Rare‐Earth Elements in Deep‐Sea Sediments in the South Pacific Gyre: Source Materials and Resource Potentials

Deep‐sea sediments enriched in rare‐earth elements and yttrium (REY) plus scandium (Sc), termed “REY‐rich mud,” have attracted attention as a possible resource for these critical industrial elements. Examples have been reported from the western North Pacific, central Pacific, low‐latitude South Pacific, and eastern South Pacific. Although previous studies of pelagic clay have reported the existence of highly REY‐rich mud in the ultraoligotrophic South Pacific Gyre, neither the source materials nor the resource potentials for REY and Sc of the sediment have been quantified. We analyzed the major‐ and trace‐element contents of bulk sediments in Integrated Ocean Drilling Program Holes U1365A to U1370D, drilled in the South Pacific Gyre. The elemental relationships suggest that the enrichment in REY and Sc reflects the accumulation of biogenic Ca phosphate in an environment with low sedimentation rates as well as the tectonic transition of depositional environments from hydrothermally influenced sites near the mid‐ocean ridge to distal basins far from hydrothermal vents. The maximum total REY content of 4,662 ppm at Hole U1366C is the highest value yet reported from the South Pacific Ocean. Although the REY‐ and Sc‐enriched sediment layers of most cores from the South Pacific Gyre are located deeper beneath the seafloor than those elsewhere in the Pacific Ocean, Hole U1367B demonstrates large resource potentials (1.21 × 104 t/km2 of REY oxides and 102 t/km2 of Sc) in the uppermost 6.5‐m interval, making it the most promising site for REY and Sc yet found in the South Pacific Gyre.

demand for which will grow significantly as the world moves toward carbon-neutral economies (European Commission, 2020;Goodenough et al., 2018). Scandium, likewise, has become a focus of attention as a critical material for novel high-performance alloys and cost-effective solid-oxide fuel cells (Ahmad, 2003;Laosiripojana et al., 2009;Riva et al., 2016). It is notable that, whereas Y behaves like lanthanoids, Sc differs from lanthanoids in its geochemical characteristics because of the smaller size of its cation (McLennan, 2018).
In recent years, deep-sea sediments containing high concentrations of REY have been discovered in wide areas of the Pacific Kato et al., 2011;Sa et al., 2018;Yasukawa et al., 2016;T. Zhou et al., 2020, Indian (Yasukawa et al., 2014Yu et al., 2021;X. Zhang et al., 2017), and Atlantic Oceans (Menendez et al., 2017;Nakamura et al., 2015). Sediments with total REY concentrations (ΣREY) exceeding 400, 2,000, and 5,000 ppm are called "REY-rich mud," "highly REY-rich mud," and "extremely REY-rich mud," respectively Iijima et al., 2016). Notably, the extremely REY-rich mud contains up to ∼1,200 ppm of heavy REE (HREE), some 20 times higher than HREE concentrations in the ion-absorption-type ore deposits of south China (Iijima et al., 2016). The extremely REY-rich mud, therefore, has attracted strong attention as a new REY resource, especially for HREE. Furthermore, the highly and extremely REY-rich muds around Minamitorishima Island in the western North Pacific Ocean contain Sc in commercial quantities and thus could be a unique resource for both HREE and Sc . So far, the area south of Minamitorishima Island is one of the most promising areas for deep-sea sediments enriched in REY and Sc. Previous studies have reported the distribution and resource potential of the highly/extremely REY-rich mud in this area (Iijima et al., 2016;Takaya et al., 2018;Tanaka et al., 2020aTanaka et al., , 2020b: the potential resource is as great as 1.1 × 10 4 t/km 2 of REY oxides (REO) for the interval from 0 to 10 m below seafloor (mbsf)  and 107 t/km 2 of Sc for the 0-6.3 mbsf interval .
Recent studies have suggested that potential areas for REY and Sc resources exist elsewhere in the Pacific Ocean. J. Ohta et al. (2021) described highly REY-rich mud with 4,489 ppm of ΣREY in the central Pacific Ocean and estimated resource potentials of up to 3.3 × 10 3 t-REO/km 2 (0-5.38 mbsf) and 31 t-Sc/km 2 (0-7.65 mbsf), but no similar study of candidate sites has been conducted in the South Pacific. Sediments with high REY concentrations have been reported in the eastern South Pacific (ΣREY = 1,500-2,230 ppm; Kato et al., 2011;T. Zhou et al., 2020 and the central South Pacific (ΣREY = 2,782 ppm; Yasukawa et al., 2016), but the findings were limited to REY concentrations and the constituents contributing to the REY enrichment.  reported that pelagic clay collected in the ultraoligotrophic South Pacific Gyre (SPG) by Integrated Ocean Drilling Program (IODP) Expedition 329 contained up to 4,324 ppm of ΣREY, but their study mainly focused on changes in the depositional environment and end-member components, specifically dust and volcanic ash, in the South Pacific during the Cenozoic.
In this study, we analyzed the major-and trace-element compositions of the bulk sediment in cores obtained by IODP Expedition 329 (Holes U1365A to U1370D) using samples collected every 0.25-1.00 m, a much higher stratigraphic resolution than that of . The resulting data set allowed us to treat in detail the end-member components related to REY and Sc enrichment in the SPG and to estimate the REY and Sc resource potentials of SPG sediments for the first time.

Samples
This study analyzed sediment samples obtained from IODP Sites U1365 to U1370 using advanced piston corers during Expedition 329. The site locations are shown in Figure 1 and listed in Table 1 ( D'Hondt et al., 2011).  reported chemical compositions of sediments at these sites that were determined at sampling intervals of 1-5 m from "squeeze cakes," or residues of the sediment samples after extracting pore water with a hydraulic press on board the drillship. Their data represented composites of multiple holes at each site so as to minimize the stratigraphic gaps in the cores to the extent possible . However, our evaluation of the resource potential required us to obtain continuous depth profiles of bulk chemical compositions. We therefore obtained bulk chemical composition data at intervals comparable to those used in the area around Minamitorishima Island (every 10-50 cm; Takaya et al., 2018;Yasukawa et al., 2018) and in the central Pacific Ocean (every 30-50 cm; J. Ohta et al., 2021).

Major Elements
The major-element analyses in this study followed the methods described by Kato et al. (1998Kato et al. ( , 2005, Yasukawa et al. (2014), and Tanaka et al. (2020a). Before the measurements, the bulk sediment samples were powdered and   , sedimentation rates based on Co flux age (Dunlea, Murray, Sauvage, Pockalny, et al., 2015), and sediment classifications used in this study (see Section 4.2). Note the different vertical scales. MCCD, maximum cumulative concentration depth; BCP, biogenic calcium phosphate.
(Rigaku ZSX Primus II, Tokyo, Japan) at the Department of Systems Innovation, the University of Tokyo. As calibration standards during the measurements, we used geochemical reference materials issued by the Geological Survey of Japan (GSJ). The analytical results were within 3% (relative percent difference) of the accepted values for GSJ reference material JB-1b (Terashima et al., 1998).

Trace Elements
The trace-element analyses in this study followed the methods described by Kato et al. (2002Kato et al. ( , 2005, Yasukawa et al. (2014), and Tanaka et al. (2020a). Subsamples of unignited powdered sediment weighing 0.050 g were dried at 110°C for ∼12 hr, then decomposed with a mixed acid of 4 mL of HF, 2 mL of HNO 3 , and 0.8 mL of HClO 4 in a tightly sealed PFA jar (Savillex, Minnesota, USA) at 130°C for 2 hr. The dissolved sample was progressively evaporated at 110°C, 160°C, and 190°C to dryness. The residue was dissolved in 2 mL of aqua regia at 90°C for ∼6 hr and progressively evaporated at 120°C and 160°C to dryness. The residue was dissolved in 10 mL of a 2 wt.% mixed acid of HNO 3 :HCl:HF = 20:5:1 at 90°C, then diluted to 1:10,000 by mass using the same mixed acid  Aries et al. (2000). Analyses were generally within 5% (relative percent difference) of the accepted values for GSJ reference materials JB-2 (Makishima & Nakamura, 2006) and JMS-2 (Takaya et al., 2014).

Maximum Cumulative Concentration Depth
For the estimation of REY and Sc resource potentials, we determined the maximum cumulative concentration depth (MCCD; J. Ohta et al., 2012Ohta et al., , 2021Yasukawa et al., 2018) for each site. Here the dry bulk density ρ, REY concentration c REY , and Sc concentration c Sc of the ith and (i+1)-th samples are presented as ρ i , ρ i+1 , c REY_i , c REY_i+1 , c Sc_i , and c Sc_i+1 , respectively. Assuming a thickness Δd i between the positions of the ith and (i+1)-th samples, the total amounts of REY (m REY ) and Sc (m Sc ) between the ith and (i+1)-th samples per unit area are as follows: The total amounts of REY (M REY ) and Sc (M Sc ) between the seafloor and the kth sample are then Defining the average value of the dry bulk density between the seafloor and the kth sample as P k , we obtain the following relationship between P k and P k+1 : In this study, we estimated the dry bulk density of every sample by linear interpolation of measurements reported in D' Hondt et al. (2011) and used the results to calculate the total amounts of REY (M REY ) and Sc (M Sc ) as well as average dry bulk density (P k ). From these values, we obtained the average contents of REY and Sc between the seafloor and the kth sample (C REY_k , C Sc_k ), termed "cumulative concentrations" (J. Ohta et al., 2012Ohta et al., , 2021Yasukawa et al., 2018), by Finally, we identified MCCDs of REY and Sc (MCCD REY and MCCD Sc ), signifying the depths that yield the maximum cumulative concentrations (MCC; C REY and C Sc ).

Depth Profiles
Depth profiles of ΣREY and Sc concentrations at our six study sites are shown in Figure 2, and the data for all samples are listed in Table S1 of Supporting Information S2.

Sc
The depth profile of Hole U1365A (

Comparison of Bulk Sediment and Squeeze Cake Samples
The REY and Sc depth profiles of five of the six holes (U1365A, U1367B, U1368B, U1369B, and U1370D) are generally consistent with those reported by  orange data in Figure 2). These congruent profiles suggest that the sedimentary and chemical sequences were horizontally correlated among the holes in each site, and there is little variation between the results from the bulk sediments in this study and in . Moreover, the general trends of the Hole U1366C profiles (this study) are similar to those of Hole U1366F ) ( Figure 2); therefore, we used data from Hole U1366F rather than those of Hole U1366D (Dunlea, Murray, Sauvage, Spivack, et al., 2015; Figure 2) or the composite core of Holes U1366D and U1366F ; Figure S1 in Supporting Information S1), for stratigraphic and geochemical comparisons between this study and . This result implies that these three holes would not be stratigraphically aligned, which can explain the offset in the REY and Sc depth profiles of these cores.

Relationships Between Sample Characteristics and ΣREY Enrichment
We classified the SPG samples into seven groups on the basis of their bulk chemical compositions: (a) biogenic Ca-carbonate-rich (Ca-rich); (b) biogenic Ca-phosphate-rich (BCP-rich); (c) biogenic Si-rich; (d)  We discuss the geochemical characteristics of each group in detail below.

Samples Enriched in Biogenic Ca, P, and Si
The CaO-P 2 O 5 diagram (Figures 3a and 3b) shows clear trends in the sample data toward calcium carbonate and BCP, indicating that these two end-members strongly affected the bulk chemical composition of the samples. The relatively low REY concentrations of the samples in the first trend, or "biogenic Ca-rich" samples (in which CaO >10 wt.%), are between 42 and 343 ppm, the result of dilution of bulk ΣREY by low-ΣREY calcium carbonate (Kato et al., 2011;Nakamura et al., 2015). The depth intervals of biogenic Ca-rich samples generally coincide with intervals of nannofossil ooze to nannofossil-bearing clay ( Figure 2). In contrast, all samples with ΣREY >2,000 ppm have both CaO >5 wt.% and P 2 O 5 >3 wt.% and form a linear trend toward BCP compositions (Figures 3b-3e), implying a significant contribution of BCP. Also, REY patterns normalized to the upper continental crust (UCC; Rudnick and Gao, 2014) show negative Ce and positive Y anomalies ( Figure 4a) similar to those of BCP ( Figure 4h; Takaya et al., 2018). Therefore, BCP strongly contributed to the REY enrichment in the highly and extremely REY-rich mud in the SPG, as it did in other areas in the Pacific (J. Ohta et al., 2021;Takaya et al., 2018). For this reason, we defined samples with CaO >5 wt.% and P 2 O 5 >3 wt.% as well as ΣREY >2,000 ppm as "BCP-rich" samples. Notably, the depth intervals of BCP-rich samples at Sites U1365 and U1366 correspond to those with (a) very low sedimentation rates (<30 cm/Myr at Site U1365 and <20 cm/Myr at Site U1366) based on the Co flux age model of Dunlea, Murray, Sauvage, Pockalny, et al. (2015;Figure 2), and (b) more than 5% apatite . Because the input of materials with low REY contents (e.g., biogenic calcareous and siliceous and detrital materials) can dilute bulk REY concentrations (Kato et al., 2011), the absence of these materials is the crucial factor for accumulation of REY in sediment (Plank & Langmuir, 1998;Toyoda et al., 1990;Yasukawa et al., 2015Yasukawa et al., , 2016. In addition, because BCP is highly resistant to dissolution (Sibert et al., 2014(Sibert et al., , 2016 and has a significant capacity to host REY (Liao et al., 2019;Takaya et al., 2018;Yasukawa et al., 2022), an environment with a low sedimentation rate favors high BCP concentrations in sediment (J. Ohta et al., 2021;Toyoda et al., 1990;Yasukawa et al., 2015). Prolonged exposure of sediment to ambient seawater and porewater in such an environment could also facilitate the incorporation of REY into BCP from these sources (Elderfield & Pagett, 1986;Plank & Langmuir, 1998;Yasukawa et al., 2015Yasukawa et al., , 2016. Thus, the combination of minimal dilution by low-REY sources, BCP concentration, and enhanced incorporation of REY from ambient seawater and porewater can explain the formation of BCP-rich samples. On the Al 2 O 3 -SiO 2 diagram (Figure 3f), the samples with SiO 2 concentrations exceeding 50 wt.% form a strong trend toward chert, suggesting a considerable input of biogenic Si by sedimentation (Sa et al., 2018) or input of soluble Si from seawater during diagenesis (Murray, 1994). The implied high sedimentation rate would have led to low ΣREY values and BCP percentages by dilution (Figures 3d and 3e). The REY patterns of these samples also displayed negative Ce and positive Y anomalies similar to those of BCP (Figures 4b and 4h). In the CaO-P 2 O 5 diagram, these samples are plotted near the origin (CaO <2.2 wt.% and P 2 O 5 <1.0 wt.%) of the linear trend toward the BCP (Figure 3b). Therefore, BCP was the main REY host of biogenic Si-rich samples, as documented in siliceous sediments in the central North Pacific (Sa et al., 2018). It should be noted that these Si-rich samples were from almost the same depth interval as the samples with >30% of excess Si reported by .

Hydrothermal and Hydrogenous Samples
The Co-MnO diagram (Figure 3g) shows that the samples with very high MnO/Co ratios form a linear trend similar to that of the hydrothermal component. These same samples form a trend toward hydrothermal sediments of the East Pacific Rise (EPR) on the ΣREY-Ba and Fe 2 O 3 *-Ba diagrams (Figures 3h and 3i). Although Fe and Mn are dissolved in reductive hydrothermal fluid as Fe 2+ and Mn 2+ , they take the oxidative forms [Fe(III) and Mn(IV) oxyhydroxides] in seawater (German & Seyfried, 2013). Thus, hydrothermal Fe and Mn enter the open ocean in plumes, then are dispersed and deposited as metalliferous sediments. In addition, it is known that Ba tends to precipitate as hydrothermal barite (BaSO 4 ) near hydrothermal vents (Dymond et al., 1992;Jarvis et al., 1985;Paytan et al., 2002). Therefore, the very high MnO/Co ratios of these samples, similar to those of hydrothermal sediments, or "hydrothermal samples," reflect the input of hydrothermal Fe-Mn oxyhydroxides. Hydrothermal samples are present at Sites U1366 and U1370, at depth intervals corresponding to those where  found large proportions of Fe-Mn oxyhydroxides (22%-77%).
Previous studies have suggested that Fe-Mn oxyhydroxides can adsorb REY in seawater and pore water (Barrett & Jarvis, 1988;German et al., 1990); thus, sediment deposited near hydrothermal vents becomes enriched in REY (Kato et al., 2011;Ruhlin & Owen, 1986;Yasukawa et al., 2016;T. Zhou et al., 2020). However, the ΣREY values of the hydrothermal samples were not especially high (400-1,150 ppm; Figure 3h), even though their very high MnO, Fe 2 O 3 *, and Ba contents suggest that they were markedly affected by hydrothermal Fe-Mn oxyhydroxides (Figures 3g-3i). These observations lead us to conclude that hydrothermal Fe-Mn oxyhydroxides alone do not account for the formation of highly and extremely REY-rich mud (ΣREY >2,000 ppm). It should be noted that, on the CaO-P 2 O 5 diagram (Figure 3b), the hydrothermal samples lie near the linear trend toward BCP. We propose that because hydrothermal Fe-Mn oxyhydroxides effectively adsorb dissolved P in the water column (Feely et al., 1991), as well as REY, while they settle to the seafloor, this adsorbed P can form authigenic apatite (Ca-phosphate) by combining with ambient dissolved Ca ions (Kashiwabara et al., 2018). This authigenic Ca-phosphate may also incorporate a portion of the REY adsorbed on Fe-Mn oxyhydroxides, resulting in a change in the host mineral phase during early diagenesis (Kashiwabara et al., 2018;Takahashi et al., 2015).
The Co-MnO diagram shows another linear trend similar to that of hydrogenous Mn crusts (Figure 3g). Generally, enrichment of Co in pelagic clay is associated with hydrogenous Mn-oxides (Dunlea, Murray, Sauvage, Pockalny, et al., 2015;L. Zhou & Kyte, 1992; and references therein). Hydrogenous Co dissolved in seawater is scavenged onto the surfaces of Mn-oxide particles through ionic force and incorporated due to oxidation of Co(II) to Co(III) by highly oxidative δ-MnO 2 (Takahashi et al., 2007). Therefore, Co from seawater accumulates constantly in deep-sea sediments containing hydrogenous Mn-oxides and may reach high concentrations in bulk sediment where sedimentation rates are low (Dunlea, Murray, Sauvage, Pockalny, et al., 2015;L. Zhou & Kyte, 1992). Because the main source of Mn is hydrothermal vents and because Co in seawater precipitates at a roughly constant rate corresponding to the sedimentation rate, we distinguished hydrogenous and hydrothermal deposits on the basis of the MnO/Co ratio: MnO/Co > 0.05 for hydrothermal samples, and MnO/Co < 0.0135 for the samples on the linear trend to hydrogenous Mn crust in the Co-MnO diagram (Figure 3g). Even though Co in deep-sea sediments may be of detrital origin (Dunlea, Murray, Sauvage, Pockalny, et al., 2015), the samples with low MnO/Co ratios plot on the mixing curve between PAAS and South Pacific Mn crust (Figure 3g), which is of hydrogenous origin . The very low Co content in PAAS means that any excess Co in these samples is presumably of hydrogenous origin, and thus we refer to the samples plotting near this trend as "hydrogenous samples" (Figure 3g).
The REY patterns of the hydrogenous samples display both positive and negative Ce anomalies (Figures 4d and 4e). The positive Ce anomalies arise at the deep-sea sediment surface, where Ce is oxidized from Ce(III) to Ce(IV) and accumulates on solid Mn-oxide phases selectively because it has a larger distribution coefficient than other trivalent REY species (Kashiwabara et al., 2018;A. Ohta & Kawabe, 2001;Takahashi et al., 2000Takahashi et al., , 2007Takahashi et al., , 2015.  Figure 3j) shows a well-defined trend in these samples between BCP and hydrogenous Mn-oxides that includes Ce/Ce* values above and below unity. Therefore, hydrogenous Mn-oxides affect the bulk REY compositions of these samples, as well as BCP. deep-sea seawater from J. Zhang and Nozaki (1996) and Nozaki and Alibo (2003); hydrothermal Fe-Mn sediment (EPR, carbonate-free basis) from Barrett and Jarvis (1988) Figure 3h). Their Ba contents exceed those of terrigenous end-members such as PAAS (Taylor & McLennan, 1985), North American Shale Composite (NASC; Gromet et al., 1984), and UCC (Rudnick & Gao, 2014), in which the Ba content is approximately 650 ppm. They have positive Ce anomalies (Ce/Ce* > 1; Figure 3j) and low Fe contents (Fe 2 O 3 * = ∼6.5-8.4 wt.%; Figure 3i), which are comparable to that of PAAS, suggesting that the influence of a hydrothermal component is small. An alternative explanation for the excess Ba content in the samples is barite input associated with high biological productivity. Because excess Ba can be associated with marine barite from decomposing organic matter in the open ocean, Ba contents are considered a useful indicator of paleoproductivity (Carter et al., 2020;Dymond et al., 1992;Eagle et al., 2003;Paytan & Griffith, 2007). In support of this interpretation, the depth intervals of these samples at Sites U1369 and U1370 have sedimentation rates more than twice as high as the depth intervals of other hydrogenous samples (Figure 2; Dunlea, Murray, Sauvage, Pockalny, et al., 2015).

Samples of Mixed Origin
Some of the samples containing 500-2,000 ppm of ΣREY cannot be categorized into the group noted above. They plot on the trend toward BCP on the CaO-P 2 O 5 diagram (Figure 3b), between the hydrogenous and hydrothermal trends on the Co-MnO diagram (Figure 3g), and in the region of moderate to high Fe contents (Fe 2 O 3 * = 7.9-42.2 wt.%; Figures 3i and 3k). These compositions can be attributed to a combination of BCP, hydrogenous Mn-oxides, and hydrothermal Fe-Mn oxyhydroxides (i.e., "mixed samples"). Specifically, on the Fe 2 O 3 *-Ba diagram (Figure 3i), several samples have more than 20 wt.% Fe 2 O 3 * but less than 1,000 ppm Ba.
On the Co-MnO diagram (Figure 3g), they lie in the area of Co >200 ppm, unlike hydrothermal samples with Co <100 ppm. These facts suggest that the samples with Fe 2 O 3 * >20 wt.% and Ba <1,000 ppm were deposited in an area with a low sedimentation rate. Because their chemical compositions (high Fe 2 O 3 * and Co contents) reflect the influence of both hydrogenous and hydrothermal material; we call them "hydrogenous-hydrothermal samples." The reason for the relatively low Ba content of these samples could be because they were deposited in the distal area from hydrothermal vents, where most hydrothermal barite is deposited. More noteworthy is that the ΣREY values of hydrogenous-hydrothermal samples (ΣREY = 1,000-2,000 ppm; Figure 3k) are higher than those of hydrothermal samples (ΣREY = 400-1,150 ppm). This difference may reflect the fact that Fe-Mn oxyhydroxides can adsorb REY from ambient water after their deposition. The higher Co concentrations in the hydrogenous-hydrothermal samples likewise indicate that their sediment could have been in contact with seawater for longer than the hydrothermal samples. Long exposures favor the accumulation of BCP and its incorporated REY (Elderfield & Pagett, 1986;Plank & Langmuir, 1998;Yasukawa et al., 2015), which would result in higher REY contents than those of the hydrothermal samples. Long exposures are also supported by the very low estimated accumulation rates (<30 cm/Myr) of the hydrogenous-hydrothermal sample interval at Site U1366 (Figure 2; Dunlea, Murray, Sauvage, Pockalny, et al., 2015).

Relationships Between Sample Characteristics and Sc Contents
The Sc-ΣREY diagram displays linear trends toward BCP, PAAS, and carbonate end-members (Figure 3l). The BCP-rich samples contain the highest Sc concentrations (>50 ppm) of the samples as well as high ΣREY levels (>2,000 ppm). Considering that BCP is the main host of both Sc and REY in the highly REY-rich mud near Minamitorishima Island , we infer that Sc concentrations in the BCP-rich samples mainly reflect the influence of BCP. Hydrogenous, hydrothermal, and mixed samples are also located on the mixing trend between PAAS and BCP, which implies that the addition of BCP to continental components controlled their Sc concentrations, as it did their REY concentrations. The samples with low Sc and REY concentrations in the Sc-ΣREY diagram correspond to the biogenic Ca samples (nannofossil ooze) at Holes U1367B and U1368B, and to the biogenic Si samples from the radiolarian-bearing interval at Hole U1365A (D'Hondt et al., 2011). These biogenic components dilute the Sc and REY contents of bulk sediment and markedly lower their resource potential.

Changes in REY and Sc Hosts and Depositional Environments in the South Pacific Gyre
We have shown that in our samples from the SPG, BCP contributes more to bulk REY concentrations than hydrothermal and hydrogenous materials, whereas biogenic Ca and Si dilute REY concentrations. We next consider the influence of depositional environments on REY and Sc enrichment, using the sample classifications of the SPG cores, specifically Holes U1370D, U1365A, and U1366C, the three longest cores in this study. The major change within these cores is the transition from hydrothermal to hydrogenous samples: hydrothermal samples are at the bottom of Holes U1366C and U1370D, and hydrogenous samples are in the upper part of all three holes ( Figure 2).
This compositional change may correspond to the gradual change in depositional environment as oceanic plate motion takes the core sites from areas near an oceanic ridge to deep-sea basins distant from hydrothermal vents (see Figure 1 of ), resulting in a decrease in the sedimentation rate. The hydrogenous samples in the three cores overlie various-type sediments: biogenic Si (Holes U1365A and U1366C); biogenic Ca (U1370D); hydrogenous-hydrothermal (Holes U1365A and U1366C); and other mixed samples (Holes U1365A and U1370D). The presence of the latter two types of mixed samples implies that these cores were located in areas affected by both hydrogenous and hydrothermal fluxes, where multiple components contributed to REY enrichment. In the present Pacific Ocean, such an area might be represented by Hole U1367B, which contains hydrogenous-hydrothermal samples near the core top. This occurrence is consistent with the "transition zone" proposed by Kashiwabara et al. (2018), the area around 0°-30°S and 120°-150°W (including Hole U1367B) that represents a transition from hydrothermal sediments to REY-rich mud.
We documented Si-rich samples at Holes U1365A and U1366C. These can be compared to modern sediments that are mainly composed of biogenic Si (e.g., diatom or radiolarian ooze), which are currently distributed in the equatorial and mid-latitude North Pacific and the Southern Ocean (Dutkiewicz et al., 2015(Dutkiewicz et al., , 2016. The growth and compositions of siliceous planktons depend on the sea-surface temperature and salinity (Anderson et al., 1989;Dutkiewicz et al., 2016;Matsuoka & Anderson, 1992). Therefore, one possibility is that Holes U1365A and U1366C experienced sea-surface salinity and temperature conditions that differed from those of the other sites. Another possibility is that the Si-rich samples reflect an increase in biogenic Si fixation at the time represented by that depth interval.  suggested that the intervals of high excess Si mass accumulation rate at Sites U1365 and U1366 could be attributed to increased deposition of biogenic Si in the form of frustules or authigenic mineral phases. Although we cannot eliminate either of these possibilities, it is safe to say that the samples indicate a lithological transition from chert to pelagic clay, because the Si-rich samples in Hole U1365A consist of metalliferous clay bearing radiolarians that overlies porcellanite and chert (see Section 2.1). In addition, because these samples are overlain by mixed samples and hydrogenous samples, their deposition could have occurred during the transition from hydrothermal to hydrogenous samples defined in this study.
Likewise, the deposition of biogenic Ca is generally controlled by the paleodepth of a site relative to the carbonate compensation depth (CCD; Pälike et al., 2012;Van Andel, 1975). Indeed, Sites U1367 and U1368 are the SPG core sites closest to the East Pacific Rise (Figure 1 of ) and thus represent relatively shallow depths. However, Alvarez Zarikian (2015) reported that ostracods, benthic organisms bearing calcareous shells, decreased in abundance during the late Oligocene warming and the late Miocene carbonate crash, as well as at the time that Sites U1367 and U1368 subsided below the CCD. Given that foraminifers have similar calcareous shells, the paleodepth as well as environmental and biotic events could have influenced the biogenic Ca deposition observed at Holes U1367B, U1368B, and U1370D ( Figure 2). In contrast, the position of BCP-rich samples in the sediment columns varies among the SPG cores: they occur within the hydrogenous samples in Hole U1365A, between the hydrogenous-hydrothermal and hydrothermal samples in Hole U1366C, and at the top of the core in Hole U1367B (Figure 2). These variations imply that the deposition of BCP and the enrichment of REY occurred under conditions that were dominated by slow sedimentation rates, but complicated by changes in various paleoceanographic conditions other than water depth, such as primary productivity, hydrothermal activity, and eolian dust supply.

Potential of REY Resources
Based on their MCCDs and total amounts of REY (M REY ; Table 2 and Table S2 in Supporting Information S2) Table 2) due to dilution by carbonate ( Figure 2). Although the main lithologies of Holes U1369B and U1370D are metalliferous zeolitic pelagic clay, zeolitic metalliferous pelagic clay, and metalliferous clay (see Section 2.1), they also have low MCCs. At Hole U1369B, the MCCD (17.46 mbsf) is comparable to that of Hole U1366C, but its MCC is low (C REY = 588 ppm). The MCCD of Hole U1370D is considerably deeper than that at the other sites (C REY = 527 ppm at MCCD = 69.52 mbsf). The low MCCs in Holes U1369B and U1370D reflect the deposition of thick hydrogenous samples with low REY concentrations or the absence of BCP-rich and hydrothermal samples.
Sites U1366C and U1365A contain the largest and second-largest REY amounts above the MCCD, respectively. These are 7.8 times and 6.7 times as large as REY amounts documented in the western central Pacific ( Because the MCCD of Hole U1367B (0.26 mbsf) is much shallower than the total thickness of elevated REY concentrations in zeolitic metalliferous pelagic clay to metalliferous clay (Figure 2), we also calculated the possible amount of total REY at Hole U1367B from the seafloor to 6.50 mbsf. The total REY amount above 6.50 mbsf at Hole U1367B could be as much as 12,106 t-REO per 1 km 2 (Table 2). This value is slightly larger than the REY amounts above 10 mbsf in the B1 area near Minamitorishima Island (11,092 t-REO; Takaya et al., 2018), making it the most promising area to date for REY resources among studied sites in the South Pacific Ocean.

Potential of Sc Resources
As was the case for REY, the MCCs for Sc of Holes U1365A, U1366C, and U1367B are notably higher than for the other three holes in this study (Table 2 and Table S2  The resulting estimates of total leachable Sc above the MCCD (M Sc ) are provided in Table 2. It is worth noting that Hole U1365A has different MCCDs for REY and Sc, because the depth profiles of C REY_k and C Sc_k ( Figure 5) have two local maxima at 21.60 mbsf and 33.96 mbsf; the MCCD of REY reflects the former, whereas that of Sc reflects the latter. When we estimate the Sc potential from the MCCD of REY, Hole U1365A has 422 t-Sc at 21.60 mbsf rather than 700 t-Sc at 33.96 mbsf. In addition, when we increase the MCCD at Hole U1367B to 6.5 mbsf instead of 0.26 mbsf (see Section 4.5.1), the total amount of leachable Sc increases from 6 t-Sc to 102 t-Sc.
Our results indicate that Holes U1365A (above 21.60 mbsf), U1366C, U1367B (above 6.5 mbsf), and U1369B have considerable resource potential for Sc, exceeding that of the western central Pacific Ocean (31 t-Sc at P161; J. Ohta et al., 2021) by 13.6, 12.9, 3.3, and 2.0 times, respectively; and exhibiting values of 3.9, 3.0, 0.95, and 0.58 times that of the most promising area around Minamitorishima Island . However, as in the case of REY, the MCCDs at Holes U1365A and U1366C are much deeper than those around Minamitorishima Island  and in the western central Pacific Ocean (J. Ohta et al., 2021) and thus would require additional technical and operational considerations. In contrast, Hole U1367B, with its high Sc resource potential in the uppermost interval (0-6.5 mbsf), appears to be a particularly promising site for a Sc resource in the SPG.

Conclusions
This study conducted major-and trace-element analyses on bulk sediment samples collected at short intervals from six cores obtained during IODP Expedition 329 in the South Pacific Gyre. The data revealed the following findings.
1. Highly REY-rich mud layers (ΣREY >2,000 ppm) exist in Holes U1365A, U1366C, and U1367B, and Sc-enriched layers (Sc >50 ppm) are present in Holes U1365A, U1366C, and U1370D. The ΣREY values are not affected by the pore-water sampling process applied during Expedition 329. 2. The SPG sediments can be categorized on the basis of bulk chemistry into seven groups: (a) biogenic Ca-rich, (b) BCP-rich, (c) biogenic Si-rich, (d) hydrothermal, (e) hydrogenous, (f) mixed hydrogenous-hydrothermal, and (g) other mixed samples. The cores generally exhibit a change from the hydrothermal to hydrogenous samples stratigraphically upward through the succession, corresponding to plate movement from near the mid-ocean ridge to deep-sea basin settings distant from hydrothermal vents. Between the hydrothermal and hydrogenous samples are transitional layers consisting of biogenic Si-rich, mixed, and hydrogenoushydrothermal samples. The characteristics of BCP-rich samples suggest that they formed at low sedimentation rates in complex depositional environments that may reflect paleoceanographic changes or biological events, whereas biogenic Ca-rich samples may be related to paleoceanographic events as well as the paleodepth relative to the local CCD. 3. On the basis of the total REY amounts above the MCCDs, Holes U1366C, U1365A, and U1367B have high resource potentials for REY in this order: 26,512 t-REO, 22,608 t-REO, and 12,106 t-REO. Similarly, Sc resource potentials are high at Holes U1365A, U1366C, U1367B, and U1369B, reaching 422 t-Sc, 451 t-Sc, 102 t-Sc, and 62 t-Sc above their respective MCCDs, respectively. However, the MCCDs of REY and Sc at Sites U1365A and U1366C are two to five times deeper than those of other promising areas in the western North Pacific and western central Pacific; thus, a simple comparison of potential resources among different sites is not straightforward. Hole U1367B, with high REY and Sc potentials in the uppermost 6.5-m interval, appears to be one of the most promising REY-rich mud sites found to date.

Conflict of Interest
The authors declare no conflicts of interest relevant to this study.

Data Availability Statement
The data used in this study were stored in the "Mendeley Data" database (https://doi.org/10.17632/ fjjy8r3xmw.1). Figure 5. Changes in C REY_k (ppm) and C Sc_k (ppm) through the SPG cores.