Rates of particle scavenging, sediment reworking, and longitudinal ripple formation at the HEBBLE site based on measurements of 234Th and 210Pb

Abstract

Between 1982 and 1986, seabed and water column samples were collected on five cruises to the HEBBLE site. Two particle-reactive radionuclides ( 234Th and 210Pb) have been used to examine rates of particle scavenging in bottom waters, sediment reworking in the surface seabed, and formation of longitudinal ripples. Settling particles at the HEBBLE site, which have been resuspended by strong bottom currents and benthic storm activity, scavenge approximately 60% of the 234Th (half life of 24 days) produced in the bottom 100 m of the water column (corresponding to removal of 15 dpm/cm 2). 234Th inventories from cores collected during a single cruise show substantial variability (e.g., 1–18 dpm/cm 2) over the 8-km 2 study area, indicating that resuspended particles are not deposited uniformly across the seabed after storm events. The activity of benthic fauna and non-uniform deposition over a topographically variable seabed are important processes affecting boundary-layer sedimentation and the development of radionuclide inventories in surface sediments. The mean 234Th seabed inventory from individual cruises to the HEBBLE area varies to a greater extent (2 to 38 dpm/cm 2) than the mean 210Pb inventory (47 to 82 dpm/cm 2), because 234Th inventories are dominated by sedimentary processes associated with an individual storm event, whereas 210Pb inventories integrate these processes over several cycles of deposition and erosion. Sediments at the HEBBLE site are biologically reworked very rapidly on a 100-day time scale with 234Th mixing coefficients ( D b = 1 to 33 cm 2/yr) comparable to values observed in nearshore environments. On time scales of months to years, 210Pb data indicate that strong currents and benthic storm activity have eroded and redeposited the upper 5–10 cm in 17 of the 20 cores examined. Reoccupying field sites during the 4-year field program provided the unique opportunity to evaluate steady-state models commonly used to determine rates of sediment reworking. 210Pb profiles and inventories showed significant temporal variations at HEBBLE sites occupied between 1982 and 1986. Steady-state models should not be used to evaluate profiles of 210Pb (or other tracers with comparable half life) in a dynamic environment like the HEBBLE area, because steady-state assumptions are not justified and rates of sediment reworking can be underestimated by nearly an order of magnitude. 234Th and 210Pb analyses of core samples from two longitudinal ripples suggest that these sedimentary features form on a time scale of months and consist of a mixture of recently deposited material from the water column and older sediment from depth. The absence of 234Th activity in one of the ripples indicates that these bedforms may be stable for periods of months without significant growth.