Thesis Format

Monograph

Calibrating Oxygen-Isotope Values of Plant Char to Pyrolysis Temperatures

Author

Kassandra Pineda, The University of Western OntarioFollow

Degree

Master of Science

Program

Geology

Supervisor

Webb, Elizabeth

2nd Supervisor

Dech, Jeff

Affiliation

Nipissing University

Co-Supervisor

Abstract

Our objective was to improve a model that relates d¹⁸O of plant char to burn temperatures to understand forest fire intensity, by considering species effects. Wood, bark and leaves from different tree species were charred for 30 min at 200-900˚C. Lignin and holocellulose were extracted from each tissue and species to determine their weight percent. Overall, d¹⁸O of char decreased with increasing temperature. This trend was not a result of exchange with water vapor because d¹⁸O of char from dried and undried wood were not significantly different. The proportions of holocellulose and lignin in different tissues or the same tissues among different species did not consistently influence the loss of ¹⁸O from char with increasing temperature. However, the differential thermal stability of biochemicals contributed to the decrease in d¹⁸O with increasing temperature. The d¹⁸O of plant char can serve as a tool to identify forest fire intensity qualitatively.

Summary for Lay Audience

Currently there is a lack of suitable methods to measure burn temperatures (intensity) of past forest fires. Knowing the intensity of past fires can be telling of environmental factors of that time. Char is formed during natural fires when biomass is burned in the absence of oxygen. The goal of this thesis was to consider the influence of plant biochemical compositions on a model that relates the d¹⁸O of plant char to burn temperatures to understand forest fire intensity. Oxygen has two main stable isotopes, ¹⁶O and ¹⁸O. The ratio of ¹⁸O/¹⁶O in a sample relative to the ratio in an international standard is denoted by d¹⁸O. The d¹⁸O value a material changes based on formation conditions which are controlled by environmental variables. Plants are made of two main biochemicals: holocellulose and lignin. The d¹⁸O of these biochemicals contribute to the overall d¹⁸O of the plant. These biochemicals thermally degrade over different temperature ranges and this impacts the d¹⁸O of plant char. Wood from nine tree species, bark from seven tree species, and leaves from one tree species with varying proportions of biochemicals, were charred for 30 min at temperatures from 200-900˚C. Lignin and holocellulose were extracted from raw samples from each tissue and species to determine their weight percent. The d¹⁸O of plant char decreased by up to 28.4‰ with increasing charring temperature. This decrease was not attributed to isotopic exchange with water vapor emitted from sample because dried and undried wood of the same species charred at three different temperatures did not have significantly different d¹⁸O values. The degradation of biochemicals partially contributed to the decrease in d¹⁸O. Although the original proportions of biochemicals explained differences in d¹⁸O between raw tissues, no consistent pattern was observed between biochemical composition and the D¹⁸O_raw-char of char from different tissues of the same species, or char from different species of the same tissue type. Although this study was not able to explain variations in the d¹⁸O of char in terms of species biochemical composition, the d¹⁸O of plant char can be used to qualitatively classify forest fire intensity even if original species is unknown.

Recommended Citation

Pineda, Kassandra, "Calibrating Oxygen-Isotope Values of Plant Char to Pyrolysis Temperatures" (2024). Electronic Thesis and Dissertation Repository. 10513.
https://ir.lib.uwo.ca/etd/10513

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