HR: 08:00h
AN: OS31C-01 INVITED    [Abstracts]
TI: Modeling Nd Isotope Composition of Continental Inputs to the Ocean
AU: * Peucker-Ehrenbrink, B
EM: behrenbrink@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States
AU: Jeandel, C
EM: Catherine.Jeandel@legos.obs-mip.fr
AF: CNRS, LEGOS, UMR 5566, Observatoire Midi-Pyrenees, 14 avenue Edouard Belin, Toulouse, 31400, France
AB: Among the long-lived radiogenic isotope systems neodymium (Nd) is of particular interest to the oceanographic community because its residence time is shorter than the oceanic mixing time, causing regional differences in the Nd isotope composition of ocean basins and water masses (Piepgras et al., 1979). The unique Nd isotope compositions of water masses are created by inputs from the geologically diverse continents and then transported by the oceanic circulation. Old cratonic landmasses along passive margins impart a uniquely unradiogenic Nd isotope signature that reflects time-integrated low Sm/Nd values. Young active margins, in contrast, are characterized by radiogenic Nd isotope values that reflect the high Sm/Nd of the depleted mantle. This systematic can, in principle, be used to reconstruct ocean circulation and/or continental inputs to the ocean in the geologic past. Prerequisite for accurate reconstructions is a good understanding of changes in the geologic makeup of the continents and related isotopic signatures of the input fluxes to the ocean in the geologic past. Here we argue that the good negative correlation between the age of bedrock, as indicated by geologic maps, and the Nd isotope composition of suspended particles in rivers can be used to parameterize the Nd isotope composition of continental inputs to the ocean. We use a global assessment of bedrock ages based on the digital geologic map of the world at a scale of 1:25M in combination with more detailed digital bedrock maps for individual river drainage basins for which good isotope data are available (e.g., the Fraser, Mississippi, Hudson, Mackenzie rivers). Both the regional and global data define a linear array of epsilon Nd = -0.023 bedrock age (Myr) + 0.1. The lack of significant fractionation of Sm from Nd during erosion and weathering indicates that the particulate Nd isotope composition can also be used as a reliable proxy for the isotope composition of dissolved Nd in rivers. We use this correlation to estimate the isotope composition of runoff from 19 large-scale drainage regions into adjacent ocean basins (Graham et al., 1999). The results agree well with independent estimates of the Nd isotope composition of lithogenic inputs to the ocean (Jeandel et al., 2007). If glacial cover on cratonic shields surrounding the North Atlantic caused detrital and dissolved Nd fluxes to the ocean to decrease, significant changes in the isotope composition of the flux, and thus the Nd isotope composition of adjacent water masses, are to be expected. The amplitude of such glacial-interglacial variations may be modulated by continuing chemical exchange of continental detritus with seawater along continental margins. Reliable high-resolution records of the Nd isotope evolution of North Atlantic Deep Water (NADW) are needed to evaluate the effects of the Northern Hemisphere glaciation on the Nd isotope composition of NADW, and possible changes in its formation rate.
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1879 Watershed
DE: 4283 Water masses
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting