Estuarine distributions of Zr, Hf, and Ag in the Hudson River and the implications for their continental and anthropogenic sources to seawater

Zirconium and Hf in seawater are derived from continental weathering and are transported to the oceans by rivers, but the modification of this flux with estuaries is largely unknown. Furthermore, there is no information as to whether there is a pollutant source of these metals that might affect their budget calculations in the modern ocean. To address the riverine flux and potential anthropogenic sources, the distributions of dissolved Zr and Hf have been measured in surface and bottom water through the Hudson River Estuary. The distribution of Ag is included because it is sensitive to urban sources and has partial similarities in its distribution to those of Zr and Hf. There is about 50% removal of Zr and Hf in surface water relative to their freshwater end-member concentrations of 243 and 3.1 pmol/kg as salinity starts to increase, but an urban source of these metals midestuary may obscure greater removal. Silver concentrations in surface water are below the detection limit until salinity of 3.5 is reached. The concentrations of Zr and Hf 0.5 m above the bottom are 1.5 to 3 times higher than their surface water concentrations throughout the estuary and may result from input of groundwater, diffusion from pore fluids, or inclusion of colloidal phases in our analyses. Downstream of the turbidity maximum, bottom water Ag concentrations are high due to release from particles that originally formed within the estuary and were moved inland following the spring freshet; this process may also affect Zr and Hf. The increase in the freshwater atomic Zr/Hf ratio from 70 to 80, characteristic of terrestrial rocks, to the elevated Zr/Hf (>110) ratio of seawater takes place at middle to high salinity due to enhanced release of Zr from resuspended particles and slower removal rates compared to Hf. A main implication of this study is that estuaries are a sink for Zr and Hf. The reduction of the predicted export of continental Zr and Hf to the oceans increases their previously calculated seawater residence by as much as 2 to 4 times, assuming rivers dominate the supply of Zr and Hf to seawater.