4:15 PM - 4:30 PM
[AOS21-10] Tracing sediment-water exchange in the shelf region off the Tone River using radium isotopes
Keywords:Pore water, Sediment-water interface, Shelf, Radium
Shelf regions serve as critical for biological production and material dynamics in the ocean, sustained by nutrient inputs from the terrigenous source via river water and groundwater. In recent years, it has been recognized that the water exchange across the sediment-water interface as a significant mechanism for nutrient transport process in shallow water areas. While this process has been implicated in influencing nutrient dynamics and biological productivity in shelf seas, our understanding of water exchange across the sediment-water interface and its role in biogeochemical cycling within regions remains limited.
In this study, we evaluated the contribution of pore water to shelf seawater and the flux of pore water-derived nutrients in the shelf region off the Tone River. Oceanic observations were conducted aboard the R/V Shinsei-Maru in August 2024. At eight stations, vertical profiles of temperature and salinity were obtained using a CTD sensor, followed by microstructure measurements with a vertical microstructure profiler (VMP). Seawater samples for nutrient analysis were collected using Niskin samplers. To investigate vertical distribution of radium isotopes (Ra) as tracers of pore water, large volume seawater samples were collected from five depth layers at three stations. At these same stations, sediment core samples were retrieved using a multiple corer for chemical analysis of both overlying seawater and pore water.
Vertical profiles of 224Ra (t1/2 = 3.66 d) activity at each station exhibited lower activity in the middle layer, with higher activity observed in both the surface and bottom layers. The activity of 224Ra above the surface pycnocline was negatively correlated with salinity, indicating that pore water from shallow sediments is transported laterally offshore while mixing with freshwater. In contrast, seawater characterized by high salinity and 224Ra activity was found beneath the pycnocline. Given that 224Ra activity in overlying seawater near the seafloor and in pore water was approximately 10-fold and 100-fold higher than that in seawater of the water column, respectively, these results suggest that seawater below the pycnocline can be influenced by pore water advected from shelf sediments.
To quantify the pore water exchange rate at the sediment-water interface, we adopted a one-dimensional vertical Ra mass balance model, focusing on two spatial domains: the near-seafloor region and the water column below the pycnocline. The results indicated that the 224Ra flux derived from pore water exchange was >10 times higher than that attributed to molecular diffusion, regardless of station or model domain. These findings demonstrate the pivotal role of pore water exchange in transporting solutes from shelf sediments to the overlying water column.
In this study, we evaluated the contribution of pore water to shelf seawater and the flux of pore water-derived nutrients in the shelf region off the Tone River. Oceanic observations were conducted aboard the R/V Shinsei-Maru in August 2024. At eight stations, vertical profiles of temperature and salinity were obtained using a CTD sensor, followed by microstructure measurements with a vertical microstructure profiler (VMP). Seawater samples for nutrient analysis were collected using Niskin samplers. To investigate vertical distribution of radium isotopes (Ra) as tracers of pore water, large volume seawater samples were collected from five depth layers at three stations. At these same stations, sediment core samples were retrieved using a multiple corer for chemical analysis of both overlying seawater and pore water.
Vertical profiles of 224Ra (t1/2 = 3.66 d) activity at each station exhibited lower activity in the middle layer, with higher activity observed in both the surface and bottom layers. The activity of 224Ra above the surface pycnocline was negatively correlated with salinity, indicating that pore water from shallow sediments is transported laterally offshore while mixing with freshwater. In contrast, seawater characterized by high salinity and 224Ra activity was found beneath the pycnocline. Given that 224Ra activity in overlying seawater near the seafloor and in pore water was approximately 10-fold and 100-fold higher than that in seawater of the water column, respectively, these results suggest that seawater below the pycnocline can be influenced by pore water advected from shelf sediments.
To quantify the pore water exchange rate at the sediment-water interface, we adopted a one-dimensional vertical Ra mass balance model, focusing on two spatial domains: the near-seafloor region and the water column below the pycnocline. The results indicated that the 224Ra flux derived from pore water exchange was >10 times higher than that attributed to molecular diffusion, regardless of station or model domain. These findings demonstrate the pivotal role of pore water exchange in transporting solutes from shelf sediments to the overlying water column.