318
19.5 Evidence of Residual Ixtoc 1 Oil-Derived Compounds
in Southern GoM Sediments >35 Years After the Spill
In 2015 the Gulf of Mexico Research Initiative-funded Center for the Integrated
Modeling and Analysis of the Gulf Ecosystem (C-IMAGE) research consortium
undertook a sampling campaign to evaluate the potential presence and impacts of
Ixtoc 1 in the southern GoM. Radović et al. (2020) discusses evidence for Ixtoc 1 oil
input to coastal environments; here, we focus exclusively on evidence for preservation in the deep sea. Sediment cores were retrieved from the seafloor at sites across
the Bay of Campeche using a multicorer deployed from the B/O Justo Sierra. Site
selection was guided by historical satellite data and observations of oiling during
and after the spill (Sun et al. 2015). Tandem organic geochemical and geochronological analyses were used to search for compositional and temporal matches
between sediment hydrocarbons and putative Ixtoc 1 inputs (Lincoln et al. 2017;
Radović et al. 2017). The results of these analyses are summarized below after a
brief explanation of the geochronological and chemostratigraphic approaches that
enabled age estimates for oil spill event horizons.
19.5.1 Temporal Constraints from Geochronology and PCB
Chronostratigraphy
Environmental oil fingerprinting can be difficult and contentious even when undertaken shortly after a spill, particularly if oils have experienced extensive weathering;
the challenge is only compounded as sedimentation buries any potential record
beneath the seafloor. Northern GoM studies have either made the implicit assumption that core tops (0–0.5 to 0–2 cm below seafloor) should contain any sedimentary
record of DWH-derived petrocarbon <5 years after the spill (e.g., Valentine et al.
2014; Chanton et al. 2014), which seems reasonable given knowledge of sedimentation rates in the region, or have taken a higher-resolution event stratigraphy approach,
sampling cores at depth intervals of as little as 2 mm and employing geochronological techniques to more tightly constrain horizons containing potential spill-related
records (e.g., Romero et al. 2015; Brooks et al. 2015; Schwing et al. 2016).
Geochronology becomes more critical as sediments deposited during and after
spills are buried and compacted over time. Short-lived radioisotopes (specifically,
the excess
210
Pb approach) have been used to resolve the DWH event (Larson et al.
2020) and to generate age models for sediment cores retrieved from the southern
GoM. At the site with the strongest terpane biomarker evidence for Ixtoc 1 input
(described in Sect. 19.5.2.1),
210
Pb dating placed the putative event horizon at a
depth of 6–7 cm below seafloor (Lincoln et al. 2017).
Polychlorinated biphenyl compound (PCB) concentration profiles provided additional chronostratigraphic constraints in the southern GoM (Lincoln et al. 2017). First
synthesized in 1881 and widely manufactured from the 1930s through the 1970s for
S. A. Lincoln et al.
19.5 Evidence of Residual Ixtoc 1 Oil-Derived Compounds
in Southern GoM Sediments >35 Years After the Spill
In 2015 the Gulf of Mexico Research Initiative-funded Center for the Integrated
Modeling and Analysis of the Gulf Ecosystem (C-IMAGE) research consortium
undertook a sampling campaign to evaluate the potential presence and impacts of
Ixtoc 1 in the southern GoM. Radović et al. (2020) discusses evidence for Ixtoc 1 oil
input to coastal environments; here, we focus exclusively on evidence for preservation in the deep sea. Sediment cores were retrieved from the seafloor at sites across
the Bay of Campeche using a multicorer deployed from the B/O Justo Sierra. Site
selection was guided by historical satellite data and observations of oiling during
and after the spill (Sun et al. 2015). Tandem organic geochemical and geochronological analyses were used to search for compositional and temporal matches
between sediment hydrocarbons and putative Ixtoc 1 inputs (Lincoln et al. 2017;
Radović et al. 2017). The results of these analyses are summarized below after a
brief explanation of the geochronological and chemostratigraphic approaches that
enabled age estimates for oil spill event horizons.
19.5.1 Temporal Constraints from Geochronology and PCB
Chronostratigraphy
Environmental oil fingerprinting can be difficult and contentious even when undertaken shortly after a spill, particularly if oils have experienced extensive weathering;
the challenge is only compounded as sedimentation buries any potential record
beneath the seafloor. Northern GoM studies have either made the implicit assumption that core tops (0–0.5 to 0–2 cm below seafloor) should contain any sedimentary
record of DWH-derived petrocarbon <5 years after the spill (e.g., Valentine et al.
2014; Chanton et al. 2014), which seems reasonable given knowledge of sedimentation rates in the region, or have taken a higher-resolution event stratigraphy approach,
sampling cores at depth intervals of as little as 2 mm and employing geochronological techniques to more tightly constrain horizons containing potential spill-related
records (e.g., Romero et al. 2015; Brooks et al. 2015; Schwing et al. 2016).
Geochronology becomes more critical as sediments deposited during and after
spills are buried and compacted over time. Short-lived radioisotopes (specifically,
the excess
210
Pb approach) have been used to resolve the DWH event (Larson et al.
2020) and to generate age models for sediment cores retrieved from the southern
GoM. At the site with the strongest terpane biomarker evidence for Ixtoc 1 input
(described in Sect. 19.5.2.1),
210
Pb dating placed the putative event horizon at a
depth of 6–7 cm below seafloor (Lincoln et al. 2017).
Polychlorinated biphenyl compound (PCB) concentration profiles provided additional chronostratigraphic constraints in the southern GoM (Lincoln et al. 2017). First
synthesized in 1881 and widely manufactured from the 1930s through the 1970s for
S. A. Lincoln et al.