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 Dissolved Black Carbon 

Contact: Aron Stubbins
Citation: Stubbins et al. 2012;Dittmar et al. 2012;Stubbins et al. 2010Jaffé et al. 2013
Video: Fire in the Ocean: a video lecture by collaborator Thorsten Dittmar on Black Carbon

Brazil's Atlantic Forest Ablaze.
Photo Credit: Gustavo Luna Peixoto, Chico Mendes Institute for Biodiversity, Rio de Janeiro, Brazil.

Citations: Stubbins et al. 2010; Stubbins et al. 2012; Dittmar et al. 2012; Jaffé et al. 2013; Stubbins et al., 2015a; Stubbins et al. 2015b

Dissolved Black Carbon in the Global Carbon Cycle

Wildfires started charring biomass shortly after land plants emerged some 480 million years ago. After wildfires a wide range of charred residues remain. Black carbon refers to organic material chemically altered by charring and it comes in many forms, ranging in chemistry from minimally altered biomolecules to highly polycyclic aromatics. The latter can only be formed at high temperatures, making them a telltale tracer of fire derived organics. Of the wide range of charred material that remains after a fire, the polycyclic aromatics are preferentially preserved, persisting in the soil for hundreds to thousands of years. When we look to the oceans, we find dissolved forms of these polycyclic aromatics and refer to them en masse as dissolved black carbon (DBC). The oceanic pool of DBC also appears to resist biodegradation and has subsequently accumulated throughout the deep ocean where it represents about 2% of oceanic organic carbon stocks. This resistance to biological decay in soils and in the oceans has resulted in polycyclic aromatics becoming the largest, identifiable and therefore quantifiable store of organic carbon on Earth.

Recent work with colleagues in GermanyFloridaFinlandBrazil has shown these two major global pools of black carbon to be connected by river export. One of these studies published in Nature Geoscience in 2012 showed that the burning of the now lost Mata Atlantica, a former extension of the Amazon Rainforest in Brasil, resulted in the generation of massive stores of black carbon. This black carbon on the land is still being transferred to the oceans some 40 years after forest clearing ceased. In 2013 we published an article in Science reporting that about 10% of the dissolved organic carbon carried by rivers globally is in the form of DBC. This suggests that dissolution into rivers may be the major way in which black carbon is lost from global soil stores and that rivers are the main source of DBC to the oceans.

Based upon radiocarbon dates and near-conservative distribution patterns, DBC appears to persist for millennia in the dark of the deep ocean. In contrast, all dissolved polycyclic aromatics identified in the Fourier transform ion cyclotron resonance mass spectra of Congo River water were lost after 57 days of irradiation under simulated sunlight (Stubbins et al. 2010) and we recently reported that greater than 95% of DBC was lost from deep ocean water when irradiated with simulated sunlight (Stubbins et al. 2012). These results suggest that sunlight driven photoreactions may be the major pathway by which DBC is mineralized back to CO2.

As we learn more about the global black carbon cycle it is becoming clear that black carbon is not just a major store of global carbon, but is also actively involved in the contemporary carbon cycle and so vulnerable in a changing climate. For instance, if black carbon export from soils, through rivers, to the oceans accelerates due to changing hydrology then black carbon becomes susceptible to photo-mineralization to the greenhouse gas CO2, precipitating a climate feedback.

 

Sugar cane burning at night. Aerial view of the Rio Paraiba do Sul, Brazil 1 Aerial view of the Rio Paraiba do Sul, Brazil 2 Aerial view of the Rio Paraiba do Sul, Brazil 3

Photo's Credit: Laboratorio de Ciencias Ambientais, Universidade Estadual do Norte Fluminense, Rio de Janeiro, Brazil.

 



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