On the Holocene evolution of the Ayeyawady megadelta

The Ayeyawady delta is the last Asian megadelta whose evolution has remained essentially unexplored so far. Unlike most other deltas across the world, the Ayeyawady has not yet been affected by dam construction providing a unique view on largely natural deltaic processes benefiting from abundant sediment loads affected by tectonics and monsoon hydroclimate. To alleviate the information gap and provide a baseline for future work, here we provide a first model for the Holocene development of this megadelta based on radiocarbon and optically stimulated luminescence-dated trench and drill core sediments collected in 2016 and 2017, together with a re-evaluation of published maps, charts and scientific literature. Altogether, this data indicates that Ayeyawady is a mud-dominated delta with tidal and wave influences that has been constructed within a vertically stable Pleistocene incised valley. The sediment-rich Ayeyawady River built meander belt alluvial ridges with avulsive characters. A more advanced coast in the western half of delta (i.e., the Pathein lobe) was probably favored by the more western location of the early course of the river. Radiogenic fingerprinting of the sediment suggest that the Pathein lobe has been built with Ayeyawady sediments alone. However, the eastern region of the delta (i.e., Yangon lobe) is offset inland and extends east into the mudflats of the Sittaung estuary. Wave-built beach ridge construction during the late Holocene, similar to other several deltas across the Indian monsoon domain, suggests a common climatic control on monsoonal delta morphodynamics through variability in discharge, changes in wave climate, or both. Correlation of the delta morphological and stratigraphic architecture information onland to the shelf bathymetry as well as its tectonic, sedimentary and hydrodynamic characteristics provide insight on the peculiar growth style of the Ayeyawady delta. The offset between the western Pathein lobe and the eastern deltaic coast appears to be driven by tectonic-hydrodynamic feedbacks as the extensionally lowered shelf block of the Gulf of Mottama amplifies tidal currents relative to the western part of the shelf. This situation probably activates a perennial shear front between the two regions that acts as a leaky energy fence helping to trap part of the sediment within the Pathein shore-attached subaquoeus clinoform. Just as importantly, the strong currents in the Gulf of Mottama act as an offshore-directed tidal pump that help build the deep mid-shelf Mottama clinoform with mixed sediments from Ayeyawady, Sittaung, and Thanlwin rivers. The highly energetic tidal, wind and wave regime of the northern Andaman Sea thus exports most sediment offshore despite the large load of the Ayeyawady river. The expected sediment deficit if dams are constructed on the river and tributaries may significantly impact the Ayeyawady delta fragile sedimentary equilibrium making it more vulnerable to the accelerating sea level rise and changes in frequency and intensity of cyclones hitting the coast.