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Australia: The Land Where Time Began |
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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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |