Stratigraphic Variations of Fe–Mn Micronodules and Implications for the Formation of Extremely REY-Rich Mud in the Western North Pacific Ocean

Abstract

The origin of deep-sea sediments in the western North Pacific Ocean, which are significantly enriched in rare-earth elements and yttrium (REY), and its paleoceanographic implications have been poorly constrained. Here, we investigated stratigraphic variations in the chemical compositions and textures of ferromanganese (Fe–Mn) micronodules separated from western North Pacific sediments. The characteristics of the micronodules of an extremely REY-rich mud layer vary from almost purely diagenetic to relatively hydrogenetic. This indicates the abundant supply of organic matter to the sediment together with fish debris that accumulates REY at the onset of the REY-enrichment of the mud, followed by the exposure of the seafloor to oxic water masses during the latter half of the formation of the REY-rich mud. These results support a previously proposed formation mechanism based on which enhanced bottom water currents caused pelagic fish proliferation via the upwelling of nutrients and fish debris was physically sorted and selectively accumulated on the seafloor. After the main REY-enrichment, the micronodules exhibit varying diagenetic signatures, suggesting changes in the bottom current intensities after the main REY-enrichment. However, the bulk REY contents do not increase. This implies that a sufficient increase in the fish productivity is an essential factor affecting the formation of REY-rich mud.

Secular Variations in Provenance of Sedimentary Components in the Western North Pacific Ocean Constrained by Sr Isotopic Features of Deep‐Sea Sediments

“…Previous research applied radiogenic isotopes to the sediments in the western North Pacific, where the sediments similar to those in deep‐sea basins around Minamitorishima exist, to discuss binary mixing between Chinese Loess and volcanic ash (Asahara, 1999; Asahara et al., 1999; Jones et al., 1994, 2000; Nakai et al., 1993). However, pelagic clay in the area around Minamitorishima Island also contains biogenic calcium phosphate (BCP) and hydrogenous Mn‐oxides (Takaya et al., 2018; Yasukawa et al., 2020, 2021). Thus, for a comprehensive understanding of the depositional history of pelagic clay, various components should be discussed, which will place the pelagic clay research in a broader context than before.…”

High‐Dimensional Chemostratigraphy of Pelagic Clay in the Western North Pacific Ocean Revealed via an Unsupervised Clustering Approach

“…In this regard, KCA is a conventional method that can classify data into a limited number of groups or clusters following a multivariate data structure (MacQueen, 1967). Notably, Iwamori et al (2017) demonstrated that applying KCA on whitened data (i.e., preprocessed data with uncorrelated variables, zero mean, and unit variance) highlights the essential structure of the data set and provides each sample with objective data labeling that reflects their inherent features (Hamada et al, 2020;Iwamori et al, 2017Iwamori et al, , 2020Yasukawa et al, 2021).…”

Editorial for Special Issue “Deep-Sea Ferromanganese Nodules and Related Mineral Resources: Genesis, Exploration, and Mining”

“…The importance of investigations about other mineral resources related to ferromanganese nodules was highlighted by Yasukawa et al [4], Hong et al [5], and Pérez et al [6]. Yasukawa et al [4] focused on stratigraphic variations in geochemistry, morphology, and surficial textures of micronodules in REY-rich mud.…”

“…The importance of investigations about other mineral resources related to ferromanganese nodules was highlighted by Yasukawa et al [4], Hong et al [5], and Pérez et al [6]. Yasukawa et al [4] focused on stratigraphic variations in geochemistry, morphology, and surficial textures of micronodules in REY-rich mud. Their results showed that micronodules are an important indicator of the environment of sedimentation in forming REY-rich mud at the seafloor surface and indicate the history of oxygen supply or consumption in response to the dynamics of bottom currents.…”

“…Although our model for the intermittent beginning of the growth of nodules [8] needs to be tested in other regions along the LCDW, we hope that our approach [7,8] will be a future standard of nodule investigation. Moreover, micronodules [4] could be a key material in investigating the genetic relationship between nodules and REY-rich muds in terms of the dynamics of bottom currents.…”