Solute Transfer Across The Sediment-

Mangroves are gradually being regard as an important player in coastal biogeochemical and hydrological cycles with their complex belowground structures(i.e.,crab burrows)and various biota species in the sediment.The former facilitates pore water exchange and submarine groundwater discharge,meanwhile the latter makes biochemical properties of mangrove sediments different from other coastal ecosystems.Spatial and seasonal variabilities of salinity,DIC,nutrients and trace metals in porewater and overlying water were investigated in a tidal section in a mangrove.Exchange and circulation of solutes across the sediment-water interface has been recognized as a significant process.In this aspect,the 224Ra/228Th disequilibrium was recently observed in coastal muddy sediments,and has been proven to be an excellent proxy for tracing the benthic processes that regulate these solutes and quantifying their fluxes across the interface.We have not only observed the deficit of 224Ra relative to 228Th,but also found the excess of 224Ra with respect to 228Th in this study area.These phenomena varied with seasons and tides.At high temperature,benthic fluxes of 224Ra reached the minimum value in the summer.Meanwhile,224Ra fluxes at the intertidal zone were greater than at the other sites.The biogeochemical behaviors of nutrients and trace metals illustrate that there is an internal exchange and circulation within the mangrove sediment.Our data indicate an obvious water transport pattern that exits in a tidal cross section in a mangrove with a two layers' loop mode---interfacial exchange and seepage flow.Irrigation and bioturbation are major processes that control solute transfer across the sediment-water interface,whereas seepage flow was driven by the advection derived from hydraulic gradients.Seepage fluxes usually were higher than interfacial flux.Flux of Mn from the mangrove is(?6390±13000)?mol m-2d-1,and if upscaled to the global area of mangrove(140,000 km2),mangrove sediments provide Mn to surroundings at a rate of?33±66 Gmol y-1.