【Frontiers in HaiChuan Chemical Technology】Chinese scientists discover new mechanisms for deep-sea rare earth cycling and enrichment

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On February 19, it was learned from the Guangzhou Marine Geology Survey Bureau of the China Geological Survey that a team led by Deng Yinan from this bureau recently published research findings in the international journal Geology, revealing new mechanisms for the cycling and enrichment of rare earth elements in the deep sea, which holds scientific significance for gaining a deeper understanding of the formation patterns of deep-sea rare earth resources. Rare earths are key strategic resources worldwide. Deep-sea sediments are potential reservoirs of giant rare earth resources, and the mechanisms of rare earth element cycling and enrichment in them represent a key focus in research on global marine biogeochemical cycles. Deng Yinan, the first author of the paper and a professor-level senior engineer at the Guangzhou Marine Geology Survey Bureau, explained that hydrothermal influence zones are areas where rare earth elements are concentrated in the deep sea, as well as key hubs in the circulation of these elements; iron-manganese oxide particles in hydrothermal plumes play a role in removing rare earth elements from seawater. However, it is still unclear how these removed rare earth elements are remobilized and enriched through early diagenetic processes after deposition, as well as their fluxes and mechanisms, which hinders an accurate assessment of the potential of deep-sea rare earth resources. In the past, academic research on the cycling patterns of deep-sea rare earths has mainly focused on the \"top-down\" process, that is, how iron-manganese oxides can remove rare earths from water during sedimentation, thereby allowing these rare earths to cycle from surface seawater to deeper seawater and sediments. After conducting systematic geochemical analyses on seawater, pore water, and biological apatite from deep-sea basins in the southeast Pacific that are significantly affected by hydrothermal activity, Deng Yinan’s team discovered that there is also a \"bottom-up\" circulation pattern for rare earth elements in the deep sea; that is, iron-manganese oxides release rare earth elements during the early diagenetic processes at the seawater-sediment interface, which then diffuse upward into the seawater. Studies have found that in the dual-cycle pattern of \"top-down\" removal and \"bottom-up\" release of marine rare earth elements, shallow seabed sediments release large amounts of rare earth elements back into the seawater, whereas the rate of rare earth regeneration in deep ocean sediments is much lower than that at continental margins; as a result, the majority of rare earth elements remain trapped in the sediments. From a new perspective based on the sources of rare earth elements, this demonstrates that deep-sea sediments are important sites for their accumulation. Further research found that the total amount of rare earths released into the seawater from deep-sea oceanic areas in hydrothermal zones, through the pore water in sediments, is significantly higher than that in non-hydrothermal zones. This is mainly because deep-sea sediments affected by hydrothermal activity contain a large number of hydrothermal iron-manganese oxide particles, and the rare earths adsorbed on their surfaces are released into the seawater at the water-rock interface. In deep environments where the sediment depth is more than 2 meters, the release of rare earths is minimal and sporadic. This finding indicates that early diagenesis at the water-rock interface is key to influencing marine rare earth cycling. Deng Yinan explained that this study reveals the key role of early diagenesis in controlling the circulation and enrichment of rare earth elements in deep-sea hydrothermal areas, thereby enhancing our understanding of the global marine rare earth element circulation process. It also provides new theoretical perspectives and scientific foundations for the scientific assessment and exploration of deep-sea rare earth resources.