Geochemistry of buried polymetallic nodules from the eastern Pacific Ocean: Implication for the depth-controlled alteration process

Polymetallic nodules are often exposed or semi-buried in surface sediments of abyssal plains, with certain older generations of nodules being deeply buried in the sediment column. Owing to the limitations of sampling, it is unclear whether these nodules have remained intact or have been altered after burial. To explore this question, we carried out a comparative study of the mineralogy and chemistry of polymetallic nodules recovered from the surface sediment and two depth intervals within a sediment column (220-223 cm and 550-555 cm) in the Clarion-Clipperton Zone of the eastern Pacific Ocean. All of the nodules examined have obvious layered structures. The main minerals in the buried nodules are 10 angstrom vernadite, 7 angstrom phyllomanganate, and Fevernadite, which are characteristic of the hydrogenetic type of polymetallic nodule. However, the in situ Mn/Fe ratios suggest that some microlayers are diagenetic type. With the burial depth, the total content of rare earth elements and yttrium decreased in the bulk nodules, whereas their positive Ce anomalies and diagenetic parts gradually increase. The nodule buried at 220-223 cm sediment depth shows no obvious alterations, whereas the one buried at 550-555 cm has an approximately 200-mu m-thick microlayer of continuous diagenetic growth on its surface. Additionally, this GC04_550-555 cm nodule has a microlayer with higher Cu/Ni and Si/Al ratios, located adjacent to its outer diagenetic microlayer, further indicating its alteration after burial. Therefore, the alterations of buried polymetallic nodules may be related to their burial depth. Compared with the sediment layers above and below, the bulk sediment at the 550-555 cm interval has a relatively lower Mn content and higher Fe content, which may be related to the observed alterations in the buried nodule found therein. Our findings on the alterations of polymetallic nodules after burial and the possible impact of the sedimentary environment on those changes provide a theoretical basis for establishing the burial events of these ferromanganese aggregates in the geological history of the study area.