For centuries, humans have mined, refined, and utilized various metals and minerals found on land and along coasts. As global supply of these finite resources dwindles, the pursuit of them intensifies; this inverse relationship has pushed countries to the ends of the earth in search of them and now, increasingly, their sights are set on the seafloor.
A Review of Alaska's Marine Mineral Resources
“Knowledge of marine minerals—where and how they are formed—has been around for more than 40 years,” says Amy Gartman, Research Oceanographer with the Pacific Coastal and Marine Science Center and the lead author of a literature review of Alaska’s marine mineral resources. “Interest waxes and wanes, depending on supply and the costs involved with potential extraction.”
The review is a “desktop” study that examines data collected on previous research expeditions to the Arctic and North Pacific oceans. In it, Gartman and colleagues expand on existing knowledge concerning “critical” minerals—defined as supporting U.S. economic and national security—that could potentially occur far offshore of Alaska.
While terrestrial and coastal mineral deposits have been exploited for centuries, the review notes, deep-sea geologic formations containing concentrations of minerals are largely untouched. Any inventory of global mineral resources is thus incomplete without considering marine minerals, which are distributed widely across the global seafloor and vary considerably in how and where they form.
The review examines evidence that coastal and deep-sea minerals exist off Alaska’s coasts and are potentially enriched with minerals of critical importance.
Ferromanganese (Fe-Mn) crusts grow slowly on the substrate, concentrating iron and manganese in layers that may take millions of years to form. Over time, Fe-Mn crusts can also become enriched in valuable and critical commodities such as cobalt, tellurium, and rare earth metals.
“Ferromanganese crusts grow layer by layer, about a millimeter per million years,” says Kira Mizell, another author of the review. “The crusts are repositories of past oceanic conditions: When you go back in time through the rock, you can see the seawater chemistry being recorded in its layers.”
A similar mineral type is the manganese nodule. “Instead of growing in one stratigraphic direction like crusts on rock surfaces and rock outcrops,” says Mizell, “manganese nodules actually form around a nucleus and grow outward radially and form concentric layers, kind of like tree rings.”
Manganese nodules are commonly associated with abyssal plains, those enormous stretches of relatively flat, bare seafloor that cover more than half of the world’s surface. Nickel, copper, cobalt, lithium, molybdenum, and manganese are among the metals concentrated in nodules from seawater.
Seafloor massive sulfide mounds form at areas where hot water beneath the seafloor erupts and mixes with cold seawater, leaving mineral-rich mounds and chimneys. As the hot water leaches through layers of rock beneath the ocean, it collects trace amounts of elements such as copper, zinc, iron, gold and silver, which then precipitate onto the seafloor as the solution cools. These sulfides are most common near tectonic plate boundaries where mantle-heated rock is exposed to seawater.
Coastal mineral deposits may include mineral deposits contained within submerged continental crust, as well as dense minerals eroded by streams, rivers, and coastal processes known as placer deposits. Placer deposits, including marine placer deposits, are well known in Alaska and have been previously exploited for minerals, including gold.
Although this review focuses on minerals within the US EEZ, many deep-sea marine-mineral deposits occur beyond U.S. jurisdiction in what’s known as “The Area”. In recognition of the economic importance of coastal and offshore resources, in 1982 the United Nations adopted a policy called Exclusive Economic Zones, whereby nations could lay claim to offshore resources 200 nautical miles off their coasts. (In the U.S., the area of seafloor within the EEZ is greater than the land area on shore.) “The Area” is the vast oceanic expanse that lies outside of EEZs.
To date, no extraction of deep-sea marine minerals occurs off the coast of Alaska or elsewhere in the world. “This review summarizes the state of our knowledge about Alaska’s marine mineral resources,” says Gartman. “There is a definite need for more samples and geochemical data from the review area.”
Gartman serves as a member of the U.S. delegation to the International Seabed Authority, which is charged with overseeing nations’ activities in “The Area” and codifying—if and when it is necessary—a set of rules for how to extract deep-sea minerals.