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.
The record we generated offers a first indication of Amazonian burning trends prior to 1980, which marked the advent of fire monitoring by satellite. Our findings of climatically influenced burning, a rapid response of fire dynamics to climate change, and the potential for protracted strong burning over many successive dry seasons will help inform future forest management efforts. Understanding the sensitivity of Amazon Basin forests to climate change and their potential role in carbon cycle feedback systems will be important to forest scientists and policy makers as atmospheric CO2 abundances increase over time. This project enabled extensive repair of the gas chromatograph/time-of-flight mass spectrometer at WHOI, which has recently been adapted for use with a FID. We have helped to establish the extent of climatic and human influences on annual burning in tropical South American forests, which represent an invaluable natural and economic resource. Our constraints on fire dynamics provide new information for forest managers and policy makers, who will benefit from an improved understanding of climate’s role in ecological change. The ability to make informed decisions about forest management will be especially important as climate changes in the future. Our study represents the first application of the MH vegetation burning proxy, supporting the use of a new organic geochemical marker for environmental reconstruction. This compound was not previously used to generate burning records because routine sample processing methods typically mask its presence. Specifically, esterification in the laboratory of alkanoic acids artificially produces the same methyl alkanoates we observed in ice. We have avoided using such methods by employing stir bar sorptive extraction and thermal desorption, and thus were able to provide evidence for a natural source of MH. We then authenticated the proxy by identifying high abundances of the compound in deciduous leaf smoke (Figure 1). Our results suggest that MH is well preserved during atmospheric transport and after deposition compared to other semi-volatile compounds, enhancing its utility.
