Chemistry in the Universe New Insights from Old Fires Insights from Old... · 2016. 6. 25. · After CO2, atmospheric black carbon (BC) is the most important contributor to climate

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Chemistry in the Universe

New Insights from Old Firesby Nancy McGuire

June 13, 2016

How do the environmental effects of slash-and-burn agriculture and smokestack industries compare with those of lightningstrikes and volcanic eruptions? The fire evidence that Nature leaves behind differs from that of human-caused fires, providingclues 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, andSavoy Mont Blanc (Chambéry, all in France) delved into 10,000 years of sedimentary history around Lake Paladru, locatedbetween Grenoble and Lyon, France. The area surrounding this small glacier-formed lake is especially well studied and hasrich archaeological and historical archives.

The authors measured magnetic susceptibility and color variations in various sediment cores to create a composite coreprofile that covered the entire time range of interest. They used X-ray fluorescence spectroscopy to track occurrences andorigins of mineral deposits. Carbon and lead isotope dating, pollen analysis, and geochemical data established thechronology 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 agoand 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. Discontinuitiesin the record suggest that plots of land were cleared and farmed until the soil was depleted. The plots were then abandonedand the forests returned.

Black Carbon Aloft

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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, comparedwith 4.4 Tg from fossil fuel combustion.

BC aerosols absorb solar radiation; and some studies suggest that they suppress convective activity and affect cloudformation. BC can enhance the transport of organic pollutants into the atmosphere. Organic aerosols stay aloft about as longas 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 theycan 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 andmetalworking shops in the 19th century used large amounts of wood for fuel and construction. Agriculture decreased from the19th century to the present; and industrial activities have declined since the mid-20th century, so forested areas areincreasing again. (J. Archaeol. Sci. Rep. DOI: 10.1016/jasrep.2016.03.040)


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Baltic black soils

Forest Fire

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

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 mixedtopsoil layers than in the overlying plowed and humic layers. This is evidence for heated or charred material below thesurface. The upper layers had a greater amide content, indicative of plant litter, bacteria, and fungi. The high amount ofnitrogen 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 relativecontributions from hotter domestic fires (B6CA-rich) and cooler grassland fires (B5CA-rich). The authors attribute the soil’scolor to these fire residues and their transformation products. They note that the BPCA patterns are not typical of extensive orrepeated vegetation fires.


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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 AgricultureDiorit

Slash-and-burn practices are still prevalent in less-developed regions of the world. In southern Africa, most biomass burningoccurs from June to September, the annual dry season. Human activity almost doubled the amount of BC emitted in thisregion 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 theUniversity of Leeds (UK) used models to establish a connection between local biomass burning and a 20–30% reduction indry-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, aperiod during which the BC burden in southern Africa increased by 60% and the OC burden by 70%. They establishedaerosols from biomass burning as a leading cause of local atmospheric drying. The accompanying CO2 emissions reduceprecipitation by a smaller amount, but over a larger area. Long-term precipitation data are sparse for this region, introducingsome 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 aresource in this effort. The group is refining modeling systems and integrating techniques such as satellite imaging to build amore complete picture.

Paleofire studies, coupled with data from other sources, can help reconstruct the environmental history of areas that are notwell documented by historical and archaeological evidence. This information, in turn, can serve as the basis for moreaccurate predictive models to help land use planners and policy makers respond to the effects of climate change.

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Chemistry in the Universe New Insights from Old Fires Insights from Old... · 2016. 6. 25. · After CO2, atmospheric black carbon (BC) is the most important contributor to climate