Mn-micronodules from the sediments of the Clarion-Clipperton zone (Pacific Ocean): Origin, elemental source, and Fe-Cu-Zn-isotope composition

Abstract Mn- micronodules and nodules of the Clarion-Clipperton zone (Pacific Ocean) are composed of 10 A and 7 A phyllomanganates, and δ-MnO2. The Mn-micronodules are built of fine concentric growth layers of three types (1, 2a, and 2b) according to their Mn/Fe ratio and Ni, Cu, and Co content. Applying previously developped geochemical discrimination approaches we found that the Mn-micronodules were diagenetic precipitates that were a result of suboxic diagenesis, whereas the paired Mn-nodules were diagenetic‑hydrogenetic formations. The most common growth layers (type 2) within the Mn-micronodules are suboxic-diagenetic, whereas the rare growth layers (type 1) are mixed diagenetic‑hydrogenetic and hydrogenetic precipitates. The suboxic diagenetic formation of the Mn-micronodules seems to be a result of the fluctuation of the oxic-suboxic front in the sediment since the Last Glacial Period (LGP). The migration of the oxic-suboxic front close to the seawater/sediment boundary during the LGP has likely resulted in suboxic reduction of Mn4+ and other elements in the sediment and their upward diffusion. Post-LGP deepening of the oxic-suboxic front has seemingly led to re-oxidation of Mn2+ in the pore waters and Mn-micronodule precipitation. The suboxic quantitative re-mobilization of seawater-derived Cesolid phase in the sediment (positive Ce anomaly) and its subsequent sequestration by Mn-micronodules resulted in positive Ce anomaly of the Mn-micronodules and Ce-deficient pore water. This Ce deficiency was recorded in the diagenetic Mn-nodules (negative or no Ce anomaly). The sediment pore waters were source of most elements in the Mn-micronodules and to the bottom seawater. The diagenetic processes were the major control on the Fe-Cu-Zn isotope composition of the Mn- micronodules and nodules. Measured Fe-isotope composition of the Mn-micronodules can equally be explained by hydrogenetic and diagenetic precipitation. Considering our mineralogical and geochemical data we would suggest a rather diagenetic than hydrogenetic control on the Fe-isotope composition of the Mn-micronodules: suboxic diagenetic reduction of the sedimentary Fe in the sediment, fractionation of Fe-isotopes that produces an isotopically light dissolved Fe pool, which leads to light Fe isotope composition of both the Mn- micronodules and nodules (−0.63 to −0.27‰). The preferential scavenging of 63Cu from seawater on the hydrogenetic Mn-Fe-oxyhydroxides accounts for the Cu-isotope composition of the hydrogenetic-diagenetic Mn-nodules (+0.21 − +0.35‰), which is lighter than that of seawater. The identical Cu-isotope composition of the diagenetic Mn-micronodules is a result of oxidative dissolution of the sedimentary Cu-containing minerals, release of isotopically heavy Cuaq2+ in the pore waters and record of this diagenetic Cu-isotope pool in the Mn-micronodules. The hydrogenetic-diagenetic Mn-nodules have Zn-isotope composition (+0.75 − +0.87‰) heavier than that of the seawater which is interpreted to be a result of equilibrium isotope partitioning between dissolved and adsorbed Zn: preferential sorption of 66Zn on Fe-Mn-oxyhydroxides surfaces. Preferential adsorption of 66Zn from the light Zn isotope pool of the pore waters on the Mn-Fe-oxyhydroxides has resulted in heavy Zn-isotope composition of the Mn-micronodules and diagenetic layers of the Mn-nodules. The lack of robust assessment of the Mn-micronodule abundance in sediment volume unit and the insufficient geochemical data for the Mn-micronodules prevents a meaningful estimation of their resource potential.