The Isotopic Composition of Neodymium in the Marine Environment: Investigations of the Sources and Transport of Rare Earth Elements in the Oceans

Citation

Piepgras, Donald John (1984) The Isotopic Composition of Neodymium in the Marine Environment: Investigations of the Sources and Transport of Rare Earth Elements in the Oceans. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/GATW-C580. https://resolver.caltech.edu/CaltechETD:etd-08152006-132531

Abstract

In this study, the isotopic composition of neodymium in the marine environment has been determined from analysis of marine ferromanganese precipitates and seawater. An initial survey of the isotopic composition of Nd in the marine environment was made utilizing the analyses of authigenic ferromanganese sediments. These included ferromanganese nodules, metalliferous sediments, and hydrothermal ferromanganese crust deposits. Large variations in ε_Nd(0) values are observed which exhibit a clear separation of the ocean basin. Nd isotopic variations within an ocean basin fall within a relatively small, well defined range which is characteristic of the ocean basin sampled. Based on these results, the following average ¹⁴³Nd/¹⁴⁴Nd ratios for the ocean basins have been determined: Atlantic Ocean, ε_Nd(0) ≈ -12; Indian Ocean, ε_Nd(0) ≈ -8; Pacific Ocean, ε_Nd(0) ≈ -3. These values are considerably lower than ε_Nd(0) values associated sources having oceanic mantle affinities, indicating that the REE in the oceans are dominated by continental sources. Therefore, the variations must reflect primarily the age and ¹⁴⁷Sm/¹⁴⁴Nd ratio of the continental masses being sampled.

Direct measurements of the isotopic composition of Nd in seawater samples from the Atlantic and Pacific are in excellent agreement with the values determined from the ferromanganese sediments indicating that these sediments accurately reflect the isotopic composition of Nd dissolved in seawater. The results clearly demonstrate the existence of distinctive Nd isotopic differences in waters of the major ocean basins. These values correspond to a difference in the absolute abundance of ¹⁴³Nd between the Atlantic and the Pacific Oceans of ~ 10(6) atoms ¹⁴³Nd per gram of seawater. In addition to the isotopic differences observed between the ocean basins, smaller but distinctive variations are observed in the water column of both the Pacific and the Atlantic, indicating different sources of REE at different levels in the water column. 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 isotopic composition of Nd was determined in seawater samples from the Drake Passage in order to monitor the exchange of REE between the Pacific and Atlantic Oceans. The Antarctic Circumpolar Current, which flows eastward through this passage, represents the primary conduit through which the major ocean basins communicate with each other. The isotopic composition of Nd is found to be uniform with depth at all stations and corresponds to ε_Nd(0) ≃ -9.0. This value 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. 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.

The concentration of Nd exhibits a regular increase with depth at all locations studied. In contrast, Nd isotopic compositions can exhibit substantial variations in the water column which vary depending on the location. Where isotopic differences in the water column occur, substantial lateral transport of REE from different sources and at different levels in the water column is required to maintain these differences. It is shown that the concentration gradients are established without significantly affecting the isotopic distribution, and that the enrichment of Nd in the deep water cannot be a result of resolution of REE scavenged from surface waters.

The isotopic distributions are compared to water mass analyses based on temperature and salinity characteristics in the water column at the various sampling locations. It is shown that differences in isotopic compositions in the water column are well correlated with changes in the temperature and salinity characteristics. Thus, the isotopic distributions are fully consistent with the circulation of major water masses. This indicates that while Nd is nonconservative in concentration, the isotopic composition is conserved and can be used as a tracer for studying the origin and circulation of water masses.

The results of these studies have provided some important contributions to the understanding of trace element transport in the oceans. First, the Nd isotopic differences in the water column clearly indicate that transport of Nd from the surface to the deep ocean cannot account for the observed increase in concentration of Nd with depth. These isotopic differences must be maintained by lateral transport of the REE and indicates that the concentration gradients of the REE and possibly other trace elements must also be related in part to lateral transport processes. Second, the close correlation observed between changes in Nd isotopic compositions and temperature-salinity relationships in the water column indicates that the lateral transport of REE in the oceans is directly related to the origin and flow of water masses. Thus, the isotopic composition of Nd in seawater is shown to be a useful tracer for studying the sources of injection and transport of trace elements in the oceans.

In addition to the seawater studies, the concentrations and isotopic compositions of Nd and Sr were determined in hydrothermal solutions emanating from hot springs on the crest of the East Pacific Rise at 21°N and at Guaymas Basin, Gulf of California. This study represents the first effort to measure the Nd isotopic compositions in hydrothermal solutions. Endmember samples (T = 350°C) from 21°N exhibit a small range in ε_Sr values from -13.4 to -15.7. Correcting to C_Mg = 0, the pure hydrothermal solutions are estimated to have ε_Sr ≃ -18. These results indicate that the fluids have undergone extensive but not complete exchange with Sr in the depleted oceanic crust (ε_Sr ≃ -30). C_Sr ranges from 5.8 to 8.7 ppm and is similar to seawater (7.6 ppm) indicating that there must be buffering. Hydrothermal solutions from Guaymas Basin (T = 315°C) rise through several hundred meters of sediment before reaching the sea floor. One sample from here has ε_Sr = +5.8, indicating that the solutions have reacted first with oceanic crust and then sediments. The high Sr concentration in this sample (19.3 ppm) is consistent with late stage interaction between the ascending fluid and carbonate rich sediments.

Nd shows a wide range in concentration and isotopic compositions in solutions from 21°N. C_Nd ranges from 20 to 659 pg/g, indicating substantial enrichments of Nd over typical seawater concentrations of ~3 to 4pg/g. ε_Nd ranges from -10.8 to +7.9. The data clearly show substantial contributions of Nd from depleted oceanic crust to many of the samples analyzed. In spite of enrichments in Nd of up to about 100 times seawater, none of the samples have ε_Nd values equal to MORB (ε_Nd ≃ +10). One sample from Guaymas Basin has ε_Nd = -11.4 consistent with leaching of Nd from sediments derived from old, continental sources. There is some inconsistency in the Nd isotopic data indicating that there is a possibility of contamination during sampling and/or handling of the solutions.

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