Using long-lived radium isotopes as water-mass tracers in the North Sea and investigating their use for tracking artificial ocean alkalinization.

The long-lived radium isotopes, ²²⁶Ra (t_1/2= 1600 yrs.) and ²²⁸Ra (t_1/2 = 5.8 yrs.), are established shelf-sea tracers, capable of discerning key water-mass compositions and distribution patterns from source to sea. Within the North Sea, radium has not only been recognized as a suitable tool for identifying water-mass characteristics, but ²²⁸Ra has also been found to effectively trace total alkalinity (AT). Within the known continental shelf pump system of the North Sea, this indirect link between radium and the carbonate system has recently enticed greater interest for climate mitigation strategies, such as Artificial Ocean Alkalinization (AOA). But, prior to initiating intentional anthropogenic perturbations on the complex coastal North Sea, it is imperative to understand the initial state of the system. In order to do just that, our study builds on the previous knowledge of water-mass distributions within the North Sea, distinguishing the sources and mixing patterns which contribute to the three main water-masses (with particular focus placed on further identification of the North Atlantic input source components). Quantitatively, these patterns are further supported through the use of inverse modelling techniques, which highlight the importance of end members for each of the water-masses. Overall this study provides a more in-depth baseline understanding of water-mass distribution and mixing within the North Sea.