Balancing the global oceanic neodymium budget: Evaluating the role of groundwater

Abstract

The distinctly different, ε Nd(0) values of the Atlantic, Indian, and Pacific Oceans requires that the residence time of Nd in the ocean (i.e., τ Nd) be on the order of, or less than, the ocean mixing time of ∼ 500–1500 yr. However, estimates of τ Nd, based on river influxes, range from 4000 to 15,000 yr, thus exceeding the ocean mixing time. In order to reconcile the oceanic Nd budget and lower the residence time by roughly a factor of 10, an additional, as yet unidentified, and hence “missing Nd flux” to the ocean is necessary. Dissolution of materials deposited on continental margins has previously been proposed as a source of the missing flux. In this contribution, submarine groundwater discharge (SGD) is examined as a possible source of the missing Nd flux. Neodymium concentrations ( n = 730) and ε Nd(0) values ( n = 58) for groundwaters were obtained from the literature in order to establish representative groundwater values. Mean groundwater Nd concentrations and ε Nd(0) values were used along with recent estimates of the terrestrial (freshwater) component of SGD (6% of river discharge on a global basis) to test whether groundwater discharge to the coastal oceans could account for the missing flux. Employing mean Nd concentrations of 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, terrestrial-derived SGD flux of Nd is estimated to range between 2.9 × 10 7 and 8.1 × 10 7 mol/yr. These estimates are of the same order of magnitude, and within a factor of 2, of the missing Nd flux (i.e., 5.4 × 10 7 mol/yr). Applying the SGD Nd flux estimates, the global average ε Nd(0) of SGD is predicted to be − 9.1, which is similar to our estimate for the missing Nd flux (− 9.2), and in agreement with the mean (± S.D.) 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 could account for the missing Nd flux. Future investigations should focus on quantifying the Nd concentrations and isotope compositions of groundwater from coastal aquifers from a variety of coastal settings, as well as the important geochemical reactions that effect Nd concentrations in subterranean estuaries in order to better constrain contributions of SGD to the oceanic Nd budget.