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

We propose an alternative scheme for the use of Ra-224/Th-228 disequilibria to investigate carbon and nutrient export from a permeable sandy seabed. Sediment profiles of dissolved Ra-224, total Ra-224 and Th-228 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 Ra-224 relative to Th-228 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 Ra-224/Th-228 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. (C) 2020 Elsevier Ltd. All rights reserved.