Records of forest fire occurrence in tropical South America are sparse prior to the advent of satellite monitoring. As a result, basic understanding of the relationships between past fire activity, climate variability, and human societies is limited. Amazonian forest fires can significantly affect the global carbon cycle through redistributing large amounts of organic carbon between the atmosphere, biosphere, and soils. Fires also impact forest health, biomass abundance, and biodiversity. This research project will relate past forest fire activity and its connections to climate variability and human activities by providing the first annually resolved tropical South American paleofire records using molecular organic carbon signatures preserved in high-altitude Andean ice fields. A late Holocene biomass burning record spanning the last 1,000 years, thereby including both the medieval warm period and little ice age, will be generated using the Quelccaya (Peru) ice core, which is well situated to receive organic carbon inputs from the Amazon Basin and Andes and affords remarkable temporal constraints. The investigators will use newly developed analytical techniques to identify and quantify numerous trace-level organic compounds in small volumes of ice, providing high-resolution, multi-molecular records that will describe fire occurrence and the type of material that burned as well as direct emissions from fresh vegetation. The project will provide information about high-altitude carbon cycle dynamics through analysis of vegetation-derived organic carbon in several other ice cores along and straddling the Andean range, which will constrain the depositional fate of aerosols generated during burning. Organic carbon sequestration achieved through incomplete burning will be investigated by characterizing higher plant-derived organic carbon associated with ice core mineral dust and black carbon particles, with its persistence determined through radiocarbon analysis. The variability observed in 20th-century fire records previously developed using this approach is pronounced and quasi-periodic and differs from that of Quelccaya ice core oxygen isotopic and dust records, suggesting that ice core organic geochemical data encodes unique climatic and anthropogenic signals. By extending these records back to roughly 1,000 AD, this project will provide valuable information about tropical South American forest health across major climate shifts and societal developments, providing a basis for determining the role of this important resource and organic carbon pool in future climate change.
This project will provide valuable new information and insights regarding biomass burning variability, tropical vegetation fire impacts on the carbon cycle, and the relationships between fire occurrence, climate, and human activity over the last 1,000 years, during which time Amazonian populations expanded and subsequently declined, global climate changed significantly, and industrialization occurred. The biomass burning information generated by this project will help infer changes in the health of Amazonian forests, which are a vital natural and economic resource subject to change from both natural and human-related processes. Amazon Basin vegetation represents a vast sink or source of atmospheric carbon dioxide. Understanding how past changes in Amazon Basin vegetation were connected to global climate variability is important in determining the role of tropical forests in future climate change.
We analyzed specific organic compounds preserved in ice cores from the Peruvian Andes in order to reconstruct South American forest fire activity over the last 1,000 years. High temporal resolution Amazon Basin burning histories were previously available only for the satellite era, and thus very little was known about trends in fire activity prior to 1980. Here, we extended the seasonal-scale burning record to 1882 AD, and investigated burning trends at lower temporal resolution as far back as 972 AD. In order to develop robust burning records, we first examined the organic matter present in the ice cores and identified a specific compound produced during burning of deciduous plants, which appears to be well preserved over centennial time scales. We confirmed its origin by identifying the compound in plant smoke but not in fresh leaf material. These experiments provide the basis for a new organic geochemical proxy for biomass burning that can be used in studies of past climate change and carbon cycling. Using this geochemical burning proxy, we generated records of past forest fire activity in ice cores collected from the Quelccaya and Coropuna ice caps situated in the eastern and western Peruvian Andes, respectively. The Quelccaya record suggests strong and variable fire activity during the Twentieth Century that is associated with climate variability, as observed through changing ocean temperature gradients. The observed burning trends likely reflect an interaction between climate and human activities in the nearby Amazon Basin, such as deforestation. Strong burning variability since 1960 was also inferred from the Coropuna ice core, although compound concentrations were lower than for Quelccaya, likely reflecting the greater distance of this site from Amazon Basin forests. For Coropuna, biomass burning aerosols may have been largely derived from sources west of the Andes. Ongoing interpretation and publication of results will continue to help refine our understanding of South American fire activity and its causes. This project also supported continuing public education efforts regarding ice core research.
