Balancing the global oceanic neodymium budget: Evaluating the role of groundwater |
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| Institution: | 1. Department of Earth and Environmental Sciences, The University of Texas at Arlington, Arlington, TX 76019-0049, USA;2. Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529-0276, USA;1. CEOAS, 104 CEOAS Admin. Bldg., Corvallis, OR 97331-5503, USA;2. GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1-3, 24148 Kiel, Germany;1. School of Ocean and Earth Sciences, University of Hawaii at Manoa, Honolulu, HI 96822, USA;2. Lamont–Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, Palisades, NY 10964, USA;3. Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA;4. Max Planck research group for Marine Isotope Geochemistry (ICBM), Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University, 26129 Oldenburg, Germany;1. Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK;2. Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA;1. IFREMER, Unité de Recherche Géosciences Marines, F-29280 Plouzané, France;2. UEB, Université Européenne de Bretagne, F-35000 Rennes, France;3. IUEM, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, CNRS UMS 3113, IUEM, F-29280 Plouzané, France;4. IFREMER, Unité de Recherche Environnements Profonds, F-29280 Plouzané, France;1. Faculty of International Resource Sciences, Akita University, Tegatagakuen-machi 1-1, Akita, Akita Pref. 010-8502, Japan;2. Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama, Kanagawa Pref. 223-8522, Japan;3. Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aoba-ku, Aramaki, Sendai 980-8579, Japan;4. Department of Geosciences, North Dakota State University (NDSU), Fargo, ND 58108, USA;1. IFREMER, Unité de Recherche Géosciences Marines, F-29280 Plouzané, France;2. Royal Museum for Central Africa, Department of Earth Sciences, B-3080 Tervuren, Belgium;3. Université Européenne de Bretagne, F-35000 Rennes, France;4. CEREGE, Université Aix Marseille, CNRS, IRD, Collège de France, UMS 34, F-13545 Aix-en-Provence Cedex 04, France;5. Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, CNRS UMS 3113, F-29280 Plouzané, France |
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| Abstract: | The distinctly different, εNd(0) values of the Atlantic, Indian, and Pacific Oceans requires that the residence time of Nd in the ocean (i.e., τNd) be on the order of, or less than, the ocean mixing time of ～ 500–1500 yr. However, estimates of τNd, 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 εNd(0) values (n = 58) for groundwaters were obtained from the literature in order to establish representative groundwater values. Mean groundwater Nd concentrations and εNd(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 × 107 and 8.1 × 107 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 × 107 mol/yr). Applying the SGD Nd flux estimates, the global average εNd(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.) εNd(0) measured in groundwaters (i.e., εNd(0) = ?8.9 ± 4.2). The similarities in the estimated SGD Nd flux and corresponding εNd(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. |
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