The essential micronutrient iron (Fe) limits productivity in much of the global ocean. Despite recent advances quantifying the oceanic inventory of Fe, our understanding of the sensitivity of marine productivity to Fe supply is limited. In particular, the rates of Fe supply from key sources such as sediments and hydrothermal vents are poorly quantified, leading to estimates of the residence time of Fe that span two orders of magnitude. A key challenge in constraining fluxes of trace metals is the lack of time information with which to accurately assess supply rates. Radium (Ra) is produced naturally from the radioactive decay of thorium, and as the parent thorium isotopes are concentrated in sediment, Ra has a strong source at ocean margins. Radium itself decays over time, making it an ideal chronometric tracer of ocean margin processes.
I will present the use of Ra isotopes to quantify flux rates at ocean margins, using the recent UK Shelf Seas Biogeochemistry programme as a case study. At the Celtic Sea shelf break, we observed intermediate nepheloid layers at depths corresponding to focal points of tidal energy conversion. These nepheloid layers were associated with excess 224Ra, evidence of interaction with slope sediments on a time scale of <2 weeks, and exhibited high soluble, dissolved, colloidal and particulate Fe concentrations. The resulting estimates of dissolved Fe flux are 10-100 times greater than previously determined slope values, suggesting that Fe residence time in the ocean is toward the low end of modelled values. I will also outline upcoming projects expanding this approach to new locations, and using Ra to quantify trace metal supply from hydrothermal vents and glacial meltwater.