Using Uranium Series Radiogenic Isotopes to Trace and Quantify Chemical Fluxes in the Atlantic Ocean

Currently the magnitude of hydrothermal trace-element flux to the oceans is poorly constrained for a number of reasons. Firstly, we have only discovered a small number of the total hydrothermal vents in our oceans and the majority of these sites are relatively large, well-established, focussed vent sites such as TAG (Trans-Atlantic Geo-traverse), Rainbow or Lost City along the Mid-Atlantic Ridge. Even at sites such as TAG, which have been studied extensively, the timescale of the loss or transformation of elements such as iron via chemical, biological or particle scavenging is poorly constrained.
Radio-isotopes provide a tool to understand the timescale of such processes. Radium (Ra) is present in the ocean as four naturally occurring radioactive isotopes: 223Ra, 224Ra, 226Ra and 228Ra, with half-lives (11.4 d, 3.66 d, 1600 y and 5.75 y, respectively) and acts conservatively in the water column. Understanding vertical or horizontal fluxes within the neutrally buoyant plume could be achieved using Ra-isotope ratios. Normalising a short-lived isotope against a long-lived one corrects for any Ra scavenging or dilution. Hence if the neutrally buoyant plume is sampled at a given distance and the end-member isotope-ratio is known the sample can be aged using the change in sample activity. This would provide a timescale that could be related to changes in trace-element concentrations between the end-member and sample-site (Kipp et al. 2017).
Oceanic radium activities are incredibly low; therefore large sample volumes are required. Such samples could be obtained via the use of Stand Alone Pumping Systems (SAPS) in conjunction with polymer-fibre filter cartridges, impregnated with manganese oxide. The water would be pumped through the filter cartridge and the manganese oxides on the fibres would strip radium from the water as it passes through, concentrating the radium on the cartridge (Moore et al., 1973). The activity of this cartridge could then be determined using a Radium Delayed-Coincidence Counter (RaDeCC) (Moore et al., 1996).
In reality, there are multiple vents that run the length of a margin such as the Mid-Atlantic Ridge as well as physical processes that cause the mixing of vent fluids. However, the timescales provided by radium-isotope measurements would be a powerful tool in improving the hydrothermal trace-element flux estimates produced by current models.