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Lithium contents and isotopic compositions of ferromanganese deposits from the global ocean

Deep-Sea Research Part II: Topical Studies in Oceanography

By:
and
DOI: 10.1016/j.dsr2.2007.04.003

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Abstract

To test the feasibility of using lithium isotopes in marine ferromanganese deposits as an indicator of paleoceanographic conditions and seawater composition, we analyzed samples from a variety of tectonic environments in the global ocean. Hydrogenetic, hydrothermal, mixed hydrogenetic–hydrothermal, and hydrogenetic–diagenetic samples were subjected to a two-step leaching and dissolution procedure to extract first the loosely bound Li and then the more tightly bound Li in the Mn oxide and Fe oxyhydroxide. Total leachable Li contents vary from <1 ppm in hydrogenetic crusts to 1422 ppm (up to 1188 ppm measured here) in hydrothermal deposits. Hydrated Li ions in seawater and hydrothermal fluids are preferentially sorbed on the negatively charged surface of MnO2 by coulombic force. Hence, the abundant Li in hydrothermal deposits is mainly associated with the dominant phase, MnO2. The surface of amorphous FeOOH holds a slightly positive charge and attracts little Li, as demonstrated by data for hydrothermal Fe oxyhydroxide. Loosely sorbed Li in both hydrogenetic crusts and hydrothermal deposits exhibit Li isotopic compositions that resemble that of modern seawater. We infer that the hydrothermally derived Li scavenged onto the surface of MnO2 freely exchanged with ambient seawater, thereby losing its original isotopic signature. Li in the tightly bound sites is always isotopically lighter than that in the loosely bound fraction, suggesting that the isotopic fractionation occurred during formation of chemical bonds in the oxide and oxyhydroxide structures. Sr isotopes also show evidence of re-equilibration with seawater after deposition. Because of their mobility, Li and Sr in the ferromanganese crusts do not faithfully record secular variations in the isotopic compositions of seawater. However, Li content can be a useful proxy for the hydrothermal history of ocean basins. Based on the Li concentrations of the globally distributed hydrogenetic and hydrothermal samples, we estimate a scavenging flux of Li that is insignificant compared to the hydrothermal flux and river input to the ocean.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Lithium contents and isotopic compositions of ferromanganese deposits from the global ocean
Series title:
Deep-Sea Research Part II: Topical Studies in Oceanography
DOI:
10.1016/j.dsr2.2007.04.003
Volume
54
Issue:
11-13
Year Published:
2007
Language:
English
Publisher:
Elsevier
Contributing office(s):
Coastal and Marine Geology Program
Description:
16 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Deep-Sea Research Part II: Topical Studies in Oceanography
First page:
1147
Last page:
1162
Number of Pages:
16