APPLICATION OF ²³⁴TH/²³⁸U DISEQUILIBRIUM TO THE STUDY OF MARINE PARTICLE CYCLING

In the past, scientists focused more on measuring ²³⁴Th activity in the surface water, but overlooked its change in the deep ocean. In the "twilight zone" and the zone below it, the marine aggregates exported from sunlit euphotic zone will be altered by zooplankton and piezophilic microorganisms. These marine particles not only connect the surface and the deep ocean, but also are the most critical part of the global carbon cycle. However, it is a great challenge to determine the flux of marine particles from the surface to the deep ocean. The disequilibrium between ²³⁴Th and ²³⁸U has been widely utilized to study vertical transport, particle cycling and other biological processes. Its half-life of 24.1 days and particle-reactive nature make ²³⁴Th an ideal tracer for these processes on time scales of days to weeks. The method of ²³⁴Th/²³⁸U disequilibrium, based on the small-volume MnO₂ co-precipitation technique, is prospective for its easy sampling and high resolution. In this paper, we reviewed the principles, history, sampling, and analytical methods involved in the ²³⁴Th/²³⁸U disequilibrium, as well as the existing problems, and the development prospects of its application to aggregates cycling and oceanography. Further, we discussed the research potential of particle organic matter remineralization processes, its extent and the underlying acting mechanisms and the deep ocean carbon cycle.