5:15 PM - 6:45 PM
[MZZ42-P04] Assessment of bioturbation of polymetallic nodule deposits in the CCZ
Keywords:bioturbation, 210Pb, 14C age, bioturbation rates, sedimentation rates, mixing depth
Events of disturbance and mixing to bottom sediments caused by activities such as respiration, migration, and feeding of benthic organisms are referred to as "bioturbation" (e.g., Lohrer et al., 2004). Fresh particles that fall from the water column to the seafloor surface are inflow and dispersed into the upper few to several tens of centimeters of the sediment by bioturbation. On the other hand, mining activities associated with the development of seafloor mineral resources such as polymetallic nodules artificially disturb the deep-sea floor and may significantly alter the environment around the mining site due to the generation and redeposition of sediment plumes and the release of chemically active substances. Therefore, it is important to quantitatively understand bioturbation to predict the behavior of material redeposited by mining activities.
The International Seabed Authority (ISA) recommends estimating bioturbation rates (Db) and mixing depths (L) using 210Pb analysis to assess bioturbation for the purpose of environmental impact assessment and predictive modeling in mining (ISBA/25/LTC/6/Rev.3). However, previous studies in the Clarion-Clipperton Fracture Zone (CCZ) have only reported data from 16 sites (e.g., DeMaster and Cochran, 1982; Yu et al, 2023). In this study, in order to obtain baseline data on bioturbation in exploration area contracted by DORD in the CCZ, the values of 210Pb and 14C were analized from seafloor sediment samples collected in FY2021. The bioturbation rates, mixing depth, and sedimentation rates were estimated from values of 210Pb and 14C.
Sedimentation rates were estimated to range from 8.0 to 26.2 mm/kyr based on 14C age for each layer by AMS. The gamma-ray spectrum of 210Pb in the sediment samples was measured, and the excess 210Pb concentration was calculated. The bioturbation rates Db, which indicates the degree of sediment disturbance, was calculated to be 0.14 - 1.9 cm2/yr using the coefficients of the approximate curve for the vertical profile of excess 210Pb, the sedimentation rates, and the steady-state diffusion model. The mixing depth L was calculated 9 - 31 cm by extrapolation of the approximate curve for the vertical profile of excess 210Pb. These were within the range of values of previous studies that reported Db and L using steady-state diffusion models and vertical profiles of excess 210Pb in the CCZ.
The International Seabed Authority (ISA) recommends estimating bioturbation rates (Db) and mixing depths (L) using 210Pb analysis to assess bioturbation for the purpose of environmental impact assessment and predictive modeling in mining (ISBA/25/LTC/6/Rev.3). However, previous studies in the Clarion-Clipperton Fracture Zone (CCZ) have only reported data from 16 sites (e.g., DeMaster and Cochran, 1982; Yu et al, 2023). In this study, in order to obtain baseline data on bioturbation in exploration area contracted by DORD in the CCZ, the values of 210Pb and 14C were analized from seafloor sediment samples collected in FY2021. The bioturbation rates, mixing depth, and sedimentation rates were estimated from values of 210Pb and 14C.
Sedimentation rates were estimated to range from 8.0 to 26.2 mm/kyr based on 14C age for each layer by AMS. The gamma-ray spectrum of 210Pb in the sediment samples was measured, and the excess 210Pb concentration was calculated. The bioturbation rates Db, which indicates the degree of sediment disturbance, was calculated to be 0.14 - 1.9 cm2/yr using the coefficients of the approximate curve for the vertical profile of excess 210Pb, the sedimentation rates, and the steady-state diffusion model. The mixing depth L was calculated 9 - 31 cm by extrapolation of the approximate curve for the vertical profile of excess 210Pb. These were within the range of values of previous studies that reported Db and L using steady-state diffusion models and vertical profiles of excess 210Pb in the CCZ.