Using 224Ra/228Th disequilibrium to quantify benthic fluxes of dissolved inorganic carbon and nutrients into the Pearl River Estuary

Abstract

The 224Ra/228Th disequilibrium that was recently observed in coastal sediments has been proven to be an excellent proxy for tracing the benthic processes that regulate solute transfer across the sediment–water interface. In order to better utilize this proxy, there is a need to understand the reaction kinetics of 224Ra in sediments. In this study, depth profiles of 224Ra and 228Th in bulk sediments were collected along a transect in the Pearl River Estuary (PRE). Together with bulk sediment measurements, dissolved 224Ra, dissolved inorganic carbon (DIC), and nutrients (NO2−+NO3−, NH4+) in pore water and in the overlying waters were also determined. A marked deficit of 224Ra with respect to 228Th with large spatial variations was observed in the PRE sediments. By use of a diagenetic model for the distributions of dissolved and adsorbed 224Ra in sediments, we infer that adsorption removes 224Ra from aqueous phase at a rate of 0.1±1.1–2000±400d−1. In addition, adsorption of 224Ra exhibits a rate sequence of oxic freshwater>anoxic freshwater>anoxic brackish water, probably reflecting the effect of the redox conditions and ionic strength on the adsorption–desorption kinetics of 224Ra.Benthic fluxes of 224Ra were estimated from the observed deficit of 224Ra in the sediments using a one-dimensional (1D) mass balance exchange model. We demonstrated that irrigation was the predominant process that controls solute transfer across the sediment–water interface, whereas molecular diffusion and sediment mixing together contributed <5% of the total 224Ra fluxes from bottom sediments. We then utilized the 224Ra/228Th disequilibrium approach to quantify the benthic fluxes of DIC and nutrients. We showed that sediment interstitial waters delivered approximately 42±6×109mol of DIC and ∼16±1×109mol of NH4+ into the PRE in the dry season. In contrast, it removed about 13±1×109mol of NO3− from the overlying water column. The benthic flux of DIC is equivalent to ∼18% of the riverine input in this season. In terms of nutrients, our results suggest that bottom sediments are a major sink of water column NO3−, and are a predominant source of NH4+ in the PRE. Overall, this study indicates that irrigation is an important process and must be considered in the mass balance of DIC and nutrients in estuaries.