96
conditions. For example, hopane, a relatively recalcitrant petroleum biomarker, was
used to delineate the seafloor impact of the DWH spill (Valentine et al. 2014). This
approach was based on the assumptions that hopane does not degrade and that the
only hopane present in the GoM sediments came from the DWH spill. However, the
GoM has many active hydrocarbon seeps which contribute hopane to the water
column and sediments (MacDonald et al. 2015), and using a single hopane compound to trace oil does not distinguish between spill and seep sources. If background hopane never degrades, 100% of the hopane ever released from the seeps
would be present in the GoM, confounding interpretations. However, hopane has
been suggested to degrade in this environment (Bagby et al. 2017), potentially complicating interpretations. As we will discuss below, the material containing the
hopane on the seafloor is certainly not chemically similar to fresh crude oil—it is
the highly degraded residue of that oil, and studies show that degradation began
before deposition (Hazen et al. 2010; Valentine et al. 2010). Assuming that the
hopane in the sediments represents fresh oil substantially overestimates the amount
of oil that reached the seafloor. Another approach uses carbon isotopes to estimate
oil flux to the seafloor based on the radiocarbon content of sediment samples as in
the approach used by Chanton et al. (Chanton et al. 2012; Chanton et al. 2014). In
these studies, the authors used the radioactive tracer
14
C to differentiate between
relatively modern inputs that contain
14
C and fossil inputs such as crude oil that no
longer contain any
14
C. However, this method does not discriminate between different fossil sources and cannot be reliably used to calculate oil mass balance and
overall sedimentation. The presence of oil-derived carbon does not necessarily indicate the presence of any oil component in the sediment and may be the result of
fully metabolized oil that has been incorporated into biomass that has sunk. This
distinction is very important because one represents a potential injury (sediments
containing toxic components of oil) and one does not (degraded oil-derived carbon
in biomass).
6.5 Fate of Oil and Gas: Understanding the Degradation
of Oil Components
One of the most significant findings of the early sampling from the DWH spill was
the discovery of deep-sea anomalies or diffuse “plumes” of oil and dissolved gas
(Camilli et al. 2010; Spier et al. 2013) and the rate of biodegradation of the dispersed and dissolved oil and gas within those plumes (Hazen et al. 2010; Kessler
et al. 2011). The existence of “plumes” was not completely unexpected since the
ocean, like the atmosphere, is often stratified, but the rate of biodegradation—oil
half-lives of 1.2–6.1 days—was substantially faster than many expected. While later
analysis suggests these rates may have been overestimated (Bagby et al. 2017),
laboratory experiments have confirmed that such rates for very dilute oil are not
unreasonable (Prince et al. 2017) and that the much slower rates measured after oil
has beached, for example, following the Exxon Valdez spill (Bragg et al. 1994), are
K. J. Murray et al.
conditions. For example, hopane, a relatively recalcitrant petroleum biomarker, was
used to delineate the seafloor impact of the DWH spill (Valentine et al. 2014). This
approach was based on the assumptions that hopane does not degrade and that the
only hopane present in the GoM sediments came from the DWH spill. However, the
GoM has many active hydrocarbon seeps which contribute hopane to the water
column and sediments (MacDonald et al. 2015), and using a single hopane compound to trace oil does not distinguish between spill and seep sources. If background hopane never degrades, 100% of the hopane ever released from the seeps
would be present in the GoM, confounding interpretations. However, hopane has
been suggested to degrade in this environment (Bagby et al. 2017), potentially complicating interpretations. As we will discuss below, the material containing the
hopane on the seafloor is certainly not chemically similar to fresh crude oil—it is
the highly degraded residue of that oil, and studies show that degradation began
before deposition (Hazen et al. 2010; Valentine et al. 2010). Assuming that the
hopane in the sediments represents fresh oil substantially overestimates the amount
of oil that reached the seafloor. Another approach uses carbon isotopes to estimate
oil flux to the seafloor based on the radiocarbon content of sediment samples as in
the approach used by Chanton et al. (Chanton et al. 2012; Chanton et al. 2014). In
these studies, the authors used the radioactive tracer
14
C to differentiate between
relatively modern inputs that contain
14
C and fossil inputs such as crude oil that no
longer contain any
14
C. However, this method does not discriminate between different fossil sources and cannot be reliably used to calculate oil mass balance and
overall sedimentation. The presence of oil-derived carbon does not necessarily indicate the presence of any oil component in the sediment and may be the result of
fully metabolized oil that has been incorporated into biomass that has sunk. This
distinction is very important because one represents a potential injury (sediments
containing toxic components of oil) and one does not (degraded oil-derived carbon
in biomass).
6.5 Fate of Oil and Gas: Understanding the Degradation
of Oil Components
One of the most significant findings of the early sampling from the DWH spill was
the discovery of deep-sea anomalies or diffuse “plumes” of oil and dissolved gas
(Camilli et al. 2010; Spier et al. 2013) and the rate of biodegradation of the dispersed and dissolved oil and gas within those plumes (Hazen et al. 2010; Kessler
et al. 2011). The existence of “plumes” was not completely unexpected since the
ocean, like the atmosphere, is often stratified, but the rate of biodegradation—oil
half-lives of 1.2–6.1 days—was substantially faster than many expected. While later
analysis suggests these rates may have been overestimated (Bagby et al. 2017),
laboratory experiments have confirmed that such rates for very dilute oil are not
unreasonable (Prince et al. 2017) and that the much slower rates measured after oil
has beached, for example, following the Exxon Valdez spill (Bragg et al. 1994), are
K. J. Murray et al.