Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China
Laboratory for Marine Geology, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266061, China
The Shandong Provincial Natural Science Foundation of China under contract No. ZR2014DP009; the China Ocean Mineral Resource Research and Development Association Research Program under contract Nos DY135-N-1-03, DY135-C1-1-04 and DY135-R2-1-03; the Fund of the Construction and Operation of Test and Technical Support System for Natural Resources Investigation and Evaluation.
Ferromanganese (Fe-Mn) crusts are potential archives of the Cu and Zn isotope compositions of seawater through time. In this study, the Cu and Zn isotopes of the top surface of 28 Fe-Mn crusts and 2 Fe-Mn nodules were analysed by MC-ICP-MS using combined sample-standard bracketing for mass bias correction. The Zn isotope compositions of the top surface of Fe-Mn crusts are in the range of 0.71‰ to 1.08‰, with a mean δ66Zn value of 0.94‰±0.21‰ (2SD, n=28). The δ65Cu values of the top surface of Fe-Mn crusts range from 0.33‰ to 0.73‰, with a mean value of 0.58‰±0.20‰ (2SD, n=28). The Cu isotope compositions of Fe-Mn crusts are isotopically lighter than that of dissolved Cu in deep seawater (0.58‰ vs. 0.9‰). In contrast, the δ66Zn values of Fe-Mn crusts appear to be isotopically heavy compared to deep seawater (0.94‰±0.21‰ vs. 0.51‰±0.14‰). The isotope fractionation between Fe-Mn crusts and seawater is attributed to equilibrium partitioning between the sorption to crusts and the organic-ligand-bound Cu and Zn in seawater. The Cu and Zn isotopes in the top surface of Fe-Mn crusts are not a direct reflection of the Cu and Zn isotopes, but a function of Cu and Zn isotopes in modern seawater. This study proposes that Fe-Mn crusts have the potential to be archives for paleoceanography through Cu and Zn isotope analysis.