Challenging the southern boundary of active rock glaciers in West Greenland

Rock glaciers (RGs) are landscape features impacting the composition and magnitude of runoff and, given their ice content, they are used as indicators for past and present climate conditions. While our knowledge of RG coverage has improved over recent years in many mountainous regions, there is very little information available for RGs in Greenland. Here, we provide evidence for an active RG in West Greenland, about 230 km south of what previously has been identified as the southern limit of active RGs. We present field evidence such as bottom temperature of the snow pack and surface displacements and indicate how these results could be utilized in further studies to better assess RG distribution or their ice content.


| INTRODUCTION
Active rock glaciers (RG) are "the visible expression of cumulative deformation by long-term creep of ice/debris mixtures under permafrost conditions". 1 They play a hydrological role in water routing 2 and store substantial volumes of water. 3 They are climatically more resilient than glaciers and their hydrological significance may increase in a changing climate. 4 RGs can be used as a indicators of past and present climates due to their hidden ice content. 5,6 While internationally coordinated efforts have led to an impressive reduction of uncertainties in the quantification of global glacier coverage 7 and volumes, 8 corresponding information on RGs is much less constrained. A recent study ( 3 ; corrected in 9 ) attempted to provide a near-global inventory of RG occurrence and found that in total 62.02 ± 12.40 Gt of water volume equivalent is stored in RGs and they derived an RG to glacier volume ratio of 1:618. This compilation relies on regional and national inventories of varying levels of detail.
RGs are known to occur in Greenland, but they have received very little attention in the literature compared to other features of the Greenland cryosphere, such as the Greenland Ice sheet and peripheral glaciers. Most studies covering RGs in Greenland have either been site-specific (e.g., 6,[10][11][12] or limited to smaller regions. 13 A compilation of the spatial coverage of RGs in Disko was made by Humlum, 13 available through, 14 putting the results in context with other known sites of active and inactive/fossil RGs in Greenland. In their figure 1 they clearly indicate that, while sporadic permafrost reaches all the way to the southern tip of Greenland, the southernmost known RG on the west coast occurs at around 66.5 N near the town of Sisimiut. More recent studies attempting to provide a worldwide inventory of RGs 3 have based their estimates for Greenland on the spatially limited study by Humlum 6 and extrapolated based on the Randolph Glacier Inventory. 7 It therefore appears appropriate to challenge the limited knowledge on the spatial coverage of RGs in Greenland.
In the following we present observational evidence for an active RG in West Greenland, about 2 latitudinal degrees or 230 km further south than previously thought to be the southern boundary. We present first monitoring results based on 5 years of air temperature near the RG snout, a bottom temperature of snow cover (BTS) survey, summer RG spring temperatures, as well as DGPS (Differential Global Positioning System)-derived surface displacement rates that support the plausibility of an active RG.

| STUDY SITE
Our study area focuses on the northern part of Qoornup Qeqertarssua (colloquially known as Bjørneø, which is Danish for

| RESULTS
We combine several independent monitoring results from the past 5 years to challenge the hitherto postulated southern boundary of active RGs in West Greenland.

| Morphological and qualitative evidence
Ridges and furrows are typical features of RGs 1 and are prevalent on BJRG. They become filled with seasonal snow cover that can remain well into, or throughout, the summer. During a visit in August 2017 we observed compacted snow in several furrows that may have remained in place for the entire summer season, possibly due to cooling from below.

| DISCUSSION AND CONCLUSION
There is evidence for a site in southwest Greenland that shows several independent indicators pointing towards the existence of an active RG at a latitude of ca. 64 30 0 N. Flow velocities are very low and on the order of other studies in the high latitudes. 19 The major part of the RG is gently inclined with slope angles below 20 (cf. Figure 1a and b). Cicoira et al. 20 revealed the dependence of RG creep rates on slope angle based on a thorough literature survey. They find very low (sub-meter) creep for slope angles <20 , which supports the fact that little shear stress develops. The strong BTS differences between the RG and its immediate surroundings lend credence to an ice body below.
The current study provides a first assessment of an active RG, potentially allowing the southern limit of active RGs in West Greenland to be extended considerably southwards. An important next step will be the deployment of additional geophysical methods such as electric resistivity tomography and ground-penetrating radar, which may shed light on the ice content of BJRG. Furthermore, relative or absolute dating approaches may allow us to assess the genesis and age of this landscape feature. On a regional and Greenland-wide scale, a state-of-the art RG inventory should be established to both provide the basis for climate/permafrost assessments and better constrain the ice volume stored in these understudied features. Rasch 21 provided an overview of neighboring areas to our study site based on qualitative analysis of optical remote sensing images. They found several candidates with surface features that could contain RGs, which we display in Figure S1 along with an oblique photograph of another potentially active RG. While their assessment is relevant for determination of further RG occurrence, the qualitative approach does not allow us to assess the activity state of the RGs. Satelliteretrieved products of high-resolution surface displacement such as synthetic aperture radar may prove powerful for assessing the activity level of RGs. 12