Use of excess 210Pb and 228Th to estimate rates of sediment accumulation and bioturbation in Port Phillip Bay, Australia

Abstract

Rates of sediment accumulation, sediment mixing and depositional particle fluxes were estimated by use of excess ²¹⁰Pb and ²²⁸Th. In central Port Phillip Bay, there was a rapidly mixed surface layer and two layers of different mixing rates at 2–20 cm and 21–45 cm depths. When the sediment profiles of excess ²¹⁰Pb and ²²⁸Th were combined and diffusive mixing was assumed, the sediment accumulation rate in the 2–20 cm layer was constrained to be <0.15 cm year^–1. The mixing coefficient in the 2–20 cm layer was 5.0 ± 0.1 cm² year^–1. Hence, mixing rather than sedimentation governs the distribution of ²¹⁰Pb and ²²⁸Th in the surficial 20 cm. Below 20 cm, the different mixing regime may be due to the dominance of deposit-feeders at these depths. Evidence for bioturbation to a depth of 50 cm was obtained from profiles of excess ²¹⁰Pb and ²²⁸Ra deficiency. The mean residence time of particles in the central bay water column was 10 ± 2 days (a normalized depositional particle flux of 0.16 ± 0.02 g cm^–2 year^–1). This flux is three times the upper estimate of the sediment accumulation rate, indicating that most of the suspended particulate matter in the water column is resuspended bottom sediment.