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Biogeochemistry

Carbon on Fire! The role dissolved black carbon plays in the ocean

Article: C. A. Masiello and P. Louchouarn (2013) Fire in the Ocean. Science. Vol. 340, 287-288. DOI: 10.1126/science.1237688

 

Background Information

 

The dissolved organic carbon (DOC) pool is one of the largest active carbon pools in the ocean (and the planet for that matter), yet it is frustratingly one of the hardest to chemically characterize. For years, biogeochemists have been trying to understand the DOC pool and its potential role as a carbon sink….and how the size of this sink may change with the changing climate.

One of the components of the DOC pool is dissolved black carbon. Black carbon is created during the combustion of organic materials, such as during forest fires or burning fossil fuels. It is that black residue left behind in a fire pit. Black carbon is chemically described as graphitic (almost the same structure as the graphite in a pencil) and has a very high ratio of carbon to nitrogen. This means that black carbon is presumed to be extremely stable and difficult for ocean critters to eat. But it does not mean that it will never break down.

 

A key to understanding the chemical nature of the DOC pool, and the role it plays in as a source and/or sink for carbon dioxide, is to understand how dissolved black carbon behaves in the ocean.

 

The Findings

 

Dissolved black carbon behaves just like DOC in the ocean. This was unexpected since it means that dissolved black carbon is not a distinctly separate pool of dissolved carbon but rather an important part of the DOC pool. A recent study by Jaffe et al. 2013 (Science, Vol. 340) determined that at least 10% of the marine DOC pool is dissolved black carbon. Thus, we are 10% closer to chemically understanding what makes up the DOC pool. This also means that DOC could be used as a tracer for dissolved black carbon. Hence, if we measure the DOC in a sample, we already know that at least 10% of that is dissolved black carbon.

 

Black carbon appears to be more stable in the terrestrial environment where it is protected from sunlight when buried in soil. The major transport of black carbon to the ocean is through rivers. Once in the rivers, and then the ocean, dissolved black carbon can undergo photo-degradation (gets broken down into smaller pieces and converted to carbon dioxide by the sun). The sunlight causes the black carbon, with its large aromatic structures, to get broken down into smaller aromatic pieces (See the figure below). These smaller dissolved black carbon pieces accumulate in the ocean and makes as many as 4 trips around the entire ocean (global conveyor belt) before they are oxidized back to carbon dioxide. This is roughly 4,000 years!

 

These dissolved black carbon “morsels” when first released into the ocean by rivers can be very reactive and may lead to a priming effect that may accelerate the turnover of the DOC pool. In biology, the priming effect is when degradation is accelerated when new organic material is added to a system. More specifically, microbes and other ocean critters have a very difficult time eating DOC (on average, DOC is 6,000 years old and very low in nutrients). But, when these freshly released black carbon “morsels” from photo-degradation are present, microbes may get extra hungry and eat some of DOC molecules as well. (The remaining black carbon that is not eaten then becomes part of the long term storage of the marine DOC pool.)

 

This newly hypothesized priming effect, that dissolved back carbon may help facilitate DOC turnover, is particularly important for the application of biochars. Biochars are charcoal-like substances (a type of black carbon) that are added to soils to increase soil fertility and mitigate climate change. When added to soil, biochars can improve the water flow, better retain nutrients, and, because black carbon is very stable in soils, reduce carbon dioxide emissions. However, it is completely possible that biochars in soils may accidentally increase the amount of dissolved black carbon that enters the ocean, thus altering the marine DOC pool and possibly, due to priming, accelerate turnover. This would decrease the size of the DOC pool as a CO2 sink and increase the amount of CO2 released back into the ocean and atmosphere.

Oceanbite_2_Science

Figure 1: Dissolved black carbon enters the ocean by river transport where sunlight can break it down into smaller molecules ( and becomes part of the dissolved organic carbon (DOC) pool). This process may act to prime the marine DOC pool, or allow microbes to eat the DOC faster than if the black carbon wasn’t there.

 

Significance

Dissolved black carbon makes up at least 10% of the DOC pool. Upon entry into the ocean, black carbon can undergo photo-degradation which causes black carbon to accumulate as smaller molecules in the DOC pool. This photo-degradation process may release more reactive dissolved compounds that can act to prime, or facilitate, DOC turnover. Understanding the fate, residence time, and priming capabilities of black carbon are important to understand as the use of large scale biochar application becomes more important.

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