Renewable energy sources and green-technologies are required to mitigate carbon-based fuel consumption, but they require increasing amounts of raw materials. Critical metals are abundant in ocean-floor iron-manganese deposits. However, the nature of their association with iron and manganese oxide minerals and exchangeability with water remain poorly understood. Advancing our knowledge in this field is important in order to understand biogeochemical metal cycling in marine environments and to develop tailored metal processing techniques. These will aid in unlocking metals through zero-waste approaches which help reduce environmental impacts of mining.