321
19.5.2.2 Alkylated Dibenzothiophenes and Phenanthrenes
Ratios of dimethylated phenanthrenes to dimethylated dibenzothiophenes (P2/D2)
and of trimethylated phenanthrenes to trimethylated dibenzothiophenes (P3/D3)
were considered most diagnostic in fingerprinting Ixtoc 1 oil in the GoM immediately after the spill (Boehm et al. 1983). Because P2/D2 and P3/D3 ratios have been
demonstrated to be useful for oil source correlation until 98% depletion of these
compounds occurs (Douglas et al. 1996), they were among the most promising tools
for identifying Ixtoc 1 input decades after the spill.
P2/D2 and P3/D3 ratios in the top 9 cm of a sediment core collected near the
Ixtoc 1 site in 2015 (Lincoln et al. 2017) were consistent with those reported in
Ixtoc 1 oil (Boehm and Fiest 1980). At IXW-250, these ratios deviated from core
background values between 6 and 9 cmbsf (Fig. 19.2). In that depth range, both P2/
D2 and P3/D3 declined, and P3/D3 fell to <1. The two ratios are closest in this horizon and diverge above it. Both the decline in the P2/D2 and P3/D3 and the decreased
difference between them (a pattern contrasting with that of sediments at depths
above 6 cmbsf) are consistent with a contribution from Ixtoc 1 oil.
Significant differences in the P2/D2 and P3/D3 ratios between sites and their
utility as an indicator of Ixtoc 1 oil in the IXW-250 sediment core support their
apparent robustness as a metric of contaminant-source correlation over multidecadal time scales.
19.5.3 Fourier Transform Ion Cyclotron Resonance-Mass
Spectrometry
Alteration of oil residues due to biotic and abiotic weathering can transform oil
compounds, particularly aromatics, to oxygen-containing residues (Aeppli et al.
2012, 2014; Radović et al. 2014; Ruddy et al. 2014), many of which are not detectable with standard GC-MS fingerprinting protocols. Fourier transform ion cyclotron
resonance-mass spectrometry (FTICR-MS) is a useful complement to conventional
GC-MS in that it extends the analytical window for petroleum analysis to include
thousands of non-GC-amenable, high-molecular-weight, polar heteroatomcontaining compounds, due to its diverse ionization modes, broad mass range, and
ultrahigh mass resolution (McKenna et al. 2013). Following its demonstrated utility
in characterizing the source Macondo well oil and the weathered residues collected
in the aftermath of the DWH spill (McKenna et al. 2013; Ruddy et al. 2014),
Radović et al. (2017) employed FTICR-MS to assess Ixtoc 1 input to southern GoM
sediments, comparing signals detected in sediments with those of a reference sample of Ixtoc 1 crude oil obtained from PEMEX.
FTICR-MS analyses focused on one sediment core in which striking visual evidence provided the first clue of petrogenic input: the organic solvent extract of sediment 70–72 mm below seafloor at site IXW-500 (Fig. 19.1, 1010 m water depth)
had a pronounced oily appearance (Radović et al. 2017). Comparison of FTICR-MS
19 Molecular Legacy of the 1979 Ixtoc 1 Oil Spill in Deep-Sea Sediments…
19.5.2.2 Alkylated Dibenzothiophenes and Phenanthrenes
Ratios of dimethylated phenanthrenes to dimethylated dibenzothiophenes (P2/D2)
and of trimethylated phenanthrenes to trimethylated dibenzothiophenes (P3/D3)
were considered most diagnostic in fingerprinting Ixtoc 1 oil in the GoM immediately after the spill (Boehm et al. 1983). Because P2/D2 and P3/D3 ratios have been
demonstrated to be useful for oil source correlation until 98% depletion of these
compounds occurs (Douglas et al. 1996), they were among the most promising tools
for identifying Ixtoc 1 input decades after the spill.
P2/D2 and P3/D3 ratios in the top 9 cm of a sediment core collected near the
Ixtoc 1 site in 2015 (Lincoln et al. 2017) were consistent with those reported in
Ixtoc 1 oil (Boehm and Fiest 1980). At IXW-250, these ratios deviated from core
background values between 6 and 9 cmbsf (Fig. 19.2). In that depth range, both P2/
D2 and P3/D3 declined, and P3/D3 fell to <1. The two ratios are closest in this horizon and diverge above it. Both the decline in the P2/D2 and P3/D3 and the decreased
difference between them (a pattern contrasting with that of sediments at depths
above 6 cmbsf) are consistent with a contribution from Ixtoc 1 oil.
Significant differences in the P2/D2 and P3/D3 ratios between sites and their
utility as an indicator of Ixtoc 1 oil in the IXW-250 sediment core support their
apparent robustness as a metric of contaminant-source correlation over multidecadal time scales.
19.5.3 Fourier Transform Ion Cyclotron Resonance-Mass
Spectrometry
Alteration of oil residues due to biotic and abiotic weathering can transform oil
compounds, particularly aromatics, to oxygen-containing residues (Aeppli et al.
2012, 2014; Radović et al. 2014; Ruddy et al. 2014), many of which are not detectable with standard GC-MS fingerprinting protocols. Fourier transform ion cyclotron
resonance-mass spectrometry (FTICR-MS) is a useful complement to conventional
GC-MS in that it extends the analytical window for petroleum analysis to include
thousands of non-GC-amenable, high-molecular-weight, polar heteroatomcontaining compounds, due to its diverse ionization modes, broad mass range, and
ultrahigh mass resolution (McKenna et al. 2013). Following its demonstrated utility
in characterizing the source Macondo well oil and the weathered residues collected
in the aftermath of the DWH spill (McKenna et al. 2013; Ruddy et al. 2014),
Radović et al. (2017) employed FTICR-MS to assess Ixtoc 1 input to southern GoM
sediments, comparing signals detected in sediments with those of a reference sample of Ixtoc 1 crude oil obtained from PEMEX.
FTICR-MS analyses focused on one sediment core in which striking visual evidence provided the first clue of petrogenic input: the organic solvent extract of sediment 70–72 mm below seafloor at site IXW-500 (Fig. 19.1, 1010 m water depth)
had a pronounced oily appearance (Radović et al. 2017). Comparison of FTICR-MS
19 Molecular Legacy of the 1979 Ixtoc 1 Oil Spill in Deep-Sea Sediments…