239
14.2.1 Time-Series Approach/High-Resolution Sampling
A time series of annually collected cores from the same sites allowed for the detection of the event in the sedimentary record, the annual-scale evolution of the sedimentary deposit over the subsequent years, and the potential burial and preservation
in the sedimentary system. In the absence of any baseline information, such as for
the DWH blowout, a time-series approach provides a post-event baseline. This
approach also helps identify how indicators of oil-contaminated sediments may
change over time in the sedimentary record. It also can assist in determining the
preservation potential of oil-contaminated sediments including influences of burial,
bioturbation/mixing, postdepositional alteration, and resuspension/erosion.
Both the siliciclastic and carbonate sedimentary regimes are represented in the
time series with sites DSH08 and DSH10 located on the siliciclastic side of the
Desoto Canyon and sites PCB06 and M04 located on the carbonate side of the
DeSoto Canyon (Fig. 14.1). High-resolution sub-sampling of cores provided high
temporal resolution to define pre-event baselines (downcore), the event, and postevent sedimentation. Sub-sampling of cores at 2 mm resolution by extrusion methods described by Schwing et al. (2016) provided the highest temporal resolution
possible for analyses.
14.2.2 Chronometers: Timing of Deposition
Defining the timing of sedimentation is critical to correlate the deviations in the
sedimentary record with the DWH event. The most commonly used chronometers
for recent deposition are short-lived radioisotopes including excess
234
Th (
234
Th xs) ,
excess
210
Pb (
210
Pb xs) ,
137
Cs, and
7
Be (Swarzenski 2014; Holmes 1998; Appleby
2001). Ideally, multiple chronometers would be utilized as they each have strengths
and limitations. Sediment cores collected from the deep-sea oceanic setting (all
analyzed to date) did not contain detectable
137
Cs or
7
Be, so they are not viable in
this environment as chronometers or for corroborating other age dating techniques.
Therefore,
234
Th xs and
210
Pb xs are the primary chronometers for defining sedimentation on monthly (
234
Th xs ) and annual/decadal (
210
Pb xs ) time scales. Sediment core
samples were analyzed for short-lived radioisotopes by gamma spectrometry on
Series HPGe (high-purity germanium) Coaxial Planar Photon Detectors for total
210
Pb (46.5 keV),
214
Pb (295 keV and 351 keV),
214
Bi (609 keV), and
234
Th (63 keV)
in order to determine
234
Th xs and
210
Pb xs activities expressed as disintegrations per
minute per gram (dpm/g) using methodology described by Brooks et al. (2015) and
Larson et al. (2018).
The short duration of the sedimentation pulse required a high temporal resolution chronometer, such as
234
Th xs , to identify the most recent sedimentation that
would be associated with the event. Uranium-238 is soluble in seawater and behaves
conservatively with salinity (Chen et al. 1986), and the decay product,
234
Th xs ,
14 Characterization of the Sedimentation Associated with the Deepwater Horizon…
14.2.1 Time-Series Approach/High-Resolution Sampling
A time series of annually collected cores from the same sites allowed for the detection of the event in the sedimentary record, the annual-scale evolution of the sedimentary deposit over the subsequent years, and the potential burial and preservation
in the sedimentary system. In the absence of any baseline information, such as for
the DWH blowout, a time-series approach provides a post-event baseline. This
approach also helps identify how indicators of oil-contaminated sediments may
change over time in the sedimentary record. It also can assist in determining the
preservation potential of oil-contaminated sediments including influences of burial,
bioturbation/mixing, postdepositional alteration, and resuspension/erosion.
Both the siliciclastic and carbonate sedimentary regimes are represented in the
time series with sites DSH08 and DSH10 located on the siliciclastic side of the
Desoto Canyon and sites PCB06 and M04 located on the carbonate side of the
DeSoto Canyon (Fig. 14.1). High-resolution sub-sampling of cores provided high
temporal resolution to define pre-event baselines (downcore), the event, and postevent sedimentation. Sub-sampling of cores at 2 mm resolution by extrusion methods described by Schwing et al. (2016) provided the highest temporal resolution
possible for analyses.
14.2.2 Chronometers: Timing of Deposition
Defining the timing of sedimentation is critical to correlate the deviations in the
sedimentary record with the DWH event. The most commonly used chronometers
for recent deposition are short-lived radioisotopes including excess
234
Th (
234
Th xs) ,
excess
210
Pb (
210
Pb xs) ,
137
Cs, and
7
Be (Swarzenski 2014; Holmes 1998; Appleby
2001). Ideally, multiple chronometers would be utilized as they each have strengths
and limitations. Sediment cores collected from the deep-sea oceanic setting (all
analyzed to date) did not contain detectable
137
Cs or
7
Be, so they are not viable in
this environment as chronometers or for corroborating other age dating techniques.
Therefore,
234
Th xs and
210
Pb xs are the primary chronometers for defining sedimentation on monthly (
234
Th xs ) and annual/decadal (
210
Pb xs ) time scales. Sediment core
samples were analyzed for short-lived radioisotopes by gamma spectrometry on
Series HPGe (high-purity germanium) Coaxial Planar Photon Detectors for total
210
Pb (46.5 keV),
214
Pb (295 keV and 351 keV),
214
Bi (609 keV), and
234
Th (63 keV)
in order to determine
234
Th xs and
210
Pb xs activities expressed as disintegrations per
minute per gram (dpm/g) using methodology described by Brooks et al. (2015) and
Larson et al. (2018).
The short duration of the sedimentation pulse required a high temporal resolution chronometer, such as
234
Th xs , to identify the most recent sedimentation that
would be associated with the event. Uranium-238 is soluble in seawater and behaves
conservatively with salinity (Chen et al. 1986), and the decay product,
234
Th xs ,
14 Characterization of the Sedimentation Associated with the Deepwater Horizon…