This grant supports research generating continuous, centennial to millennial-scale records of mountain glaciation in Peru spanning the Holocene (~12 ka to present) that tests hypotheses concerning the causes of abrupt climate change in the tropics. Radiocarbon and 210Pb dating of glacial flour flux provides precise ages of Holocene ice advances/retreats and records of abrupt climatic transitions, using proglacial lake sediment cores from multiple lakes along the steep East-West moisture gradient across the central Peruvian Andes. The flux of glacial flour is 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 demonstrates 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. This approach has 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 is coupled with detailed moraine mapping and the targeted application of cosmogenic radionuclide dating to select Holocene moraines located up-valley from coring localities. The strategic pairing of glacial flour records with dated moraines provides both the timing and magnitude of ice margin changes. Inverse glacier mass balance-ice flow simulations from multiple watersheds in different precipitation regimes provide validation for the dynamical down-scaling of global-scale circulation and paleoclimate models. Intellectual Merits and Broader Impacts: Relating the changes in tropical glacier mass to climate dynamics is a priority for global climate modeling efforts to predict future changes based on accurately simulating past changes under different greenhouse scenarios. The long-term perspective of temperature and precipitation changes on tropical glacier mass and energy balance is still poorly understood, primarily because of a lack of detailed and continuous glacial records from both the humid and more arid parts of the tropical Andes. This work substantially improves our understanding of the link between alpine glacial variability, water resources and mass and energy fluxes in the tropical "heat engine" of the planet. These records are needed to better understand the timing, magnitude, and spatial extent of high elevation (5000 - 6000 masl) tropical atmospheric change during the Holocene to evaluate the role of the low latitude hydrologic cycle in abrupt global climatic shifts. This, in turn, will enable testing of the following hypotheses: (1) Glacier margin fluctuations in the tropical Andes during the Holocene were driven by changes in the strength of the South American Summer Monsoon (2) A broad regional pattern of Holocene glacial variability was punctuated by periods of rapid ice advances every ~1000 to 1500 years (3) Abrupt (centennial to millennial-scale) Holocene glacier variations in the southern tropics were nearly synchronous with those in the Northern Hemisphere (i.e. The Little Ice Age)
This project was undertaken to determine the chronology of glaciation and climate change in the tropical Andes Mountains of Peru. Becasue glaciers are sensitive to climate change, variations in the former aerial extent of glaciers can tell us about changes in regional climate. Critical to this research is the successful dating of former ice positions. For this, we used a combination of cosmogenic radionuclide dating of the exposure of boulders on moraine crests, and radiocarbon dating the variation in flux of glacially derived sediment to proglacial lakes. Both of these methods have been widely applied around the world to document the former extent of alpine glaciers. We undertook a similar set of procedures in both the eastern and western cordilleras of Peru in order to deconvolve the effects of changes in temperature and precipitation on the histroy of ice cover in the tropical Andes. Because the bulk of the moisture that nourishes glaciers in the tropical Andes comes from the tropical Atlantic Ocean, there is a steep E-W precipitation gradient across the tropical Andes. On the western side of the Andes, mean annual precipitation (MAP) is less than 50 cm, and glaciers only exist on the very highest peaks today. From a paleoclimatic standpoint, this region is highly sensitive to precipitation change, given that precipitation is the glacier-limiting climatic variable in this region. In contrast, on the eastern side of the Andes, MAP can exceed 300 cm, and glaciers can exist at much lower elevations. This region is far more sensitive to changes in temperature than the western side of the Andes, and taken together our glacial geologic studies in both regions will delineate records of both temperature change and precipitation change in the tropics. Our results reveal that the two cordillera share a similar history of glaciation with several exceptions. The last glacial maximum in both cordillera occurred ~25,000 years ago; glaciers retreated rapidly beginnning as early as 20,000 years ago in response to a naturally-induced interval of warming. Glaciers stabilized and/0or readvanced between 13,000 and 15,000 years ago in both coridllera before retreatin grapidly during the interval from about 13,000- 11,000 yr BP. The early Holocene was marked by extensive glacier readvances in both cordillera that lasted from about 10,000 to 8,000 years ago. The interval from about 8,000 to 500 yr BP is seen in the eastern cordillera as an interval of reduced ice extent, but in the western cordillera this interval was punctuated by glacier readvances during the last past 2000 years. This difference between the eastern and western cordillera during the last 2000 years may indicate that this interval of expanded ice cover may ave been driven more by increased precipitation than by decreased temperature. As results are proceessed more fully, we will be able to make additional hypotheses regarding the climatological significance of differences in glacier history between the eastern and western cordillera. At this, time, however, we have generated several well-dated glacier chronologies for the tropical Andes and we have identified intervals during which glaciers were not waxing and waning in unison.
