Radium isotopes as tracers of element cycling at ocean boundaries

Trace elements (TEs) play crucial roles in regulating ocean processes including marine biogeochemical cycles, and are therefore vital to support marine life. Understanding the biogeochemical cycling of TEs requires knowledge of their sources/ sinks and transport in the oceans. Radium isotopes are suitable tools to study inputs of elements from the continental margins, as they are produced by the decay of thorium isotopes in sediments, and are soluble in seawater. Therefore, in this Ph.D. thesis, I used Ra isotopes to determine boundary TE fluxes from two diverse environments that constrain the major TE sources, including shelf sediments in an Arctic shelf region and in an eastern boundary system off the western African coast, as well as rivers such as the River Congo. First, the distributions of the dissolved and total dissolvable TEs (Cd, Fe, Ni, Cu, Zn, Pb, Mn, and Co) were investigated in the Chukchi Sea and 228Ra isotope was applied as tracer of benthic TE inputs. The results show that elevated benthic TE inputs on the Chukchi shelf provided suitable conditions for phytoplankton blooms. The 228Ra activities in spring appear to be 2-fold higher than in summer, which was a surprising observation and requires further investigation. Next, 228Ra was used to investigate the influence of the Congo River on surface ocean TE concentrations in the South Atlantic Ocean. The results show that the Congo River plume constitutes a large and unexpected input of TEs (Fe, Mn, and Co) into the South Atlantic. Finally, Ra isotopes were used to investigate a number of key ocean boundary processes in the Southeast Atlantic Ocean. Upwelling in the Benguela region is visible in the Ra distributions, and elevated Ra isotopes, Fe (II) and Si concentrations were observed in the Benguela oxygen minimum zone, possibly due to inputs from the reducing shelf sediments, or submarine groundwater discharge along the Namibian shelf