Jump to: Reconstructing the past extents of Rwenzori glaciers • Modeling the past extents of Rwenzori glaciers • Reconstructing past temperatures
Overview
Our research has focused on reconstructing past temperature in the Rwenzori since the end of the last ice age (over the last approximately 30,000 years ago). We have done this using three primary methods:
- Reconstructing the past extents of Rwenzori glaciers using mapping and dating.
- Applying glacial modeling to infer climate conditions associated with past glacial extents.
- Reconstructing past temperatures using organic molecules in lake sediments.
Here we describe these methods in more detail and provide some of the materials we have produced.
We are grateful for funding for this research from the US National Science Foundation and the Comer Family Foundation
1) Reconstructing the past extents of Rwenzori glaciers
We reconstruct past extents of glaciers based on deposits such as moraines (ridges of sediment that mark the melting edge of a glacier). We map moraines using satellite imagery and field observations. You can explore some of the satellite imagery in a 3D model here (opens in new tab).
These images show moraines in the lower Mubuku valley and our mapping and interpretations of past glacial extents:
We determine the ages of moraines and other glacial features using surface exposure dating. This method uses the concentration of a cosmogenic nuclide in the surface of a rock at Earth’s surface to calculate the amount of time that the rock has been exposed to cosmogenic radiation (i.e., how long the rock has been at Earth’s surface). The cartoon below illustrates how the surface exposure dating method is used to date glacial moraines.
Maps and cartoon by Margaret Jackson
We have published a few papers about the timing and implications of past glaciation in the Rwenzori Mountains using 10Be surface exposure ages. These include Kelly et al. (2014), Jackson et al. (2019) and Jackson et al. (2020). If you cannot access these papers, please contact Meredith Kelly for a copy.
Here is a news release about the Jackson et al. (2019) paper.
As Comer Fellows through the Comer Family Foundation, Alice Doughty and Meredith Kelly worked with graduate students at the Northwestern University Medill School of Journalism to create these Medill Reports:
All of the 10Be ages we determined for the Rwenzori Mountains are available on the NOAA National Centers for Environmental Information here.
Here is an Interactive map of the Rwenzori Mountains, with geospatial data layers that we have created (opens in new tab):
Here are our mapping data for download:
- Digital elevation model and shaded relief (90 MB zip file). These are 30-meter resolution raster layers (GeoTIFF format) in the UTM zone 35 coordinate system.
- Vector geospatial data layers (0.5 MB zip file). These are shapefiles of surficial geology and other landscape features. These layers are also shown in the interactive map above.
For our mapping and general interest, we found these photographs of Rwenzori glaciers helpful:
Aerial photos taken by Mary Meader in 1937
Day’s Edge Productions Rwenzori film and photographs (see film “Snows of the Nile”)
2) Modeling the past extents of Rwenzori glaciers
We use glacial modeling to estimate a range of climate conditions that may have influenced the past (mapped and dated) Rwenzori glacial extents. We use a coupled two-dimensional ice-flow and mass-balance model, which includes a spatially distributed energy-balance model (Oerlemans, 1992; Anderson et al., 2010; Doughty et al., 2013). Model input includes a 100-m resolution digital elevation model (SRTM_v4; Jarvis et al., 2008) to represent the land surface and modern climate data from a range of sources (e.g., temperature data are from a weather station on Mt. Stanley (Lentini et al., 2011)) and precipitation data are from annual measurements from 1951-1954 by Osmaston (1989). We then change the climate conditions (e.g., temperature, precipitation and lapse rate) from the modern to “grow” glaciers and evaluate which conditions result in glaciers reaching the mapped and data extents.
Here is an example of a model run in which modeled glaciers reached near the glacial extents dated to ~21,000 years ago (at the end of the last Ice Age):
The figure above shows (left) mapped Rwenzori glacial extents with the targeted extent marked by a yellow start and (right) modeled glacial extents assuming a decrease in temperature of 2.7°C and a decrease in precipitation of 10% (both relative to modern) and lapse rates of 5.8, 6.7 and 7.0°C/km.
The figure above shows the range of temperature and precipitation changes (relative to modern) needed to reach the mapped and dated glacial extent with lapse rates of 5.8, 6.7 and 7.0 degrees C/km.
The model data and results are presented in a few papers. These include Doughty et al., 2020 and Doughty et al. (in review). If you cannot access these papers, please contact Meredith Kelly for a copy.
Here are some climate data we found useful for our modeling:
Rwenzori temperature data
Download spreadsheet (Doughty et al., in review)
Rwenzori precipitation data
The figure above shows annual precipitation across the Rwenzori Mountains based on rain gauge data from Osmaston (1989). The x- and y-axes are UTM coordinates and the colors show annual precipitation in mm/year. We made the figure by georeferencing the original precipitation contour map, and tracing and converting it into a raster in ArcGIS. These data are also available as a MATLAB structure file (rwen_100_annualmeanpptnsurface.mat) at 100-m resolution.
Determination of modern and past lapse rates for the Rwenzori was done by Loomis et al. (2017)
3) Reconstructing past temperatures using organic molecules in lake sediments
We use organic molecules known as “branched glycerol dialkyl glycerol tetraethers (brGDGTs)” as a proxy for past temperatures. These are membrane lipids produced by bacteria and are preserved in lake sediments. We applied this methodology in Lake Mahoma, located near the mapped and dated glacial moraines in the lower Mubuku valley. This methodology provides a means to develop continuous temperature records over time and is independent from the glacial-based temperature reconstructions. The brGDGT-based temperature records from Rwenzori lakes can be combined with brGDGT-based temperature records from lower elevations to calculate past lapse rates and evaluate lapse-rate changes over time.
The images below show coring on Lake Mahoma (top)and Lake Africa (bottom left) and the huge effort it was to transport the coring gear to the sites (bottom right).
The left image below shows an example of a sediment core we retrieved from Lake Mahoma. The brown material is organic-rich mud (known as “gyttja”) and the gray material is ground-up rock (clastic sediment). The top of the core is to the left in the image. The transition from the gray to brown in the core marks the time when glaciers retreated out of the lake basin. Right image below shows organic remains in the lake sediments that we use for radiocarbon dating.
We have published a few papers about brGDGT-based temperature reconstructions. These include Loomis et al. (2017) and Garelick et al. (2022). If you cannot access these papers, please contact Meredith Kelly for a copy.
All of the lake sediment data are available on the NOAA National Centers for Environmental Information here.
Please see our Research Team page to learn more about our research and contact us if you have questions!