sedimentary sequences along the Noli river between Mangrol and Chorwad and carried out geomorphology, sedimentology and optical-dating studies.

The team found four distinct significant layers. The bottom-most layer of two metres is composed of miliolite limestone with lot of oyster shells. There is also evidence of bioturbations, indicating there was a marine environment around the time when the layer was formed. Miliolite Limestone is a rock typically formed under shallow, warm, agitated waters of tropical sea shores and oysters are salt-water molluscs.

According to scientists, the layer perhaps represented a 3-5 m higher sea level than now and related to the last inter-glacial high sea level—about 1.5-1.2 lakh years ago.

The layer above (one-metre-thick) has a lot of miliolitic sand, pebbles and oyster shells. It was found to be 1.14 lakh years old. The Indian summer monsoon had been the most intense during this period. The miliolitic sand seems to have been transported by what is called fluvial activity or a flow of water. It was overlaid by a 1.5-m-thick layer with pebbles, cobble stone and lithoclasts of limestone.

The top most layer is a metre thick and had miliolitic sand with crude cross-laminations. It is found to be about 23,000 years old and represented an increase in aridity due to the lowering of sea level.

According to the study, these top sediments were related to the Last Glacial Maxima period, between 28,000 and 13,000 years ago, when the level was lower by about 100 metres from present level. During this period, miliolites from continental shelf of the Arabian Sea were transported by wind, This aeolian activites extended upto the Thar Desert.

"The oscillations in the sea levels are recorded from the sedimentary sequence. Such oscillations are also recorded in the form of tidal notches carved out into the rocky, coastal cliffs made of miliolite limestone on the southern coast of Saurashtra from Somnath—Diu to Gopnath area. The study has found that the range of fluvially reworked miliolitesmay be even as far back as 1.14 lakhs years ago, against the previously thought of limit of 75,000 years ago," Nilesh Bhatt, a member of the research team, said.

The findings of the study are expected to be of help in better land use planning of the Saurashtra region, as it has led to a new understanding of sea-level fluctuations and land-sea interactions at the local scale. 

The research team included S P Prizomwala and Gunjan Yadav of Institute of Seismological Research, Gandhinagar, and Komal Sharma of Kangwon National University, Republic of Korea, besides Bhat. The study has been published in journal Current Science. (India Science Wire)

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