Location of natural oil seep and chemical fingerprinting suggest alternative explanation for deep sea coral observations.
- [Show abstract] [Hide abstract]
ABSTRACT: The sinking of the Deepwater Horizon in the Gulf of Mexico led to uncontrolled emission of oil to the ocean, with an official government estimate of ∼5.0 million barrels released. Among the pressing uncertainties surrounding this event is the fate of ∼2 million barrels of submerged oil thought to have been trapped in deep-ocean intrusion layers at depths of ∼1,000-1,300 m. Here we use chemical distributions of hydrocarbons in >3,000 sediment samples from 534 locations to describe a footprint of oil deposited on the deep-ocean floor. Using a recalcitrant biomarker of crude oil, 17α(H),21β(H)-hopane (hopane), we have identified a 3,200-km(2) region around the Macondo Well contaminated by ∼1.8 ± 1.0 × 10(6) g of excess hopane. Based on spatial, chemical, oceanographic, and mass balance considerations, we calculate that this contamination represents 4-31% of the oil sequestered in the deep ocean. The pattern of contamination points to deep-ocean intrusion layers as the source and is most consistent with dual modes of deposition: a "bathtub ring" formed from an oil-rich layer of water impinging laterally upon the continental slope (at a depth of ∼900-1,300 m) and a higher-flux "fallout plume" where suspended oil particles sank to underlying sediment (at a depth of ∼1,300-1,700 m). We also suggest that a significant quantity of oil was deposited on the ocean floor outside this area but so far has evaded detection because of its heterogeneous spatial distribution.Proceedings of the National Academy of Sciences of the United States of America. 10/2014;
- [Show abstract] [Hide abstract]
ABSTRACT: On April 20, 2010, the Deepwater Horizon (DWH) blowout occurred, releasing more oil than any accidental spill in history. Oil release continued for 87 d and much of the oil and gas remained in, or returned to, the deep sea. A coral community significantly impacted by the spill was discovered in late 2010 at 1,370 m depth. Here we describe the discovery of five previously unknown coral communities near the Macondo wellhead and show that at least two additional coral communities were impacted by the spill. Although the oil-containing flocullent material that was present on corals when the first impacted community was discovered was largely gone, a characteristic patchy covering of hydrozoans on dead portions of the skeleton allowed recognition of impacted colonies at the more recently discovered sites. One of these communities was 6 km south of the Macondo wellhead and over 90% of the corals present showed the characteristic signs of recent impact. The other community, 22 km southeast of the wellhead between 1,850 and 1,950 m depth, was more lightly impacted. However, the discovery of this site considerably extends the distance from Macondo and depth range of significant impact to benthic macrofaunal communities. We also show that most known deep-water coral communities in the Gulf of Mexico do not appear to have been acutely impacted by the spill, although two of the newly discovered communities near the wellhead apparently not impacted by the spill have been impacted by deep-sea fishing operations.Proceedings of the National Academy of Sciences of the United States of America. 07/2014;
- Proceedings of the National Academy of Sciences 10/2012; 109(40):2648-E2648. · 9.81 Impact Factor
Location of natural oil seep and
chemical fingerprinting suggest
alternative explanation for deep
sea coral observations
In the article by White et al. (1) we find that the authors pre-
maturely linked the condition of a single coral community to the
Deepwater Horizon oil spill, without evaluating other plausible
explanations, including the presence of natural oil and gas seeps
near the coral, and the potential for underwater landslides to
carry seep sediments onto the coral.
The single coral location in question, (28.67211° N,
−88.47641° W), of 11 studied, is on the western slope of the faulted
west flank of the Biloxi salt dome, an area of active oil and gas
seepage. The site is some 21 m down slope and between 310 and
350 m southwest of proven, active areas of gas and probable oil
seeps (Fig. 1). It is plausible that episodic oil seepage has impacted
this coral colony and also that slope instability and subsequent
turbid, sediment-laden flows could have impacted it.
The authors claim the coral colony was in the path of the
“...100-m-thick deep-water plume of neutrally buoyant water
enriched with petroleum hydrocarbons from the Macondo....”
However, the base of this water layer in the vicinity of the coral
was located by a conductivity-temperature-depth cast on June 2,
2010 (Gordon Gunter 019, http://www.nodc.noaa.gov/General/
DeepwaterHorizon/oceanprofile.html) at 1,230 m,some 140 mabove
the corals, which are on the seabed at 1,370 m. In other words, the
corals were below the measured layer that contained Macondo oil.
In addition, extensive measurements of hydrocarbon concen-
trations (2) in the deep layer within 20 km of the wellhead in-
dicate no more than 0.1–1.0 mg/L (ppm) of oil and 1–10 μg/L
(ppb) of polycyclic aromatic hydrocarbons (PAHs) was present,
hardly the concentrations that would produce the brown floc-
culent material observed and sampled.
Additionally, the detailed geochemical and statistical analyses
required in this deep sea environment are absent from the article.
Similar PAH compositions and biomarker signatures occur in oils
from the South Louisiana Sweet Crude “oil family” area, which
includes Biloxi Dome as well as the Macondo oil well. The pub-
lished GC×GC qualitative comparisons and/or the simplistic sin-
gle biomarker maturity ratio [Ts/(Ts+Tm); Ts, 18 alpha (H)
22,29,30 trisnorhopane; Tm, 17 alpha (H) 22,29,30 trisnorhopane]
are not sufficient to establish a match. Pairwise statistical com-
parisons to biomarker concentrations and biomarker ratios in the
purported source (Macondo oil) and the floc are required.
The authors report that floc and the oil from the Macondo well
have consistent fingerprints. However, a rigorous correlation
analysis (R2) applied to 21 hopanoid biomarker ratios comprising
30+ individual hopanes delivers a strong correlation (R2> 0.97)
between Macondo oil and a produced oil sample collected from
the Kepler field at the southern end of the Biloxi Dome before
Macondo was drilled. High correlation of oil samples collected
from different wells at different times show the importance of
using all relevant data to establish the origin of an oil sample.
Biloxi oil seeps and Macondo oil are difficult to distinguish and
will not be distinguished by the method used by the authors.
We therefore respectfully suggest that the authors fully con-
sider other relevant data and plausible explanations for the
condition of this particular coral community.
ACKNOWLEDGMENTS. This research was supported by BP Exploration &
Paul D. Boehma,1and Peter D. Carragherb
aExponent, Inc., Maynard MA; and
bRose & Associates, Houston TX
1. White HK, et al. (2012) Impact of the Deepwater Horizon oil spill on a deep-water coral
community in the Gulf of Mexico. Proc Natl Acad Sci USA, 10.1073/pnas.1118029109.
2. Boehm PD, Cook LL, Murray KJ (2011) Aromatic hydrocarbon concentrations in
seawater: Deepwater Horizon Oil Spill. Proceedings 2011 International Oil Spill
Conference (American Petroleum Institute, Washington, DC). Available at: http://www.
iosc.org/papers_posters/IOSC-2011-371-file001.pdf. Accessed on July 27, 2012.
hardgrounds and seabed seeps. AUV, autonomous underwater vehicle; BP-NRDA, BP Natural Resource Damage Assessment; NOAA, National Oceanic and
Atmospheric Administration; WD, water depth.
Seabed location of corals downslope from the local high and oil seeps on the western side of the Biloxi Dome. Complex seafloor is the site of
Author contributions: P.D.B. and P.D.C. analyzed data and wrote the paper.
The authors declare no conflict of interest.
1To whom correspondence should be addressed. E-mail: email@example.com.
| October 2, 2012
| vol. 109
| no. 40