336
mangrove forests of Yucatan Peninsula, which host residues related to the 1979
Ixtoc 1 spill as demonstrated by analyses of recalcitrant biomarkers (Sect. 20.3.1
GC-MS/MS-MRM), there is an apparent preservation of partially weathered oil.
The samples from these sites, particularly the ones buried in the shallow subsurface, still had a characteristic oily appearance and smell and are relatively enriched
in hydrocarbon content (e.g., homohopanes C27–C32). They also contain abundant oxygen- bearing species, likely oxidized metabolites of parent oil compounds,
and an indication of incomplete oil weathering. Potential spill history leading to
oil residues of such chemistry might have included initial biotic and abiotic transformations of the Ixtoc 1 surface slick, in the immediate aftermath of the spill,
which produced “oxyhydrocarbon” products, analogous to the DWH spill fate
(Aeppli et al. 2012; Ruddy et al. 2014; Ward et al. 2018). Based on the apparent
relative preservation of TAS, photooxidation of surfaced Ixtoc 1 oil might have
been less pronounced than in the DWH case (Radović et al. 2014). According to
slick trajectory reconstructions by Sun et al. (2015), surface oil weathering might
have been taking place over a period of ~3.5 months since the beginning of the
spill, after which, by mid-September 1979 and later, the slicks reached the mangrove forests of the western Yucatan Peninsula. In these shaded, low-energy, and
often hypoxic environments, oil degradation processes would slow down, and
partially weathered Ixtoc 1 oil would have a good likelihood to be preserved over
prolonged periods. As indicated by our results, this oil still contains appreciable
amount of parent high molecular weight (HMW) PAHs, which can pose a chronic
source of toxicity.
If we try to draw an analogy to the DWH case, it could be reasonably argued
that the MW oil buried in the environments similar to sGoM mangrove forests,
such as the salt marshes of Louisiana (Zengel et al. 2015; Nixon et al. 2016), has
the potential to be preserved over multi-decadal timescales – an occurrence
already reported in some historic spill cases, such as the 1969 West Falmouth spill
(Reddy et al. 2002).
On the contrary, GC-MS/MS-MRM analysis of oil residues found exposed on
supratidal rocks at sites such as Punta Delgado, Isla En Medio, and Montepio indicates possible modern oil sources. A similar conclusion could be drawn from the
higher sulfur content (as detected by FTICR-MS), observed in these samples, compared to the mangrove tar samples. Contributions from concurrent, contemporary
oil releases are not an unlikely possibility given the extent of oil exploration and
the number of natural oil seep sites in the area (Garcia et al. 2009). On the other
hand, this might also point to a very advanced stage of weathering, relatively enriching the content of sulfur species, including the occurrence of multiple sulfurized
compound classes, possibly indicating S-crosslinking (natural vulcanization).
Sulfur heteroatom oil fractions have been previously shown to be more recalcitrant
to photooxidation (Radović et al. 2014) and biodegradation (Larter and Head 2014;
Oldenburg et al. 2017).
J. R. Radović et al.
mangrove forests of Yucatan Peninsula, which host residues related to the 1979
Ixtoc 1 spill as demonstrated by analyses of recalcitrant biomarkers (Sect. 20.3.1
GC-MS/MS-MRM), there is an apparent preservation of partially weathered oil.
The samples from these sites, particularly the ones buried in the shallow subsurface, still had a characteristic oily appearance and smell and are relatively enriched
in hydrocarbon content (e.g., homohopanes C27–C32). They also contain abundant oxygen- bearing species, likely oxidized metabolites of parent oil compounds,
and an indication of incomplete oil weathering. Potential spill history leading to
oil residues of such chemistry might have included initial biotic and abiotic transformations of the Ixtoc 1 surface slick, in the immediate aftermath of the spill,
which produced “oxyhydrocarbon” products, analogous to the DWH spill fate
(Aeppli et al. 2012; Ruddy et al. 2014; Ward et al. 2018). Based on the apparent
relative preservation of TAS, photooxidation of surfaced Ixtoc 1 oil might have
been less pronounced than in the DWH case (Radović et al. 2014). According to
slick trajectory reconstructions by Sun et al. (2015), surface oil weathering might
have been taking place over a period of ~3.5 months since the beginning of the
spill, after which, by mid-September 1979 and later, the slicks reached the mangrove forests of the western Yucatan Peninsula. In these shaded, low-energy, and
often hypoxic environments, oil degradation processes would slow down, and
partially weathered Ixtoc 1 oil would have a good likelihood to be preserved over
prolonged periods. As indicated by our results, this oil still contains appreciable
amount of parent high molecular weight (HMW) PAHs, which can pose a chronic
source of toxicity.
If we try to draw an analogy to the DWH case, it could be reasonably argued
that the MW oil buried in the environments similar to sGoM mangrove forests,
such as the salt marshes of Louisiana (Zengel et al. 2015; Nixon et al. 2016), has
the potential to be preserved over multi-decadal timescales – an occurrence
already reported in some historic spill cases, such as the 1969 West Falmouth spill
(Reddy et al. 2002).
On the contrary, GC-MS/MS-MRM analysis of oil residues found exposed on
supratidal rocks at sites such as Punta Delgado, Isla En Medio, and Montepio indicates possible modern oil sources. A similar conclusion could be drawn from the
higher sulfur content (as detected by FTICR-MS), observed in these samples, compared to the mangrove tar samples. Contributions from concurrent, contemporary
oil releases are not an unlikely possibility given the extent of oil exploration and
the number of natural oil seep sites in the area (Garcia et al. 2009). On the other
hand, this might also point to a very advanced stage of weathering, relatively enriching the content of sulfur species, including the occurrence of multiple sulfurized
compound classes, possibly indicating S-crosslinking (natural vulcanization).
Sulfur heteroatom oil fractions have been previously shown to be more recalcitrant
to photooxidation (Radović et al. 2014) and biodegradation (Larter and Head 2014;
Oldenburg et al. 2017).
J. R. Radović et al.