HOLOCENE GLACIATION IN THE AFRICAN AND SOUTH AMERICAN TROPICS

The ongoing retreat of tropical glaciers is an emblem of modern climate warming. Yet there exist few records of past Holocene glacier fluctuations in the low latitudes that we can use to contextualize modern glacier recession. This is particularly true for the early- and middle-Holocene period, when tropical temperatures were as warm as or warmer than today. We use surface-exposure dating to assess the timing of Holocene glacier fluctuations in the Rwenzori Mountains, Uganda, and compare these results with similar glacial chronologies from elsewhere in the tropics. In the Bujuku valley, ¹⁰Be surface-exposure ages from moraines and rockfall deposits suggest that glaciers retreated rapidly up valley prior to ~11.0 ka. The historical ice extent at the head of the valley is marked by a rampart moraine that dates to ~0.3 ka. No glacial landforms are preserved between the ~11.0 ka rockfall and the historical rampart moraine. In the more southern Nyamugasani valley, perched boulders on bedrock indicate that ice retreated rapidly after ~11.2 ka. In addition, three bedrock samples collected from an unoccupied cirque near the valley head yield ¹⁰Be concentrations equivalent to ~5.2 ka of exposure. Aerial imagery indicates the cirque site was covered by ice in 1936. These bedrock samples therefore indicate that the cirque was ice free for at least ~5 ka during the Holocene, and that ice was inboard of its early 20th century extent during this time. Comparison of these data with previously reported Holocene glacial chronologies from tropical South America suggest early-Holocene deglaciation and late-Holocene readvance occurred in both regions. Moreover, paired in-situ ¹⁰Be/¹⁴C analysis of bedrock samples from both the Rwenzori and from Quelccaya Ice Cap, Peru, indicate that ice extents at both locations were restricted for much of the early and middle Holocene. These similarities suggest that glaciers in both tropical South America and Africa responded to a common forcing during the Holocene, most likely temperature. Lake-sediment-based temperature reconstructions from the African tropics suggest a period of middle-Holocene warmth was followed by late-Holocene cooling, coherent with the pattern of glacier fluctuations inferred from surface-exposure chronologies.