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Biogeochemistry

A sticky situation: Old black carbon and sinking particulate organic carbon

 

Article: Alysha I. Coppola, Lori A. Ziolkowski, Caroline A. Masiello, Ellen R. M. Druffel (2014). Aged black carbon in marine sediments and sinking particles. Geophysical Research Letters, 41. DOI:10.1002/2013GL059068

 

Background Information

Think back to the last time you lit a candle or had a campfire. Remember all that black residue that got left behind? Maybe it collected on the glass walls of the candle or was scattered near the site of the fire. You probably can even remember the hazy dark gray particles in the smoke of that candle or fire. That dark residue is collectively called black carbon.

 

Black carbon is a byproduct of combustion. When any fuel (this could be oil, wood, leaves, coal…) is set on fire, the heat of the fire combines with oxygen in the air to convert that fuel to energy, water, and carbon dioxide. We use this reaction to heat our homes, power a car, turn on a light…any many more things. This combustion reaction, however, is not 100% efficient and will produce a few unintentional, but unavoidable, byproducts. One of these byproducts is black carbon.

 

Black carbon is difficult to measure in the environment because it is an umbrella term for many different types of incomplete combustion. These ‘types’ range from charred particles that you could hold (think about the leftover logs in a fire that are black and cracked) to the smaller-than-your-eyes-can-see soot.

 

Because it’s so small, soot and other types of black carbon, can travel great distances in the atmosphere or in rivers and groundwater. This transport is how black carbon from your backyard fire can end up in the ocean. Once in the ocean, black carbon can exist in the water as a particle or be dissolved. Isotopic analyses suggest that a major loss must occur to this dissolved black carbon pool in the open ocean, however, little research has invested this further. There are two ideas of what happens to dissolved black carbon in the open ocean waters: 1) it gets degraded by sunlight and is converted to carbon dioxide or 2) it attaches itself to particles and gets transported to the sediments.

 

Many other studies have detected black carbon in marine sediments. In some coastal areas, black carbon can make-up 50% of the total organic carbon in the sediments! So it’s obvious that at least some black carbon ends up in the sediment. Additionally, the age of the black carbon in the sediments can be up to 5000 years older than the other (non-black carbon) organic matter. This suggests that black carbon is aged before it is transported to the marine sediments (either in the water or in soils on land).

 

Coppola et al. (2014) set out to measure black carbon in the pelagic (open ocean) Pacific Ocean to show that black carbon is removed from the seawater by attaching to particles and is then deposited to the deep underlying sediments.

The Approach

The researchers collected deep (4100 m) sediments from the Pacific Abyssal plain off the coast of California. They also collected sinking particulate organic matter from sediments traps (positioned 650 m from the bottom of the ocean).

 

Examples of benzene polycarboxylic acid molecules. Source: Mark Foreman's Blog.

Examples of benzene polycarboxylic acid molecules. Source: Mark Foreman’s Blog.

Black carbon was measured using the BPCA method, where BPCA stands for benzene polycarboxylic acid. In this method, sediment samples are partially oxidized to aromatic carboxylic acids using nitric acid under high temperature and pressure. The aromatic structures in the black carbon are oxidized to carboxylic acids (the greater the amount of aromatics in the black carbon, the more carboxylic acids produced). These oxidized samples are then analyzed with gas chromatography mass spectrometry. All produced BPCAs with >3 carboxylic acid groups were defined as black carbon.

 

This method, although chemically involved, is beneficial since it can analyze more black carbon structures. In other words, the BPCA method can measure the char and soot forms of black carbon to accurately depict the overall black carbon concentration. Additionally, it can also aid in determining the black carbon source. For example, higher temperature combustion is typically associated with fossil fuel combustion and will produce black carbon with more aromatic structures. So, the BPCA method would show if fossil fuels (compared to a wildfire) was a likely source of the black carbon by measuring a greater number of carboxylic acids.

 

The age of the black carbon and bulk organic carbon was measured using radiocarbon14C). Δ14C is the radioactive isotope of carbon that has a half-life of 5730 years. The decay of radiocarbon can be measured by beta detection in reference to the concentration of carbon-12 (the stable and most abundant form of carbon on earth). Δ14C measurements can be used to date carbon-based materials up to 50,000 years old. Coppola et al. (2014) used Δ14C to date the black carbon in the sediment and sinking particulate organic carbon samples, as well as the bulk (total) organic carbon in both those media.

The Findings

The ratio of black carbon (BC) to the bulk organic carbon (OC) was an average of 6% for both the deep surface sediments and sinking particulate organic matter (Figure 1). Additionally, the BC in the sediment mixed layer was older than the OC, suggesting that it had to age before being deposited to the sediments. The OC (non-BC) was modern and mostly from biological production (phytoplankton). The BPCA analysis had a lower number of carboxylic acids, suggesting that biomass burning, rather than fossil fuel combustion, was the original source of the BC.

 

(a) The ratio of black carbon (BC) to the sedimentary organic carbon (SOC) with sediment depth.  The closed circles are data from Coppola et al. (2014) and the open circles are data from Masiello and Druffel (1998) from the same region using a different measurement approach. (b) The radiocarbon (Δ14C) of the black carbon. (c) The Δ14C of the organic carbon (non-black carbon fraction).

(a) The ratio of black carbon (BC) to the sedimentary organic carbon (SOC) with sediment depth. The closed circles are data from Coppola et al. (2014) and the open circles are data from Masiello and Druffel (1998) from the same region using a different measurement approach. (b) The radiocarbon (Δ14C) of the black carbon. (c) The Δ14C of the organic carbon (non-black carbon fraction).

Coppola et al. (2014) suggest three sources of this aged black carbon to the particulate organic carbon to explain how the BC could be older than the bulk organic carbon.

 

1) One source of this BC could be from resuspended sediment. Bottom currents in the ocean can transport suspended sediment laterally. Thus some BC and OC already in the sediments can be moved deeper into the abyssal plain with these bottom currents. A previous study found that ~35% of the particulate organic carbon can be due to the recycling of material by currents.

 

2) A second source of aged BC could be from the atmospheric deposition of fossil fuel carbon. Fossil fuel BC is radiocarbon dead, meaning that it is so old, all of the Δ14C has decayed away. Having some fossil fuel-BC could “dilute” the overall BC age to make it older. However, Coppola et al. (2014) concluded that fossil fuel BC is unlikely to be present since the sedimentation rate is too slow (fossil fuel BC is still sinking and not much has reached the sediment yet) and their BPCA analysis suggests biomass burning is the main source.

 

3) The third, and mostly likely source of this aged BC, is from the dissolved organic carbon pool. Dissolved BC could cycle in the ocean for thousands of year before attaching to sinking particles. They propose two mechanisms for this “attachment” to occur: aggregation when exposed to sunlight or by the formation of microgels (which occur due to hydrophobic bonding).

Significance

The flux of BC to marine sediments can now be better understood if indeed the sorption of dissolved BC to sinking particles is the main export of BC to deep sediments. Coppola et al. (2014) estimated that up to 0.016 Gigatons per year of BC can be transported to the sediments. This could account for 8-16% of the global burial of organic matter. It also means that 6-32% of biomass-created-BC is buried in the abyssal plains of the ocean.

 

Although more work is needed to further our understanding of BC fluxes (such as repeating this study in another ocean), Coppola et al. (2014) reports that BC fluxes to sediments is driven by sorption to sinking particulate organic matter and that BC fluxes to the sediment are actually quite large.

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