# Water Reservoirs in the Lower Mantle Beneath Northeastern Asia

Shiwen Li, Junhao Guo, Yanhui Zhang, Yabin Li, Zikun Zhou, Wenliang Xu, Geeth Manthilake, Aihua Weng

► To cite this version:

Shiwen Li, Junhao Guo, Yanhui Zhang, Yabin Li, Zikun Zhou, et al. Water Reservoirs in the Lower Mantle Beneath Northeastern Asia. Geophysical Research Letters, 2025, 52 (8), pp.e2024GL114180. 10.1029/2024gl114180. hal-05034956

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Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2024GL114180

**Key Points:**

* An electrical conductivity model reveals a step-like structure related with water crossing the 660-km interface beneath northeastern Asia

* Water may store in the lower mantle mainly in the form of hydrated stishovite and liebermannite

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

W. Xu and A. Weng,
xuwl@jlu.edu.cn;
wengah@jlu.edu.cn

**Citation:**

Li, S., Guo, J., Zhang, Y., Li, Y., Zhou, Z., Xu, W., et al. (2025). Water reservoirs in the lower mantle beneath northeastern Asia. *Geophysical Research Letters*, *52*, e2024GL114180. https://doi.org/10.1029/2024GL114180

Received 18 DEC 2024
Accepted 20 MAR 2025

**Author Contributions:**

**Conceptualization:** Shiwen Li,
Wenliang Xu, Aihua Weng

**Data curation:** Shiwen Li, Yanhui Zhang,
Geeth Manthilake

**Funding acquisition:** Shiwen Li,
Wenliang Xu, Aihua Weng

**Investigation:** Shiwen Li, Yabin Li,
Zikun Zhou

**Methodology:** Shiwen Li,
Geeth Manthilake

**Project administration:** Wenliang Xu,
Aihua Weng

**Software:** Shiwen Li, Junhao Guo,
Yanhui Zhang

**Supervision:** Wenliang Xu, Aihua Weng

**Visualization:** Shiwen Li, Junhao Guo,
Yanhui Zhang

## Water Reservoirs in the Lower Mantle Beneath Northeastern Asia

<figure><img src="image_2.png" /></figure>

Shiwen Li<fnref n="1" />, Junhao Guo<fnref n="1" />, Yanhui Zhang² ID, Yabin Li<fnref n="1" /> ID, Zikun Zhou<fnref n="1" /> ID, Wenliang Xu³ ID, Geeth Manthilake⁴ ID, and Aihua Weng<fnref n="1" /> ID

**Abstract** Water in the mantle controls mantle convection and chemical transport processes within Earth's interior. However, whether the water in the lower mantle is primordial or recycled remains an open question. Here we report an electrical structure consisting of two stepped high-conductivity anomalies crossing the 660-km interface beneath northeastern Asia. The sheet-like first-step conductor locates in the mantle transition zone that can be best interpreted as hydrated stagnant Pacific slab, whereas the funnel-like second-step anomaly occurs in the uppermost lower mantle that is inferred to be a water reservoir replenished by the collapsed stagnant slab. The pivotal mechanism of this water transport process could be the dehydration reaction of dense hydrous magnesium silicates and the rehydration of stishovite and liebermannite in the transition zone. Our findings provide evidence for recycled lower-mantle water reservoirs, and together with seismological and mineralogical studies, illustrate the deep-mantle water plumbing system operated by cold slab subduction.

**Plain Language Summary** Water plays a critical role in modulating the dynamics of Earth because it reduces mantle viscosity and favours extensive melting. Although the mantle transition zone is considered a barrier to the sinking of water into the lower mantle, subduction slabs are able to transfer water from the surface to the deep mantle down to the core-mantle boundary. Here, we obtain a mantle electrical conductivity model converted from geomagnetic data in northeastern Asia. We identify two sheet-like step-wise conductive anomalies across the 660-km discontinuity. We interpret the anomaly in the mantle transition zone as a water channel in terms of the hydrated stagnant Pacific slab, whereas the anomaly in the uppermost lower mantle is a water reservoir. We argue the dehydration of dense hydrous magnesium silicates and the rehydration of stishovite and liebermannite in the mantle transition zone are responsible for the water transport into the lower mantle across the mantle transition zone from the surface.

### 1. Introduction

Large contrasts in water storage capacity between ringwoodite and bridgmanite at the 660 km depth discontinuity, along with the extremely low water storage capacity of bridgmanite, is supposed to construct the mantle transition zone (MTZ) to be a water filter (Bercovici & Karato, 2003; Fei et al., 2017; Schmandt et al., 2014), forming a water barrier preventing the input of water deeper into Earth, leaving a nominally dry lower mantle (Hirschmann, 2006). However, the discovery of ice-VII in diamonds from the lower mantle (Tschauner et al., 2019) evidences the possible storage of water in the lower mantle. Although primordial water in the deep mantle has been substantiated (Hallis et al., 2015), the continuous replenishment of deeper reservoirs of oceanic island basalt (Hauri, 2002) and the existence of recycled water in deep mantle (Timmerman et al., 2019) suggest that the water has broken through the MTZ barrier and been transported to lower-mantle depths. This process has been experimentally illustrated by a number of high-temperature-high-pressure (HTHP) studies that prefers cool subducting slabs acting as bridges efficiently transporting water across the MTZ, thus allowing closure of the water cycle (Lin et al., 2019; Ohtani, 2020). However, there is a lack of geophysical observations capable of supporting this hypothesis.

The western Pacific plate is subducting below the islands of Japan at a rate of ~10 cm/yr. Deep-focused earthquakes (Engdahl et al., 2020) suggest the slab represents a water carrier that continuously releases fluids along its subducting pathway and thereby supplies water to the upper mantle (Halpaap et al., 2019; Zhao et al., 2009). This water accounts for the relatively high water content (up to 3 wt.%) of Cenozoic intraplate basalts

<footnote n="1">¹College of Geo-Exploration Science and Technology, Jilin University, Changchun, China, ²School of Safety Engineering and Emergency Management, Shijiazhuang Tiedao University, Shijiazhuang, China, ³College of Earth Sciences, Jilin University, Changchun, China, ⁴Laboratoire Magmas et Volcans CNRS, IRD, OPGC, Université Clermont Auvergne, Clermont-Ferrand, France</footnote>

<footnote>© 2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
in northeastern Asia (Xia et al., 2019). In the MTZ, the presence of a high-velocity zone ($\delta V > 3\%$) in seismic imaging shows that the slab becomes stagnant and extends westward beneath NE Asia for ~1,000 km (Fukao et al., 2001; Fukao & Obayashi, 2013; Tao et al., 2018; Zhao et al., 2009). The flat portion of the slab is inferred to contain up to ~1 wt.% water (Kelbert et al., 2009). However, the transition-zone stagnation of the Pacific slab is transient and generally lasts for less than 50 My (X. Liu et al., 2017; Mao & Zhong, 2018). This implies that parts of the slab must have collapsed into the lower mantle. Northeastern Asia, therefore, is an ideal area in identifying lower-mantle water reservoirs and exposing their trans-MTZ plumbing system.

Existence of water in mantle minerals can increase their electrical conductivity (Gardes et al., 2014; Karato, 2011; H. Liu et al., 2021; Yoshino et al., 2008). Electromagnetic induction in the Earth that can detect these conductivities thus represents a valuable method for identifying water storage zones in Earth's deep interior (Kelbert et al., 2009). As the electromagnetic field used by the method has periods as long as 100 days, the possible conductive bodies at depths more than 1,000 km into Earth's interior can be sounded (Li et al., 2020; Semenov & Kuvshinov, 2012). Here, we use the method to recover the electrical structure at depths of the MTZ and upper lower mantle beneath Northeastern Asia. Our electrical conductivity features identify a water reservoir in the lower mantle, and together with seismic tomography data and very recent HTHP results, can gain insights into the mechanisms transporting water across the MTZ into the lower mantle by cold subduction slabs.

## 2. Methods

### 2.1. Data and Materials

Geomagnetic depth sounding (GDS) is a passive electromagnetic method that can detect the electrical conductivity in and around the mantle transition zone (Kelbert et al., 2009; Li et al., 2020, 2023; Semenov & Kuvshinov, 2012; Zhang et al., 2022). With the $P_1^0$ assumption that the inducing current system in magnetosphere can be described by a ring current around the geomagnetic equator (Banks, 1969), C-response of GDS for observatories in low- and mid-latitude can be defined as

$$C(\omega) = -\frac{a_0 \tan \theta H_r(\omega)}{2 H_{\theta}(\omega)}$$

to link the vertical ($H_r$) and horizontal north-oriented ($H_{\theta}$) components of the geomagnetic field with mantle conductivity, where $\omega$ is angular frequency, $\theta$ is geomagnetic colatitude, and $a_0$ is Earth's radius. Here, to estimate the responses we adapted the $L_1$-norm smoothing method (Zhang et al., 2020), which is based on the BIRRP software (Chave & Thomson, 2004). We collected the records of geomagnetic field at more than 100 observatories in northeastern Asia, not only from the World Data Centre but also from the Geomagnetic Network of China. After careful selection and processing, we obtained 41 observatories located in low- and mid-latitude (Figure 1) with stable C-responses for further consideration. The distribution of the selected observatories is much denser compared with previous studies (Kelbert et al., 2009; Semenov & Kuvshinov, 2012; Sun et al., 2015; Zhang et al., 2020) due to the added collection from Geomagnetic Network of China. These observatories constitute a dense GDS network with an average 250-km station spacing. The instruments captured time series of magnetic fields at the surface over 15 years. We processed these data using a particular robust method (Zhang et al., 2020) and finally obtained high-quality C-response curves covering periods from 1.3 to 113.8 days (Figure S1 in Supporting Information S1).

### 2.2. Modelling and Inversion

The numerical modelling of GDS is solving by the conservative staggered-grid finite difference method (Uyeshima & Schultz, 2000), and the L-BFGS inversion method is used to convert the observed C-responses to 3-D electrical conductivity model (Li et al., 2020). We parameterized Earth using curved rectangular prisms in 13 layers covering the core (Kelbert et al., 2009), with the topmost 12.65-km-thick shell containing oceans and lands. This shell is set to consider the large conductivity contrast between ocean and land that has been known from the conductance data from https://marineemlab.ucsd.edu/~steve/projects/MDAT/sedmap.html website and therefore should not be changed during inversion. In contrast to global GDS inversion, where the horizontal grid is uniform, in our inversions the study area, with its dense coverage of observatories, is covered by a dense grid with a cell size of about $3^\circ \times 3^\circ$. This cell size increases progressively outside the study area, reaching about $10^\circ \times 10^\circ$ to cover the whole
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Geomagnetic observatories in northeastern Asia. Circles indicate geomagnetic stations (more details in Table S1 in Supporting Information S1), with circle size inversely proportional to data-fitting quality measured by root-mean-square (RMS). Red, blue, and white colours indicate a good data fitting at small, large, and whole periods. The dashed black rectangle outlines the imaging area in Figure 2, and the solid black line indicates the location of the cross-section in Figure 2c. Thin grey lines are depth contours (in km) of the Wadati-Benioff zone (Zhao et al., 2009). NSGL: north-south gravity lineament; CBV: Changbaishan volcano.</figcaption></figure>

Earth (Figure S2 in Supporting Information S1). The initial conductivity of each prism is from ref (Kelbert et al., 2009) and was inverted directly to obtain a sufficiently high conductivity contrast and resolution (Li et al., 2020). A smoothing strategy is used to balance the conductivity variations between neighboring prisms. At the phase transition boundaries (410, 520, and 660 km), model conductivity is allowed to jump vertically (Kelbert et al., 2009). We gradually reduced initial regularization parameter from an initial value of 1.0 to a relatively small value. The inversion is triggered by a set of optimized parameters (Kelbert et al., 2009; Semenov & Kuvshinov, 2012), and produces the preferred model with a normalized root-mean-square (RMS) misfit of approximately 1.2 (Figure S3 in Supporting Information S1).

Comparing to previous studies that used part of our observations to obtain the large-wave-length spatial distribution of mantle conductivity beneath the study area (Kelbert et al., 2009; Li et al., 2020; Semenov & Kuvshinov, 2012), our preferred model, owing to incorporating denser observatory arrangement, utilizing heterogeneously fine grid, and directly updating cell-conductivity, has an enhanced resolution of as high as 500 km × 500 km (Figures S4–S7 in Supporting Information S1), thus exposing more detailed features of the mantle conductivity beneath the study area (Figure 2). The reliability of observed electrical structure had been verified by several numerical experiments (Text S1 and Figures S9–S11 in Supporting Information S1).

## 3. Results

The most striking feature in the preferred electrical conductivity model is a two-stepped-stair high-conductivity anomaly that crosses the 660-km discontinuity beneath Northeastern Asia (Figure 2). The upper-step anomaly is located in the lower MTZ (hereafter “anomaly G”) and better resolved in the region of a larger anomaly discovered previously (Kelbert et al., 2009; Zhang et al., 2020) (Figure 2a). This anomaly occurs in correspondence with the 600-km depth of the Wadati-Benioff zone (Hayes et al., 2018; Zhao et al., 2009). Off the belt front, it stretches ~800 km westward beneath the Korean Peninsula and the Yellow Sea, reaching the eastern margin of the China mainland, where it touches a low-conductivity zone beneath the north-south gravity lineament (NSGL). Anomaly G thickens to >140 km above the 660-km discontinuity and has a conductivity reaching ~1.0 Sm⁻¹, about 0.5 log units higher than the global average model (Kelbert et al., 2009).

The lower-step conductivity anomaly (“anomaly H”) occurs in the uppermost lower mantle mainly below eastern China, with its western edge spatially close to the NSGL while the eastern edge touching anomaly G (Figure 2b). Anomaly H shapes in an ellipse whose conductivity is ~2.5 Sm⁻¹ in the central area measuring 800 × 600 km. This conductivity is significantly greater than the average mantle (1 Sm⁻¹) (Kelbert et al., 2009). Vertically, anomaly H is funnel shaped and extends 500 km in the lower mantle. We note that the lower portion of this feature, which is located at a depth greater than 900 km, may not be fully constrained by our data (Figures S6 and S7 in Supporting Information S1) due to the screening effect from the top of the anomaly. Taken together, anomalies G and H form a structure that seems to distribute high-conductivity materials into a funnel sitting in the lower mantle (Figure 2c). The co-occurrence and geometry of the anomalies are verified to be well constrained (Figures S8–S13 in Supporting Information S1). While anomaly G has been revealed by a previous study (Sun et al., 2015), anomaly H, together with its spatial connection to anomaly G, is first discovered by this study.

## 4. Discussion

### 4.1. Hydrous Stishovite and Liebermannite in the Lower Mantle

High-conductivity anomalies in the mantle can be explained by variations in temperature, fluid content, and chemical composition (Karato, 2011; Yoshino, 2010; Yoshino et al., 2008). The geodynamic setting of the study area (Fukao et al., 2001), laboratory results from HTHP experiments (Lin et al., 2019), and theoretical studies of HP mineral physics (Ohtani, 2020) can provide useful data to better identify the physical and/or chemical parameters responsible for these anomalies. High temperatures could be the first-order approximation in interpreting
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Electrical conductivity images for the region beneath northeastern Asia. (a, b) show slices of conductivity in the lower-mantle transition zone (520–660 km) and the uppermost lower mantle (660–900 km). Blue polygons denote zones of high seismic velocity (δV<sub>P</sub> > 1.5%) (Fukao et al., 2001). G and H signify the identified high-conductivity anomalies. At the top-right corner, σ<sub>0</sub> is the conductivity of background model (Kelbert et al., 2009) and X is the maximum value of color bar. (c) Cross-section of conductivity along transect W–E in Figure 1. Red- and blue-shaded triangles are the positive and negative amplitude phases of seismic receiver functions (Wang et al., 2020). Contours represent the increment in seismic S-wave velocity (Tao et al., 2018). White circles represent deep earthquakes from the ISC–EHB catalogue (Engdahl et al., 2020). EC: East China; YS: Yellow Sea; KP: Korean Peninsula; JS: Japan Sea. (d) Electrical conductivity–depth profiles across the 660 km discontinuity. Conductivity values of anomalies G and H are delimited by brown rectangles, with yellow lines indicating their average values. The hatched blue rectangle shows the conductivity beneath northeastern Asia from ref (Karato, 2011). Thick black lines specify the global average conductivity (Kelbert et al., 2009). Thin black lines are conductivity values of bridgmanite at different temperatures (Sinmyo et al., 2014). Blue lines represent the variation in bulk conductivity with water content of stishovite at specified temperatures (numbered beside lines). Shaded bands constrain possible water contents of stishovite in the MTZ and uppermost lower mantle according to the observed conductivity. The inset shows the mineral compositions of slab used in discussing anomalies G and H. Mineral colours schematically correspond to water contents. Numbers denote volume percent of each mineral in the slab. See details in Texts S2–S4 in Supporting Information S1.</figcaption></figure>

the high conductivity. For instance, anomaly H in the lower mantle can be ascribed to be (Al, Fe)-bearing
perovskite with a temperature excess of about 100–500 K compared with normal mantle (Figure 2d) (Sinmyo
et al., 2014). However, this explanation does not fully agree with the observed seismic-wave velocity increments
(Fukao & Obayashi, 2013; Tao et al., 2018), which instead require temperatures not exceeding those of the mantle
at the depth of anomaly H (Karato & Karki, 2001). A primitive mantle with enriched water should not account for
the anomaly as perovskites are water incompatible (Hirschmann, 2006).

Alternatively, compositional variation could be a best contributor in forming the anomaly. According to the high velocities similar to those recorded corresponding to anomaly H, the variation may result from subducted slab material (Waszek et al., 2018). The continental and oceanic crustal component of this slab (Table S2 in
Supporting Information S1) should have not completely transformed into CaSiO₃ perovskite (Gréaux et al., 2019), most likely involving 7 vol.% stishovite and 3 vol.% liebermannite (Lin et al., 2019; Manthilake et al., 2020) (Text S2 in Supporting Information S1).

The latest conductivity models from laboratory show that presence of water can enhance dramatically the conductivity of stishovite and liebermannite (Manthilake et al., 2020; Yoshino et al., 2014) (Figure S1b in Supporting Information S1). To reproduce the observed high conductivity, it is required to add ~3–5 wt.% and 0.1 wt.% water to stishovite and liebermannite, respectively (Manthilake et al., 2020; Yoshino et al., 2014) (Figure 2d, and Texts S2 and S4 in Supporting Information S1). Although the stability of hydrous stishovite in the deep mantle remains debated (Takaichi et al., 2024), most experimental studies suggest that stishovite possesses a high water storage capacity at HTHP places (Lin et al., 2019; Nisr et al., 2020), enabling it to accommodate significant water contents. The presence of hydrous stishovite can also get supported by the pronounced seismic attenuation in the region around anomaly H (Lawrence & Wysession, 2006). Here, we rule out the contribution from the dense hydrous magnesium silicates (DHMSs), because the temperature (1600–1700 K estimated from the velocity increment of up to 1.5% (Karato, 2011)) has exceeded the stable field of these minerals (Yang & Faccenda, 2020), and they should have transformed into bridgmanite, a water-incompatible less-conductivity mineral (Yoshino, 2010). We also rule out high FeO in the subducted oceanic crust even its presence could increase electrical conductivity a lot (Karato & Wang, 2013), since increased FeO content means a velocity drop that is incompatible to the observations. As a result, we conclude that anomaly H represents a water reservoir in the lower mantle, with stishovite and liebermannite being the hosts of the water.

## 4.2. Hydrating Stishovite and Liebermannite in the MTZ

Anomaly G most likely is the stagnant portion of the Pacific slab because it spreads from the front of the Wadati-Benioff zone (Engdahl et al., 2020; Hayes et al., 2018; Zhao et al., 2009) that represents the subducting Pacific slab and overlaps a high seismic-wave-velocity zone that reflects the cold stagnant Pacific slab in the MTZ (Fukao et al., 2001; Fukao & Obayashi, 2013; Tao et al., 2018) (Figures 2b and 2c). This slab may contain DHMSs, as well as stishovite and liebermannite (Manthilake et al., 2020; Yoshino et al., 2014). DHMSs have high electrical conductivity (Guo & Yoshino, 2013), but DHMSs alone cannot account for the observed conductivity (Figure S1a in Supporting Information S1). Considering the temperature distribution in the slab, we estimate that the conductivity requires stishovite and liebermannite containing approximately 1 wt.% and 0.1 wt.% water, respectively (Figure 2d; Texts S2 and S3 in Supporting Information S1). As stishovite and liebermannite are almost dry before entering the MTZ (Lee et al., 2021; Ohtani, 2020; Van Keken et al., 2011), their water enrichment implies a subsequent rehydration, which most likely occurs following the dehydration of DHMSs in regions close to the boundary layers of the stagnant slab, where the stability field of DHMSs has been destroyed (Yang & Faccenda, 2020). This provides aqueous fluids that may generate a locally supersaturated zone in the slab. Because of positive buoyancy, the water favors upwelling possibly through the mechanism such as Rayleigh-Taylor instability (Iwamori & Nakakuki, 2013). The upwelling gives stishovite and liebermannite a chance to undergo rehydration as the water infiltrate through the crust (Lin et al., 2019; Manthilake et al., 2020), however, deprives the underlying slab lithosphere mainly in the form of ringwoodite of the opportunity for rehydration. Furthermore, due to the detachment of the oceanic crustal material (Wang et al., 2020), stishovite and liebermannite may have mixed with DHMSs, also promoting the rehydration of stishovite and liebermannite. We rule out the possibility of this water absorbing from the MTZ itself, as diffusion is inefficient for transporting water in Earth's interior (Freitas & Manthilake, 2019).

## 4.3. Cool Slab Pumping Surface Water Into the Lower Mantle

In contrast to steep subduction, where the slab directly penetrates the MTZ (Goes et al., 2017; Hayes et al., 2018; Sigloch et al., 2008), the spatial structure defined by anomalies G and H, if combined with the subducting slab imaged from earthquake data (Engdahl et al., 2020), illustrates a textbook-like example of the process involving slab subduction, stagnation, and foundering (Goes et al., 2017). This slab, during its early phases of subduction, incorporated water as a result of extensive plate faulting (Emry & Wiens, 2015; Fujie et al., 2018), which allowed water to percolate deep into the subducting plate and thereby hydrate the basaltic crust as well as lithospheric mantle (Cai et al., 2018; Tsuji et al., 2008). In subduction channel, the hydrated crust releases water continuously (Lee et al., 2021; Van Keken et al., 2011), leaving behind a nominally dry basaltic crust that can further transform into stishovite and liebermannite (Ohtani, 2020). In contrast, the serpentinized lithospheric mantle can retain
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Schematic model of the trans-MTZ water-transport system beneath northeastern Asia. The cold subducting Pacific slab transports DHMSs and stishovite to the MTZ, where persistent heating triggers the dehydration of DHMSs in the lithospheric slab (as indicated by diamonds in colour from blue to white). The released water, as well as a local supersaturated MTZ induced by this process, enables the rehydration of stishovite and liebermannite in the slab crust (as indicated by triangles from white to blue). Mixing the crust with the underlying lithospheric slab reinforces the rehydration of the crust (as shown by triangles deep into the slab). After a period of stagnation, the hydrous stishovite- and liebermannite-bearing slab founders into the lower mantle (LM), forming a giant water reservoir. See the text for details. The size of arrows with V-tails is proportional to the intensity of dehydration of DHMSs.</figcaption></figure>

water in hydrous minerals, mainly in the form of DHMSs. In the MTZ, these silicates involve phase D and super-
hydrous phase B (Shy B) (Ohtani, 2020) because the slab interior is still cool (<1400 K) (Yang & Fac-
cenda, 2020). However, before entering the lower mantle, their stable field gets destroyed, vomiting the contained
water to the MTZ. This water, through rehydration, enables stishovite and liebermannite to accommodate weight-
percent levels of water even down to lower-mantle depths (Lin et al., 2019). A pivotal mechanism of this process
could be the dehydration reaction of DHMSs and the rehydration of stishovite and liebermannite in the MTZ (Lin
et al., 2019; Ohtani, 2020) (Figure 3). In this scenario, anomaly H may be interpreted as a giant water reservoir in
the lower mantle, with the water being transported downward from the surface by the cold subducting slab.

The electrical conductivity images reported here clearly demonstrate that water in cold slabs can break through the water barrier of the 660-km discontinuity (Bercovici & Karato, 2003) to enter the lower mantle, enabling the water cycle to operate in deep Earth. Comparing to anomaly G in the MTZ, the stishovite and liebermannite in the lower mantle (corresponding to anomaly H) contain more water, implying further rehydration of these minerals before entering the lower mantle. Even so, the average water concentration (~0.35 wt%) of the slab in the lower mantle is still lower than that of the stagnant slab (~3 wt%) in the MTZ (Texts S3 and S4 in Supporting Information S1). Therefore, a large amount of water in the stagnant slab has discharged into the MTZ prior to entering the lower mantle. These water may have subsequently migrated into the upper mantle, thereby hydrating the large mantle wedge beneath the Sea of Japan and China mainland (Lee et al., 2021; Litasov et al., 2013; Zhao et al., 2009).

**Data Availability Statement**

Time series of observed geomagnetic data at observatories can be downloaded from the Geomagnetic Network of China (https://data.earthquake.cn/D13000/index.html) and World Data Centre (ftp://ftp.nmh.ac.uk/wdc/obsdata/hourval/single_obsv/). Details of geomagnetic field data we used from the Geomagnetic Network of China, the processed responses and inversion result can be found in Li. (2025).
Acknowledgments

We thank Anna Kelbert at US Geological Survey, and Xinzhuan Guo from Institute of Geochemistry, Chinese Academy of Sciences, and Guido Ventura from Istituto Nazionale di Geofisica e Vulcanologia for insightful comments. We are grateful for the computational resources of Jilin Kingti Geoexploration Tech. This work is supported by the National Natural Science Foundation of China (Grant 42074080 to AHW, Grant 42204076 to SWL, and Grant 42130302 to WLX and AHW). WLX and AHW acknowledge support from the Program for Jilin University Science and Technology Innovative Research Team Grant 2021TD-05.

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