Location of natural oil seep and chemical fingerprinting suggest alternative explanation for deep sea coral observations.
- SourceAvailable from: Robert K Nelson[show abstract] [hide abstract]
ABSTRACT: To assess the potential impact of the Deepwater Horizon oil spill on offshore ecosystems, 11 sites hosting deep-water coral communities were examined 3 to 4 mo after the well was capped. Healthy coral communities were observed at all sites >20 km from the Macondo well, including seven sites previously visited in September 2009, where the corals and communities appeared unchanged. However, at one site 11 km southwest of the Macondo well, coral colonies presented widespread signs of stress, including varying degrees of tissue loss, sclerite enlargement, excess mucous production, bleached commensal ophiuroids, and covering by brown flocculent material (floc). On the basis of these criteria the level of impact to individual colonies was ranked from 0 (least impact) to 4 (greatest impact). Of the 43 corals imaged at that site, 46% exhibited evidence of impact on more than half of the colony, whereas nearly a quarter of all of the corals showed impact to >90% of the colony. Additionally, 53% of these corals' ophiuroid associates displayed abnormal color and/or attachment posture. Analysis of hopanoid petroleum biomarkers isolated from the floc provides strong evidence that this material contained oil from the Macondo well. The presence of recently damaged and deceased corals beneath the path of a previously documented plume emanating from the Macondo well provides compelling evidence that the oil impacted deep-water ecosystems. Our findings underscore the unprecedented nature of the spill in terms of its magnitude, release at depth, and impact to deep-water ecosystems.Proceedings of the National Academy of Sciences 03/2012; · 9.74 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