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: Our original study (1) used visual inspection as well as biological and geochemical analyses of corals and the surrounding sediment to provide complementary and compelling evidence linking the Deep-water Horizon (DWH) oil spill to the presence of damaged deep-water corals and brittle stars 11 km from the site of the leaking oil. The probability that the impact to this coral community oc-curred independently of the DWH spill can be estimated on the basis of three facts. (i) This is the only site among 20 deep-water coral communities associated with authigenic seep carbonates in the northern Gulf of Mexico where visual inspection over the past decade has revealed evidence of notable damage to corals. (ii) The presence of dead and dying tissue and the attachment of living ophiuroids to the corals indicate that the impact was recent (Fig. 1). (iii) The average age of four coral colonies sampled from the site is 460 ± 35 y [according to radio carbon dating as in Prouty et al. (2)]. Assuming that an independent event had an equal chance of occurring at any of the other seep-related coral sites (1 in 20) and during any of the past 460 y at this site (1 in 460), this yields a probability of this happening coincidently at this place and time of approximately 0.0001. In addition, there is no evidence from Bureau of Ocean and Energy Management seismic data, National Oceanic and Atmo-spheric Administration multibeam data, or high-resolution AUV Q:6 multibeam data to indicate slope failure and the underwater landslide suggested by Boehm and Carragher (3) as an alternate explanation for the damage to this site. It is also noteworthy that this community is on top of a discrete ridge where no known mechanism would gather material from such an event and not also be apparent in the surrounding area. The coral community examined in our original article is 11 km to the southwest of the Macondo well at a depth of 1,370 m (1), placing it in the path of a documented deep-water plume enriched with petroleum hydrocarbons. A maximum of oil constituents centered at ∼1,400 m was observed within 2 km of these corals between June 23 and 27, 2010 (4), and levels of polycyclic aro-matic hydrocarbons considered to be toxic to marine organisms were measured up to a distance of 13 km to the southwest of the Macondo well, at depths between 1,000 and 1,400 m between May 9 and 16, 2010 (5). Both of these studies support our findings (1) and describe discrete measurements that reflect a minimum of petroleum hydrocarbons that may have impacted coral commu-nities over the 86 d of the DWH spill. The consistent biomarker ratios between coral samples and the oil from the DWH spill were determined using comprehen-sive 2D gas chromatography coupled to a time-of-flight mass spectrometer (GC×GC-TOF-MS). Although oil samples in the area are indeed difficult to distinguish, GC×GC-TOF-MS is capable of separating, identifying, and quantifying compounds that may not be resolved by the 1D chromatographic techniques (6) referred to by Boehm and Carragher (3). ACKNOWLEDGMENTS. We thank James Rosenberger, PSUProceedings of the National Academy of Sciences. 01/2012;
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: firstname.lastname@example.org.
| October 2, 2012
| vol. 109
| no. 40