Suspended Sediment Composition of the Irrawaddy and Salween Rivers: Grain-size Dependence and Spatiotemporal Variations

Studies of the dissolved and sediment composition of global rivers provide crucial insights into the relationship between climate, weathering, and landscape dynamics. The chemical composition of suspended riverine sediment constrains contemporary erosion and chemical weathering, and is critical to the interpretation of sedimentary records, such as continental shelf deposits.

Here we present suspended sediment flux and chemical composition data from the Irrawaddy (Ayeyarwady) and Salween (Thanlwin) rivers in Myanmar, from samples collected in the wet and dry seasons in 2017-2018. Analyses of major element, ^87/86Sr, and ɛ_Nd composition are combined with a multi-step leaching protocol to determine the ccompositions of silicate, carbonate, and iron oxide components in the bulk sediment. Sources of the organic matter are determined by analyses of carbon and nitrogen isotopes.

We show that, as in other large rivers, suspended sediment concentration and composition in the river channel is depth-dependent due to hydrodynamic sorting. Depth profile sampling is therefore required for the complete characterization of the sediment flux and composition in these rivers. Having accounted for hydrodynamic sorting, we demonstrate that there are key differences in the composition of the Irrawaddy and the Salween rivers at the mouth. We further show how the composition of the Irrawaddy sediments evolves along a downstream transect from the northern headwaters to the delta, due to inputs from tributaries draining distinct lithologies and the continued weathering of sediments transported through the floodplain.

Finally, we discuss the implications of our findings for the interpretation of offshore sedimentary deposits and for quantifying the regional chemical weathering and carbon budgets.