HR: 1340h
AN: OS33A-0988    [Abstracts]
TI: Modeling the distribution of Nd isotopes in the oceans using an offline Ocean General Circulation Model
AU: * Jones, K M
EM: kjones@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Khatiwala, S P
EM: spk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: van de Flierdt, T
EM: tina@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AB: The authigenic (seawater-derived) Nd isotopic composition of marine archives is increasingly used to study changes in ocean circulation on tectonic to millennial time-scales. Such applications for Nd isotopes assume that water masses are "tagged" with distinct Nd isotopic compositions in source regions and mix quasi- conservatively thereafter. However, there are a number of possible sources and sinks of Nd within the ocean that may complicate quasi-conservative behavior, including input from rivers, dissolution of dust, exchange at continental margin boundaries, and fluxes from the ocean-sediment interface. We use an offline ocean general circulation model (OGCM) to model the distribution of Nd isotope variability in seawater. A major obstacle to a thorough understanding of the marine Nd cycle is the lack of a truly global dataset of Nd isotopes in the modern oceans--most data are focused in only a few regions of the ocean. However, even within the constraints of sparse data, a better understanding of sources, sinks, and internal cycling of Nd and its isotopes can be reached through ocean modeling. We take a simple approach, treating the Nd isotopic composition of seawater as a conservative tracer, neglecting the effect of variable Nd concentrations on mixing. Nd isotope data from modern surface waters are used to generate a map of Nd isotope compositions for the entire surface ocean. This map is treated as a fixed boundary condition, and Nd isotope compositions of the surface are transported and mixed according to the flow characteristics of the OGCM until the interior ocean reaches a steady state. This simple approach produces Nd isotope estimates for North Atlantic Deep Water that are consistent with the observations, but produces values lower than observed in the deep Pacific and Southern Oceans. However, by introducing an additional source of Nd in the deep Pacific with higher Nd isotope ratios, the model output agrees well with the data. With the addition of radiogenic Nd to the deep Pacific, more than half of the measured data from the interior ocean fall within one ε-unit of the model output from the nearest OGCM grid point. These results indicate that for the currently available seawater data, the distribution of Nd in the ocean can be largely explained by quasi-conservative behavior of Nd in all ocean basins except for the Pacific, where a significant internal source of radiogenic Nd must exist.
DE: 1040 Radiogenic isotope geochemistry
DE: 4255 Numerical modeling (0545, 0560)
DE: 4875 Trace elements (0489)
DE: 4924 Geochemical tracers
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting