Oceanic Melting Research Articles

How Does the Ocean Melt Antarctic Ice Shelves?

The present-day state and future of the Antarctic Ice Sheet depend on the rate at which the ocean melts its fringing ice shelves. Ocean heat must cross many physical and dynamical barriers to melt ice shelves, with the last of these being the ice-ocean boundary layer. This review summarizes the current understanding of ice-ocean boundary-layer dynamics, focusing on recent progress from laboratory experiments, turbulence-resolving numerical simulations, novel observations, and the application to large-scale simulations. The complex interplay between buoyant meltwater and external processes such as current shear leads to the emergence of several melting regimes that we describe, as well as freezing processes. The remaining challenges include developing new parameterizations for large-scale ice-ocean models based on recent advances and understanding the coevolution of melt and basal topography.

Evolution of sub-ice-shelf channels reveals changes in ocean-driven melt in West Antarctica

Abstract Basal channels, which are troughs carved into the undersides of ice shelves by buoyant plumes of water, are modulators of ice-shelf basal melt and structural stability. In this study, we track the evolution of 12 large basal channels beneath ice shelves of the Amundsen and Bellingshausen seas region in West Antarctica using the Landsat record since its start in the 1970s through 2020. We observe examples of channel growth, interactions with ice-shelf features, and systematic changes in sinuosity that give insight into the life cycles of basal channels. We use the last two decades of the record, combined with contemporary ice-flow velocity datasets, to separate channel-path evolution into components related to advection by ice flow and those controlled by other forcings, such as ocean melt or surface accumulation. Our results show that ice-flow-independent lateral channel migration is overwhelmingly to the left when viewed down-flow, suggesting that it is dominated by Coriolis-influenced ocean melt. By applying a model of channel-path evolution dominantly controlled by ice flow and ocean melt, we show that the majority of channels surveyed exhibit non-steady behavior that serves as a novel proxy for increased ocean forcing in West Antarctica starting at least in the early 1970s.

High-Mg andesites and Nb-enriched basalts in the Dulate arc, East Junggar (NW China): Evidence of ocean ridge subduction

Abstract The Dulate arc, located in East Junggar (NW China) in the southern Central Asian orogenic belt, records a Devonian magmatic arc evolution, offering a window to understanding the orogenic processes of the Central Asian orogenic belt. Here we present new geochemical and isotopic data for Late Devonian high-Mg andesite (HMA) and Nb-enriched basalt (NEB) suites from the Qiakuerte area, East Junggar. The HMA samples are typical subduction-related volcanic rocks. They have SiO2 contents ranging from 53.30 to 54.59 wt%, high MgO (5.0–5.26 wt%), and high Mg# values (~55) and show enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The HMA samples have high (La/Yb)N ratios and Sr/Y (~6.5 and 50–59, respectively) with no Eu anomalies. The HMA samples have high Na2O (~3.3 wt%) and low K2O (~2.5 wt%) and Th (~2.4 ppm) contents, combined with positive εNd(t) and low (87Sr/86Sr)i values. These characteristics suggest that the samples were formed mainly through interactions between subducted oceanic melts and mantle peridotites. Compared to normal arc basalts, the NEB samples have higher concentrations of Nb (~20 ppm), higher primitive mantle–normalized Nb/La (0.50–0.58), and higher ratios of Nb/U (9.4–14.6). The NEB samples also have positive εNd(t) and low (87Sr/86Sr)i values, indicating that their source was mantle wedge that had been metasomatized by slab melt. Considering the widespread presence of A-type granites, the abnormally high heat flow, and the tectonic characteristics of East Junggar, we conclude that a slab window created by the subduction of an ocean ridge was responsible for the melting of slab and the formation of the NEB-HMA suites. These processes may have also played a key role in the tectonic evolution processes of East Junggar during the Late Devonian.

Strong Ocean Melting Feedback During the Recent Retreat of Thwaites Glacier

AbstractAccelerating ice loss from Thwaites Glacier is contributing approximately 5% of global sea‐level rise, and could add tens of centimeters to sea level over the coming centuries. We use an ocean model to calculate sub‐ice melting for a succession of Digital Elevation Models of the main trunk of Thwaites Glacier from 2011 to 2022. The ice evolution during this period induces a strong geometrical feedback onto melting. Ice thinning and retreat provides a larger melting area, thicker and better‐connected sub‐ice water column, and steeper ice base. This leads to stronger sub‐ice ocean currents, increasing melting by over 30% without any change in forcing from wider ocean conditions. This geometrical feedback over just 12 years is comparable to melting changes arising from plausible century‐scale changes in ocean conditions and subglacial meltwater inflow. These findings imply that ocean‐driven ice loss from Thwaites Glacier may only be weakly influenced by anthropogenic emissions mitigation.

What Determines the Shape of a Pine‐Island‐Like Ice Shelf?

AbstractIce shelf shape directly controls ocean heat intrusions, melting near the grounding line, and buttressing. Little is known about what determines ice‐shelf shape because ice‐ocean coupled simulations typically aim at projecting Antarctica's contribution to sea‐level rise and they do not resolve small‐scale ice‐ocean interactive processes. We conduct ice‐ocean coupled simulations for an idealized high‐resolution, Pine‐Island‐like model configuration. We show that ocean melting and ice stretching caused by acceleration thin the ice shelf from the grounding line toward the ice shelf front, consistent with previous studies. In the across‐flow direction, ocean melting and ice advection cancel each other out and flatten the ice shelf. More than one‐third of the ice thinning from grounding line to ice front can be attributed to ocean melting at depths shallower than 500 m. Our results emphasize the importance of interactive processes between the entire ice shelf and the ocean for determining the ice shelf shape.

Modeling the thermal processes within the ice shelf–ocean boundary current underlain by strong pycnocline underneath a cold-water ice shelf using a 2.5-dimensional vertical slice model

Oceanic melting beneath ice shelves is the main driver of the current mass loss of the Antarctic ice sheet. The resultant meltwater plumes influence the basal melting of ice shelves, and contribute to the development of the unique two-layer stratified ice shelf–ocean boundary currents underlain by warmer, saltier, and stationary source waters. However, knowledge of the dynamics and thermodynamics within these meltwater plumes, controlling the heat available for melting ice, remains outstanding. Here we investigate that important issue by developing a 2.5-dimensional nonhydrostatic vertical slice model with 1.5 m vertical resolution, and conduct the reference run based on a representative Ice Shelf Water (ISW)-High Salinity Shelf Water (HSSW) boundary current beneath the Amery Ice Shelf, East Antarctica. Based on that we identify two dominating vertical thermal processes regulating the local temperature: the turbulent diffusion and the shear instabilities-induced convection (i.e., vertical advection), and carry out a quantitative thermal budget analysis in the framework of plume model, including deriving an analytical expression for the entrainment-induced heat flux. Moreover, after evaluating the entrainment parametrizations, it is suggested that the common assumption of neglecting the velocity at the lower boundary of meltwater plume potentially leads to a considerable deviation from the real entrainment. The sensitivity of the simulated results to model configuration and model resolution are also investigated. The findings in this study imply that we need to improve the model resolution of current ocean cavity models to capture the interfacial processes between lighter–denser waters.

Petrogenesis and tectonic setting of Late Devonian high-Mg andesites in the Beishan orogen, NW China

AbstractHigh-Mg andesites (HMAs) are crucial for the reconstruction of plate tectonics, continental margin formation and lithospheric evolution. In this study, we present new fossil age, whole-rock geochemical and Sr–Nd isotope data on the newly discovered Dundunshan Group HMAs in the Dundunshan area of the Beishan orogen (central-southern Central Asian Orogenic Belt). The Dundunshan HMA samples are characterized by high MgO (6.47–7.02 wt%) contents and high Mg# values (67.27–68.77), with SiO2 (58.57–62.13 wt%), Al2O3 (14.49–16.07 wt%) and CaO (5.05–6.24 wt%) resembling typical HMAs. The Dundunshan HMA samples are calc-alkaline and strongly enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), with slightly negative Eu anomaly and high-field-strength element (HFSE) depletions. Their (87Sr/86Sr)i ratios (0.7041–0.7057) and ϵNd(t) (3.73–5.59) indicate that the Dundunshan HMAs were mainly formed by the interactions between subducted oceanic sediment-derived melts and mantle peridotites. Fossil evidence and published radiometric age data constrain the formation of the Dundunshan HMAs to early Late Devonian time. Sedimentological features of the Middle Devonian Sangejing Formation and regional tectonic correlation suggest that the Hongliuhe–Niujuanzi–Xichangjing Ocean in the Dundunshan area was likely closed during late Middle Devonian time, and that the Dundunshan HMAs were formed in a post-collision extensional setting.

Brief communication: Thwaites Glacier cavity evolution

Abstract. Between 2014 and 2017, ocean melt eroded a large cavity beneath and along the western margin of the fast-flowing core of Thwaites Glacier. Here we show that from 2017 to the end of 2020 the cavity persisted but did not expand. This behaviour, of melt concentrated at the grounding line within confined sub-shelf cavities, fits with prior observations and modelling studies. We also show that acceleration and thinning of Thwaites Glacier grounded ice continued, with an increase in speed of 400 m a−1 and a thinning rate of at least 1.5 m a−1, between 2012 and 2020.

Review article: Earth's ice imbalance

Abstract. We combine satellite observations and numerical models to show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes), mountain glaciers (6.1 trillion tonnes), the Greenland ice sheet (3.8 trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass. Just over half (58 %) of the ice loss was from the Northern Hemisphere, and the remainder (42 %) was from the Southern Hemisphere. The rate of ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion tonnes per year – owing to increased losses from mountain glaciers, Antarctica, Greenland and from Antarctic ice shelves. During the same period, the loss of grounded ice from the Antarctic and Greenland ice sheets and mountain glaciers raised the global sea level by 34.6 ± 3.1 mm. The majority of all ice losses were driven by atmospheric melting (68 % from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet surface mass balance), with the remaining losses (32 % from ice sheet discharge and ice shelf thinning) being driven by oceanic melting. Altogether, these elements of the cryosphere have taken up 3.2 % of the global energy imbalance.

An Empirical Algorithm for Mitigating the Sea Ice Effect in SMAP Radiometer for Sea Surface Salinity Retrieval in the Arctic Seas

The L-band radiometer onboard the Soil Moisture Active Passive (SMAP mission is used to retrieve sea surface salinity (SSS over global ocean. In the Arctic seas, one of the major challenges of SSS remote sensing is the presence of sea ice. This paper proposes a data-driven ice correction (IC algorithm which extracts emission from the water portion of measured brightness temperature (TB in scenes mixed with water and ice. Emission of the ice portion was removed based on estimation according to the ice fraction (fice in the satellite footprint and ice signature derived from surrounding pixels. The IC algorithm is applied to SMAP TB data to obtain TB with ice correction (TBIC, which are used for SSS retrieval using the standard JPL SMAP CAP processing system. We show that the algorithm is most effective near the ice edge, thereby increasing the fice threshold for possible SSS retrieval to 15% from the current 3% without IC. SMAP SSS are validated using in situ salinity collected during NASAs Ocean Melting Greenland (OMG mission from 2016 to 2020 along the Greenland coast. The number of collocations between OMG and SMAP daily gridded salinity increased by more than 30% with IC. The statistical analysis shows a similar retrieval accuracy with or without IC, with the standard deviation of the difference between OMG and SMAP of 1.41 psu (with IC and 1.42 psu (without IC . The bias adjusted SMAP SSS depicts salinity patterns and gradients around Greenland consistent with OMG measurements.

The Impact of the Amundsen Sea Freshwater Balance on Ocean Melting of the West Antarctic Ice Sheet

AbstractThe Amundsen Sea has the highest thinning rates of ice shelves in Antarctica. This imbalance is caused by changes in ocean melting induced by warm Circumpolar Deep Water (CDW) intrusions. The resulting changing freshwater balance could affect the on‐shelf currents and mixing. However, a clear understanding of the sources and sinks of freshwater in the region is lacking. Here we use a model of the Amundsen Sea, with passive freshwater tracers, to investigate the relative magnitudes and spatial distributions of the different freshwater components. In the surface layer and as a depth average, all freshwater tracer concentrations are of comparable magnitude, though on a depth average, sea ice and ice shelf are largest. The total freshwater tracer distribution is similar to that of the ice‐shelf tracer field. This implies a potential for ice‐shelf meltwater feedbacks, whereby abundant ice‐shelf meltwater alters the ocean circulation and stratification, affecting melting. Ice‐shelf and sea‐ice freshwater fluxes have the largest interannual variability. The effect of including grounded icebergs and iceberg freshwater flux are studied in detail. The presence of icebergs increases CDW intrusions that reach the base of ice shelves. This suggests another possible feedback mechanism, whereby more icebergs induce greater ice‐shelf melting and hence more icebergs. However, the strength of this potential feedback is dependent on poorly constrained sea‐ice model parameters. These results imply that poorly constrained parameters relating to the ocean freshwater balance, such as those relating to icebergs and sea ice, impact predictions for melting of the West Antarctic Ice Sheet.

How Oceanic Melt Controls Tidewater Glacier Evolution

AbstractThe recent rapid retreat of many Arctic outlet glaciers has been attributed to increased oceanic melt, but the relationship between oceanic melt and iceberg calving remains poorly understood. Here, we employ a transient finite element model that simulates oceanic melt and ice break‐off at the terminus. The response of an idealized tidewater glacier to various submarine melt rates and seasonal variations is investigated. Our modeling shows that for zero to low oceanic melt, the rate of volume loss at the front is similar or higher than that for intermediate oceanic melt rates. Only very high melt rates lead to increasing volume losses. These results highlight the complex interplay between oceanic melt and calving and question the general assumption that increased submarine melt leads to higher calving fluxes and enhanced retreat. Models for tidewater glacier evolution should therefore consider calving and oceanic melt as tightly coupled processes rather than as simple, additive parameterizations.

The Arctic Ocean Melt and the Impacts in Traditional Populations: Possibilities for an International Guarantee

The research intends to show how the melting of the Arctic Ocean, caused by the climatic collapse, affects the life of the traditional populations that inhabit the region and its ways of being influenced by the changes of the landscape and of the own structure and environmental availability. In view of this, it aims to present the environmental balance as a Human Right and, in view of this position, demonstrate the need for international mobilization to protect traditional communities that are historically more vulnerable. In order to do so, the study questions the possibility of universal jurisdiction of the Inter-American Court of Human Rights, since the litigation previously raised by the Inuit people was frustrated due to the unavailability of the non-jurisdictional countries. In addition, the possibility of litigation is also raised in the International Criminal Court, on the grounds of Ecocide. Thus, through the hypothetical-deductive method and the questions raised by Cloutier SW, et al. [1], the impacts of climate collapse on vulnerable populations will be demonstrated, with a dissertation on the imperative need of effective and sensitive international tutelage - which also justifies the research.

Nonlinear response of the Antarctic Ice Sheet to late Quaternary sea level and climate forcing

Abstract. Antarctic ice volume has varied substantially during the late Quaternary, with reconstructions suggesting a glacial ice sheet extending to the continental shelf break and interglacial sea level highstands of several meters. Throughout this period, changes in the Antarctic Ice Sheet were driven by changes in atmospheric and oceanic conditions and global sea level; yet, so far modeling studies have not addressed which of these environmental forcings dominate and how they interact in the dynamical ice sheet response. Here, we force an Antarctic Ice Sheet model with global sea level reconstructions and transient, spatially explicit boundary conditions from a 408 ka climate model simulation, not only in concert with each other but, for the first time, also separately. We find that together these forcings drive glacial–interglacial ice volume changes of 12–14 ms.l.e., in line with reconstructions and previous modeling studies. None of the individual drivers – atmospheric temperature and precipitation, ocean temperatures, or sea level – single-handedly explains the full ice sheet response. In fact, the sum of the individual ice volume changes amounts to less than half of the full ice volume response, indicating the existence of strong nonlinearities and forcing synergy. Both sea level and atmospheric forcing are necessary to create full glacial ice sheet growth, whereas the contribution of ocean melt changes is found to be more a function of ice sheet geometry than climatic change. Our results highlight the importance of accurately representing the relative timing of forcings of past ice sheet simulations and underscore the need for developing coupled climate–ice sheet modeling frameworks that properly capture key feedbacks.

A Transient Coupled Ice Flow‐Damage Model to Simulate Iceberg Calving From Tidewater Outlet Glaciers

AbstractIceberg calving, the detachment of an ice block at the glacier front, is the main process responsible for the dynamic mass loss from the ice sheets to the ocean. Understanding this process is essential to accurately predict ice sheet response to the future climate. We present a transient multiphysics finite‐element model to simulate iceberg break‐off and geometry evolution of a marine‐terminating glacier. The model solves the coupled equations of ice flow, damage mechanics, oceanic melt, and geometry evolution on the same Lagrangian computational grid. A modeling sensitivity analysis shows that the choice of stress measure used for damage evolution strongly influences the resulting calving front geometries. Our analysis suggests that the von Mises stress measures produce the most realistic calving front geometry evolutions for tidewater glaciers. Submarine frontal melt is shown to have a strong impact on the calving front geometry. The presented multiphysics model includes all processes thus far shown to be relevant for the evolution of tidewater glaciers and can be readily adapted for 3‐D and arbitrary bedrock geometries.

West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing

Recent ice loss from the West Antarctic Ice Sheet has been caused by ocean melting of ice shelves in the Amundsen Sea. Eastward wind anomalies at the shelf break enhance the import of warm Circumpolar Deep Water onto the Amundsen Sea continental shelf, which creates transient melting anomalies with an approximately decadal period. No anthropogenic influence on this process has been established. Here, we combine observations and climate model simulations to suggest that increased greenhouse gas forcing caused shelf-break winds to transition from mean easterlies in the 1920s to the near-zero mean zonal winds of the present day. Strong internal climate variability, primarily linked to the tropical Pacific, is superimposed on this forced trend. We infer that the Amundsen Sea experienced decadal ocean variability throughout the twentieth century, with warm anomalies gradually becoming more prevalent, offering a credible explanation for the ongoing ice loss. Existing climate model projections show that strong future greenhouse gas forcing creates persistent mean westerly shelf-break winds by 2100, suggesting a further enhancement of warm ocean anomalies. These wind changes are weaker under a scenario in which greenhouse gas concentrations are stabilized. Anthropogenic changes in Antarctic shelf-break winds from the 1920s onwards have contributed to ice loss in the Amundsen Sea, along with natural variability, suggests an analysis of observations and model simulations.

Assessment of sub-shelf melting parameterisations using the ocean–ice-sheet coupled model NEMO(v3.6)–Elmer/Ice(v8.3)

Abstract. Oceanic melting beneath ice shelves is the main driver of the current mass loss of the Antarctic ice sheet and is mostly parameterised in stand-alone ice-sheet modelling. Parameterisations are crude representations of reality, and their response to ocean warming has not been compared to 3-D ocean–ice-sheet coupled models. Here, we assess various melting parameterisations ranging from simple scalings with far-field thermal driving to emulators of box and plume models, using a new coupling framework combining the ocean model NEMO and the ice-sheet model Elmer/Ice. We define six idealised one-century scenarios for the far-field ocean ranging from cold to warm, and representative of potential futures for typical Antarctic ice shelves. The scenarios are used to constrain an idealised geometry of the Pine Island glacier representative of a relatively small cavity. Melt rates and sea-level contributions obtained with the parameterised stand-alone ice-sheet model are compared to the coupled model results. The plume parameterisations give good results for cold scenarios but fail and underestimate sea level contribution by tens of percent for warm(ing) scenarios, which may be improved by adapting its empirical scaling. The box parameterisation with five boxes compares fairly well to the coupled results for almost all scenarios, but further work is needed to grasp the correct number of boxes. For simple scalings, the comparison to the coupled framework shows that a quadratic as opposed to linear dependency on thermal forcing is required. In addition, the quadratic dependency is improved when melting depends on both local and non-local, i.e. averaged over the ice shelf, thermal forcing. The results of both the box and the two quadratic parameterisations fall within or close to the coupled model uncertainty. All parameterisations overestimate melting for thin ice shelves while underestimating melting in deep water near the grounding line. Further work is therefore needed to assess the validity of these melting parameteriations in more realistic set-ups.

Ross Ice Shelf response to climate driven by the tectonic imprint on seafloor bathymetry

Ocean melting has thinned Antarctica’s ice shelves at an increasing rate over the past two decades, leading to loss of grounded ice. The Ross Ice Shelf is currently close to steady state but geological records indicate that it can disintegrate rapidly, which would accelerate grounded ice loss from catchments equivalent to 11.6 m of global sea level rise. Here, we use data from the ROSETTA-Ice airborne survey and ocean simulations to identify the principal threats to Ross Ice Shelf stability. We locate the tectonic boundary between East and West Antarctica from magnetic anomalies and use gravity data to generate a new high-resolution map of sub-ice-shelf bathymetry. The tectonic imprint on the bathymetry constrains sub-ice-shelf ocean circulation, protecting the ice shelf grounding line from moderate changes in global ocean heat content. In contrast, local, seasonal production of warm upper-ocean water near the ice front drives rapid ice shelf melting east of Ross Island, where thinning would lead to faster grounded ice loss from both the East and West Antarctic ice sheets. We confirm high modelled melt rates in this region using ROSETTA-Ice radar data. Our findings highlight the significance of both the tectonic framework and local ocean–atmosphere exchange processes near the ice front in determining the future of the Antarctic Ice Sheet. The boundary between West and East Antarctica is a tectonic feature that bisects the Ross Ice Shelf. This boundary constrains ocean circulation under the ice, which affects ice stability, according to airborne survey data and ocean simulations.

Brief communication: PICOP, a new ocean melt parameterization under ice shelves combining PICO and a plume model

Abstract. Basal melting at the bottom of Antarctic ice shelves is a major control on glacier dynamics, as it modulates the amount of buttressing that floating ice shelves exert onto the ice streams feeding them. Three-dimensional ocean circulation numerical models provide reliable estimates of basal melt rates but remain too computationally expensive for century-scale projections. Ice sheet modelers therefore routinely rely on simplified parameterizations based on either ice shelf depth or more sophisticated box models. However, existing parameterizations do not accurately resolve the complex spatial patterns of sub-shelf melt rates that have been observed over Antarctica's ice shelves, especially in the vicinity of the grounding line, where basal melting is one of the primary drivers of grounding line migration. In this study, we couple the Potsdam Ice-shelf Cavity mOdel (PICO, Reese et al., 2018) to a buoyant plume melt rate parameterization (Lazeroms et al., 2018) to create PICOP, a novel basal melt rate parameterization that is easy to implement in transient ice sheet numerical models and produces a melt rate field that is in excellent agreement with the spatial distribution and magnitude of observations for several ocean basins. We test PICOP on the Amundsen Sea sector of West Antarctica, Totten, and Moscow University ice shelves in East Antarctica and the Filchner-Ronne Ice Shelf and compare the results to PICO. We find that PICOP is able to reproduce inferred high melt rates beneath Pine Island, Thwaites, and Totten glaciers (on the order of 100 m yr−1) and removes the “banding” pattern observed in melt rates produced by PICO over the Filchner-Ronne Ice Shelf. PICOP resolves many of the issues contemporary basal melt rate parameterizations face and is therefore a valuable tool for those looking to make future projections of Antarctic glaciers.

The participation of ilmenite-bearing cumulates in lunar mantle overturn

The ilmenite-bearing cumulates (IBC) formed from the solidification of the lunar magma ocean are thought to have significantly affected the long-term evolution of the lunar interior and surface. Their high density is considered to trigger Rayleigh–Taylor instabilities which allow them to sink into the solidified cumulates below and drive a large-scale overturn in the lunar mantle. Knowledge of how the IBC participate in the overturn is important for studying the early lunar dynamo, chemistry of surface volcanism, and the existence of present-day partial melt at the lunar core–mantle boundary. Despite early efforts to study this process as Rayleigh–Taylor instabilities, no dynamical models have quantified the degree of IBC sinking systematically. We have performed quantitative 2-D geodynamical simulations to measure the extent to which IBC participate in the overturn after their solidification, and tested the effect of a range of physical and chemical parameters. Our results show that IBC overturn most likely happened when the magma ocean had not yet fully solidified, with the residual melt decoupling the crust and IBC, resulting in 50–70% IBC sinking. Participation of the last dregs of remaining magma ocean melt is unlikely, leaving its high concentrations of radiogenic elements close to the surface. Our simulations further indicate that foundered IBC can stay relatively stable at the core–mantle boundary until the present day, at temperatures consistent with the presence of a partially molten zone in the deep mantle as inferred from geophysical data. 30–50% of the primary IBC remain at shallow depths throughout lunar history, enabling their assimilation by rising magma to form high-Ti basalts.