Neodymium isotope ratios

Rutberg R. L., Hemming S. R., and Goldstein S. L. (2000) Reduced North Atlantic deep water flux to the glacial Southern Ocean inferred from neodymium isotope ratios. Nature 405, 935-938. 

REEs in Seawater Neodymium-isotope Ratios in Seawater Where does Seawater Neodymium Come From ... 

The first neodymium-isotopic analyses aimed at characterizing the oceans closely followed the initial development of neodymium isotopes as a chronometer and tracer (Richard et al., 1976 DePaolo and Wasserburg, 1976a O Nions et al., 1977). O Nions et al. (1978) was the first to report neodymium (along with lead and strontium) isotopes in manganese nodules and hydrothermal sediments. They confirmed the distinction between continental and mantle provenances of lead in hydrogenous and hydrothermal manganese sediments, respectively. Consistent with the previous studies, they found that strontium in these deposits is derived from seawater. All of the neodymium-isotope ratios in their samples from the Pacific were similar and lower than the bulk... 

Earth value (s d = —1.5 to —4.3). They concluded correctly that neodymium in the oceans is mainly derived from the continents, and incorrectly that neodymium-isotope ratios in the oceans are like strontium isotopes in the sense that they are globally well-mixed and display this restricted range. In order to reach this conclusion they ignored a sample from the Indian Ocean having high strontium-isotope ratios and a lower, even more continent-like neodymium-isotope ratio of 8n(J = —8.5. 

Neodymium-isotope ratios in deep seawater show the same general geographical pattern as in Fe-Mn nodules and crusts. The global geographical pattern is illustrated in the Fe-Mn nodule-crust map (Figure 3), and by some depth profiles (Figure 5). The North Atlantic is characterized by... 

Surface waters show more variable neodymium-isotope ratios than intermediate and deep waters (Figure 5). In contrast to the dominance... 

The limited seawater data from the circum-Antarctic display uniform neodymium-isotope ratios intermediate to the Atlantic and Pacihc (sNd = 8 to —9). The only neodymium-isotope data available are from two profiles in the Drake Passage and one in the eastern Pacific sector (Piepgras and Wasserburg, 1982). 

Pacific intermediate and deep water dominantly display high values, of s d = to —4 (Piepgras and Wasserburg, 1982 Piepgras and Jacobsen, 1988 Shimizu et al., 1994). Near southern Chile, and in the west-central Pacific, where circumpolar deep water moves northward, neodymium-isotope ratios display lower values closer to those typical of circumpolar seawater. In the central Pacific the circumpolar neodymium-isotope signal can be detected as far north as —40° N (Shimizu et al., 1994). 

Neodymium-isotope ratios of intermediate and deep water in the Indian Ocean are intermediate to the Atlantic and Pacific. They generally fall between s d = 1 to —9, and are consistent with domination by northward flowing circumpolar water (Bertram and Elderfield, 1993 Jeandel et al., 1998). A depth profile east of southern Africa (Figure 7) displays the same zig-zag pattern as South Atlantic intermediate and deep water, reflecting advection of NADW into the western Indian Ocean (Bertram and Elderfield, 1993). 

The broadest characteristics can be concisely summarized. Neodymium-isotope ratios in seawater vary systematically with location throughout the oceans, high in the Pacific, low in... 

In the Pacific, the mechanism through which seawater obtains its high neodymium-isotope ratios is not well identified. The source is clearly not from the surrounding continental masses. The major Pacific sediment sources from the surrounding... 

In the cases, increases in neodymium-isotope ratios along water advection pathways are attributed to local volcanic sources. Thus, it is likely that addition of neodymium from volcanic particles imparts the distinctive neodymium-isotopic fingerprint to Pacific seawater, which is ultimately intermediate to values of recent volcanic and old continental sources. 

These observations, taken together, indicate that the neodymium-isotope ratios of intermediate and deep seawater are imprinted mainly in the Atlantic and Pacihc oceans. The circum-Antarctic is fed by both and its intermediate neodymium-isotope ratio reflects those of the Atlantic and Pacihc water sources. Indian Ocean intermediate and deep water is fed primarily by the circum-Antarctic and tends to retain a circum-Antarctic neodymium-isotope ratio. 

The highest neodymium-isotope ratios in the South Atlantic at depths greater than 2,500 m reach... 

Passage values (Figure 7). This may indicate a source of neodymium with high isotope ratios in the South Atlantic. However, it is premature to conclude that deep South Atlantic neodymium-isotope ratios overstep the Southern Ocean values, for the following reasons. The maxima for all of the deep South Atlantic waters are between SNd = 7 to —9, more variable than presently available data from the Drake Passage but still quite similar. This range is also similar to circumpolar Fe-Mn sediments (Albare(c)de et al., 1997). Depth profiles from the western Indian Ocean near southern Africa are similar to the South Atlantic and Drake Passage, but like the South... 

Taking all of these considerations into account, the available data strongly indicate that neodymium-isotope ratios in deep water are... 

The previous section demonstrated that neodymium-isotope ratios in the deep-ocean trace mixing between Southern and Northern Ocean... 

Considered separately, the neodymium-isotope ratios and the neodymium concentrations have very different implications, (i) Neodymium-isotope ratios are variable in the different oceans and within an ocean they fingerprint the advective paths of water masses. This requires that the residence time of neodymium is shorter than the ocean mixing time of 10 yr (e.g., Broecker and Peng, 1982). (ii) Neodymium concentrations appear to mimic silicate, implying orders of magnitude longer residence times of 10" yr... 

In the Atlantic, like the Pacihc, neodymium-isotope ratios are consistent with mixing of... 

Northern and Southern end-members using salinity or silicate as conservative water-mass mixing proxies (Figure 10). Comparing neodymium-isotope ratios and concentrations, the compositions of many samples are also consistent with North-South water-mass mixing (Figure 14(a)). However, a substantial portion of the data is outside any... 

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