Tide pumping drives Fe and Mg isotopic dynamic in the subterranean esturay of Tolo Harbour, Hong Kong

Subterranean estuaries (STE) are defined as region where groundwater charge from land mixes with invading seawater in the aquifer (Moore, 1999). Ground water charge (SGD) contributes huge dissolved chemical species budget in a global scale, even though it composes only a minor fresh water flux (Santos et al., 2008). Significant dissovled inputs, like nutrients, are strongly regulated by the biogeochemical cycles in STE, making STE be an important term to the ecosystem of coastal ocean. Invesitagions have been developed on the nutrient dynamics in STE, such as Fe, Si and N, while the nutrient metal isotopes data are very limited. Mg is one of substantial elements for photosynthesis of phytoplankton and the second most metal element in saline ocean water, and Fe impacts on the productivity of ocean organisms. We thus investigate the dynamics and cycles of Mg and Fe in STE from a case at Tolo Harbour, Hong Kong. Nutrient distribution has been known as tightly related with distribution of salinity at STE, which can be influenced by tide. Santos et al., 2009 has reported tide pumping would insert an important control on nutrients, such as nitrogen and organic carbon, in subterrean estury. Effect of tide on dynamics of dissolved metal nutrients in STE is unknown. In this study, we will first explore tidal effect in diverse coastal wet seasons on Fe and Mg isotopic dynamics in a seaward STE cross section profile. We will employ standard estuarine model and a non-steady-state box model to simulate the Fe and Mg isotopic dynamics over the variation in the distribution of salinity that is caused by tide. We will also combine radium isotope data to estimate offshore fluxes in our study case. We expect Mg and Fe would behave like nonmetal nutrients, showing a large variability over tidal and seasonal time scales. This data input would help understand fates and biogeochemicals of dissolved Fe and Mg in STE. References 1.Moore W. S. (1999). Marine Chemistry 65, 111-126. 2.Santos I. R., et al., (2008). Aquatic Geochemistry 14, 133-146.3.Santos, Isaac R., et al. (2009).Geochimica et Cosmochimica Acta