The behaviour of uranium and radium in an inverse estuary

Abstract

Spencer Gulf in South Australia is an inverse estuary, where an excess of evaporation over precipitation and the absence of major rivers lead to a well defined horizontal salinity gradient from 36‰ near its entrance to about 46‰ at its head. The excess of salt is exported annually from the gulf as a gravity current moving along the sea floor and across the shelf. Water balance is maintained by an inflow of less saline shelf water at the surface. The inverted salinity gradient which persists throughout the year can be used to advantage in the study of processes which govern the cycling of radionuclides of the uranium and thorium decay series in the nearshore marine environment. Radionuclides investigated include 234U, 238U 226Ra and 228Ra. The results obtained indicate that uranium isotopes in Spencer Gulf have a typical marine signature, but that uranium is being removed from the water column at a rate of about 0.2 μg cm −2 y −1, based on conservation of salt. The 234U/ 238U ratio of uranium in the predominantly carbonate sediments of Spencer Gulf are consistent with uranium input from seawater, and the measured accumulation rate of authigenic uranium in the sediments supports the model removal rate, indicating th at Spencer Gulf is a net sink for uranium. A systematic increase in 228Ra/ 226Ra with salinity, coupled with a general increase in 226Ra above that predicted from evaporation of open ocean sea water demonstrate that both radium isotopes enter the gulf. In order to maintain the excess radium in the water column, diffusive fluxes of 0.3 dpm cm −2 y −1 and 0.05 dpm cm −2 y −1 for 228Ra and 226Ra, respectively, from the seafloor would be required. The standing crop of 232Th in the surface mixed layer of the sediment as a potential source for 228Ra via decay and recoil processes is sufficient to support the excess 228Ra in the water column, but there appears to be insufficient 230Th in the sediment to generate the required 226Ra flux, suggesting an external source for some of the excess 226Ra, such as periodic surface runoff after rainfall events and/or submarine discharge of groundwater from granitic basement rocks.