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17.1 Introduction
Marine sediments can provide a valuable record of natural and human-induced
historical events that have affected the water column. For example, after the
Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM), analyses of sediment and water column samples demonstrated multiple processes for weathered oil
deposition on the seafloor, as oil-associated marine snow (MOS), oil-mineral aggregates (OMAs), and dissolved compounds (Ryerson et al. 2012; Passow et al. 2012;
Daly et al. 2016; Romero et al. 2017). Oil-associated deposition was observed to
occur mostly as a combination of sticky material of weathered oil mixed with plankton, bacteria (MOS), and/or sediment particles (OMAs), referred to as MOSSFA
(Marine Oil Snow Sedimentation and Flocculent Accumulation; Daly et al. 2016).
Also, based on sedimentological, geochemical, and biological measurements, deposition of weathered oil onto the seafloor seems to have occurred over a ~5-month
period after the DWH oil spill (Brooks et al. 2015). Furthermore, results from studies using sediment traps indicate deposition of oil-based petrocarbon for as long as
3 years following the spill (Yan et al. 2016). As a consequence, this short-term
transport of weathered oil to depth led to the deposition of an unprecedented amount
of oil residues over ~110,000 km
2
of the seafloor (~0.8–1.9 million barrels), from
coastal (including bays and estuaries) to deep-sea areas (down to 2600 m water
column depth) (Romero et al. 2017). Specifically for the deep sea, several studies
using different analytical methods have quantified similar amounts of deposited
weathered oil (~0.1–0.4 million barrels) (Valentine et al. 2014; Chanton et al. 2015;
Stout et al. 2016; Romero et al. 2017). The accumulation of oil residues on surface
sediments can be still found years after the spill on beaches (Hayworth et al. 2015;
Yin et al. 2015; White et al. 2016) and deep-sea sediments (Stout et al. 2016).
However, changes in the chemical composition of the deposited oil residues have
been observed with biodegradation and transformation processes as the primary
drivers of oil weathering in the environment (Aeppli et al. 2014; Stout et al. 2016).
In addition, chemical analyses have indicated the persistence of molecular signatures, recalcitrant to weathering processes, serving as long-term indicators of the
deposition and accumulation of oil residues on sediments (Aeppli et al. 2014;
Rosenheim et al. 2014; Chanton et al. 2015; Stout et al. 2016; Romero et al. 2017).
The goal of this chapter is to summarize the chemical methods targeting recalcitrant
molecules and transformed organic material in sediments as indicators of the DWH
event. Monitoring of these recalcitrant molecules and transformed material will
help to elucidate the long-term fate of the DWH weathered oil in sedimentary
environments of the GoM.
I. C. Romero et al.
17.1 Introduction
Marine sediments can provide a valuable record of natural and human-induced
historical events that have affected the water column. For example, after the
Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM), analyses of sediment and water column samples demonstrated multiple processes for weathered oil
deposition on the seafloor, as oil-associated marine snow (MOS), oil-mineral aggregates (OMAs), and dissolved compounds (Ryerson et al. 2012; Passow et al. 2012;
Daly et al. 2016; Romero et al. 2017). Oil-associated deposition was observed to
occur mostly as a combination of sticky material of weathered oil mixed with plankton, bacteria (MOS), and/or sediment particles (OMAs), referred to as MOSSFA
(Marine Oil Snow Sedimentation and Flocculent Accumulation; Daly et al. 2016).
Also, based on sedimentological, geochemical, and biological measurements, deposition of weathered oil onto the seafloor seems to have occurred over a ~5-month
period after the DWH oil spill (Brooks et al. 2015). Furthermore, results from studies using sediment traps indicate deposition of oil-based petrocarbon for as long as
3 years following the spill (Yan et al. 2016). As a consequence, this short-term
transport of weathered oil to depth led to the deposition of an unprecedented amount
of oil residues over ~110,000 km
2
of the seafloor (~0.8–1.9 million barrels), from
coastal (including bays and estuaries) to deep-sea areas (down to 2600 m water
column depth) (Romero et al. 2017). Specifically for the deep sea, several studies
using different analytical methods have quantified similar amounts of deposited
weathered oil (~0.1–0.4 million barrels) (Valentine et al. 2014; Chanton et al. 2015;
Stout et al. 2016; Romero et al. 2017). The accumulation of oil residues on surface
sediments can be still found years after the spill on beaches (Hayworth et al. 2015;
Yin et al. 2015; White et al. 2016) and deep-sea sediments (Stout et al. 2016).
However, changes in the chemical composition of the deposited oil residues have
been observed with biodegradation and transformation processes as the primary
drivers of oil weathering in the environment (Aeppli et al. 2014; Stout et al. 2016).
In addition, chemical analyses have indicated the persistence of molecular signatures, recalcitrant to weathering processes, serving as long-term indicators of the
deposition and accumulation of oil residues on sediments (Aeppli et al. 2014;
Rosenheim et al. 2014; Chanton et al. 2015; Stout et al. 2016; Romero et al. 2017).
The goal of this chapter is to summarize the chemical methods targeting recalcitrant
molecules and transformed organic material in sediments as indicators of the DWH
event. Monitoring of these recalcitrant molecules and transformed material will
help to elucidate the long-term fate of the DWH weathered oil in sedimentary
environments of the GoM.
I. C. Romero et al.