EVALUATING THE ROLE OF GROUNDWATER IN THE GLOBAL OCEANIC NEODYMIUM BUDGET

Distinct ε_Nd(0) values of the Atlantic, Indian, and Pacific Oceans necessitate that the oceanic residence time of Nd, τ_Nd be on the order of, or less than, the ocean mixing time of ~ 500 - 1500 years. Nonetheless, estimates of τ_Nd, based on river influxes, range from 4000 to 15,000 years. To reconcile the oceanic Nd budget and lower the residence time by roughly a factor of 10, an additional, “missing Nd flux” (i.e., 5.4 x 10⁷ moles of Nd/year) has been proposed [K. Tachikawa, V. Athias, C. Jeandel, J. Geophys. Res 108 (2003) 3253 doi:10.1029/1999JC000285]. We demonstrate that submarine groundwater discharge (SGD) can account for this missing Nd flux. Neodymium concentrations (n = 730) and ε_Nd(0) values (n = 58) for groundwaters, obtained from the literature, were used along with recent estimates of SGD (6% of river discharge on a global basis) to examine if SGD to the coastal oceans could account for the missing flux. Employing mean Nd concentrations from the compiled data base (i.e., 31.8 nmol/kg for all 730 analyses and 11.3 nmol/kg for 141 groundwater samples from a coastal aquifer), the global SGD flux of Nd is estimated to range between 2.9 x 10⁷ and 8.1 x 10⁷ moles/year. These estimates are of the same order of magnitude, and within a factor of 2, of the missing Nd flux. Applying the SGD Nd flux estimates, the global average ε_Nd(0) of SGD is calculated to be -9.1, which is similar to our estimate for the missing Nd flux (-9.2), and in agreement with the mean (± SD)ε_Nd(0) measured in groundwaters (i.e., ε_Nd(0) = -8.9 ± 4.2). The similarities in the estimated SGD Nd flux and corresponding ε_Nd(0) values to the magnitude and isotope composition of the missing Nd flux are compelling, and suggest that discharge of groundwater to the oceans can account for the missing Nd flux. Our SGD flux estimates for Nd suggest an average oceanic Nd residence time of 600 years.