Application of 224Ra/228Th Disequilibrium Method for Investigating Benthic Fluxes and Hydrothermal Influence

Hydrothermal systems form when seawater flows through cracks and faults on the seafloor into the deeper layers of the Earth's crust, where it is heated by geothermal sources and reacts with host rocks, forming high-temperature, acidic fluids. These fluids rise and exit through seafloor vents, playing an important part in the Earth's geochemical cycles. During circulation, radium isotopes (²²⁴Ra, ²²⁶Ra) are enriched in hydrothermal fluids through rock—hydrothermal alteration, making the radium isotopes useful tracers for studying hydrothermal systems.

In previous studies, a method for measuring ²²⁴Ra and ²²⁸Th activities in sediments based on a delayed-coincidence counting system was developed, and ²²⁴Ra depth profiles were modelled using the general diagenetic equation to evaluate material transport processes at the sediment-water interface. Thus, in this study, we used the ²²⁴Ra/²²⁸Th disequilibrium in sediments to evaluate the impact of hydrothermal activity on the bottom sediments. Furthermore, the same ²²⁴Ra/²²⁸Th disequilibrium analysis method is applied across general oceanic areas to elucidate differences in radium fluxes caused by various factors in hydrothermal and non-hydrothermal regions.

The ²²⁴Ra/²²⁸Th disequilibrium analysis of cores collected from the Mienhua submarine hydrothermal system in the southernmost Okinawa Trough showed that ²²⁴Ra_ex increases at a depth of 10-15 cm below the surface, then decreases below 17 cm. However, ²²⁴Ra_ex decreases downward from the core top, which was collected at the margin of the hydrothermal zone. It is inferred that in the upper layer of cores from the hydrothermal system, numerous gaps created by hydrothermal fluids in the sediments facilitate direct exchange of pore water with seawater, resulting in lower ²²⁴Ra_ex values within the top 10 cm of sediments. Whereas cores from the margin of the hydrothermal system show fluctuating ²²⁴Ra_ex values in the upper 10 cm due to dynamic changes in the hydrothermal fluid. Additionally, ²²⁴Ra_ex results in cores from non-hydrothermal areas showed lower values than those from the hydrothermal area, assuming a rapid depositional environment in these areas, which causes the rapid dissipation of ²²⁴Ra_ex.