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The isotopic composition of neodymium in different ocean masses
Samarium-neodymium data for authigenic ferromanganese
sediments from the oceans indicate that the Atlantic, Pacific, and
Indian Oceans each have a distinct range in Nd isotopic composition,
all of which are far less than ¹⁴³Nd/¹⁴⁴Nd ratios of source rocks with
oceanic affinities. Direct measurements of the Nd isotopic composition
of seawater presented here support the view that REE in ferromanganese
sediments are derived by the direct precipitation of these elements
out of seawater. Nd isotopic variations in ferromanganese sediments
cannot be explained by contributions from continental detritus. It is
therefore believed that the Nd isotopic variations found for ferromanganese
sediments represent true variations in the isotopic composition
of Nd dissolved in seawater in various ocean masses. These variations
reflect primarily the age and ¹⁴⁷Sm/¹⁴⁴Nd of the continental masses
being sampled, which is believed to be the major source of REE in
seawater. These variations indicate that the residence time of Nd in
seawater must be very short relative to the mixing rates between ocean
masses. Nd isotopic studies, both in seawater and sediments should,
therefore, be useful as a monitor of ocean currents and interocean
mixing over the past several million years
Neodymium isotopic variations in seawater
New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average ε_(Nd)(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean,ε_(Nd)(0) ≅ −12 ± 2; Indian Ocean,ε_(Nd)(0) ≅ −8 ± 2; Pacific Ocean,ε_(Nd)(0) ≅ −3 ± 2. These values are considerably less than ε_(Nd)(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of ^(143)Nd between the Pacific and Atlantic Oceans corresponds to ∼106 atoms ^(143)Nd per gram of seawater. The correspondence between the ^(143)Nd/^(144)Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography.
Distinctive differences in ε_(Nd)(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition.
The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans
(Table 4, page 134) Results of Nd isotopic measurements in two layers of a Pacific Ocean manganese nodule (MN139)
New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average eNd(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean, eNd(0) = -12 ± 2; Indian Ocean, eNd(0) = -8 ± 2; Pacific Ocean, eNd(0) = -3 ± 2. These values are considerably less than eNd(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of 143Nd between the Pacific and Atlantic Oceans corresponds to ab. 10**6 atoms 143Nd per gram of seawater. The correspondence between the 143Nd/144Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography. Distinctive differences in eNd(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition. The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans
Isotopic Composition of Neodymium in Waters from the Drake Passage
The isotopic composition of neodymium has been determined in seawaters from the Drake Passage. The Antarctic Circumpolar Current, which controls interocean mixing, flows through this passage. The parameter ε_(Nd)(0) which is a function of the ratio of neodymium-143 to neodymium-144, is found to be uniform with depth at two stations with a value which is intermediate between the values for the Atlantic and the Pacific and indicates that the Antarctic Circumpolar current consists of about 70 percent Atlantic water. Cold bottom water from a site in the south central Pacific has the neodymium isotopic signature of the waters in the Drake Passage. By using a box model to describe the exchange of water between the Southern Ocean and the ocean basins to the north together with the isotopic results, an upper limit of approximately 33 million cubic meters per second is calculated for the rate of exchange between the Pacific and the Southern Ocean. Concentrations of samarium and neodymium were also determined and found to increase approximately linearly with depth. These results suggest that neodymium may be a valuable tracer in oceanography and may be useful in paleo-oceanographic studies