290
radiocarbon was produced in the atmosphere due to nuclear testing and warfare.
This additional radiocarbon is slowly mixing into the earth’s pool of surface carbon
so that contemporary atmospheric CO 2 at the time of the oil spill was about +30 to
40‰ (Graven et al. 2012). At present (2018), the value of atmospheric CO 2 has been
further reduced, and because of continual and growing fossil fuel combustion and
the addition of that CO 2 to the atmosphere, organic matter fixed in 2050 may be
indistinguishable from organic carbon fixed in the year 1050 (Graven 2015).
Oil and methane from fossil carbon reserves have characteristic isotopic values
that allow them to be followed in the environment and in food webs (Chanton et al.
2012; Wilson et al. 2016). These compounds formed millions of years ago and have
been locked away in the subsurface so they are radiocarbon-free with an isotopic
signature of −1000‰ on the Δ
14
C scale. Carbon fixed at the time of the oil spill by
photosynthetic processes had a value of about +40‰ (Chanton et al. 2012, 2018). Oil
and methane are also depleted in δ
13
C (−27‰ to roughly −60‰, respectively) relative to modern photosynthetic marine production (−20 to −22‰). Away from seep
sites, sedimentary organic matter in the Gulf has a δ
13
C value that reflects marine
production and a surface
14
C value of −200‰ ± 29‰ (Chanton et al. 2012, 2015).
Significant transformations can accompany oil weathering and biodegradation.
The hydrocarbons can be oxygenated (Aeppli et al. 2012, 2014), while saturated
and lighter hydrocarbons are lost due to evaporation and biodegradation. The oxygenated hydrocarbons are apparently rather persistent in the environment (Aeppli
et al. 2018). The products of the transformed petroleum in the environment may be
defined generically as “petrocarbon.” Despite these reactions, the fossil-based carbon retains its distinctive
14
C-depleted isotopic composition relative to background
carbon values allowing its detection in the environment. The larger range for Δ
14
C
relative to the δ
13
C variation makes
14
C a significant tracer for finding evidence of
petrocarbon in the sediment after the release of oil. The evidence for oil or methane
contamination in the sediments consists of a surficial Δ
14
C-depleted layer overlying
relatively Δ
14
C-enriched background layers (Fig. 17.2, Chanton et al. 2015).
Sediments in the surface-most layer were as depleted as −500‰ in 2010 following
the oil spill. Over time,
14
C depletion at some sites was ameliorated, returning
toward background values of −200‰ but sediments as depleted as −420‰ were
still observed in surface layers closer to the oil spill site in 2015. Both the distance
of a site from the source of oil contamination and the quantity of that contamination
influence the recovery rate of an oil-contaminated site (Bagby et al. 2016; Stout and
Payne 2016; Roger et al. unpublished). Greater levels of petroleum contamination
may reduce oil biodegradation rates, potentially due to larger particle sizes, which
fall through the oxic water column more rapidly (Bagby et al. 2016). Less exposure
to the oxic water column results in less weathering to the oil in transit, and in this
manner both distance and particle size influence the state of the hydrocarbons upon
their arrival on the suboxic sea floor. Consistent with this idea, elevated levels of
PAHs were found at distances less than 35 km from the wellhead 3 years after the
blowout; however, sediment farther than 35 km had returned to background levels
(Adhikari et al. 2016). Biodegradation of hydrocarbon is more rapid in the oxygenated water column prior to deposition on the seafloor, so the more the oil traveled in
the water column, the more it was degraded (Stout and Payne 2016).
I. C. Romero et al.
radiocarbon was produced in the atmosphere due to nuclear testing and warfare.
This additional radiocarbon is slowly mixing into the earth’s pool of surface carbon
so that contemporary atmospheric CO 2 at the time of the oil spill was about +30 to
40‰ (Graven et al. 2012). At present (2018), the value of atmospheric CO 2 has been
further reduced, and because of continual and growing fossil fuel combustion and
the addition of that CO 2 to the atmosphere, organic matter fixed in 2050 may be
indistinguishable from organic carbon fixed in the year 1050 (Graven 2015).
Oil and methane from fossil carbon reserves have characteristic isotopic values
that allow them to be followed in the environment and in food webs (Chanton et al.
2012; Wilson et al. 2016). These compounds formed millions of years ago and have
been locked away in the subsurface so they are radiocarbon-free with an isotopic
signature of −1000‰ on the Δ
14
C scale. Carbon fixed at the time of the oil spill by
photosynthetic processes had a value of about +40‰ (Chanton et al. 2012, 2018). Oil
and methane are also depleted in δ
13
C (−27‰ to roughly −60‰, respectively) relative to modern photosynthetic marine production (−20 to −22‰). Away from seep
sites, sedimentary organic matter in the Gulf has a δ
13
C value that reflects marine
production and a surface
14
C value of −200‰ ± 29‰ (Chanton et al. 2012, 2015).
Significant transformations can accompany oil weathering and biodegradation.
The hydrocarbons can be oxygenated (Aeppli et al. 2012, 2014), while saturated
and lighter hydrocarbons are lost due to evaporation and biodegradation. The oxygenated hydrocarbons are apparently rather persistent in the environment (Aeppli
et al. 2018). The products of the transformed petroleum in the environment may be
defined generically as “petrocarbon.” Despite these reactions, the fossil-based carbon retains its distinctive
14
C-depleted isotopic composition relative to background
carbon values allowing its detection in the environment. The larger range for Δ
14
C
relative to the δ
13
C variation makes
14
C a significant tracer for finding evidence of
petrocarbon in the sediment after the release of oil. The evidence for oil or methane
contamination in the sediments consists of a surficial Δ
14
C-depleted layer overlying
relatively Δ
14
C-enriched background layers (Fig. 17.2, Chanton et al. 2015).
Sediments in the surface-most layer were as depleted as −500‰ in 2010 following
the oil spill. Over time,
14
C depletion at some sites was ameliorated, returning
toward background values of −200‰ but sediments as depleted as −420‰ were
still observed in surface layers closer to the oil spill site in 2015. Both the distance
of a site from the source of oil contamination and the quantity of that contamination
influence the recovery rate of an oil-contaminated site (Bagby et al. 2016; Stout and
Payne 2016; Roger et al. unpublished). Greater levels of petroleum contamination
may reduce oil biodegradation rates, potentially due to larger particle sizes, which
fall through the oxic water column more rapidly (Bagby et al. 2016). Less exposure
to the oxic water column results in less weathering to the oil in transit, and in this
manner both distance and particle size influence the state of the hydrocarbons upon
their arrival on the suboxic sea floor. Consistent with this idea, elevated levels of
PAHs were found at distances less than 35 km from the wellhead 3 years after the
blowout; however, sediment farther than 35 km had returned to background levels
(Adhikari et al. 2016). Biodegradation of hydrocarbon is more rapid in the oxygenated water column prior to deposition on the seafloor, so the more the oil traveled in
the water column, the more it was degraded (Stout and Payne 2016).
I. C. Romero et al.