HR: 0830h
AN: PP51B-0931    [PDF]
TI: Significance of Dust Inputs and Arc Fluxes for the Nd and Hf Budgets in the North Pacific
AU: van de Flierdt, T
EM: tina@erdw.ethz.ch
AF: Lamont-Doherty Earth Observatory of Columbia University, Geochemistry Building 61 Route 9W Palisades, New York, NY 10964 United States
AU: van de Flierdt, T
EM: tina@erdw.ethz.ch
AF: Institute of Isotope Geology and Mineral Resources, ETH-Zentrum Sonnegstrasse 5, Zurich, CH-8092 Switzerland
AU: * Reynolds, B
EM: reynolds@erdw.ethz.ch
AF: Institute of Isotope Geology and Mineral Resources, ETH-Zentrum Sonnegstrasse 5, Zurich, CH-8092 Switzerland
AU: Frank, M
EM: frank@erdw.ethz.ch
AF: Institute of Isotope Geology and Mineral Resources, ETH-Zentrum Sonnegstrasse 5, Zurich, CH-8092 Switzerland
AU: Halliday, A N
EM: Halliday@erdw.ethz.ch
AF: Institute of Isotope Geology and Mineral Resources, ETH-Zentrum Sonnegstrasse 5, Zurich, CH-8092 Switzerland
AB: Neodymium and Hf isotope time-series from Neogene ferromanganese crusts in the Pacific Ocean indicate a long-term balance of these radiogenic isotopes over the entire Pacific basin. The two main factors controlling this balance have been (i) advection of water masses from the Southern Ocean and (ii) erosion and supply of Nd and Hf from young circum-Pacific island arcs. It has, however, been suggested that Asian dust is of major importance for the Nd (and Hf) budget of the North Pacific. Looking back into the past there was an order of magnitude increase in dust flux to the North Pacific at about 3.5 Ma. The timing of this increase in dust flux coincides with a slight decrease in Nd and Hf isotope ratios in ferromanganese crust records from the equatorial and North Pacific, which renders it possible that the release of Nd and Hf from dust particles had some effect on the seawater budget, at least over the past 3.5 Myr. In order to evaluate the importance of Nd and Hf fluxes from different sources to the North Pacific, a simple box model was developed. Assuming steady state, a self-consistent model is presented, in which the fluxes and isotope compositions into and out of North Pacific seawater are balanced. The modelled fraction of dust dissolving in the North Pacific is less than 3 % and the total dust-derived Nd contributes less than 13 % to the dissolved Nd budget of the North Pacific. Prior to the ten-fold increase in dust flux the effect of dissolved dust on the deep water isotope composition must have been negligible for Nd. Despite large uncertainties we obtained similar results for the Hf budget (less than 2.4 % dust is being dissolved; maximal contribution of dust to the Hf budget of 23 %). The results for the Nd budget are considered robust because, forced by the isotopic results derived from the ferromanganese crusts, variations of the frame parameters within realistic ranges only lead to minor changes in the results. The box model also provides an estimate of the flux of arc-derived Nd (and Hf). Assuming $\epsilon _{Nd}$ for the radiogenic arc-endmember to be +5, the arc-derived flux of Nd cannot have been less than 4 x 10$^{8}$ g/yr in order to meet the requirement to balance the isotopic composition of advected Nd. The results of the model strongly support the view that island arcs have been a very important input source for the Nd (and Hf) budget of the Pacific. Concerning the input mechanisms for arc-derived Nd and Hf it is suggested that either small riverine systems or exchange processes between particles and seawater at the ocean margins or a combination of both have been responsible for supplying these elements to Pacific deep water.
DE: 1045 Low-temperature geochemistry
DE: 4808 Chemical tracers
DE: 4875 Trace elements
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting