The Amazon River is the largest river by discharge in the world. It carries terrestrial nutrients into the Western Tropical North Atlantic via a buoyant freshwater plume, conveying water hundreds to thousands of kilometers away and driving critical biogeochemical cycles near the coast. Factors controlling the delivery of nutrients offshore by the plume are complicated and interconnected, yet these nutrients impact the foundation of phytoplankton community structures across the entire plume ecosystem. To better understand the temporal dynamics of this massive, highly influential region, we employ naturally occurring radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) to analyze mixing and transport behaviors through two separate research cruises to the Amazon River plume during high discharge seasons in 2018 and 2019. Radium is uniquely suited for this task because it displays elevated activities at the low end of a salinity gradient and decreases offshore as a function of dilution for all isotopes and radioactive decay for only the short-lived isotopes (223Ra and 224Ra). Known half-lives of these isotopes allow us to calculate apparent radium \u27ages\u27 to assess dissolved material transport scales and rates, and presumably also represent those of dissolved nutrients as well. Our results from 224Ra/226Ra apparent ages suggest that low salinity plume waters travel at a rate of 77 - 136 cm/s and are influenced by river discharge. Transport rates across the plume boundary were found to be significantly lower, ranging between 13 and 44 cm/s. Examining the horizontal mixing in this boundary region indicates that advection is the dominant process. An estimated eddy diffusion coefficient for the core of the plume mixing vertically with ambient waters was found to be 3.9 x 10 -4 ± 1.5 x 10-4 m2/s, varying significantly with respect to salinity profiles. Determining the temporal scale of dissolved material (e.g., nutrients) movement across this region is a valuable first step in examining the foundation of planktonic food web dynamics of the Amazon River plume
