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

Pascal Bohleber,Nicolas J Cullen,Douglas R Hardy,Mariusz Potocki,Paul Mayewski,Leo Sold,Patrick Klenk,Margit Schwikowski,Pascal Sirguey,Helene Hoffmann,Andrea Fischer

doi:10.5194/tc-11-469-2017

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.

Highlights

The ice masses on top of Kilimanjaro (Tanzania, East Africa), the “white roof of Africa”, are the most recognized among the sparse glaciers in Africa
The following discussion of results focuses on three main features of the radar profiles, namely (i) bed reflection and ice thickness estimation, (ii) internal layer architecture within the Northern Ice Field (NIF) central flat area and (iii) meltwater disturbance
At NIF, the tracing of internal reflection horizons (IRH) provides a quantitative link between isochrone depths at existing sampling sites, thereby revealing important constraints for future efforts at integrating age– depth information obtained from the NIF ice cores and the vertical wall

Summary

Introduction

The ice masses on top of Kilimanjaro (Tanzania, East Africa), the “white roof of Africa”, are the most recognized among the sparse glaciers in Africa. Mapping ice thickness complements monitoring glacier decline and glaciological modelling of the past and future response of Kilimanjaro’s glaciers to climate variability. This especially concerns the Northern Ice Field (NIF, Fig. 1) because of two competing interpretations that exist regarding the maximum basal ice age and the mechanism of glacier formation. Basal melting features attributed to isolated fumarole activities have been observed under plateau ice (Kaser et al, 2004), making stratigraphic disturbance by basal melting a possibility It is not a priori evident to what degree stratigraphic integrity is preserved at NIF

Methods

Results

Conclusion