HR: 1340h
AN: PP43B-0675    [Abstracts]
TI: The role of calcite dust in the marine Nd cycle
AU: * Jacobson, A D
EM: adj@earth.northwestern.edu
AF: Northwestern University, Department of Geological Sciences 1850 Campus Drive, Evanston, IL 60208 United States
AU: Holmden, C M
EM: holmden@duke.usask.ca
AF: University of Saskatchewan, Saskatchewan Isotope Laboratory Department of Geological Sciences 114 Science Place, Saskatoon, SK S7N5E2 Canada
AB: Because different water masses in the world's oceans have different neodymium isotope ratios (¹⁴³Nd/¹⁴⁴Nd), the residence time of Nd in seawater (τ_Nd) must be less than the oceanic mixing time. While it is generally agreed that τ_Nd should be on the order of 200 - 500 yr, dividing the total oceanic Nd inventory by the global input from rivers yields a value for τ_Nd that is significantly higher (~10⁴ yr). Therefore, it is necessary to identify additional sources of Nd to seawater. To address this problem, we investigated if the atmospheric supply of dissolved calcite dust to the oceans influences the marine Nd cycle. We primarily focused on the Japan Sea, because it receives large quantities of dust from the Chinese Loess Plateau, and it has a known Nd isotope composition and a simple hydrologic budget. Our methods consisted of laboratory analyses of Chinese loess as well as modeling. On average, calcite dust contains ~150 nmol/g of Nd with ε_Nd = -8.6. A steady-state atmospheric transport model suggests that wet deposition supplies 7.5 x 10¹² g/yr of dissolved calcite dust to the Japan Sea surface, which corresponds to a dissolved Nd flux of 1.1 x 10¹⁸ pmol/yr. Additional model results indicate that the Nd input from the dissolution of silicate dust is about two orders of magnitude less than the input from dissolved calcite dust. A steady-state two-box model for the Japan Sea demonstrates that the calcite dust input is required to produce the observed concentration and isotope composition of Nd in the Japan Sea basin. The residence times of Nd in the Japan Sea surface and deep, with respect to particle scavenging, are ~14 and 530 yr, respectively. Approximately 65% of the Nd flowing from the Japan Sea surface into the western North Pacific originates directly from the input of dissolved calcite dust. Downwelling surface water and the remineralization of settling particles containing Nd scavenged from surface water contribute ~77% of the Nd in the Japan Sea deep, which suggests that ~50% of the deep Nd originates from calcite dust. The isotope composition of deep water is balanced by a benthic flux, but the input is a factor of ~10 lower than the calcite dust flux and accounts for only ~23% of the deep Nd. Extension of the model to the entire North Pacific suggests that the flux of Nd from dissolved calcite dust is 1.2 x 10¹⁹ pmol/yr, which is a factor of ~10 higher than the riverine input. The data show that τ_Nd in the North Pacific is ~565 yr. These findings reveal that the atmospheric supply of dissolved calcite dust to seawater is a more important component of the marine Nd cycle than previously recognized.
DE: 1040 Radiogenic isotope geochemistry
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005