323
weathering (Aeppli et al. 2014). Further evidence of weathered oil in the 70–72 mm
below seafloor horizon was provided by two important differences in compound
class distribution, as compared with the Ixtoc 1 reference oil: (1) a lower abundance
of HCrad and Srad compound classes (defined in Oldenburg et al. 2014), suggesting
preferential removal of lighter, abundant hydrocarbon ends and some sulfur heterocycles from sediments, and (2) a higher abundance of S 2 rad and S 3 rad compounds,
likely reflecting the corresponding enrichment of highly sulfurized, recalcitrant
compounds over the decades since the spill (Radović et al. 2017).
19.6 Summary and Discussion
Long-established oil fingerprinting parameters such as terpane and PAH ratios have
provided evidence that traces of recalcitrant Ixtoc 1 oil compounds persist in seafloor sediments underlying deep waters (>500 m) of the southern Gulf of Mexico
(Lincoln et al. 2017), and more recently developed, non-targeted, ultrahighresolution FTICR-MS techniques have revealed the presence of weathered petroleum residues that nonetheless share spectral and compound class characteristics
with the Ixtoc 1 reference oil (Radović et al. 2017).
In this chapter we have presented the strongest evidence for residual Ixtoc 1
input to sediments that we detected in the southern GoM. We note that several other
sediment cores collected to the west of the Ixtoc 1 wellhead showed no evidence of
a signal that could be correlated with Ixtoc 1, possibly due to overprinting by the
intense hydrocarbon background (Lincoln et al. 2017). However, deep-sea deposition of oil spill-derived hydrocarbons may simply be very heterogeneous and spotty,
particularly if the primary mode of delivery to sediments is via sinking particles
formed during MOSSFA events (Schwing et al. 2020). A single oiled particle may
be adequate to impart a strong, diagnostic petroleum fingerprint, while nearby sediments are left unimpacted.
Considering the short half-life estimates of MW oil-derived petroleum compounds in primarily oxic northern GoM sediments underlying deep waters (e.g.,
Bagby et al. 2017), preservation of Ixtoc 1 compounds in southern GoM sediments
>35 years since the spill may seem surprising. The preservation/biodegradation balance, however, is largely controlled by factors such as oxygen exposure time and
sedimentation rates (Bubenheim et al. 2020); as sediments become anoxic, biodegradation slows and the likelihood that recalcitrant residues will remain increases. We
conclude that, under optimal preservation conditions, a significant fraction of refractory oil spill-derived petroleum compounds can remain in the deep sea for decades,
overprinting background signals and contributions from recent organic matter. A
molecular legacy of the 2010 DWH oil spill will likely be detectable in deep GoM
sediments in the year 2045 and beyond.
Funding Information This research was made possible by grants from the Gulf of Mexico
Research Initiative through the Center for the Integrated Modeling and Analysis of the Gulf
Ecosystem (C-IMAGE).
19 Molecular Legacy of the 1979 Ixtoc 1 Oil Spill in Deep-Sea Sediments…
weathering (Aeppli et al. 2014). Further evidence of weathered oil in the 70–72 mm
below seafloor horizon was provided by two important differences in compound
class distribution, as compared with the Ixtoc 1 reference oil: (1) a lower abundance
of HCrad and Srad compound classes (defined in Oldenburg et al. 2014), suggesting
preferential removal of lighter, abundant hydrocarbon ends and some sulfur heterocycles from sediments, and (2) a higher abundance of S 2 rad and S 3 rad compounds,
likely reflecting the corresponding enrichment of highly sulfurized, recalcitrant
compounds over the decades since the spill (Radović et al. 2017).
19.6 Summary and Discussion
Long-established oil fingerprinting parameters such as terpane and PAH ratios have
provided evidence that traces of recalcitrant Ixtoc 1 oil compounds persist in seafloor sediments underlying deep waters (>500 m) of the southern Gulf of Mexico
(Lincoln et al. 2017), and more recently developed, non-targeted, ultrahighresolution FTICR-MS techniques have revealed the presence of weathered petroleum residues that nonetheless share spectral and compound class characteristics
with the Ixtoc 1 reference oil (Radović et al. 2017).
In this chapter we have presented the strongest evidence for residual Ixtoc 1
input to sediments that we detected in the southern GoM. We note that several other
sediment cores collected to the west of the Ixtoc 1 wellhead showed no evidence of
a signal that could be correlated with Ixtoc 1, possibly due to overprinting by the
intense hydrocarbon background (Lincoln et al. 2017). However, deep-sea deposition of oil spill-derived hydrocarbons may simply be very heterogeneous and spotty,
particularly if the primary mode of delivery to sediments is via sinking particles
formed during MOSSFA events (Schwing et al. 2020). A single oiled particle may
be adequate to impart a strong, diagnostic petroleum fingerprint, while nearby sediments are left unimpacted.
Considering the short half-life estimates of MW oil-derived petroleum compounds in primarily oxic northern GoM sediments underlying deep waters (e.g.,
Bagby et al. 2017), preservation of Ixtoc 1 compounds in southern GoM sediments
>35 years since the spill may seem surprising. The preservation/biodegradation balance, however, is largely controlled by factors such as oxygen exposure time and
sedimentation rates (Bubenheim et al. 2020); as sediments become anoxic, biodegradation slows and the likelihood that recalcitrant residues will remain increases. We
conclude that, under optimal preservation conditions, a significant fraction of refractory oil spill-derived petroleum compounds can remain in the deep sea for decades,
overprinting background signals and contributions from recent organic matter. A
molecular legacy of the 2010 DWH oil spill will likely be detectable in deep GoM
sediments in the year 2045 and beyond.
Funding Information This research was made possible by grants from the Gulf of Mexico
Research Initiative through the Center for the Integrated Modeling and Analysis of the Gulf
Ecosystem (C-IMAGE).
19 Molecular Legacy of the 1979 Ixtoc 1 Oil Spill in Deep-Sea Sediments…