Characteristics of ferromanganese micronodules in surface sediments of the tropical North Pacific Ocean

In addition to the widely acknowledged marine ferromanganese deposits, i.e., polymetallic nodules and cobalt-rich crusts, Fe-Mn (oxyhydr)oxides ubiquitously precipitate as micronodules in the oxic aquatic environments. Due to their micrometer-scale or even smaller size, ferromanganese micronodules have received limited attention until they were found to be one of the important rare earth elements and yttrium (REY)-holding phases in the deep-sea REY-rich mud, and noteworthily estimated to preserved a ~1.28-7.62 Tt Mn budget, exceeding that of nodules and crusts by at least two orders of magnitude. Combined with the increasing demands for critical elements in high-tech industries, the economic and scientific potential of micronodules, which could strongly scavenge trace elements from ambient environments, deserve to be revised.

In our study, Fe-Mn micronodules were hand-picked from surface sediments of the North-Western (NW) Pacific and the North-Eastern (NE) Pacific Clarion-Clipperton Fracture Zone (CCFZ) for morphological and in-situ geochemical analyses with field-emission scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS). The characteristic microbial-like mineralization structures, including the irregular aggregates of rod microbial-like particulates, biofilm, and the phalanxes of ellipsoid microorganisms, were commonly observed in micronodules. Their Mn/Fe ratios most frequently fall in the range of < 10 (46.1%), with a maximum value (reaching 698) that is much higher than the published EMPA data of polymetallic nodules. Unlike the continuous variation of elemental contents in nodules, the concentrations of some elements in micronodules, such as Al, Ca, K, Co, Ni, Cu, Sr, Mo and REYs other than Ce, showed a mutative tendency, being slowed down or even reversed, with the increase of Mn/Fe ratio in the ranges of Mn/Fe < 10 and Mn/Fe > 10. Whereas, the declining tendencies in Ce contents and Ce anomaly remained generally stable.

Considering the loose structure of Fe-Mn (oxyhydr)oxides, and after multiple comparisons, Mn/Fe < 5 is used here to distinguish hydrogenetic and mixed hydrogenetic-early diagenetic type (grouped into hydrogenetic type hereafter) from diagenetic type. Amounts of geochemical data of hydrogenetic Fe-Mn deposits (including micronodules, (macro)nodules and crusts) were collected. Comparative analyses reveal that both the hydrogenetic crusts (n = 289) and nodules (n = 159) have weaker positive Ce anomalies (avg. 2.23 and 2.97, respectively) than our hydrogenetic micronodules (n = 204, avg. 6.27). Combined with the similar Ce content in all three hydrogenetic ferromanganese deposit types, and the lower concentrations of other REYs in hydrogenetic micronodules, the migration of REYs (except for Ce) from micronodules to porewater is indicated.

All these findings demonstrated that the precipitation and aggregation of micronodules were predominantly controlled by the redox state of ambient pore water, even in surface sediments, and were largely influenced by microbial activities. The potential of ferromanganese micronodules to serve as a buffer in the enrichment of REYs and critical elements in pelagic sediments and an archive of the ambient redox conditions enhanced their value of further comprehensive and in-depth studies.