Lacustrine and Moraine records of Holocene Glaciation in the Central Peruvian Andes
What: We will generate continuous records of mountain glaciation in Peru that span the Holocene (~12 ka to present) through an approach that combines the acquisition and analysis of lake sediment cores with moraine dating using both lichenometry and cosmogenic radionuclides (10Be and 26Al). The pairing of paleolimnology with moraine dating will enable determination of both the timing and magnitude of ice margin changes through the Holocene with sufficient precison to enable rigorous testing of hypotheses concerning the causes of abrupt climate change in the tropics.
When: approximately June 15-July 15
Where: we will travel to Lima Peru where we will spend several days purchasing maps and organizing field equipment. We will then travel via 4X4 pickup truck across the central Peruvian Andes into the upper Amazon Basin. We will work initially in the easternmost glaciated mountain range in the central Andes where we will spend about 1 week coring lakes, mapping moraines, and collecting samples for cosmogenic radionuclide dating of moraines. We will then return westward to the high western Andes where we will conduct similar projects.
Who: A team of 3 students, Professor Donald Rodbell from Union College, Professor Nathan Stansell from Ohio State University, and two Peruvian field assistants.
Project Overview and Goals
Radiocarbon and 210Pb dating of continuous records of glacial flour flux will provide precise ages of Holocene glacier advances/retreats, and will document abrupt climatic transitions. Proglacial lake sediment cores from multiple lakes along the steep east-west moisture gradient across the central Peruvian Andes will be obtained. The flux of glacial flour will be determined based on multiple proxies at a resolution sufficient to enable comparison with existing stable isotope records of paleoclimate variability from the region. Previous work in the tropical Andes has demonstrated that the glacial-flour approach can provide a record of glaciation that is both consistent with and far more continuous than radiometrically-dated moraine records. However, this approach has yet to be focused specifically on the Holocene yet it holds the potential to resolve several glacial geologic uncertainties, such as the timing of early Holocene glacial advances, and the possible time-transgressive nature of ice margin fluctuations during the neoglacial. The glacial-lacustrine approach described here will be coupled with detailed moraine mapping, lichenometry, and the targeted application of cosmogenic radionuclide dating to select Holocene moraines located upvalley from coring localities. The strategic pairing of glacial flour records with dated moraines will provide both the timing and magnitude of ice margin changes. This research has three key goals:
- Produce centennial-scale records of glacial flour flux using geochemical analyses of proglacial lake sediment cores from sites that span the steep precipitation gradient in the central Peruvian Andes.
- Determine the age of moraines using cosmogenic radionuclide (CRN) dating methods to provide information about both the timing and extent of major Holocene ice advances.
- Test the scale and climatic forcing of Holocene glacier variability by using inverse modeling of valley-specific paleoglaciers and comparing results from this with available regional paleoclimate proxy data.
Geologic Background
Mountain glaciers are one of the best recorders of atmospheric change over the continents, and numerous workers have highlighted the importance of glacial deposits in tropical paleoclimate studies. Glaciers are also an important water resource in the tropics, and documenting the timing and causes of past variability is needed to predict future runoff changes. Studies of ice volume change in the tropics, the heat engine of Earth, provide useful information about past shifts in atmospheric water vapor content, and such studies are important for understanding the role of the low latitude hydrologic cycle in modulating global temperature and moisture-balance fluctuations. However, these studies are currently confounded by a discontinuous record of Holocene glacial variability, one that is only broadly defined: restricted ice cover early in the Holocene, followed by a regionally complicated glacial history during the middle and late Holocene.
While there are no current studies on modern sediment yields in glaciated catchments in the tropical Andes, climate and topographic conditions suggest that rates could be relatively high and closely related to climate-mediated glacier mass balances changes. Given a homogeneous temperature regime, tropical glaciers are highly sensitive to moisture related fluxes and variables such as accumulation, albedo, cloudiness, atmospheric long wave emission, and sublimation. Tropical Andean glaciers are warm-based, have strong vertical mass balance gradients, and are marked by year-round ablation. Seasonally, maximum rates of ablation are coincident with maximum accumulation, as melt rates increase 30% in the wet season. With high annual precipitation, mass turnover is sizeable and glaciers have short response times to changes in net balance, as demonstrated by the synchrony of hydrologically-derived mass balance with observations of terminus positions. Variations in glacial flour flux should, therefore, closely correspond to variations in ice extent with a lag time of less than a few years.
Recent studies focusing on Holocene glaciation in the Southern Hemisphere have illustrated both the exciting potential for better moraine chronologies to elucidate global climate dynamics, and also the limitations of inherently discontinuous moraine records for discerning the relative scale (“footprint”) of local and global forcing on glacier changes. There remains uncertainty about the relative timing of glacier advances across the globe during the Holocene, and whether these events are globally synchronous or if significant leads and lags took place. Our proposed research will advance the scientific understanding of climate change in the tropical Andes by integrating lake sediment core analyses, moraine mapping and dating, and glacier mass-balance studies. Our research will improve the knowledge of the timing, extent and causes of abrupt low latitude temperature and moisture-balance changes during the Holocene, and the role of the tropical hydrologic cycle in global climate dynamics. This study will produce multiple high-resolution (centennial-scale) records of glacial flour flux in the Central Peruvian Andes spanning the Holocene.
Possible Student Led Projects
There are numerous possible student-initiated projects that could be developed. Selection of projects will depend on student background and available analytical facilities at students’ home institutions. Student projects could include:
- Lake Coring. Radiocarbon and 210Pb dating of continuous records of glacial flour flux will provide precise ages of Holocene ice advances/retreats, and will document abrupt climatic transitions. Proglacial lake sediment cores from multiple lakes along the steep east-west moisture gradient across the central Peruvian Andes will be obtained. The flux of glacial flour will be determined. Although the primary objective of the project is the development of glacial flour records from proglacial lakes, the cores and the radiocarbon and 210Pb-based chronology developed for them can be used for the development of other proxy paleoclimatic indicators. These include charcoal, pollen, biogenic silica, and sediment provenance studies using geochemical and mineral magnetic indicators. In addition, carbonate lakes containing authigenic calcite could be cored for the development of oxygen isotope records of hydrologic balance.
- Mapping and lichenometric dating of late Holocene moraines. Detailed late Holocene moraine maps based on aerial photographs and field mapping will be essential to this project. Lichenometric dating of late Holocene moraines is receiving renewed interest as a viable chronometer in the tropical Andes. Because this project involves field work along the steep E-W climate gradient in Peru, regional growth curves for Rhizocarpon geographicum will ultimately need to be developed in order to derive numeric age estimates for Holocene moraines from lichen data. CRN dating of moraines coupled with limiting radiocarbon dates could be the basis for growth curve development.
- Mapping and cosmogenic dating of early Holocene moraines. Early Holocene moraines will be mapped in the field using differentially corrected global positioning systems, detailed topographic maps, and aerial photographs. At least 4 samples per moraine will be collected, and we anticipate dating between 5 and 10 moraines. Boulders of granodiorite, quartize and felsic volcanic rocks are found throughout the proposed field sites, and will be targeted for sampling because they are high in quartz and are ideal for CRN analyses. Surface exposure ages of boulders will be dated using the concentration of cosmogenic 10Be and 26Al in quartz.
- Bedrock mapping and sediment provenance studies. The majority of our targeted watersheds have bedrock types with compositions that vary up-valley, and clastic sediment in these systems are derived from both glacial and non-glacial processes. It is therefore important in our glacial flour studies to accurately map and “fingerprint” the possible sources of sediments that are deposited in the lake basins and moraine deposits. Till matrix and bedrock samples representing the exposed units in the field area will be collected in order to geochemically identify source materials for clastic lake sediments. Students will crush, pulverize, and digest using “near total” methods and measure for elemental chemistry using the ICP-MS at Union College. Lake sediment samples from the collected cores will also measured using the same methods. Discrimination plots, or similar methods, will then be used to illustrate the relative abundances of these elements that characterize different sediment provenances.
Logistics/Special Field Conditions
For much of the Project, we will be living in tents at elevations between 14,000 and 16,000 feet above sea level. At these elevations, diurnal temperature variations are large; nighttime temperatures are below freezing and daytime temperatures can exceed 70° F. We will be working during the austral winter, which is also the dry season, so we can expect clear cold nights and clear, sunny days. However, snowfall and especially hail can occur at any time. We will be working around the margin of active glaciers, but not on the glaciers themselves. Fieldwork will be punctuated by overnights in inexpensive Andean hotels that lack many of the amenities to which we are accustomed.
Physical fitness, and experience and comfort living in tents for extended periods are essential. Although “at altitude, attitude is everything”, some basic field gear is essential. The Project will provide tents and sleeping mattresses, but students must have their own sleeping bags (rated -15°C or lower), Gore Tex (or equivalent) rain gear, hiking boots, and backpacks.
Recommended Courses/Prerequisites
In order to be considered for this project, students will need to have had a course in Geomorphology and/or surface processes; courses in Glacial Geology, Paleolimnology, and GIS are strongly recommended. Outdoor experience in an alpine setting, especially at high altitude would be beneficial, and proficiency in Spanish would be very useful.
