Non-traditional stable isotope geochemistry of marine ferromanganese crusts and nodules

Marine ferromanganese crusts and nodules, which contain a variety of metals, are potential seabed mineral resources. Given their low growth rates, they are regarded as condensed stratigraphic sections that archive millions of years of paleoceanographic information. Ferromanganese crusts and nodules incorporate trace elements like Cu, Zn, Mo, Tl and Ni during growth. The non-traditional isotopic systems of these metals are increasingly being developed as powerful tracers in the modern ocean and as proxies for the paleo-ocean, due to their tendency to be fractionated by redox-related and/or biological processes. In recent years, both the global variations of metal stable isotopes in ferromanganese crust/nodule surface scrapings and some depth profiles through the ferromanganese crusts were systematically analysed. These studies established the isotopic variability present in ferromanganese crusts, nodules and seawater, explored the isotopic fractionation mechanisms associated with the formation of ferromanganese deposits, and determined whether these ferromanganese crusts can be used as documents of deep water metal isotope compositions and long-term seawater isotope variations. In addition, some isotopes of ferromanganese deposits have been successfully applied to constrain the metal sources and geochemical cycles in the ocean, reconstruct paleo-oceanic redox conditions and seawater isotope record, and reveal continental weathering and climate changes. Nevertheless, it is worth noting that a few limitations of current applications of some non-traditional isotopes as paleoceanographic proxies still remain. Therefore, there is still a great need for a community effort to develop and enhance non-traditional isotope geochemistry of marine ferromanganese crusts and nodules.