Radiocarbon dating of glacial dust layers and soils at Kilimanjaro’s Northern Ice Field

Abstract

As the African climate history recorded in Kilimanjaro’s Northern Ice Field (NIF) is rapidly lost, the age of the glacier remains disputed. Current age estimates from ice core data and glacial dynamics modeling disagree by an order of magnitude (11,700 vs ~1000 years old). We present radiocarbon dates of glacial dust and proximal soil samples collected from in and around the edge of the NIF that support the hypothesis that the peripheral portions of the NIF glacier are younger and more dynamic than the center. Samples of a dust-rich ice layer, 1.5 m above the base at the edge of the ~40 m high glacier ice cliff were found to be 1600–790 years old. These dates agree with the prediction of significant growth and shrinkage in glacial coverage over the last millennium. The variation in radiocarbon ages likely arises from heterogeneity in the summit volcanic soils, the main source of the dust recovered from the glacier, and cryoconite microbial activity. Sediment samples of a supraglacial pond had modern radiocarbon ages, further demonstrating the impact of cryoconite microbial communities on radiocarbon measurements of glacial samples. After accounting for the variability in the sample ages and the effects of microbial activity, we contend that the lower (i.e. younger) limit of the radiocarbon age range is the most reliable assessment of the age of the ice in which it was trapped.