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ABSTRACT Accelerated climate warming is causing significant reductions in the volume of Arctic glaciers, such that previously ice‐capped bare ground is uncovered, harboring soil development. Monitoring the thermal and hydrologic characteristics of soils, which strongly affect microbial activity, is important to understand the evolution of emerging terrestrial landscapes. We instrumented two sites on the forefield of a retreating Svalbard glacier, representing sediment ages of approximately 5 and 60 years since exposure. Our instrumentation included an ERT array complemented by adjacent point sensor measurements of subsurface temperature and water content. Sediments were sampled at each location and at two more additional sites (120 and 2000 years old) along a chronosequence aligned with the direction of glacial retreat. Analysis suggests older sediments have a lower bulk density and contain fewer large minerals, which we interpret to be indicative of sediment reworking over time. Two months of monitoring data recorded during summer 2021 indicate that the 60‐year‐old sediments are stratified showing more spatially consistent changes in electrical resistivity, whereas the younger sediments show a more irregular structure, with consequences on heat and moisture conductibility. Furthermore, our sensors reveal that young sediments have a higher moisture content, but a lower moisture content variability. 

Abstract. Arctic regions are under immense pressure from a continuously warming climate. During the winter and shoulder seasons, recently deglaciated sediments are particularly sensitive to human-induced warming. Understanding the physical mechanisms and processes that determine soil liquid moisture availability contributes to the way we conceptualize and understand the development and functioning of terrestrial Arctic ecosystems. However, harsh weather and logistical constraints limit opportunities to directly observe subsurface processes year-round; hence automated and uninterrupted strategies of monitoring the coupled heat and water movement in soils are essential. Geoelectrical monitoring using electrical resistivity tomography (ERT) has proven to be an effective method to capture soil moisture distribution in time and space. ERT instrumentation has been adapted for year-round operation in high-latitude weather conditions. We installed two geoelectrical monitoring stations on the forefield of a retreating glacier in Svalbard, consisting of semi-permanent surface ERT arrays and co-located soil sensors, which track seasonal changes in soil electrical resistivity, moisture, and temperature in 3D. One of the stations observes recently exposed sediments (5–10 years since deglaciation), whilst the other covers more established sediments (50–60 years since deglaciation). We obtained a 1-year continuous measurement record (October 2021–September 2022), which produced 4D images of soil freeze–thaw transitions with unprecedented detail, allowing us to calculate the velocity of the thawing front in 3D. At its peak, this was found to be 1 m d−1 for the older sediments and 0.4 m d−1 for the younger sediments. Records of soil moisture and thermal regime obtained by sensors help define the conditions under which snowmelt takes place. Our data reveal that the freeze–thaw shoulder period, during which the surface soils experienced the zero-curtain effect, lasted 23 d at the site closer to the glacier but only 6 d for the older sediments. Furthermore, we used unsupervised clustering to classify areas of the soil volume according to their electrical resistivity coefficient of variance, which enables us to understand spatial variations in susceptibility to water-phase transition. Novel insights into soil moisture dynamics throughout the spring melt will help parameterize models of biological activity to build a more predictive understanding of newly emerging terrestrial landscapes and their impact on carbon and nutrient cycling. 

Surface and subsurface hydrogeochemical measurements collected from three sites in the Midtre Lovenbreen (ML) glacier forefield (Svalbard), from 31 Jul 20211 - 05 Oct 2022. Measurements consist of soil volumetric water content (meter^3/meter^3), soil temperature in Celsius (deg C), soil electric conductivity in microsiemens per centimeter (uS/cm), soil dielectric permittivity (m^3/m^3) at multiple sensor depths, surface snow depth in millimeters (mm), and near-surface air temperature at 2m (deg C). Subsurface time domain reflectometry (TDR) measurements are repeated for 24 sensors at sites 1 and 2, with four boreholes of six sensors each, with the following convention: sensorOne is the uppermost sensor in borehole 1, sensorSix is the lowermost sensor in borehole 1, sensorSeven is the uppermost sensor in borehole 2, sensorTwelve is the lowermost sensor in borehole 2. Site 3 has only 1 borehole with six sensors.Installation depths for the sensors can be found in the borehole notes file. Snow depth sensor measurements contain error from damaged sensors and are unreliable. 

### Overview This data release includes surface nuclear magnetic resonance (sNMR) data collected as part of the SUN-SPEARS project. The project is funded by the National Science Foundation (Award number 2015329) and is concerned with studying soil evolution in high Arctic environments post glacial retreat. Within SUN-SPEARS, data are collected on a chronosequence from very recently deglaciated to older locations which have been exposed for decades to centuries. In this data release, sNMR data from two sites are included: site 1 which was collected approximately 15 meters (m) from the snout of the glacier, and site 2 which was located approximately 1000 m from the snout of the glacier, Global Positioning System (GPS) coordinates are included for more precise locations. ### Access Data files can be accessed via: [https://arcticdata.io/data/10.18739/A23X83N25](https://arcticdata.io/data/10.18739/A23X83N25)