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

Núria Trilla-Prieto,Gemma Casas,Begoña Jiménez,Jordi Dachs,Maria Vila-Costa

doi:10.1021/acs.estlett.1c00658

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.

Highlights

Black carbon originates from the heating and combustion of organic matter, such as during forest fires or use of fossil fuels.Semivolatile polycyclic aromatic hydrocarbons (PAHs) are produced during these combustion processes, even thoughPAHs can be released to the environment due to oil spills or even from natural sources
Particulate black carbon has been extensively studied in soils and the atmosphere due to its role in Earth radiation balance, health effects, and as an environmental sorbent for PAHs and other pollutants.[1−3] During recent years, there has been an increasing interest in dissolved black carbon (DBC) in the ocean.[3]
The objective of this work was to compare the marine occurrence of semivolatile aromatic-like compounds (SALCs) and DBC, assess how the known biogeochemical cycle of PAHs can shed light to the sources and fate of oceanic DBC by addressing the feasibility of DBC photo- and biodegradation, the latter derived from oceanic metagenomes, and the implications of semivolatile DBC for this important pool of refractory dissolved organic carbon (DOC)

Summary

Introduction

Black carbon originates from the heating and combustion of organic matter, such as during forest fires or use of fossil fuels.Semivolatile polycyclic aromatic hydrocarbons (PAHs) are produced during these combustion processes, even thoughPAHs can be released to the environment due to oil spills or even from natural sources. Particulate black carbon has been extensively studied in soils and the atmosphere due to its role in Earth radiation balance, health effects, and as an environmental sorbent for PAHs and other pollutants.[1−3] During recent years, there has been an increasing interest in dissolved black carbon (DBC) in the ocean.[3] Oceanic DBC accounts for between 0.1% and 4−7% of dissolved organic carbon (DOC).[3] DBC is old carbon, as determined by Δ14C isotopic determinations,[4,5] and it has been suggested that DBC contributes to refractory organic matter and plays a key role in the slow cycle of DOC in the ocean.[5] Even though the major identified sources of DBC are riverine inputs, followed by atmospheric deposition of aerosol-bound black carbon,[3,6] the δ13C signal of oceanic DBC is different than that of rivers, suggesting unaccounted sources of oceanic DBC.[7]

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