95
and their hydrodynamic diameter along with the ambient water temperature. Typical
settling velocities are between 10 and 150 m day
−1
in marine environments
(McDonnell and Buesseler 2010), leading to close coupling between the surface and
deep waters (Asper et al. 1992). The magnitude of this process has been debated,
and no clear formula is available to estimate the likely flux from a given spill,
although it is likely to be small compared to the other attenuation processes.
Several field measurement techniques have been employed to quantify the flux,
density, and settling velocity of marine snow. Sediment traps are the most direct
way to measure flux, as they are deployed for a known time and collect settling
particles in the trap for analysis. However, sediment traps can be affected by degradation, mobile animals, and hydrodynamic forces (Buesseler et al. 2007). Marine
snow flux can also be measured using a photographic apparatus in which a camera
captures images at regular intervals and the images are processed to determine the
number of unique particles in each image. More than 800 studies of marine snow
flux measurements using sediment traps, filtration systems, or other techniques
were published between 1979 and 2015 (Turner 2002, 2015). Laboratory studies
have attempted to quantify the production and flux of marine snow in the presence
of oil. For example, Passow and her colleagues (Passow 2016) used filtered seawater and natural bacterial assemblages in a bottle experiment to generate marine snow
in the presence of weathered crude. However, other similar experiments were not
able to produce marine snow, although oil was degraded (Ziervogel et al. 2014),
highlighting the importance of the specific conditions. Additionally, in laboratory
experiments, the long incubation times (weeks) and relatively high concentrations
may not be relevant to field conditions, where mixing may prevent marine snow
formation.
Measuring the amount of oil that has reached the sediments in an offshore oil
spill provides information on the overall mass balance of the spill, as well as the
potential injury to natural resources, but can be logistically challenging. For example, several different approaches were used to estimate the amount of oil reaching
the sediments during and after the DWH spill, resulting in different estimates of the
impact of oil on the deep sediments. In all cases, the analyses relied on obtaining
sediment data from deep-sea locations, a process which is prone to uncertainty due
to the patchiness of the oil transported to the seafloor and the very large number of
samples required to understand this distribution over such a large geographical area.
Direct approaches to estimating the oil sedimentation used comprehensive fingerprinting of sediment oil residues to identify locations containing oil and to confirm
that the oil present was consistent with oil released from the Macondo wellhead.
The oil fingerprinting was necessary to separate the spilled oil from other petroleum
products present in these areas, such as seep oil or spilled oil from other sources.
Independent analyses using this approach and a common, publicly available data set
found very similar footprints of oil on the seafloor (Stout et al. 2016; Murray et al.
2017). Other approaches used proxies for the oil to estimate the seafloor impact.
These methods can be less analytically intensive and can provide an estimate of the
impact of oil, but differ from the fingerprinting methods in their specificity. The use
of proxy measurements relies on assumptions that may not be applicable under all
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…
and their hydrodynamic diameter along with the ambient water temperature. Typical
settling velocities are between 10 and 150 m day
−1
in marine environments
(McDonnell and Buesseler 2010), leading to close coupling between the surface and
deep waters (Asper et al. 1992). The magnitude of this process has been debated,
and no clear formula is available to estimate the likely flux from a given spill,
although it is likely to be small compared to the other attenuation processes.
Several field measurement techniques have been employed to quantify the flux,
density, and settling velocity of marine snow. Sediment traps are the most direct
way to measure flux, as they are deployed for a known time and collect settling
particles in the trap for analysis. However, sediment traps can be affected by degradation, mobile animals, and hydrodynamic forces (Buesseler et al. 2007). Marine
snow flux can also be measured using a photographic apparatus in which a camera
captures images at regular intervals and the images are processed to determine the
number of unique particles in each image. More than 800 studies of marine snow
flux measurements using sediment traps, filtration systems, or other techniques
were published between 1979 and 2015 (Turner 2002, 2015). Laboratory studies
have attempted to quantify the production and flux of marine snow in the presence
of oil. For example, Passow and her colleagues (Passow 2016) used filtered seawater and natural bacterial assemblages in a bottle experiment to generate marine snow
in the presence of weathered crude. However, other similar experiments were not
able to produce marine snow, although oil was degraded (Ziervogel et al. 2014),
highlighting the importance of the specific conditions. Additionally, in laboratory
experiments, the long incubation times (weeks) and relatively high concentrations
may not be relevant to field conditions, where mixing may prevent marine snow
formation.
Measuring the amount of oil that has reached the sediments in an offshore oil
spill provides information on the overall mass balance of the spill, as well as the
potential injury to natural resources, but can be logistically challenging. For example, several different approaches were used to estimate the amount of oil reaching
the sediments during and after the DWH spill, resulting in different estimates of the
impact of oil on the deep sediments. In all cases, the analyses relied on obtaining
sediment data from deep-sea locations, a process which is prone to uncertainty due
to the patchiness of the oil transported to the seafloor and the very large number of
samples required to understand this distribution over such a large geographical area.
Direct approaches to estimating the oil sedimentation used comprehensive fingerprinting of sediment oil residues to identify locations containing oil and to confirm
that the oil present was consistent with oil released from the Macondo wellhead.
The oil fingerprinting was necessary to separate the spilled oil from other petroleum
products present in these areas, such as seep oil or spilled oil from other sources.
Independent analyses using this approach and a common, publicly available data set
found very similar footprints of oil on the seafloor (Stout et al. 2016; Murray et al.
2017). Other approaches used proxies for the oil to estimate the seafloor impact.
These methods can be less analytically intensive and can provide an estimate of the
impact of oil, but differ from the fingerprinting methods in their specificity. The use
of proxy measurements relies on assumptions that may not be applicable under all
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…