As erosion eats away at Earth's surface, some types of rocks release carbon they contain back into the atmosphere -- and now a new study suggests that microbes play a substantial role in this release. The finding may help scientists better understand how carbon is cycled between the ground and atmosphere over million-year time scales. Processes including the burial of animals and the transformation of silicate rocks into carbonate rocks consume some atmospheric carbon dioxide; at the same time, organic carbon within rocks can be released as erosion exposes certain types of rocks to the air, creating carbon dioxide. However, the latter of these processes - the release of carbon during erosion - has been less studied, despite its potential to negate the effects of geological carbon dioxide consumption. Here, Jordon Hemingway and colleagues sought to better constrain the amount of carbon emitted from rocks, particularly in settings of rapid erosion, by studying mountain ranges in Taiwan where erosion rates are particularly high. Notably, lithospheric, or "petrogenic," organic carbon trapped within rocks and soil has a distinct isotope signature (or molecular weight). Using this distinctive signature, as well as calculations of the energy required for conversion to carbon dioxide, the researchers estimate that roughly 67% of organic carbon within bedrock in the region they studied is released during erosion. Furthermore, the isotope signature of petrogenic organic carbon is reflected in the fatty acids of microbes that reside along the weathering rock, indicating that the microbes use the carbon for nourishment, the authors say, thus greatly facilitating the process. Based on landslide and erosion rates across the Central Mountain range of Taiwan, Hemingway and colleagues estimate that this region emits a median range of 6.1 to 18.6 metric tons of carbon per square kilometer each year.
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Science