Dissolved Black Carbon and Semivolatile Aromatic Hydrocarbons in the Ocean: Two Entangled Biogeochemical Cycles?

Abstract

Dissolved black carbon (DBC) plays a role in the oceanic carbon cycle. DBC originates from the heating and incomplete combustion of organic matter, including fossil fuels, a shared origin with polycyclic aromatic hydrocarbons (PAH). DBC is quantified using the benzene polycarboxylic acids produced by oxidation of the organic extract, a fraction of which derive from PAHs and other semivolatile aromatic-like compounds (SALCs). However, the current view of the DBC cycle does not take into account the inputs and sinks known for PAHs, such as diffusive air−water exchange and degradation. A meta-analysis of oceanic PAHs, SALCs, and DBC concentrations shows that SALCs account for 16% of DBC (ranging from 5% to 31%). Such a large contribution of semivolatile aromatic hydrocarbons to DBC is consistent with the large atmospheric input of SALCs (estimated as 400 Tg C y −1 ). Furthermore, photodegradation at the surface ocean and microbial degradation in the water column of semivolatile DBC can be important sinks, consistent with the ubiquitous occurrence of the degradation genes of the metabolic routes for aromatic hydrocarbons. Future work should focus on the characterization of semivolatile DBC and its degradation in order to constrain its contribution to refractory organic matter and the anthropogenic perturbation of the carbon cycle.

The black carbon cycle and its role in the Earth system

“…A newly identified, allochthonous source of DBC to oceanic surface waters 124 is the large input of polyaromatic hydrocarbons and other semi-volatile aromatic-like compounds by diffusive air-water exchange. This source could contribute to the δ 13 C signatures observed for oceanic DBC, but the isotopic composition of these semi-volatile aromatic-like compounds is unknown.…”

Provenance of Aerosol Black Carbon over Northeast Indian Ocean and South China Sea and Implications for Oceanic Black Carbon Cycling

“…In addition, the minutes to centennial degradation cycle will have little effect on the Δ 14 C values and apparent 14 C age. Notwithstanding, the diffusive air–water exchange of semi-volatile aromatic-like compounds (SALC) was suggested to contribute significantly to oceanic DBC and SALC may share similar isotope signatures to aerosol BC owing to their co-emitted nature . Overall, we suggest that marine aerosol BC may be a potentially important source of DBC in surface seawater to a considerable degree but is not a direct contributor to DBC in deep seawater.…”

Snow-Dependent Biogeochemical Cycling of Polycyclic Aromatic Hydrocarbons at Coastal Antarctica

“…The vast majority of PAHs entering the ocean are degraded in the photic zone. Even though photodegradation can occur, the overall PAH sink is dominated by microbial degradation. , Despite this relevance, the study of microbial degradation of PAHs under field conditions has been mainly centered under scenarios of high concentrations, such as those found in oil spills, , with few field studies addressing the degradation of PAHs when these occur at their background levels …”

“…Even though photodegradation can occur, the overall PAH sink is dominated by microbial degradation. 11,13 Despite this relevance, the study of microbial degradation of PAHs under field conditions has been mainly centered under scenarios of high concentrations, such as those found in oil spills, 14,15 with few field studies addressing the degradation of PAHs when these occur at their background levels. 16 Hydrocarbon-degrading bacteria are known as hydrocarbonoclastic bacteria (HCB), which include obligate HCB, that is, bacteria that only grow using hydrocarbons as carbon and energy sources, and facultative HCB, those able to grow with alternative carbon sources.…”