<jats:title>Abstract</jats:title><jats:p>In the western Antarctic Peninsula region, micronutrient injection facilitates strong plankton blooms that support productive food webs, unlike large areas of the low-productivity Southern Ocean. We use naturally occurring radioisotopes of radium to constrain rates of chemical fluxes into Ryder Bay (a small coastal embayment in northern Marguerite Bay), and hence to evaluate possible sources of sediment-derived micronutrients and estimate sediment-ocean mixing rates. We present the first coupled, short-lived radium isotope (<jats:sup>223</jats:sup>Ra and <jats:sup>224</jats:sup>Ra) measurements from Antarctic waters, both present at very low activities (mean 0.155 and 3.21 dpm m<jats:sup>-3</jats:sup>, respectively), indicating much lower radium inputs than in other coastal environments. Longer-lived <jats:sup>228</jats:sup>Ra activity was also lower than existing nearshore values, but higher than open ocean waters, indicating some degree of coastal radium input on timescales exceeding the week-to-month range reflected by <jats:sup>223</jats:sup>Ra and <jats:sup>224</jats:sup>Ra. Using a simple diffusion model along a shore to mid-bay transect, effective horizontal eddy diffusivity estimates ranged from 0.22–0.83 m<jats:sup>2</jats:sup> s<jats:sup>-1</jats:sup> from <jats:sup>223</jats:sup>Ra and <jats:sup>224</jats:sup>Ra, respectively, much lower than already-low mixing estimates for the Southern Ocean. Significant radium enrichment and much faster mixing (18 m<jats:sup>2</jats:sup> s<jats:sup>-1</jats:sup>) was found near a marine-terminating glacier and consequently any sediment-derived micronutrient inputs in this location are more probably dominated by glacial processes than groundwater, land runoff, or marine sediment sources.</jats:p>