Author: Elizabeth Denis

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Published: 2016

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The storage and release of organic carbon from the biosphere are influenced by temperature and precipitation through changes in plant productivity and in oxidative loss, such as fire and microbial respiration. The long-term fate of soil organic carbon during global warming is important because soil carbon is the largest terrestrial organic carbon reservoir and soil can serve as a sink or a source for atmospheric CO2. Soil carbon degradation is multifaceted as different pools of organic carbon in soils (e.g., fresh biomass, refractory soil organic matter, and thermally mature fossil organic matter) have different reactivity. Fire, an important component of ecosystems at a range of spatial and temporal scales, affects vegetation distribution, the carbon cycle, and climate. Because there are several variables and mechanisms are complex, it is difficult to predict future and infer past changes in both soil degradation and fire activity based on climate and environmental conditions. Examining changes in soil organic carbon, climate, and fire during past warming events, such as the Paleocene-Eocene Thermal Maximum (PETM), should help elucidate climate-carbon cycle relationships, especially effects that are expressed over long durations (e.g., 100 10,000 years).Abrupt global warming during the PETM dramatically altered vegetation and hydrologic patterns, and, likely, terrestrial organic carbon production and preservation. The PETM coincided with a negative carbon isotope excursion (CIE), signifying a large release of 13C-depleted carbon to the biosphere and a major perturbation to the carbon cycle. Bulk organic carbon isotopes (13Corg) are often used to identify the CIE, but in terrestrial sections the 13Corg CIE can be highly variable and distorted. It has been suggested that 13Corg values were highly variable because of soil carbon degradation by microbes and allochthonous (pre-PETM) fossil carbon inputs. Constraining the degree and extent of degradation is critical in identifying the 13C-depleted carbon source and understanding carbon cycling processes and possible underlying organic carbon destabilization mechanisms during the PETM. At three Paleocene-Eocene fluvial sites in the western USA, my co-authors and I test the hypothesis that there were increased degradation (soil carbon loss) and refractory (allochthonous) carbon inputs during the PETM. Clay minerals stabilize organic carbon, but we hypothesize decreased clay content and changes in mineralogy destabilized organic carbon during the PETM. If soil moisture was a control on soil organic carbon degradation, then sites with similar soil moistirue conditions would have a similar loss of organic carbon. Using polycyclic aromatic hydrocarbons (PAHs), combustion byproducts that are relatively resistant to degradation, as a proxy for intermediate refractory carbon helped to discern the relative preservation of different carbon pools in the soils. I developed a novel molecular metric of degradation by calculating the percent loss of PAHs relative to total organic carbon (TOC) to estimate the extent of organic carbon loss and proportion of refractory allochthonous carbon during the PETM. All forms of soil carbon decreased during the PETM, and PAH concentrations decreased even more than TOC, which suggests a more refractory phase was present, such as allochthonous fossil carbon. Positive correlations between elemental oxide weight percents (e.g., Al2O3 and TiO2) and TOC suggests organic carbon preservation was associated with clay minerals. Wetter sites had a greater percent loss of organic carbon during the PETM than drier sites. Reduced soil organic matter preservation during the PETM was due to a combination of increased temperatures (which increased microbial decomposition rates), decreased clay content and changes in mineralogy (which inhibited stability of fresh carbon), and fluctuations in soil moisture (which destabilized older, refractory carbon). Soil carbon degradation, even of intermediately refractory carbon, was not just a local phenomenon and was regional, and potentially global, in scope.In the marine sediments of the Arctic, where organic carbon was well-preserved during the PETM, we used PAHs as an indicator for fire and plant biomarkers, as well as published pollen data, to decipher the dynamics between fire, precipitation, and vegetation changes in the paleoecosystem. In modern ecosystems, climate influences fuel availability (e.g., vegetation), fuel flammability (e.g., precipitation and temperature), and ignitions (i.e., lightning). In the paleorecord, authors often invoke drier conditions as a cause of increased fire occurrence. During the PETM, Arctic sediments exhibit higher PAH concentrations, and they both increased relative to plant input and tracked the increase in angiosperms (inferred from plant biomarker ratios and pollen). Our results suggest wetter conditions, followed by increased temperature, favored angiosperms and enhanced fire occurrence. Like modern fire dynamics, shifts in past fire patterns reflect a balance of variability in precipitation and sufficiently flammable vegetation. Increased fire in a wetter Arctic suggests PETM precipitation was seasonal, or variable on a longer timescale, and that hotter temperatures and angiosperm-dominated forests further facilitated burning.Overall, we used PAHs as a primary signal of production (i.e., fire occurrence) in marine sediments and as a secondary signal of preservation (e.g., organic carbon degradation) in ancient soils. Our results highlight that terrestrial organic carbon was better preserved in the marine section than the fluvial sections. Increased temperatures, decreased clay content, changes in mineralogy, and variations in soil moisture destabilized carbon on millennial timescales and, with sustained higher temperatures across the PETM (~150 thousand years), increased soil carbon degradation persisted for tens of thousands of years. As temperatures warmed and remained warmer than the Paleocene, soils served as a sustained source of CO2 to the atmosphere rather than a sink. Although CO2 released from microbial respiration enhanced the greenhouse warming, increased organic carbon preservation in the marine realm may have counteracted the increased carbon output from soils.