Masters Theses

Date of Award

8-2006

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Claudia Mora

Committee Members

Theodore Labotka, Michael McKinney

Abstract

Atmospheric oxygen concentrations may have dramatically fluctuated throughout the Phanerozoic. Because oxygen is critical to fire, major oxygen fluctuations may have significantly affected biomass burning occurrences, suggesting that wildfires would have been more prevalent at times of elevated oxygen. Fossil charcoal abundances in the geologic record qualitatively reflect this relationship. Land plants evolved by the late Silurian, and the earliest true forests evolved by the mid-Devonian, while oxygen levels were still low. Fuel accumulations and oxygen content probably did not reach sufficient levels to sustain burning until the late-Devonian. There are only one or two isolated examples of fossil charcoal of this age. Oxygen levels rose to ~35% in the Carboniferous, and fossil charcoal of this age is globally abundant. During the Triassic, oxygen levels fell to ~15%, and fossil charcoal is nearly absent from the rock record. Based on the empirical association between modeled oxygen and charcoal abundance, an implicit relationship between biomass burning and oxygen level exists. By targeting a set of combustion-derived organic compounds, polycyclic aromatic hydrocarbons (PAH), the qualitative nature of this relationship can be approached quantitatively. During times of elevated oxygen level, biomass burning events are facilitated, and therefore, a greater proportion of PAH are expected to be produced and preserved in the geologic record.

To test the hypothetical relationship between atmospheric oxygen level and PAH production, charcoals were collected from Carboniferous- and Triassic- age rocks, which relate to the purportedly highest (~35%) and lowest (~15%) levels of atmospheric oxygen (Berner, 2001). To isolate the role of atmospheric oxygen in the development of PAH, sampling localities were chosen based upon their similarities to the most-probable vegetation source for charcoals, in the context of the ancient depositional environment. The occurrence and abundance of 16 PAH species extracted from these samples suggest that a relationship exists between atmospheric oxygen level and PAH content, but undoubtedly, a larger empirical data set, in conjunction with combustion experiments to produce PAH under controlled oxygen levels, are necessary to fully delineate the role of oxygen relative to other parameters such as humidity, fuel moisture, length of burn, combustion temperatures, and fuel geometry.

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