274
16.3.2 Post-impact: Organic Geochemistry
Following the blowout event, the MOSSFA event and accumulation of marine snow
resulted in a substantial sedimentation pulse. In addition to an increase in sedimentation rate, the quality of the organic carbon associated with the sediment pulse and
the oil-associated marine snow was likely more labile and would be remineralized
more quickly than the relatively low-quality partially degraded fecal pellets that
make up most of the organic carbon rain. “High-quality,” fresh diatom-derived
organic carbon is remineralized >300% faster than “low-quality” carbon from fecal
pellets, showing that different organic matter makes a significant difference in respiration rates and residence time of carbon on the ocean floor (Mayor et al. 2012).
At least 50% of the recently deposited aliphatic hydrocarbons in each of the three
times series cores degraded between the December 2010 and February 2011 sampling (Romero et al. 2017, 2020).
16.3.3 Post-impact: Mn Geochemistry
Our approach is to describe general trends and present downcore results that are
representative of elemental profiles at different stations, rather than present data
from all the downcore metal profiles from the different sites we sampled. One of the
most interesting changes in the geochemistry of surface sediments after the event is
the occurrence of multiple Mn oxide peaks. For example, two distinct Mn peaks are
present at 5 and 20 mm at DSH10 (1520 m) in August 2010 with a modest Re
enrichment of 0.1–0.2 ppb coincident with the Mn minimum between the two peaks
(Fig. 16.3a). Below the second Mn peak, Re increases to a distinct relative maximum of 2.1 ppb. The DSH10 profile sampled two years later is another illustration
of the double Mn peaks. As before, there is a large Mn peak close to the surface
(5 mm), and a smaller Mn peak at 20 mm (Fig. 16.3b). There is also a Re peak at the
base of the second Mn peak at 29 mm, just below the Mn redox boundary.
Double Mn oxide peaks in the top 40 mm are observed in virtually every depth
profile at the three time series stations (DSH10, DSH08, and PCB06) as well as
many other sites around the wellhead sampled after the event. The Mn profile is
characterized by a substantial Mn peak close to the sediment-water interface
(~5 mm) and a smaller peak, deeper in the sediment, about 20–35 mm deep. The
shallower peak is a result of the shoaled redoxcline following the large sedimentation pulse associated with the event with the relic Mn peak beneath it. See Hastings
et al. (2016) for more results and depth profiles demonstrating these multiple Mn
peaks.
Do these double Mn peaks persist over time? We sampled these three sites annually for 8 years following the event. This time series reveal that double Mn peaks or
a broad peak spanning 10–20 mm persists for 3–5 years following the event. Over
time, the shallow Mn oxide peak gradually decreases in magnitude, while the deeper
D. W. Hastings et al.
16.3.2 Post-impact: Organic Geochemistry
Following the blowout event, the MOSSFA event and accumulation of marine snow
resulted in a substantial sedimentation pulse. In addition to an increase in sedimentation rate, the quality of the organic carbon associated with the sediment pulse and
the oil-associated marine snow was likely more labile and would be remineralized
more quickly than the relatively low-quality partially degraded fecal pellets that
make up most of the organic carbon rain. “High-quality,” fresh diatom-derived
organic carbon is remineralized >300% faster than “low-quality” carbon from fecal
pellets, showing that different organic matter makes a significant difference in respiration rates and residence time of carbon on the ocean floor (Mayor et al. 2012).
At least 50% of the recently deposited aliphatic hydrocarbons in each of the three
times series cores degraded between the December 2010 and February 2011 sampling (Romero et al. 2017, 2020).
16.3.3 Post-impact: Mn Geochemistry
Our approach is to describe general trends and present downcore results that are
representative of elemental profiles at different stations, rather than present data
from all the downcore metal profiles from the different sites we sampled. One of the
most interesting changes in the geochemistry of surface sediments after the event is
the occurrence of multiple Mn oxide peaks. For example, two distinct Mn peaks are
present at 5 and 20 mm at DSH10 (1520 m) in August 2010 with a modest Re
enrichment of 0.1–0.2 ppb coincident with the Mn minimum between the two peaks
(Fig. 16.3a). Below the second Mn peak, Re increases to a distinct relative maximum of 2.1 ppb. The DSH10 profile sampled two years later is another illustration
of the double Mn peaks. As before, there is a large Mn peak close to the surface
(5 mm), and a smaller Mn peak at 20 mm (Fig. 16.3b). There is also a Re peak at the
base of the second Mn peak at 29 mm, just below the Mn redox boundary.
Double Mn oxide peaks in the top 40 mm are observed in virtually every depth
profile at the three time series stations (DSH10, DSH08, and PCB06) as well as
many other sites around the wellhead sampled after the event. The Mn profile is
characterized by a substantial Mn peak close to the sediment-water interface
(~5 mm) and a smaller peak, deeper in the sediment, about 20–35 mm deep. The
shallower peak is a result of the shoaled redoxcline following the large sedimentation pulse associated with the event with the relic Mn peak beneath it. See Hastings
et al. (2016) for more results and depth profiles demonstrating these multiple Mn
peaks.
Do these double Mn peaks persist over time? We sampled these three sites annually for 8 years following the event. This time series reveal that double Mn peaks or
a broad peak spanning 10–20 mm persists for 3–5 years following the event. Over
time, the shallow Mn oxide peak gradually decreases in magnitude, while the deeper
D. W. Hastings et al.