The role of the tropics in climate change has important implications for understanding both orbital-scale and abrupt climate variations. Yet our ability to assess tropical behavior during major climate events, such as the last glacial maximum (LGM), is limited by poor spatial coverage and insufficient control on sample ages. This project will address this problem by developing well-dated records of glacial fluctuations from the LGM through the termination and late-glacial period at Nevados Coropuna and Allinccapac in southern Peru and use these data in numerical simulations of glacier mass balance and local climate. These sites allow an examination of glacier variations, as well as coeval snowline changes, along a transect from the arid (Coropuna) to the humid (Allinccapac) Andes and thus document how major climate events may have been expressed in areas with distinctly different environments. This work consists of detailed mapping of moraines; precise surface-exposure age dating (3He and 10Be) of carefully selected boulders from moraine crests and drift edges; basal 14C ages of bogs interspersed among moraines; calculation of former snowline depression; and modeling of the relationship between glacier mass-balance changes and climate. The work will be an important step towards understanding tropical behavior and will finally allow a thorough testing of the Milankovitch hypothesis of ice ages in the tropics.
Broader Impacts: This research educates and trains students, a postdoc, and a recent female Ph.D. There is a lecture series on local geology and global climate change given to eco-tourism students at the University of Arequipa, as well as lectures to the Lima archaeology department. K-12 students benefit through a long-standing associations with classrooms in rural, commonly economically disadvantaged Maine. The project maintains a website and is exploring a learning module. In addition, the research is part of a joint initiative to understand land use and settlement patterns of the first Americans in the Peruvian highlands.
High-altitude glaciers in tropical regions are important and vulnerable elements of the environmental system (see photo below). But most importantly, these glaciers provide meltwater during the dry season, and, in turn, help to feed large down-valley cities like LaPaz, Bolivia, water the fields for agricultural production and turn the turbines of hydropower plants in this part of the planet. Almost all tropical glaciers are shrinking, but to plan strategically for the well-being of society in the region, we need to know the rate of glacier change in response to climate. Existing instrumental records of glacier change only span a few decades, which is too short to evaluate the physical coupling between glaciers and climate in this region. This collaborative NSF proposal, lead by scientists from the University of Maine and Columbia's Lamont-Doherty Earth Observatory, had the main goal to put the current glacier trends into a wider, longer-term context by studying rocks that record the ebb and flow of ice since the last ice age, over the past 20,000 years. and by evaluating the variability of different glacier systems, arid and wet, to similar climate forcing. To achieve this goal, we reconstructed amplitude and timing of glacier fluctuations during and after the Last Glacial Maximum at Nevados Coropuna (photo below) and Allinccapac in southern Peru. This approach affords an opportunity to examine glacier variations, as well as coeval snowline changes, along a transect from the arid (Coropuna) to the humid (Allinccapac) Andes and thus documents how major climate events may have been expressed in areas with distinctly different environments. Moreover, this work afforded sensitivity tests of glacier changes under rapidly changing boundary conditions during the last termination. We mapped the former glacier configurations in detail and measured cosmogenic nuclides in the surfaces of glacially deposited boulders to date the glacier margin positions. This new dating tool, referred to as 'Surface Exposure Dating' relies on the built-up of cosmogenic nuclides in near-surface rocks by interaction of secondary cosmic ray particles and target atoms in the rocks surface as a function of time. Analyzing the cosmogenic nuclide inventory in these rock surfaces allows for calculation of an exposure ages if the rate of cosmogenic nuclide production in the surfaces is known. One significant outcome of this project was an advance of cosmogenic nuclide method and an increase in robustness of these chronologies by application of a new cosmogenic nuclide production rate calibration in Peru. The produced glacier chronologies are the most comprehensive to date in this part of the world. The timing of the Last Glacial Maximum and its termination in Peru is consistent with detailed reconstructions from glaciers in southern mid-latitudes, namely New Zealand and Patagonia, making the case for hemispheric- or even inter-hemispheric climate drivers of mountain glaciers during this interval. Further, the chronologies from the dry and the wet glaciers in Peru overlap within uncertainties, indicating that the different glacier systems responded in tune to the changing climate. This project educated an early career scientist in the wide spectrum of scientific disciplines involved in this project. For the interested non-experts, we produced a video featuring this project, that is available under "www.ldeo.columbia.edu/news-events/climate-peruvian-andes-early-humans-modern-challenges"; and also documented as a 'Columbia Earth Institute - State of the planet blog' in detail the exciting field campaigns in this remote areas that are so critical for people in the down-valley areas (available here: http://blogs.ei.columbia.edu/tag/peru-glaciers/)
