"Particle Radio-sensor": Development of in situ particulate radioactivity sensor

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Carbon fixation by marine autotrophs represents a significant (~26%) sink of the carbon released to the atmosphere from the cement industry and burning of fossil fuels. Inorganic carbon (CO2) is assimilated into biological material via photosynthetic uptake by plankton. This biological material flocculates and sinks, and is either remineralised within the ocean mixed layer or exported to deeper layers of the ocean for long term sequestration (the 'biological carbon pump'). Understanding how much of this particulate material is transferred to the deep ocean is important in quantifying the role the oceans will play in ameliorating anthropogenic atmospheric emissions of CO2, and hence climate change. These particulate export fluxes can be quantified using a number of approaches including radiochemical tracers and sediment trapping. Of the radiochemical techniques available, the use of 234Th is the most commonly applied. The 234Th readily adheres to particles, and combined with its short half-life (24.1 days) make it an ideal tracer of particles sinking through the ocean column. Traditionally, large volume (owing to the low particulate 234Th activities typically found in the oceans) in situ pumps are employed to collect particles making this method very labour intensive, and also requiring long periods of static ship during filtration periods. This leads to gross under-sampling in the oceans of this variable, leading to inconsistencies reported for the magnitude of the biological carbon pump. In situ sensors such as those used on Argo floats and gliders have been transformative for marine physics and chemistry, but analogous particulate sensors and samplers do not exist. This technology proposal seeks to address this data gap by taking the concept of an in situ particulate flux instrument to TRL 4. This will be achieved by developing an in situ filtration system and coupling this with a novel deployable beta detection module to measure particulate 234Th activity. This will provide the foundations for further work to raise the TRL of this device and to apply the innovative technology principles to other oceanic variables (i.e. dissolved phase radionuclides; other radionuclides). The ability to easily sample open ocean fluxes using autonomous vehicles at high spatial and temporal resolution would constitute a step-change in our ability to measure carbon export in a changing ocean (deoxygenation, acidification) and help scientists assess the long term effect of increasing atmospheric CO2 concentration.