Carbon and nutrient export from intertidal sand systems elucidated by 224Ra/228Th disequilibria

Abstract

We propose an alternative scheme for the use of 224Ra/228Th disequilibria to investigate carbon and nutrient export from a permeable sandy seabed. Sediment profiles of dissolved 224Ra, total 224Ra and 228Th were determined at two different intertidal sand systems - an intertidal sandy beach near Weitou Bay in Fujian (China), and a tidal sand flat in the Wadden Sea near Cuxhaven (Germany). Dramatic deficit of total 224Ra relative to 228Th was identified in the upper 20 or 30 cm sand layer over the sand systems. We construct a simple two-dimensional advective cycling model to simulate interfacial fluid transport in a sand system that is subject to periodic tidal inundation and swash actions. Based on the 224Ra/228Th disequilibria in the sediment, the model gives estimates of 20.3, 9.1, and 1.9 L m−2 h−1 for water exchange flux at the high tide, mid-tide, and low tide position over the sandy beach at Weitou Bay, respectively. In comparison, the model provides an estimate of 7.2 L m−2 h−1 for water exchange flux at the tidal sand flat in the Wadden Sea.The production of dissolved inorganic carbon (DIC) in porewater is the rate-limiting step for DIC export from the sandy beach into the sea, and can be reasonably simulated as a first-order kinetic reaction. The pattern of interfacial fluid transport over the beach facilitates a horizontal zonation of redox condition in the sediment, which evolves progressively from a fully oxic state at the high tide position to a suboxic state at the low tide position. There is clear evidence of nitrogen loss via denitrification in the suboxic status, and we estimate a nitrogen removal rate of 3.3 mmolN m−2 d−1 at this site. For the two intertidal sand systems, DIC export fluxes range from 20.1 to 89.4 mmolC m−2 d−1, comparable in magnitude to fluxes determined in organic rich estuarine sediments. In the meantime, export fluxes of dissolved inorganic nitrogen (DIN) change from 0.8 to 18.6 mmolN m−2 d−1. Overall, this study suggests that the role of sandy sediments in the biogeochemical cycling of carbon and nutrients needs to be revisited.