Genesis of shallow-water manganese nodules with uniquely high Mn/Fe ratios

• We report the discovery of shallow-water Mn nodules with remarkably high Mn/Fe ratios. • Pacific Water inflow may have facilitated Mn-rich nodule growth since the Holocene. • The nodules serve as new paleoproxy of hydro-chemical evolution of the Arctic Ocean. While manganese (Mn) nodules are authigenic metal concretions that form predominantly on deep-sea seafloor, they have also been found along shallow seafloors. The formation environments of these nodules — deep sea vs. shallow water — often result in distinct chemical and morphological characteristics. As Mn is one of the essential components of energy-storing technologies, assessing the proper estimation of Mn and metal contents in both deep- and shallow-water Mn nodules is critical. It has been found that the Mn content of shallow-water nodules is often lower than that from deep-sea environments. Here, we report the discovery of shallow-water Mn nodules with exceptionally high Mn/Fe ratios on the continental slope of the East Siberian Sea, Arctic Ocean. Despite their shallow-water origin, Mn nodules show morphological and chemical characteristics that are typically unique to deep-sea nodules. These distinctive features, including exceptionally high Mn/Fe ratios, may reflect suboxic diagenesis and the preferential remobilization and re-precipitation of Mn from the adjacent continental shelf. The formation of high Mn/Fe nodules may reflect unique ocean circulation patterns that provided oxygenated bottom waters to the study area. Particularly, Pacific Water entering through the Bering Strait, which overlaps with the nodule formation depth (160 – 240 m) and is enriched in dissolved oxygen, could facilitate Mn-rich nodule growth under suboxic diagenetic conditions since the Holocene. Shallow-water Mn nodules with uniquely high Mn/Fe ratios may offer a novel paleo-environmental proxy for reconstructing paleohydrology and bio-geochemical evolutions in shallow marine environments.