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 work combines high resolution pro-glacial lake sediment records with cosmogenic ages on moraines to provide detailed and continuous archives of glacial variability during the Holocene. The results of our work on lake sediments thus far were summarized in the recent review paper that was published in Quaternary Science Reviews. Here we discuss how records from the southern tropical Andes provide supporting evidence that the early Holocene (between 12 and 8 ka) was relatively warm and dry, and the middle Holocene (between 8 and 4 ka) was marked by a shift to cooler, and possibly wetter conditions in certain regions, leading to glacial advances. This paper also highlights that glaciers in multiple valleys generally retreated from ~4.0 ka through the Medieval Climate Anomaly (1.0 to 0.7 ka). This late Holocene pattern of ice retreat occurred during a period when lake level studies, and both lacustrine and speleothem stable isotopic records indicate wetter conditions relative to the middle Holocene, suggesting that higher temperatures contributed to the pattern of ice retreat. Following this period of glacial retreat, multiple proxy records suggest that the start of the Little Ice Age (~0.6 to 0.1 ka) was a colder and wetter time throughout much of the tropical Andes. In this paper we concluded that there are two primary synoptic-scale climatic controls on temperature and precipitation linked to insolation dynamics that drive changes in ice cover in the southern tropical Andes during the Holocene: 1) the strength of the South America Summer Monsoon, which is linked to Northern Hemisphere temperatures and the mean position of the Intertropical Convergence Zone over the Atlantic, and 2) sea surface temperature distributions in the tropical Pacific Ocean and its influence on atmospheric temperature, precipitation and circulation patterns. Our most substantial findings from the 2011 fieldseason and subsequent lab work came from the Yanacocha record that will be submitted for publication in 2014. Lake Yanacocha (10°33.590 S, 75°55.815 W, 4,360 m a.s.l.) is located on the relatively wet eastern slope of the Central Peruvian Andes in valley watershed adjacent to Nevado Huaguruncho. Early results of these records tentatively suggest that glaciers advanced during the early Holocene, but more work involving the CRN samples is in progress to document the presence of ice at that time. These data also suggest that most of the remaining Holocene was a period of restricted, or absent, ice cover until the very late Holocene (last ~1000 years). Another interesting aspect of this work came from evidence in the lake sediments of pronounced aridity in the region during the mid-Holocene. We are currently working on stable isotope records from those lake sediments to compare to other archives in the region in order to further evaluate the pattern of shifting climate over the last several millennia. Training and Development We have trained several undergraduates through this project. This work has also provided early career development for the co-PI Stansell Outreach The Byrd Polar Research Center (BPRC) hosted outreach events to provide updates of changes in Peruvian glaciers over the past twelve thousand years. Bryan Mark presented his studies of the timing and impacts of changes in glacierized landscapes and the perceptions of people who depend on glacier meltwater downslope. Nathan Stansell provided his studies of longer-term environmental changes in the region as determined from lake sediments and methods that can be used to date the exposure of rocks in glaciated terrains. In addition to the guests in the audience, the events were streamed live on the Internet. The presentations were recorded, and the videos are available on the BPRC homepage. Publications A review paper was published in Quaternary Science Reviews in February, 2013. This article summarizes our work in the western Cordillera of Peru, in the Cordillera Blanca, Cordillera Raura and Cordillera Huayhuash. We also incorporated our previous work in the tropical Andes involving stable isotope archives of past precipitation changes. A follow-up review paper will be submitted by Stansell and coauthors in August, 2013. This second review focuses on a comparison of the Peru records to a composite of records from the northern Andes of Venezuela. Other papers are in preparation regarding the Yanacocha cosmogenic ages and lake sediment data.
