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42808-G8
Testing the 'Wildfire Hypothesis': Fossil-Fuel Burning as the Cause of Dramatic Global Warming 55 Million Years Ago
The Paleocene/Eocene Thermal Maximum (PETM) was a period of rapid global warming that occurred approximately 55 million years ago. A global carbon isotope excursion (CIE) at the onset of this event indicates that the addition of massive amounts of carbon to the ocean and atmosphere caused the warming, as this carbon was released as, or converted to, the greenhouse gas carbon dioxide. It has been suggested that one possible mechanism for the release of this carbon is the combustion of enormous Paleocene-aged peat and coal deposits. The goal of the research funded by this grant is to investigate this hypothesis by measuring the concentration of combustion-derived black carbon (BC) in marine sediments and using this as a proxy for global wildfire rates.
The initial research funded by this grant (conducted prior to 9/1/2006) included verifying that BC can reliably and accurately be extracted from marine sediments and measured. Following the recommendation of a steering committee of black carbon experts, we used a marine sediment standard produced by the National Institute of Standards and Technology to test our method of BC extraction and measurement. Our measured BC concentrations were consistent across samples and agreed with previous analyses of this standard published by the black carbon steering committee. Next, we performed this extraction and measurement on sediments from two ODP sites: 1210B in the central Pacific and the Bass River site on the eastern continental shelf of North America. Site 1210B was chosen because its remote location prevents the possibility of locally derived BC overprinting any global record of wildfire activity. The Bass Rver site was chosen because at the time of the PETM, it was downwind of extensive North American Paleocene peat and coal deposits – a possible location of PETM wildfires – and because this site contains an expanded and exceptionally well-studied record of the PETM which has been extensively studied and described by previous researchers.
During the period beginning 9/1/2006 a number of additional research goals were attained. First, additional samples from each site were measured for the purpose of enhancing and verifying our existing records of BC accumulation at these two sites. Second, the carbon isotope ratio (δ13C) of BC extracted from the Bass River samples was measured. This represents a vital component of our research, as the wildfire hypothesis predicts that BC produced by the combustion of fossil fuels such as peat and coal would not show the same CIE shown in bulk carbonates, while BC produced by the combustion of living and recent biomass would.
Data on BC accumulation at the two sites was presented at the 2006 Fall Meeting of the American Geophysical Union. These results showed vanishingly small amounts of BC in sediments from site 1210B, compared to several hundred ppm of BC in sediments from Bass River. The δ13C of BC extracted from Bass River sediments showed a CIE of similar magnitude to the excursion seen in foraminiferal data from these sediments, indicating that BC at this site is derived from the combustion of living and recent biomass, rather than peat or coal. The much higher concentrations of BC at Bass River probably result from the site's location, where sediments would be exposed to BC derived from local or regional combustion events, while site 1210B would only receive BC from wildfires thousands of kilometers away.
In July of 2007, work began to prepare a manuscript for publication detailing the results of our research. At the time of this report, this manuscript is nearing completion. We expect to submit it for review before the end of 2007, and for it to be published in 2008.
