Chemistry in the Universe

New Insights from Old Fires

by Nancy McGuire

June 13, 2016

How do the environmental effects of slash-and-burn agriculture and smokestack industries compare with those of lightning strikes and volcanic eruptions? The fire evidence that Nature leaves behind differs from that of human-caused fires, providing clues to the fires’ origins.

Ten thousand years of French history

E. Doyen and coauthors at the Universities of Burgundy–Franche Comté (Besançon), Orleans, Toulouse–Jean Jaurès, and Savoy Mont Blanc (Chambéry, all in France) delved into 10,000 years of sedimentary history around Lake Paladru, located between Grenoble and Lyon, France. The area surrounding this small glacier-formed lake is especially well studied and has rich archaeological and historical archives.

The authors measured magnetic susceptibility and color variations in various sediment cores to create a composite core profile that covered the entire time range of interest. They used X-ray fluorescence spectroscopy to track occurrences and origins of mineral deposits. Carbon and lead isotope dating, pollen analysis, and geochemical data established the chronology of the samples.

Charcoal Fire

Analyses of microscopic and macroscopic charcoal particles in the soil helped to establish charcoal accumulation rates (CHARs), fire frequency, and burning trends. During periods with little to no evidence of human settlement (>6000 years ago and 2900–2700 years ago), large fires were relatively rare; and micro-CHAR values, indicative of wood charcoal burning, were low and relatively constant.

Periods of human occupation corresponded with more frequent fires. The accumulation rate of macroscopic charcoal particles (macro-CHAR) and changes in pollen types indicate that forests were cleared to make room for cereal crops. Discontinuities in the record suggest that plots of land were cleared and farmed until the soil was depleted. The plots were then abandoned and the forests returned.

Black Carbon Aloft

After CO2, atmospheric black carbon (BC) is the most important contributor to climate change. (Bond, T. C., et al. DOI: 10.1002/jgrd.50171) In 2000, vegetation fires sent an estimated 50–385 Tg (billion kg) of BC into the atmosphere, compared with 4.4 Tg from fossil fuel combustion.

BC aerosols absorb solar radiation; and some studies suggest that they suppress convective activity and affect cloud formation. BC can enhance the transport of organic pollutants into the atmosphere. Organic aerosols stay aloft about as long as BC aerosols, but they scatter sunlight much more strongly.

One small positive note: BC particles remain aloft for only a few days. Then they precipitate and mix with the soil, where they can sequester carbon for millennia. (Wang, G., et al. Geoderma DOI: 10.1016/j.geoderma.2016.03.021; Hodnebrog, Ø., et al. Nat. Commun. DOI: 10.1038/ncomms11236)

Micro-CHAR values increased beginning in the 14th century with the advent of ironworks and sawmills. Paper mills and metalworking shops in the 19th century used large amounts of wood for fuel and construction. Agriculture decreased from the 19th century to the present; and industrial activities have declined since the mid-20th century, so forested areas are increasing again. (J. Archaeol. Sci. Rep. DOI: 10.1016/jasrep.2016.03.040)

Baltic black soils

Forest Fire

Peter Leinweber and colleagues at the Universities of Rostock, Bonn, and Tübingen (all in Germany); the State Authority for Mining, Energy and Geology of Lower Saxony (Hannover, Germany); and Canadian Light Source Inc. (Saskatoon, SK) looked into the origins of unusual black soils from three islands off the Baltic coast of Germany. These soils are more typical of grassland regions, including the Russian steppes and North American prairies, than of coastal regions. The Baltic coast has seen slash-and-burn forest clearing and agricultural burning practices. Its soils contain black carbon (BC) combustion residues.

The authors used X-ray absorption near-edge structure (XANES) spectroscopy, pyrolysis-field ionization mass spectrometry (Py-FIMS), and soil color measurements (diffuse reflectance chromametry) to quantify benzenepolycarboxylic acids (BPCAs), which are more typical of combustion than biological processes.

The XANES and Py-FIMS data showed more aromatic carbon and nitrogen-heterocyclic compounds in the biogenically mixed topsoil layers than in the overlying plowed and humic layers. This is evidence for heated or charred material below the surface. The upper layers had a greater amide content, indicative of plant litter, bacteria, and fungi. The high amount of nitrogen relative to organic carbon could reflect fertilizer use.

Grass Fire

The ratios of penta- to hexacarboxylic benzoic acids (B5CA/B6CA) of 0.9:1–2.2:1, with a mean of 1.2:1, revealed the relative contributions from hotter domestic fires (B6CA-rich) and cooler grassland fires (B5CA-rich). The authors attribute the soil’s color to these fire residues and their transformation products. They note that the BPCA patterns are not typical of extensive or repeated vegetation fires.

The authors point to the patchy distribution of the black soils as further evidence that humans contributed to their formation. (Geoderma Regional DOI: 10.1016/j.geodrs.2016.04.001)

Fire and Rain in Africa

Slash-and-Burn Agriculture

Slash-and-burn practices are still prevalent in less-developed regions of the world. In southern Africa, most biomass burning occurs from June to September, the annual dry season. Human activity almost doubled the amount of BC emitted in this region during the 20th century, with the greatest increase occurring since 1950.

Øivind Hodnebrog and coauthors at the Center for International Climate and Environmental Research–Oslo and the University of Leeds (UK) used models to establish a connection between local biomass burning and a 20–30% reduction in dry-season precipitation in southern Africa over the past century.

They gathered data on global CO2 volume mixing ratios, BC burden, and organic carbon (OC) burden from 1850 to 2000, a period during which the BC burden in southern Africa increased by 60% and the OC burden by 70%. They established aerosols from biomass burning as a leading cause of local atmospheric drying. The accompanying CO2 emissions reduce precipitation by a smaller amount, but over a larger area. Long-term precipitation data are sparse for this region, introducing some uncertainty about this assessment. (Nat. Commun. DOI: 10.1038/ncomms11236)

Looking Ahead

The Global Paleofire Working Group is working to fill data gaps in sparsely sampled areas. Its Global Charcoal Database is a resource in this effort. The group is refining modeling systems and integrating techniques such as satellite imaging to build a more complete picture.

Paleofire studies, coupled with data from other sources, can help reconstruct the environmental history of areas that are not well documented by historical and archaeological evidence. This information, in turn, can serve as the basis for more accurate predictive models to help land use planners and policy makers respond to the effects of climate change.