What are ferro-manganese nodules?
Spherical or ellipsoidal Fe–Mn concretions (1–20 cm) that grow extremely slowly (mm per Myr) around a nucleus such as a shark tooth, rock fragment, or older nodule debris.
What environmental conditions favour nodule formation?
Sedimentation rates <10 mm per 1,000 years, moderate surface-water productivity, small-scale seafloor topography, and available nucleation material.
What are the two main processes that form Fe–Mn nodules?
Hydrogenetic precipitation and diagenetic precipitation.
Where does hydrogenetic precipitation occur?
At the sediment–water interface under oxic conditions.
What drives hydrogenetic precipitation chemically?
Oxidation of dissolved Mn²⁺ and Fe²⁺ in seawater to δ-MnO₂ (vernadite) and ferrihydrite (FeOOH), forming colloids that scavenge metals.
Which ions attach to MnO₂ surfaces in hydrogenetic layers?
Cations such as Cu²⁺, Ni²⁺, and Co²⁺.
Which ions attach to FeOOH surfaces in hydrogenetic layers?
Anions such as UO₂(CO₃)₂²⁻.
Which neutral species bind covalently in hydrogenetic nodules?
Ti(OH)₄⁰ on FeOOH surfaces.
What chemical signatures characterize hydrogenetic layers?
Mn/Fe ~1; high Co, Te, Ce, Pt; low Y/Ho; high Th/U; strong surface oxidation on δ-MnO₂.
Where does diagenetic precipitation occur?
In suboxic sediment pore waters beneath the sediment–water interface.
What process mobilises Mn²⁺ for diagenetic nodule growth?
Organic matter decomposition consumes O₂, creating a suboxic front where Mn²⁺ is released from sediments and diffuses upward.
What minerals form during diagenetic precipitation?
7-Å birnessite and 10-Å todorokite manganates. (PHYLLOMANGANATES)
What cations are commonly incorporated into diagenetic Mn phases?
Na⁺, K⁺, Mg²⁺, and Ca²⁺.
What chemical signatures characterize diagenetic layers?
High Mn/Fe (>5); high Ni, Cu, Li; low Co, Te, Ce, Pt; high Y/Ho; low Th/U.
Why is diagenetic growth favoured in the Peru Basin?
The oxic–suboxic front lies shallow (~10 cm), allowing strong Mn²⁺ flux from pore waters.
Why are nodules in the CCZ often mixed-type rather than purely diagenetic?
The oxic–suboxic front is deeper (2–3 m+), so both hydrogenetic and diagenetic processes contribute.
How might past glacial periods have influenced diagenesis?
Higher organic flux and/or reduced deep-water ventilation likely expanded suboxic conditions, enhancing Mn mobilization.
What biological role contributes to nodule formation?
Microbial oxidation of Mn²⁺ and Fe²⁺; bacteria with S-layer surfaces promote colloid formation and Mn oxide deposition within biofilms.
What drives differences between hydrogenetic and diagenetic pore-water chemistry?
Suboxic pore waters contain Mn²⁺ at 10,000–100,000 nmol/L, whereas oxic waters contain <10 nmol/L.
What defines mixed-type nodules?
Combined hydrogenetic and diagenetic layers with transitional chemistry, including higher Mn/Fe in diagenetic zones.
What are the three main types of deep-ocean mineral resources targeted for mining?
Ferro-manganese nodules, ferro-manganese crusts, and massive sulfides.
What metals are typically found in ferro-manganese nodules?
Mn, Fe, Ni, Cu, Co, and rare earth elements (REEs).
What metals are enriched in ferro-manganese crusts?
Cobalt, tellurium, platinum, and REEs.
What metals occur in massive sulfides (SMS deposits)?
Copper, zinc, gold, silver, and other base metals.
