289
The significant reduction of both LMW and recalcitrant compounds over an
extensive area indicates that not only biodegradation but also redistribution
(downslope transport) processes may have affected the fate of the hydrocarbons
deposited after the spill in deep-sea sediments. If only biodegradation of hydrocarbon compounds is taken into account, results from in situ observations and degradation models support the hypothesis of a long-term persistence (~ decade) from
coastal to deep-sea environments of the most recalcitrant oil-derived hydrocarbons
deposited in 2010–2011 (Gros et al. 2014; Turner et al. 2014; Romero et al. 2017).
In addition, the formation of oxygenated products from weathering of both aromatic
and saturated compounds suggest an even longer persistence of DWH-derived compounds in the environment (Aeppli et al. 2012; Hall et al. 2013; Lewan et al. 2014;
White et al. 2016). However, the fact that also a significant reduction was observed
for the most recalcitrant compounds (e.g., hopanoids) indicates potential downslope
transport of bottom sediments. This is supported by a large-scale assessment of
hydrocarbons in the GoM (Romero et al. 2017) showing that (1) the deep-sea area
serves as a repository system for hydrocarbons in the northern GoM and (2) a gradient of oil-derived hydrocarbons toward deeper depths was observed (>2400 m)
along bottom drainage paths. Also small-scale resuspension events in the GoM may
occur more frequently than previously thought, perhaps as frequently as daily
(Diercks et al. 2018). Therefore, biodegradation as well as redistribution of surface
sediments containing oil residues should be considered as critical processes for
modeling the long-term fate of oil spills at depth.
17.2.2 Bulk Carbon Isotopes
Regardless of chemical change, isotopic signatures from oil residues can be maintained in the sediment throughout the weathering process as long as the end result is
not dissolved inorganic carbon (DIC). Oil from Miocene source rock, such as the
light, sweet Louisiana crude oil that leaked after the DWH, has two important properties: (1) the stable carbon isotope composition is lighter than marine organic matter, and (2) it is essentially devoid of radiocarbon (
14
C). Stable isotopes for
sedimentary organic carbon (δ
13
C) are reported relative to VPDB, an international
standard carbonate reference material as δ
13
C = (R sam /R std − 1) X1000, where
R =
13
C/
12
C, R sam refers to the ratio of heavy to light isotopes in a sample, and R std
refers to the ratio in the international standard. Radiocarbon values can be reported
according to the Δ notation as described by Stuiver and Pollach (1977). The Δ notation normalizes the radiocarbon content of any sample to a common δ
13
C value
(−25‰) and a common time interval. This
13
C correction means that mass- dependent
isotope effects (isotopic fractionation) that occur in nature are removed from comparisons between
14
C values that are made using this notation. Another benefit of this
notation is that it is a linear scale starting at −1000‰ when a sample has no radiocarbon content (McNichol and Aluwihare 2007). Modern carbon prior to atmospheric weapon testing had a value of 0‰ on this scale, but in the 1960s additional
17 Long-Term Preservation of Oil Spill Events in Sediments: The Case…
The significant reduction of both LMW and recalcitrant compounds over an
extensive area indicates that not only biodegradation but also redistribution
(downslope transport) processes may have affected the fate of the hydrocarbons
deposited after the spill in deep-sea sediments. If only biodegradation of hydrocarbon compounds is taken into account, results from in situ observations and degradation models support the hypothesis of a long-term persistence (~ decade) from
coastal to deep-sea environments of the most recalcitrant oil-derived hydrocarbons
deposited in 2010–2011 (Gros et al. 2014; Turner et al. 2014; Romero et al. 2017).
In addition, the formation of oxygenated products from weathering of both aromatic
and saturated compounds suggest an even longer persistence of DWH-derived compounds in the environment (Aeppli et al. 2012; Hall et al. 2013; Lewan et al. 2014;
White et al. 2016). However, the fact that also a significant reduction was observed
for the most recalcitrant compounds (e.g., hopanoids) indicates potential downslope
transport of bottom sediments. This is supported by a large-scale assessment of
hydrocarbons in the GoM (Romero et al. 2017) showing that (1) the deep-sea area
serves as a repository system for hydrocarbons in the northern GoM and (2) a gradient of oil-derived hydrocarbons toward deeper depths was observed (>2400 m)
along bottom drainage paths. Also small-scale resuspension events in the GoM may
occur more frequently than previously thought, perhaps as frequently as daily
(Diercks et al. 2018). Therefore, biodegradation as well as redistribution of surface
sediments containing oil residues should be considered as critical processes for
modeling the long-term fate of oil spills at depth.
17.2.2 Bulk Carbon Isotopes
Regardless of chemical change, isotopic signatures from oil residues can be maintained in the sediment throughout the weathering process as long as the end result is
not dissolved inorganic carbon (DIC). Oil from Miocene source rock, such as the
light, sweet Louisiana crude oil that leaked after the DWH, has two important properties: (1) the stable carbon isotope composition is lighter than marine organic matter, and (2) it is essentially devoid of radiocarbon (
14
C). Stable isotopes for
sedimentary organic carbon (δ
13
C) are reported relative to VPDB, an international
standard carbonate reference material as δ
13
C = (R sam /R std − 1) X1000, where
R =
13
C/
12
C, R sam refers to the ratio of heavy to light isotopes in a sample, and R std
refers to the ratio in the international standard. Radiocarbon values can be reported
according to the Δ notation as described by Stuiver and Pollach (1977). The Δ notation normalizes the radiocarbon content of any sample to a common δ
13
C value
(−25‰) and a common time interval. This
13
C correction means that mass- dependent
isotope effects (isotopic fractionation) that occur in nature are removed from comparisons between
14
C values that are made using this notation. Another benefit of this
notation is that it is a linear scale starting at −1000‰ when a sample has no radiocarbon content (McNichol and Aluwihare 2007). Modern carbon prior to atmospheric weapon testing had a value of 0‰ on this scale, but in the 1960s additional
17 Long-Term Preservation of Oil Spill Events in Sediments: The Case…