Carbonate sediment production, redistribution and accumulation in ongoing climate warming – The case of Lakshadweep Archipelago

Isolated carbonate platforms in tropical regions exhibit varying geomorphic characteristics as a consequence of differences in carbonate factories, hydrodynamic and global climatic controls. The stability of islands is dependent on the interplay between local and global factors and thus, ascertaining these factors is significant. Determining the facies and their correlation with lagoon filling is significant in determining the future of the islands to sea level rise. In the Indian context, the Lakshadweep Archipelago forms the best repository to understand the variability in sediment production to local hydrodynamics and global sea level change.

The Lakshadweep Archipelago is in the Arabian Sea off the western coast of the Indian Peninsula with an average elevation of 0.5m to 6m. This study investigates two islands, viz Agatti and Kavaratti and the adjacent lagoons utilizing sedimentological data; supplemented with satellite geomorphological studies. Geomorphic units have been identified by satellite and field studies. 42 samples have been collected from Agatti lagoon and 24 samples from the Kavaratti lagoon along E-W transects. The samples have been characterized texturally based on Dunham classification. Furthermore, petrographic analysis has been carried out for the soft sediments, representing different grain size classes to characterize the major sediment producers and grain types. These datasets are utilized to develop the depth and the first facies maps of these lagoons.

Both, the islands are approximately 4 sq km in size and are present on the eastern part of the atoll. The lagoon is about 17 sq.km for the Agatti that is more than thrice of Kavaratti that is 5 sq km. The average depth is 2m and maximum observed depth is 4m for both the lagoons. Satellite studies indicate well developed patch reefs (Acropora and Porites) and lobate sand features. The sediments are mostly coarse to medium-grained carbonate sand. No correlation between sediment size and lagoon depths has been identified. Petrographic analyses illustrate corals and foraminifera as predominant sediment producers for the coarse-grained fraction along with algae (Halimeda) and some gastropods. Based on the annual hydrodynamic patterns it is observed that the SW monsoon plays a dominant role in sediment distribution. With significant warming, the sediment production could be lowered/reduced thereby affecting the lagoon infilling and the future of the islands. The study, thus, emphasizes the necessity of acknowledging local factors and quantifying the different carbonate factories and how they will be influenced by warming seas will be critical in determining the stability of an island.