This collaborative grant is collecting paleoclimate proxy data from locations in Southern Hemisphere tropical and mid-latitudes and using these data to constrain global circulation model (GCM) simulations of climate change. Through this coordinated effort, the research enhances our knowledge of potential mechanisms which influenced Holocene climate events such as the Little Ice Age. The investigators have three specific aims: 1) developing a continuous record of Holocene climate change near the largest tropical ice mass, Peru's Quelccaya Ice Cap, using multiple paleoclimate proxies, including chironomid and diatom assemblages in lake sediments; 2) tracking Little Ice Age climatic conditions over a broader area of the Andes, from ~13 to 40°S latitude; and 3) using a GCM to evaluate which mechanistic hypotheses explain the geographical and temporal patterns of reconstructed paleoclimatic fluctuations. This research establishes a detailed multi-proxy record of the Holocene paleoclimate and paleoenvironment in the southern tropics near the Quelccaya Ice Cap. Defined by radiocarbon ages and supplemented by surface exposure (10Be) ages, this chronology is comparable with higher-latitude records from both Northern and Southern Hemispheres. In addition to the temporally detailed record from Quelccaya, the research provides a spatially resolved reconstruction of climate during the Little Ice Age in the southern tropics and mid-latitudes. The broader pattern of Little Ice Age Andean glacier fluctuations in multiple climatic regimes also provides valuable data for examining the relative influences of temperature and precipitation on such fluctuations. These efforts interface with ongoing NSF-funded research (EAR-0902363) investigating Holocene glacier fluctuations in Patagonia, enhancing both research projects. This research provides both paleoclimate proxy data and modeling results to further the understanding of Holocene and recent climate change in the southern tropics and mid-latitudes. The research tests various mechanisms for the Little Ice Age. For example, it examines results from modeling of solar forcing during the Little Ice Age which suggest that there were changes in tropical circulation in addition to Northern Hemisphere climate changes. Since the paleoclimate data from near Quelccaya Ice Cap spans a time period influenced by differing boundary conditions, such as high and low austral summer insolation, it should be possible to examine various mechanisms for rapid climate events. Possible mechanisms include a latitudinal shift of the Intertropical Convergence Zone and the strengthening/weakening of the El Niño Southern Oscillation (ENSO).
The broader impacts of the research include (1) applying and evaluating a promising proxy (chironomid assemblages) method in a new area (tropical Andes) and archiving sediment cores; (2) strengthening and establishing new international and interdisciplinary collaborations among scientists including Dr. Pedro Tapia of Peru and Chilean scientists; (3) training graduate and undergraduate students; and (4) ensuring outreach and dissemination to the general public of information regarding climate change.
Intellectual Merit: This collaborative project was designed to increase our understanding of the natural mechanisms of climate change. We focused our research in the Southern Hemisphere tropics and mid-latitudes, where less is known about past climate than in the Northern Hemisphere or the Polar Regions. The project used past fluctuations of glaciers in South America to investigate and define past climate conditions. We studied Peru’s Quelccaya Ice Cap, the largest tropical ice mass, and determined its past extents using both mapping and 10Be surface exposure dating of glacial deposits as well as analyses of sediment cores obtained from nearby lakes. The research is important because an understanding of natural climate change will help us evaluate the possible effects of human-induced influences on the climate system. Our detailed mapping and dating of glacial moraines deposited by Qori Kalis glacier, a large outlet glacier of Quelccaya Ice Cap, show that it reached its Holocene maximum extent just prior to ~AD 1490 and experienced retreat from ~AD 1490 to 1710 to within its AD 1963 position (Fig. 1). Comparison of this ice-marginal record with ice core records from atop Quelccaya shows no correlation between ice-cap fluctuations and net accumulation (Fig. 2). Therefore, we suggest that the late Holocene fluctuations of Quelccaya were influenced by temperature. This result is consistent with our project collaborators’ modeling results that indicate temperature as a primary factor influencing ice-cap fluctuations. The late Holocene fluctuations of Quelccaya Ice Cap, as well as other glaciers in tropical South America, are similar in timing to glaciers in the northern and southern mid-latitudes, suggesting common global forcing. However, whereas glaciers in Alaska and Switzerland reached their late Holocene maximum extents in the past few hundred years, Qori Kalis and New Zealand glaciers reached their maximum extents earlier, and experienced an overall pattern of retreat through late Holocene time. Since most evidence for prior Holocene ice extents was eroded or covered by the late Holocene advance, we obtained and analyzed sediment cores from two nearby lakes to develop a longer record of Quelccaya Ice Cap extents. One lake, Challpacocha, receives meltwater directly from Quelccaya Ice Cap and, thus, registers changes in Quelccaya Ice Cap extents reflected by sediment input. The other lake, Yanacocha has not received meltwater from Quelccaya Ice Cap since at least ~12.6 ka, and provides a record of ambient climate conditions. By comparing the Challpacocha sediment record with the late Holocene extents of Quelccaya Ice Cap defined by 10Be dated glacial moraines, we built a model for sedimentation associated with ice cap advance and retreat. Interestingly, we show that glacial sediments were deposited primarily during periods of ice cap recession. This result is important for other scientists who use lake sediments to interpret past ice extents. Using this knowledge and our collaborators’ work on nearby glacial deposits, we suggest that Quelccaya was significantly reduced in extent during early Holocene time, and that ice-cap expansion occurred during the middle to late Holocene. This pattern of Holocene ice-cap fluctuations is similar to that of glaciers in the northern hemisphere mid-latitudes and suggests climate mechanisms that link these regions. Broader Impacts: Results from this project provide information about the geographical and temporal patterns of past climate change. Understanding these patterns provides a means for testing various mechanisms for climate change and predicting what may happen to existing Andean glaciers. We demonstrated that, at least during late Holocene time, one tropical glacier (Quelccaya Ice Cap) likely responded sensitively to temperature changes. This result was highlighted in an article in the New York Times in February 2014. Our results also provide insight into interpreting sedimentation in glacially fed lakes which is beneficial for other scientists using similar methodology. This project developed new collaborations among a diverse group of scientists in the United States, Peru and Chile. It was interdisciplinary in nature and brought together experts in the fields of geology, biology and computer modeling. At Dartmouth College, the project facilitated the development of the PI, an early career scientist, a laboratory technician, two Dartmouth graduate students and multiple undergraduate students. One graduate student completed his Ph.D. research in May 2014 and one will finish his Ph.D. in spring 2015. One undergraduate student participated in fieldwork and conducted her Senior Honors Thesis research associated with the project. She is currently enrolled in an earth sciences graduate program. PI Kelly discussed project data and interpretations in her courses, numerous lectures at various universities and at public presentations such as one for a local science café.
