Estimates of Metals Contained in Abyssal Manganese Nodules and Ferromanganese Crusts in the Global Ocean Based on Regional Variations and Genetic Types of Nodules

Abstract

Deep-ocean ferromanganese crusts and manganese nodules are important marine repositories for global metals. Interest in these minerals as potential resources has led to detailed sampling in many regions of the global ocean, allowing for updated estimates of their global extent. Here, we present global estimates of total tonnage as well as contained metal concentrations and tonnages for ferromanganese crusts and manganese nodules using the most extensive compilation of geochemical data collected to date, along with updated boundaries of regions of interest for these minerals. We present results from mean composition calculated in two ways: first, a global flat average of regional mean compositions, and second, a regionally weighted average that considers differences in chemistry among genetic types and/or oceanographic and geologic settings for these mineral occurrences. For nodules, we use the three genetic types: (1) hydrogenetic, typified by nodules from the West Pacific Nodule Field and Penrhyn Basin; (2) diagenetic, typified by nodules from the Peru Basin; (3) mixed hydrogenetic-diagenetic, typified by nodules from the Clarion–Clipperton Zone and the Central Indian Ocean Basin, and Atlantic Ocean regional type hydrogenetic nodules. All crusts considered here are of hydrogenetic origin, which we divide into seven regional types that reflect a combination of ocean basin and other source inputs. Crust types include Arctic Ocean, Atlantic Ocean, Indian Ocean, Continental Margin, Prime Crust Zone (PCZ), North Pacific (non PCZ), and South Pacific. Based on our areal estimates, we find that abyssal regions likely to contain hydrogenetic-type nodules are by far the most widespread in the global ocean (47% of total area), Atlantic Ocean (28%) are next, followed by mixed diagenetic-hydrogenetic (22%) and diagenetic (3%) types. For crusts, the Prime Crust Zone is the most extensive global region (27% of total area) followed by South Pacific (20%), Indian Ocean (18%), North Pacific (12%), Continental Margins (11%), Atlantic Ocean (10%), and Arctic Ocean (2%) types. The global total tonnage estimates that we calculated from this method are 21 × 10¹⁰ dry tons for manganese nodules, within the range of previous estimates, and 93 × 10¹⁰dry tons for ferromanganese crusts, which is 4.5 times higher than the 20 × 10¹⁰dry tons reported by Hein et al. (2003). This geology and oceanography driven approach to marine mineral quantification contrasts with estimates typically carried out for terrestrial mineral resource deposits. Nevertheless, these estimates and the data that support them demonstrate that marine minerals are an impressive repository for global metals.

Annexure: Definitions

The terms polymetallic nodule , ferromanganese nodule , manganese nodule , and the shorthand nodule are sometimes used interchangeably. However, without context, do not have identical meanings. The term nodule is a morphologic term that generally refers to rounded mineral concretions. The term ferromanganese is a description of composition, indicating the presence of iron and manganese, which are the framework metals for marine nodules. Ferromanganese is often used as a descriptor for crusts, which have approximately equal Fe and Mn contributions by weight (this paper), and manganese is often used as a descriptor for nodules, which may have Mn contents ~2× that of Fe, although hydrogenetic nodules have a Fe/Mn ratio of approximately 1 (this paper). Polymetallic nodule , as defined by the International Seabed Authority (ISA), refers to “any deposit or accretion of nodules on or just below the surface of the deep seabed, which contain manganese, nickel, cobalt and copper.” For instance, whereas all polymetallic nodules are also manganese nodules and in shorthand may be referred to as nodules, the opposite is not true—not all nodules are manganese nor polymetallic nodules, since nodule is a morphologic term that may be applied in other contexts. For instance, carbonate nodules, phosphorite nodules, and micronodules are some examples of nodules that occur elsewhere in the oceans and are entirely distinct from abyssal plain nodules.

Likewise, the terms cobalt-rich ferromanganese crust designates a specific subset of the mineral occurrence discussed as ferromanganese crusts for which the shorthand crust is often used. As defined by the ISA, cobalt crusts also known as cobalt-rich ferromanganese crusts are “hydroxide/oxide deposits formed from direct precipitation of minerals from seawater on to hard substrates containing minor but significant concentrations of cobalt, titanium, nickel, platinum, molybdenum, tellurium, cerium, other metallic, and rare earth elements.” The terms cobalt crusts and cobalt-rich ferromanganese crusts both refer to minerals that are predominantly composed of iron and manganese; the cobalt contents average greater than 6500 ppm (0.65 wt. %) in the Prime Crust Zone and greater than 4000 ppm (0.40 wt. %) globally averaged. Ferromanganese crust refers to a chemical sedimentary rock containing iron and manganese, with trace metal contents that vary by region (e.g., Aplin and Cronan 1985; Wen et al. 1997; Hein et al. 2000; this paper); a crust is a morphologic description.