Investigating 210Pb boundary scavenging on a high-energy, river-dominated margin

On high-energy, river-dominated continental shelves, the hydrodynamic conditions of the receiving basin can modulate biogeochemical fluxes via the scavenging of particle-reactive species, and frequent resuspension and re-oxidation of the seabed. This makes an understanding of marine exchange and hydrodynamics in these shelf systems critical for determining the fate of particle-reactive species in the ocean and for deriving global geochemical budgets. In this context, we explore the boundary scavenging of ²¹⁰Pb off the Ayeyarwady-Thanlwin River system in Myanmar, one of the largest and relatively unmodified point source contributors of sediment and organic carbon to the global ocean. Together, these rivers supply 485 (+169/-121) Mt y⁻¹ of sediment and 1.9 (+1.4/-0.9) Mt y⁻¹ of organic carbon to the offshore delta (Baronas et al., 2020), where energetic conditions on the shallow inner shelf produce extensive perennial turbidity, and frequent seabed resuspension. Here, a mass balance of excess ²¹⁰Pb in shelf sediments is utilized to characterize the land-ocean mixing regime and indirectly quantify the volume and rate of open-ocean water advection onto the shelf. Our budget indicates that on average ∼71 % (with a range of 64–76 %) of the excess ²¹⁰Pb flux in shelf sediments is delivered from offshore, equating to a flow of ∼6.5 × 10⁵ m³ s⁻¹ of marine water onto the shelf which is nearly 12 times that of the combined Ayeyarwady and Thanlwin river discharge. We suggest that in the Ayeyarwady, and perhaps in other high-energy delta systems, substantial advection of ocean water onto the shelf along with intense tidal mixing and long water-column residence times increase particle scavenging across the shelf. This likely makes these systems large repositories for particle-reactive species such as marine pollutants and trace metals, and further demonstrates the importance of high-flux river-dominated margins in modulating geochemical fluxes in the global ocean.