International study quantified and characterized charcoal and soot produced by incomplete burning of trees and transported by river to the Atlantic.

Molecular markers

Carbon-14 levels and contents in samples were measured using molecular markers, such as the polycarboxylic acid released by oxidation of aromatic polycyclical hydrocarbons in black carbon. Quantitative measurement of the markers was combined with molecular characterization of the samples using ultra-high-resolution mass spectrometry. Samples collected in localities relatively distant from the Atlantic, such as Óbidos in Pará State, were younger, while those collected farther downstream were older. “This suggests the black carbon may age as it moves from dry land to the river and then flows on to the sea. Also, more reactive components may be removed during the transportation of this material,” Richey said. “The more recent material may be submitted to a process of mineralization in the river as it flows to the sea. This could cause a change in its molecular profile so that it emits an ‘older’ signal. There are still various aspects of the storage and transportation of this material from dry land to rivers and then the ocean that we need to understand better.” In a new project, also supported by FAPESP, the researchers plan to perform a larger number of measurements for comparison with the data for 2015 in an effort to find out whether the production of “young” black carbon and hence the frequency of forest fires have increased in recent years.

Largest source of organic matter

A large and refractory component of the global carbon cycle, black carbon in particulate form acts as a biospheric carbon sink by removing carbon from faster atmosphere-biosphere processes and storing it in sedimentary reservoirs. Knowledge of the origin, dynamics, and fate of this material are essential to the development of models for predicting how the global carbon cycle may interact with climate change, Richey stressed. “Our understanding of the role of black carbon at the regional and global scales is inadequate, owing largely to limited knowledge of the processing, quality, and fate of dissolved black carbon during its exportation by rivers to the ocean,” he said.