Ground-penetrating radar reveals ice thickness and undisturbed englacial layers at Kilimanjaro's Northern Ice Field

Abstract

Abstract. Although its Holocene glacier history is still subject to debate, the ongoing iconic decline of Kilimanjaro's largest remaining ice body, the Northern Ice Field (NIF), has been documented extensively based on surface and photogrammetric measurements. The study presented here adds, for the first time, ground-penetrating radar (GPR) data at centre frequencies of 100 and 200 MHz to investigate bed topography, ice thickness and internal stratigraphy at NIF. The direct comparison of the GPR signal to the visible glacier stratigraphy at NIF's vertical walls is used to validate ice thickness and reveals that the major internal reflections seen by GPR can be associated with dust layers. Internal reflections can be traced consistently within our 200 MHz profiles, indicating an uninterrupted, spatially coherent internal layering within NIF's central flat area. We show that, at least for the upper 30 m, it is possible to follow isochrone layers between two former NIF ice core drilling sites and a sampling site on NIF's vertical wall. As a result, these isochrone layers provide constraints for future attempts at linking age–depth information obtained from multiple locations at NIF. The GPR profiles reveal an ice thickness ranging between (6.1 ± 0.5) and (53.5 ± 1.0) m. Combining these data with a very high resolution digital elevation model we spatially extrapolate ice thickness and give an estimate of the total ice volume remaining at NIF's southern portion as (12.0 ± 0.3) × 106 m3.

Brief communication: Glacier thickness reconstruction on Mt. Kilimanjaro

“…For KG no thickness measurements are available, but the consensus estimate (Farinotti et al 2019) provides approximate information for the year 2000 where KG showed a mean ice thickness of 27 m. A similar value was found for NIF, which is in fair agreement with the Bohleber et al (2017) observations considering that the consensus was not informed by the available thickness measurements.…”

“…Ice thickness measurements on Kilimanjaro are limited to NIF, where apart from three ice cores drilled to bedrock in 2000, with lengths from 49 m (C1) to 52 m (C2, C3) (Thompson et al 2002; Fig. 2A for borehole locations), GPR profiles from September 2015 show a mean (maximum) ice thickness of 21.2 ± 1.0 m (53.5 ± 1.0 m) for NIF's central flat area (Bohleber et al 2017).…”

“…The modern glacier recession on Kilimanjaro has been well documented and mapping approaches have shown that from an estimated ice extent of 11.4 km² in 1912, only 1.76 km² remained in 2011, constituting to a severe 85% reduction in glacier area (Cullen et al 2013). While glaciological research on Kilimanjaro has been focused on mapping glacier area and glacier retreat (Kaser et al 2004, Cullen et al 2013, as well as mass and energy balance studies (Mölg et al 2003(Mölg et al , 2008(Mölg et al , 2009, the research on the ice thickness of different glaciers on Kilimanjaro has been comparably sparse (Bohleber et al 2017). However, in light of severe glacier recession, an assessment on current glacier thickness is important to better determine future recession.…”

A full Stokes ice-flow model to assist the interpretation of millennial-scale ice cores at the high-Alpine drilling site Colle Gnifetti, Swiss/Italian Alps

“…Sub-meter-scale features of the bedrock can also affect the basal flow field, but the 50 m mesh used here for the FE implementation is too coarse to resolve such irregularities. Likewise, the bedrock detection with a 200 MHz GPR system (deployed extensively at CG) has typical uncertainties of ∼1-2 and 0.5 m in the vertical and horizontal directions, respectively (Bohleber and others, 2017). In case of a strong surface or bed inclination (e.g.…”

Global Disappearance of Tropical Mountain Glaciers: Observations, Causes, and Challenges

“…Two different glacier systems can be identified at the mountain: (1) on the plateau above 5700 m, where the retreat of the vertical glacier wall is predominantly controlled by direct solar radiation [11,12] and temperature increase; and (2) at the slopes below 5700 m, where the rapid retreat since the early 20th century can only be explained by a change in climate characteristics such as a reduction in specific humidity and an increase in net shortwave radiation accompanied by a decrease in cloudiness and precipitation [140]. Some studies predicted the complete disappearance of all ice at Mt Kilimanjaro by 2020 [139][140][141][142][143].…”