Lithology, monsoon and sea-surface current control on provenance, dispersal and deposition of sediments over the Andaman continental shelf

Abstract

Sediments deposited on the Northern and Eastern Andaman Shelf along with a few sediments from the Irrawaddy and the Salween Rivers are studied for their elemental, Sr and Nd concentrations and their isotope composition to identify their sources, constrain their transport pathways and to assess the factors influencing the erosion in the catchment and their dispersal and deposition over the Andaman Shelf region. Major elemental compositions of the shelf sediments suggest mafic lithology such as ophiolites, ultrabasic rocks, and andesites in the Irrawaddy drainage and over Indo–Burman–Arakan (IBA) ranges as their dominant source. ⁸⁷Sr/⁸⁶Sr ratios in sediments of the Northern and Eastern Andaman Shelf range between 0.712245 and 0.742183 whereas, εNd varie from −6.29 to −17.25. Sediments around Mergui have the highest ⁸⁷Sr/⁸⁶ and the lowest εNd values. Sr and Nd isotope compositions of these sediments along with that in the potential sources suggest four major sources of these sediments to the Andaman Shelf, (i) the Irrawaddy River, (ii) the Salween River, (iii) Rivers draining the IBA ranges and (vi) Rivers draining the Western/Central granitic ranges of the Southern Myanmar and Western Thailand such as the Tavoy and the Tanintharyi Rivers. Erosion in the catchment is controlled by the precipitation and topography. Intensely focused precipitation over the higher relief of the western slopes of the IBA and Western/Central granitic ranges causes higher erosion over this mountainous region, supplying huge sediments through the Kaladan, Irrawaddy, Salween, and the Tanintharyi Rivers to the Western Myanmar Shelf, Northern, and Eastern Andaman Shelves respectively. The majority of the sediments produced in the drainage are delivered to the shelf during the south–west monsoon which is dispersed eastward by sea-surface circulation from the mouth of the Irrawaddy Rivers toward the Gulf of Martaban and further southward. The Andaman Shelf receives very little sediment, if any, from the Ganga–Brahmaputra Delta. Higher erosion over the Western/Central granitic belt of the Southern Myanmar and Western Thailand and its importance in delivering sediments through the rivers draining its western slope such as the Tanintharyi and the Tavoy Rivers to the Eastern Andaman Shelf around the Mergui Archipelago are highlighted for the first time in this study.