Geology, v. 42, p. 519–522, doi:10.1130/G35421.1

There was an error in the beryllium carrier concentration reported in this paper. The carrier concentration has been corrected and the 10Be concentrations and ages recalculated. Please see the corrected Figure 1, and Table DR1 in the GSA Data Repository (http://www.geosociety.org/datarepository/2014/2014184.pdf). The 10Be ages and mean moraine ages are corrected below in the Abstract and Results sections. The overall interpretations of the paper remain unchanged.

Abstract

Glaciers on the world’s highest tropical mountains are among the most sensitive components of the cryosphere, yet the climatic controls that influence their fluctuations are not fully understood. Here, we present the first 10Be ages of glacial moraines in Africa and use these to assess the climatic conditions that influenced past tropical glacial extents. We applied 10Be surface exposure dating to determine the ages of quartz-rich boulders atop moraines in the Rwenzori Mountains (∼1°N, 30°E), located on the border of Uganda and the Democratic Republic of Congo. The 10Be ages document expanded glaciers at ca. 24.9 and 21.5 ka, indicating that glaciers in equatorial East Africa advanced during the global Last Glacial Maximum (LGM; ca. 26–19.5 ka). A comparison of these moraine ages with regional paleoclimate records indicates that Rwenzori glaciers expanded contemporaneously with dry and cold conditions. Recession from the moraines occurred subsequent to ca. 21.5 ka, similar in timing to a rise in air temperature documented in East African lake records. Our results suggest that, on millennial time scales, past fluctuations of Rwenzori glaciers were strongly influenced by air temperature.

RESULTS

We measured eight high-precision 10Be surface exposure ages of boulders on the Lake Mahoma Stage moraines (Fig. 1; also see Table DR1). Four 10Be ages from the Mahoma-2 moraine range from 24,480 ± 470 to 25,650 ± 580 yr ago (10Be ages ± 1σ measurement uncertainties) with a mean age of 24,940 ± 500 yr ago (arithmetic mean ± 1σ). Four 10Be ages from the right-lateral Mahoma-1 moraine range from 20,530 ± 390 to 21,910 ± 750 yr ago (10Be ages ± 1σ measurement uncertainties), with a mean age of 21,500 ± 650 yr ago (arithmetic mean ± 1σ).

In general, uncertainties in 10Be dating arise from 10Be production rate and measurement uncertainties as well as geological uncertainties such as boulder surface erosion, cover by sediment, snow or vegetation, and post-depositional boulder movement. The 10Be ages presented here are calculated using a 10Be production rate determined at a similarly low-latitude, high-altitude location (Kelly et al., 2013) with an estimated uncertainty of ∼6% (http://cosmognosis.wordpress.com/). Measurement uncertainties are <3.5% (Table DR1). The 10Be ages show excellent internal consistency on individual moraines and suggest that the geological uncertainties are small. For example, two boulders from <10 m apart on the Mahoma-2 moraine (RZ-2, RZ-3) yield nearly identical ages of ∼24,480 ± 470 and 24,790 ± 480 yr ago (Figs. 1 and 2). Moreover, the 10Be ages are consistent with the crosscutting relationship of the moraines showing that the Mahoma-1 moraine is younger than the Mahoma-2 moraine. Our 10Be moraine chronology is also consistent with a radiocarbon age on organic material in a sediment core from Lake Mahoma (uncalibrated age is 14,750 ± 290 14C yr B.P.; Livingstone, 1962) This material (comprising ∼20 cm of organic-rich sediment located 60 cm above gravelly till at the base of the core) provides a minimum-limiting age on glacial retreat of 17.2–18.6 kyr B.P. (2σ calibrated age range based on IntCal13; Reimer et al., 2013).