Carbon Cycle - A hump in Ocean-Air Exchange

Australia: The Land Where Time Began

Carbon Cycle - A hump in Ocean-Air Exchange

Semivolatile organic compounds originating from fossil fuels or incomplete combustion are ubiquitous. Large carbon fluxes through the deposition of these compounds were revealed by a suite of circumglobal measurements of their concentrations in the ocean and the atmosphere.

A circumnavigation was carried out by the Malaspina 2010 expedition involving a 7-leg research cruise with the aim of documenting global change and the inventory of biodiversity in the ocean (Duarte, 2015). The expedition was named in honour of Alessandro Malaspina, a Spanish naval officer of Italian birth who sailed the ocean in the 1700s to collect marine samples and observations that were subsequently lost for 200 years. Scientists on the 2010 expedition took 108 atmospheric samples and 68 seawater samples in the tropical and subtropical Atlantic, Pacific and Indian oceans. A paper on the results of the expedition was published in Nature Geoscience by González-Gaya et al. (González-Gaya et al., 2016), which has provided an invaluable global assessment of the relative abundance of pollutants they gathered from areas adjacent to established nations that are rapidly industrialising or developing nations. Also identified are fluxes of carbon to the surface waters of the ocean that have not been reported previously, which are large enough to complicate budgets of the carbon cycle that are already complex.

It has long been recognised that polycyclic aromatic hydrocarbons (PAHs) are harmful to humans and marine life. These pollutants are released to the atmosphere from fossil fuels and in the emissions from the combustion of fossil fuel. Meticulous and arduous techniques are essential when carrying out open ocean analyses because of the ultralow trace levels of PAHs in the samples when working on or near the research vessel that is also emitting these compounds at levels that are hundreds of times or more higher. Yet, it is possible to quantify PAHs with excellent certainty as they possess physical and chemical properties that are well known, and biotic and abiotic reactions acting on them are constrained reasonably.

The global abundance of a complex mixture of Semivolatile Aromatic Compounds (SALCs), by contrast, has to a large extent been unexplored. SALCs are composed of a complex distribution of hydrocarbons that originate from hydrocarbons that are leaked or spilled before they have been combusted, as well as released as unburned fuel, and from products of incomplete combustion. Also contributed to these compounds, though at lower inputs, is contemporary combustion, such as burning biomass (White et al., 2013). It is ironic that the relatively simple CO₂ molecule is linked by combustion to the thousands of compounds that are present in SALCs; CO₂ is the product of combustion while SALCs can be present in the fuel as well as in the products of combustion.

González-Gaya et al. analysed paired samples from the lower atmosphere and the ocean surface (González-Gaya et al., 2016) and estimated that the flux of these compounds from the atmosphere to the ocean was about 0.09 Tg/month, which is approximately equivalent to the emissions from a vehicle driven ~10¹⁴ km/month in Shanghai, China (Liu et al., 2015). They also reported on the concentrations of SALCs.

It was not surprising that SALCs were detected as these compounds have been observed in thousands of Earth and environmental samples (White et al., 2013; Farrington & Quinn, 2015). SALCs are another term for an aromatic component of the complex mixture that has not been resolved (White et al., 2013; Farrington & Quinn, 2015), which has been observed as broad humps in a wide range of sample analyses with gas chromatography – as opposed to the peaks representing PAHs which are well-resolved and discrete, and the PAHs are a trace component of SALCs. The hump of compounds can be quantified by simply treating them as a single parameter but this has been hampered by their complexity when attempts are made to identify their structures (White et al., 2013; Farrington & Quinn, 2015; Booth et al., 2008; Frysinger et al., 2003). Predictions of the fate, impacts and recalcitrance of these compounds without knowing a structure of them are challenging (White et al., 2013; Farrington & Quinn, 2015; Frysinger et al., 2003).

Reddy suggests it is this uncertainty that probably made earlier investigators hesitant to study SALCs in tandem with PAHs. González-Gaya et al. estimated, on a mass basis of carbon, that global atmospheric deposition of SALCs to the ocean equals about 15 % of the oceanic uptake of CO₂, which Reddy says is a staggering and surprisingly large flux. Reddy believes that the sizeable and newly recognised input of SALCs is the most interesting finding of this study, as the PAHs were 100-1,000 times less abundant than the SALCs and have previously been studied in the open ocean (Jaward et al., 2004).

Reddy suggests there is an array of research questions that present themselves as potential follow-ups from this work.

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