The Earth’s core as a reservoir of water

Abstract

Current estimates of the budget and distribution of water in the Earth have large uncertainties, most of which come from the lack of information about the deep Earth. Recent studies suggest that the Earth could have gained a considerable amount of water during the early stages of Earth's evolution from the hydrogen-rich solar nebula, and that a large amount of the water in the Earth may have partitioned into the core. Here, we calculate the partitioning of water between iron and silicate melts at 20-135 gigapascals and 2800-5000 kelvin, using ab initio molecular dynamics and thermodynamic integration techniques. Our results indicate a siderophile nature of water at core-mantle differentiation and core-mantle boundary conditions, which can be weakened with increasing temperature; nevertheless, we find that water always partitions strongly into the iron liquid at both reducing and oxidising conditions. The siderophile nature of water is also verified by an empirical-counting method showing the distribution of hydrogen in an equilibrated iron and silicate melt. We therefore conclude that the Earth's core may act as a large reservoir containing most of the Earth's water. In addition to constraining accretion models and water distribution, the findings may partially account for the mismatch between mineral physics and seismic observations in the Earth's core.

Constraining the Volume of Earth's Early Oceans With a Temperature‐Dependent Mantle Water Storage Capacity Model

“…However, these possibilities are less likely. The water content of the core remains controversial (e.g., Clesi et al., 2018; Hirose et al., 2019; Iizuka‐Oku et al., 2017; Li et al., 2020; Okuchi et al., 1997; Terasaki et al., 2012; Yuan & Steinle‐Neumann, 2020), but it is most likely not decreasing over time, making it an unlikely source of an increase in mantle water. Possible hydrous partial melts in the early mantle would have likely been too buoyant to remain gravitationally stable at depth (Mookherjee et al., 2008) and would have erupted to release their water onto the surface.…”

Chemical Reaction Between Metallic Iron and a Limited Water Supply Under Pressure: Implications for Water Behavior at the Core‐Mantle Boundary

“…Liu et al (2017) elucidated that the sound velocity in FeO 2 H x can explain the seismic structure of ultralow velocity zones (ULVZs), which represent dense domains that are characterized by variable thicknesses in the range 5–40 km at the base of the lower mantle (Garnero & Helmberger, 1998; McNamara et al, 2010). Moreover, a recent theoretical study revealed that water at the base of the mantle would partition strongly into the core, and the ULVZs would be dry (Li et al, 2020).…”

On the evolution of thermally stratified layers at the top of Earth's core

“…Naively we might expect the temperature drop from 5300 K to 4800 K to produce a similar O(100) km change in stable layer thickness to that found for our calculations at 5900 K and 5300 K; however, this assumes that partitioning of H and its effect on thermal conductivity are similar to that of O, for which there is as yet no evidence. Furthermore, Li et al (2020) suggest from partitioning calculations that the hydrogen concentrations considered by Umemoto and Hirose (2020) are too large to be compatible with the estimated present-day mantle water content. We therefore conclude that our calculations provide plausible uncertainties on the composition-dependence of stable layer thickness given the presently available information.…”