The authors declare that that the funding source (Gulf of Mexico Research Initiative) does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.
Conceived and designed the experiments: ICR GRB DWH DJH. Performed the experiments: ICR DJH. Analyzed the data: ICR PTS GRB RAL EAG DJH. Contributed reagents/materials/analysis tools: ICR PTS GRB RAL GE EAG. Wrote the paper: ICR.
The Deepwater Horizon (DWH) spill released 4.9 million barrels of oil into the Gulf of Mexico (GoM) over 87 days. Sediment and water sampling efforts were concentrated SW of the DWH and in coastal areas. Here we present geochemistry data from sediment cores collected in the aftermath of the DWH event from 1000 – 1500 m water depth in the DeSoto Canyon, NE of the DWH wellhead. Cores were analyzed at high-resolution (at 2 mm and 5 mm intervals) in order to evaluate the concentration, composition and input of hydrocarbons to the seafloor. Specifically, we analyzed total organic carbon (TOC), aliphatic, polycyclic aromatic hydrocarbon (PAHs), and biomarker (hopanes, steranes, diasteranes) compounds to elucidate possible sources and transport pathways for deposition of hydrocarbons. Results showed higher hydrocarbon concentrations during 2010-2011 compared to years prior to 2010. Hydrocarbon inputs in 2010-2011 were composed of a mixture of sources including terrestrial, planktonic, and weathered oil. Our results suggest that after the DWH event, both soluble and highly insoluble hydrocarbons were deposited at enhanced rates in the deep-sea. We proposed two distinct transport pathways of hydrocarbon deposition: 1) sinking of oil-particle aggregates (hydrocarbon-contaminated marine snow and/or suspended particulate material), and 2) advective transport and direct contact of the deep plume with the continental slope surface sediments between 1000-1200 m. Our findings underline the complexity of the depositional event observed in the aftermath of the DWH event in terms of multiple sources, variable concentrations, and spatial (depth-related) variability in the DeSoto Canyon, NE of the DWH wellhead.
Sediments serve as repository systems for a large range of organic matter sources and hydrocarbons that can be used to assess historical impacts to the environment. In the northern Gulf of Mexico (GoM), the composition of organic matter deposited in deep-sea sediments is controlled by physical sorting of particles (regional hydrodynamics) and the off-shore movement of the less dense material from terrigenous sources transported by the Mississippi River [
It is estimated that an average of 95,500 tons oil enters the GoM annually from natural seeps (73%), oil and gas extraction activities (3%), transportation activities (4%), and oil combustion byproducts (~16%) [
Following the blowout of the DWH drilling rig, an unusually large marine snow event was observed [
The primary objectives of this study are to contribute to the overall understanding of hydrocarbon geochemistry in deepwater sediments by providing information on the concentration and composition of sediments samples collected from the DeSoto Canyon, NE of the DWH; and to interpret these data within the context of the possible sources and transport pathways of hydrocarbons to the deep sea during the period of the study.
Sediment cores were collected in December 2010 and February 2011 on board Florida of Institute of Oceanogrpahy’s (FIO) R/V WeatherBird II during oil spill response cruises WB1111 and WB1114. Three sediment-coring sites were located in the DeSoto Canyon in the northeastern GoM (
Insert show the location of the sites within the Gulf of Mexico.
A multi-corer (Ocean Instruments MC-800) was used to collect sediment cores with minimal disturbance to the sediment-water interface and surficial sediment intervals. Whole cores were sliced on board or in the laboratory at the University of South Florida’s College of Marine Science Paleo-Laboratory (USFCMS-PL) at 2 mm (0–20 mm) and 5 mm (>20 mm) intervals downcore. A modified version of the Engstrom [
Short-lived radionuclide geochronology samples were counted for 48 hours on a Canberra Series HPGe (High-Purity Germanium) Coaxial Planar Photon Detectors to determine excess 210Pb and excess 234Th activities for age dating. Samples were counted within 120 days of collection to account for the short half-life (24.1 days) of 234Th. Raw activities were corrected for counting time, detector efficiency, as well as for the fraction of the total radioisotope measured, yielding activity in disintegrations per minute per gram (dpm/g) with error generally <5% of activity. Age dates were assigned to each sample analyzed using the Constant Rate of Supply (CRS) Model as described previously [
Bulk analyses for total organic carbon (TOC), nitrogen (N), and carbon and nitrogen isotopic values (δ13C, δ15N) were carried out at USFCMS-PL. Prior to analysis of TOC and δ13C, pre-weighed subsamples were placed in glass containers and acidified (80% 1.0N HCl) to remove inorganic carbon [
We followed modified EPA methods [
The aliphatic fraction was quantified in a gas chromatograph/flame ionization detector (GC/FID) by the external standard method in splitless injections of 5μL. A VF-1ms (15m x 0.25mm x 0.25μm) capillary column was used with a GC oven temperature programming of 80°C held for 0.5 min, then increased to 320°C at a rate of 10°C min-1 and held for 5.5 min. Injector temperature was set to 280°C. Identification and quantification of
The aromatic fraction was quantified in a Gas Chromatograph/mass spectrometric detector (GC/MS) in full scan mode (
Biomarkers (hopanes and steranes) were quantified using GC/MS/MS multiple reaction monitoring (MRM) on a Varian 320 triple quadrupole MS. Splitless injections of 1μL of the sample were conducted. Chromatographic separation of biomarker compounds were conducted using a RXi®5sil column (30 m x 0.25 mm x 0.25 μm) with a GC oven temperature programming of 80°C held for 1 min, then increased to 200°C at a rate of 40°C/min, to 250°C at 5°C/min, to 300°C at 2°C/min, to 320°C at 10°C/min, and held for 2 min. The GC was operated in constant-flow mode (1ml/min) with an inlet temperature of 275°C and a transfer line temperature of 320°C. Ion source temperature was 180°C and source electron energy was 70eV. Mass transitions targeted appropriate parent molecular ion masses on Q1 and monitored mass 191.2 (hopanes) or 217.0 (steranes) on Q3, with collision energy held at 10 volts throughout. Targeted hopanes (and relevant Q1 masses) included C27, C28, and C29 norhopanes (370.5, 384.5, and 398.5, respectively), C30 hopanes (412.5), and C31 through C35 homohopanes (426.5, 440.5, 454.5, 468.5, and 482.5), while sterane targets included C27- C29 steranes and diasteranes (372.7, 386.7, and 400.7). An additional transition (376.7 to 221) was monitored to quantify the internal standard (cholestane 2,2,4,4 D4; CDN Isotopes). Argon at a pressure of 1 millitorr was used as a collision gas. Concentrations of biomarkers compounds were calculated using response factors by comparison with a known standard mixture (Hopane/Sterane calibration mix, Chiron, S-4436-10-IO) and the internal standard. When no commercial reference standard was available, compounds were quantitated using the response factor for the nearest available homologue in the same compound class. Concentrations were corrected for the recovery of the surrogate standard (d50-Tetracosane) for the F1 fraction. Recoveries from spiked samples included with each batch were generally within QA/QC criteria of 60–80%. Biomarker compounds are expressed as sediment dry weight concentrations.
Replicate hydrocarbon analyses were done in selected samples from the cores collected in 2010. Our depth resolution of the core samples allowed us to replicate hydrocarbon analyses at the 65–85 mm depth interval. The relative standard deviations (RSDs) of replicates (N = 4) for PAHs analysis were 10%, 17% and 19% for DSH10, DSH08 and PCB06 sites, respectively. RSDs of replicates (N = 4) for aliphatic analysis were 12%, 6% and 10% for DSH10, DSH08 and PCB06 sites, respectively. And, RSDs of replicates (N = 4) for biomarker analysis were 17%, 22% and 4% for DSH10, DSH08 and PCB06 sites, respectively. RSDs values for all compound groups were lower than the variability observed in concentration from surface sediment layers to downcore in all sites (see
Diagnostic ratios were calculated to discriminate hydrocarbon sources in the samples collected. Diagnostic ratios use isomer pairs that are abundant in different PAH sources but with similar dissolution and adsorption properties as they have comparable thermodynamic partitioning and kinetic mass transfer coefficients [
For comparison to potential hydrocarbon sources, diagnostic ratios were calculated for DWH crude oil (Macondo oil, MC252 block) obtained from British Petroleum (BP, sample No. SOB-20100622-084) and from crude oil standard reference material collected from the insertion tube that was receiving oil directly from the Macondo well (NIST 2779). We also used analyses from two sediment samples collected at the DWH wellhead that contain high concentration of 17α(H)21β(H)-hopane (60 ng g-1). In addition, we used GoM sediment for PAH ratios from sites with evidence of pyrogenic (HMW) PAH inputs reported by the Operational Science Advisory Team in 2010 [
In the DeSoto Canyon, recently deposited sediment sections were established in each sedimentary record indicating variable and larger deposition in 2010 compared to previous years (
Data shown are for cores collected in 2010 (interval: 2010 to 2006) and 2011 (only 2011 data). 2-ring: Naphthalene and alkylated homologues; 3-ring: Acenaphthylene, Acenaphthene, Fluorene, Dibenzothiophene, Phenanthrene, Anthracene, and alkylated homologues; 4-ring: Fluoranthene, Pyrene, Benz[a]anthracene, Chrysene, and alkylated homologues; 5-ring: Benzo[b]fluoranthene, Benzo[k]fluoranthene, Benzo[a]pyrene, Dibenz[a,h]anthracene, and alkylated homologues; 6-ring: Indeno[1,2,3-cd]pyrene, Benzo[ghi]perylene.
Site | Year | Depth Interval (mm) | TOC (%) | δ13C (‰) | δ15N (‰) | UCM (μg g-1) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
C17/Pr | C18/Phy | Total CPI14-35 | CPI14-23 | CPI26-35 | C≤25 (μg g-1) | C ≥26 (μg g-1) | |||||||
2011 | 0–2 | 1.8 | -20.3 | 6.1 | 3.8 | 1.0 | 2.6 | 1.8 | 2.7 | 0.2 | 4.4 | 28.5 | |
2011 | 2–4 | 1.8 | -23.1 | 6.0 | n.d. | 0.8 | 2.4 | 2.6 | 2.3 | 0.1 | 1.8 | 65.9 | |
2010 | 0–2 | 1.3 | -20.6 | 5.5 | 0.5 | 0.0 | 2.4 | 1.4 | 2.9 | 0.3 | 1.4 | 41.3 | |
2010 | 2–4 | 1.2 | -20.5 | 5.3 | 0.2 | 1.2 | 3.4 | 2.4 | 3.6 | 0.3 | 2.6 | 11.9 | |
2010 | 4–6 | 1.2 | -20.6 | 5.3 | 0.5 | 1.4 | 3.8 | 2.3 | 4.1 | 0.3 | 3.3 | 8.5 | |
2009 | 6–8 | 1.2 | -20.6 | 5.1 | 0.1 | 1.4 | 3.4 | 2.6 | 3.6 | 0.2 | 2.6 | 5.8 | |
2009 | 8–10 | 1.2 | -20.5 | 5.1 | 0.0 | n.d. | 0.6 | 0.8 | 0.5 | 0.4 | 1.3 | 3.5 | |
2008 | 10–12 | 1.2 | -20.7 | 5.0 | 0.3 | 1.1 | 4.3 | 3.0 | 4.6 | 0.2 | 2.4 | 6.0 | |
2007 | 12–14 | 1.3 | -20.9 | 5.0 | 0.0 | 4.3 | 3.1 | 2.1 | 3.3 | 0.2 | 1.6 | 6.7 | |
2007 | 14–16 | 1.3 | -20.9 | 4.4 | 0.1 | 0.5 | 2.3 | 1.4 | 2.5 | 0.3 | 2.0 | 6.4 | |
2006 | 16–18 | 1.4 | -21.0 | 4.7 | 0.2 | 1.5 | 4.5 | 2.8 | 4.9 | 0.0 | 0.4 | 8.9 | |
2006 | 18–20 | 1.5 | -20.8 | 4.7 | 0.2 | 0.7 | 4.6 | 2.8 | 5.0 | 0.2 | 2.0 | 8.1 | |
2005 | 20–22 | 1.5 | -20.9 | 4.7 | 0.9 | 1.0 | 4.6 | 2.6 | 5.1 | 0.2 | 1.3 | 4.8 | |
2005 | 22–24 | 1.4 | -21.0 | 4.6 | 0.7 | 1.7 | 4.4 | 2.6 | 4.8 | 0.2 | 3.0 | 6.0 | |
2004 | 24–26 | 1.3 | -20.9 | 4.5 | 0.0 | n.d. | 5.8 | 2.5 | 7.2 | 0.0 | 0.3 | 7.4 | |
2004 | 26–28 | 1.3 | -20.9 | 4.6 | 1.5 | 1.1 | 4.6 | 2.9 | 4.9 | 0.3 | 3.5 | 4.3 | |
2003 | 28–30 | 1.4 | -20.9 | 4.5 | 0.9 | 1.7 | 4.5 | 2.7 | 4.9 | 0.2 | 2.2 | 3.8 | |
2003 | 30–35 | 1.5 | -21.0 | 4.4 | 1.5 | 1.5 | 1.2 | 2.4 | 0.9 | 1.1 | 5.1 | 0.2 | |
2001 | 35–40 | 1.4 | -20.8 | 4.4 | 0.7 | 0.5 | 3.2 | 3.1 | 3.2 | 0.2 | 1.9 | 5.5 | |
1999 | 40–45 | 1.2 | -20.3 | 4.4 | 1.3 | 1.3 | 3.0 | 2.5 | 3.1 | 0.9 | 7.7 | 0.0 | |
1996 | 50–55 | 1.4 | -20.4 | 4.6 | 1.5 | 1.3 | 0.8 | 1.7 | 0.1 | 1.4 | 2.0 | 3.2 | |
1994 | 55–60 | 1.4 | -20.5 | 4.4 | 1.1 | 0.4 | 3.6 | 3.2 | 3.7 | 0.2 | 1.5 | 5.2 | |
1992 | 60–65 | 1.4 | n.d. | 4.4 | 1.3 | 1.4 | 0.8 | 2.3 | 0.3 | 0.9 | 2.6 | 9.4 | |
1976 | 85–90 | 1.4 | n.d. | 4.4 | 1.3 | 1.4 | 2.0 | 2.3 | 1.9 | 1.1 | 3.9 | 0.0 | |
2011 | 0–2 | 1.9 | -21.1 | 6.1 | 3.8 | 1.0 | 4.4 | 2.6 | 4.7 | 0.3 | 3.7 | 41.1 | |
2011 | 2–4 | 1.6 | -20.7 | 6.0 | n.d. | 0.8 | 4.1 | 2.4 | 4.4 | 0.3 | 2.2 | 12.8 | |
2010 | 0–2 | 2.0 | -20.1 | 5.5 | 1.0 | 1.7 | 1.3 | 1.9 | 1.3 | 0.4 | 5.1 | 325.7 | |
2010 | 2–4 | 1.9 | -20.4 | 5.4 | 0.5 | 1.9 | 2.5 | 1.0 | 3.4 | 0.7 | 3.5 | 48.0 | |
2010 | 4–6 | 2.0 | -22.5 | 5.7 | n.d. | 3.1 | 3.0 | 1.1 | 4.1 | 0.2 | 1.3 | 73.9 | |
2010 | 6–8 | 2.0 | -19.1 | 5.6 | 0.7 | 1.8 | 2.6 | 1.2 | 3.2 | 0.7 | 4.5 | 41.1 | |
2010 | 8–10 | 2.0 | -20.7 | 5.6 | 0.5 | 1.5 | 2.8 | 1.2 | 3.8 | 0.9 | 3.3 | 36.8 | |
2009 | 10–12 | 2.0 | -20.4 | 5.5 | 1.1 | 1.7 | 2.5 | 3.5 | 1.4 | 4.5 | 42.9 | ||
2008 | 12–14 | 2.0 | -20.6 | 5.4 | 0.8 | 1.5 | 2.9 | 3.7 | 1.0 | 5.9 | 42.8 | ||
2007 | 14–16 | 2.0 | -20.3 | 5.5 | 0.8 | 2.1 | 3.0 | 3.6 | 0.7 | 5.5 | 43.9 | ||
2006 | 16–18 | 2.0 | -20.7 | 5.4 | 0.4 | 30.0 | 2.9 | 4.9 | 0.6 | 2.2 | 54.5 | ||
2005 | 18–20 | 1.9 | -20.6 | 5.4 | 0.6 | 1.7 | 2.9 | 4.5 | 0.6 | 2.7 | 43.5 | ||
2003 | 20–25 | 1.9 | -20.7 | 5.2 | 0.9 | 1.5 | 3.5 | 1.7 | 4.0 | 1.3 | 11.4 | 17.9 | |
2000 | 25–30 | 2.0 | -20.6 | 5.1 | 0.6 | 2.0 | 3.1 | 1.3 | 3.8 | 0.6 | 3.5 | 31.5 | |
1996 | 30–35 | 1.8 | -20.8 | 5.2 | 1.0 | 1.8 | 3.1 | 1.5 | 3.7 | 1.4 | 10.0 | 13.3 | |
1993 | 35–40 | 1.9 | -20.7 | 5.2 | 0.0 | 2.6 | 3.3 | 1.5 | 3.7 | 0.9 | 10.9 | 16.7 | |
1990 | 40–45 | 1.9 | -20.7 | 4.9 | 0.0 | 3.2 | 3.5 | 1.8 | 3.9 | 0.6 | 8.7 | 15.1 | |
1985 | 45–50 | 1.8 | -20.9 | 5.0 | 0.0 | 2.9 | 3.5 | 1.9 | 3.9 | 0.8 | 10.0 | 11.4 | |
1983 | 50–55 | 1.8 | -20.9 | 5.0 | 0.0 | 3.3 | 3.6 | 4.3 | 0.7 | 7.5 | 12.4 | ||
1979 | 55–60 | 1.8 | -20.8 | 5.1 | 1.2 | 2.1 | 3.3 | 4.2 | 1.0 | 6.5 | 7.0 | ||
2011 | 0–2 | 1.9 | -20.9 | 6.1 | 0.0 | 3.4 | 1.9 | 1.2 | 1.9 | 0.3 | 0.4 | 24.4 | |
2010 | 0–2 | 1.3 | -20.5 | 4.5 | n.d. | 1.7 | 3.0 | 1.3 | 3.5 | 0.2 | 1.4 | 33.4 | |
2010 | 2–4 | 1.2 | -20.6 | 4.0 | 2.0 | 1.7 | 3.8 | 1.8 | 4.5 | 0.2 | 1.3 | 20.6 | |
2010 | 4–6 | 1.2 | -20.5 | 4.2 | 1.1 | 1.3 | 2.5 | 1.4 | 2.8 | 0.2 | 1.7 | 27.3 | |
2010 | 6–8 | 1.2 | -20.4 | 4.3 | n.d. | n.d. | 2.4 | 1.4 | 2.7 | 0.1 | 1.1 | 18.9 | |
2009 | 8–10 | 1.2 | -20.5 | 4.3 | 0.3 | 2.9 | 4.0 | 1.8 | 4.7 | 0.3 | 3.3 | 13.2 | |
2008 | 10–12 | 1.2 | -20.6 | 4.4 | 0.6 | 1.6 | 2.7 | 1.6 | 3.1 | 0.4 | 2.6 | 17.6 | |
2006 | 12–14 | 1.3 | -20.6 | 4.1 | 0.1 | 2.9 | 2.9 | 1.7 | 3.1 | 0.3 | 3.4 | 15.9 | |
2005 | 14–16 | 1.3 | -20.3 | 4.1 | 0.4 | 1.7 | 2.6 | 1.9 | 2.7 | 0.3 | 3.5 | 17.2 | |
2004 | 16–18 | 1.4 | -20.7 | 4.2 | n.d. | 1.3 | 4.3 | 2.0 | 5.0 | 0.3 | 3.4 | 12.3 | |
2002 | 18–20 | 1.5 | -20.7 | 3.8 | n.d. | 1.1 | 3.0 | 1.7 | 3.3 | 0.2 | 2.2 | 14.8 | |
2001 | 20–22 | 1.5 | -20.5 | 5.1 | n.d. | 1.8 | 3.3 | 1.5 | 4.1 | 0.5 | 3.3 | 9.6 | |
2000 | 22–24 | 1.4 | -20.5 | 5.1 | n.d. | 2.0 | 2.2 | 1.1 | 2.7 | 0.3 | 1.5 | 9.2 | |
1998 | 24–26 | 1.3 | -20.3 | 4.7 | n.d. | 2.3 | 3.4 | 1.7 | 4.0 | 0.2 | 2.0 | 8.8 | |
1997 | 26–28 | 1.3 | -20.5 | 4.9 | n.d. | 1.8 | 3.5 | 1.5 | 4.5 | 0.1 | 0.9 | 7.5 | |
1996 | 28–30 | 1.4 | -20.4 | 4.9 | n.d. | n.d. | 2.6 | 1.5 | 2.9 | 0.2 | 1.8 | 12.3 | |
1993 | 30–35 | 1.5 | -20.5 | 4.8 | n.d. | 1.2 | 3.8 | 2.0 | 4.3 | 0.6 | 5.6 | 2.4 | |
1989 | 36–38 | 1.4 | -20.5 | 4.6 | n.d. | 1.3 | 3.6 | 2.1 | 4.0 | 0.6 | 6.0 | 0.8 | |
1986 | 40–45 | 1.7 | -20.6 | 4.2 | n.d. | 1.5 | 3.9 | 2.0 | 4.4 | 0.4 | 4.6 | 2.3 | |
1982 | 45–50 | 1.5 | -20.3 | 4.4 | 2.6 | 3.3 | 2.6 | 1.7 | 2.8 | 0.3 | 3.5 | 2.0 | |
1978 | 50–55 | 1.3 | -20.2 | 4.5 | n.d. | 1.2 | 4.6 | 2.0 | 5.5 | 0.4 | 4.5 | 0.9 | |
1974 | 55–60 | 0.9 | -20.1 | 4.2 | 1.4 | 3.5 | 3.2 | 1.1 | 4.5 | 1.3 | 7.4 | 0.0 | |
1970 | 60–65 | 1.4 | -20.4 | n.d. | n.d. | 1.9 | 3.0 | 1.5 | 3.6 | 0.4 | 2.3 | 2.3 | |
Averages shown as arithmetic mean ± CI. CPI (Carbon preference index) = ∑ odd Cn / ∑ even Cn, for each specific range of
The δ13C of organic carbon in the sediment samples showed a higher variation in 2010 at DSH08 (from -19‰ to -23‰), and in 2011 at DSH10 (-20‰ to -23‰) than in previous years (
Downcore depth intervals for each year are in
Total concentration of aliphatics in the DeSoto Canyon varied from 5 to 337 μg g-1. Higher concentrations were observed in 2010 for shallower sites (up to 337 and 35 μg g-1 for DSH08 and PCB06, respectively;
Long-chain (C ≥ 26) and short-chain (C ≤ 25)
Isoprenoids (Pr and Phy) are more resistant to degradation than
The general trends observed in the aliphatic fraction indicate a mixture of organic carbon sources and increased concentrations in 2010 (DSH08 and PCB06) and 2011 (DSH10) in contrast to previous years. In contrast, similar organic carbon sources and low concentrations were observed in all depth intervals (2011 to 1990) in the control site, NT1200 (
Total PAH concentration in the DeSoto Canyon varied from 70 to 524 ng g-1. Higher concentrations were observed in 2010 for the shallower sites (up to 524 and 329 ng g-1 for DSH08 and PCB06, respectively) and in 2011 only for the deeper site (up to 373 ng g-1 for DSH10) (
Site | Year | Sed. Depth (mm) | PAHs | |||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
N | ACL | ACE | F | D | P | AN | FL | PY | BAA | C | BBF | BKF | BAP | ID | DA | BGP | N (C1-C4) | P/AN (C1-C4) | FL/PY (C1-C4) | BAA/C (C1-C4) | BP/PER (C1-C4) | D (C1-C2) | Total PAH | |||
2011 | 0–2 | 0.0 | 0.8 | 0.0 | 2.0 | 0.0 | 17.5 | 1.6 | 14.7 | 10.9 | 17.4 | 15.8 | 26.7 | 0.0 | 0.1 | 15.4 | 0.0 | 18.3 | 29.2 | 41.0 | 14.2 | 113.3 | 14.3 | 1.6 | 354.7 | |
2011 | 2–4 | 0.0 | 1.4 | 0.0 | 2.7 | 1.4 | 31.3 | 79.3 | 13.1 | 8.7 | 13.7 | 14.6 | 19.9 | 0.0 | 9.7 | 19.9 | 0.0 | 21.8 | 13.3 | 55.4 | 6.5 | 55.1 | 2.8 | 2.0 | 372.8 | |
2010 | 0–2 | 1.0 | 0.6 | 0.0 | 0.7 | 0.7 | 7.8 | 0.8 | 6.5 | 7.5 | 8.2 | 7.3 | 14.8 | 0.0 | 4.0 | 10.1 | 0.0 | 10.9 | 11.1 | 22.1 | 7.0 | 66.4 | 2.8 | 0.0 | 190.2 | |
2010 | 2–4 | 0.0 | 0.7 | 0.0 | 0.7 | 0.0 | 7.5 | 0.9 | 5.9 | 7.1 | 6.9 | 7.4 | 13.6 | 0.0 | 3.0 | 9.5 | 0.0 | 8.7 | 10.7 | 18.2 | 3.4 | 26.2 | 0.5 | 0.0 | 130.9 | |
2010 | 4–6 | 1.0 | 0.6 | 0.0 | 0.5 | 0.6 | 7.7 | 0.9 | 0.0 | 8.0 | 6.5 | 6.0 | 14.4 | 0.0 | 3.3 | 10.4 | 0.0 | 8.6 | 10.4 | 17.3 | 5.4 | 30.9 | 0.0 | 0.0 | 132.6 | |
2009 | 6–8 | 0.0 | 0.2 | 0.0 | 0.4 | 0.4 | 4.6 | 0.6 | 0.1 | 6.2 | 6.5 | 6.0 | 14.2 | 0.0 | 2.6 | 9.7 | 0.0 | 7.9 | 6.5 | 16.0 | 3.2 | 24.3 | 1.3 | 0.0 | 110.6 | |
2007 | 14–16 | 0.0 | 0.9 | 0.0 | 0.2 | 0.5 | 6.9 | 0.8 | 5.0 | 7.4 | 5.0 | 5.0 | 12.9 | 0.0 | 3.2 | 10.5 | 0.0 | 10.2 | 11.2 | 15.5 | 1.6 | 7.4 | 0.0 | 0.0 | 104.0 | |
2004 | 24–26 | 2.8 | 0.4 | 0.0 | 0.2 | 0.4 | 2.8 | 0.6 | 3.4 | 5.1 | 3.3 | 3.4 | 9.3 | 0.0 | 2.4 | 8.1 | 0.0 | 6.2 | 7.2 | 12.8 | 1.5 | 0.6 | 0.3 | 0.0 | 70.6 | |
2011 | 0–2 | 0.0 | 0.2 | 0.0 | 1.2 | 0.2 | 4.6 | 0.5 | 3.6 | 5.2 | 1.3 | 4.9 | 5.2 | 0.0 | 0.9 | 1.9 | 0.0 | 2.7 | 9.1 | 38.4 | 7.7 | 10.8 | 0.8 | 0.0 | 99.3 | |
2011 | 2–4 | 0.0 | 0.3 | 0.0 | 0.9 | 0.0 | 7.4 | 0.5 | 3.6 | 4.4 | 0.4 | 3.6 | 4.1 | 0.0 | 1.3 | 1.8 | 0.0 | 3.1 | 7.1 | 47.7 | 2.5 | 5.8 | 0.6 | 0.0 | 95.2 | |
2010 | 0–2 | 0.0 | 0.8 | 0.0 | 1.8 | 1.0 | 14.2 | 2.1 | 12.8 | 22.4 | 3.1 | 20.4 | 0.0 | 9.2 | 4.8 | 6.2 | 1.1 | 5.7 | 15.3 | 326.0 | 10.6 | 62.6 | 2.6 | 1.4 | 524.3 | |
2010 | 2–4 | 0.0 | 0.8 | 0.0 | 2.5 | 1.0 | 17.5 | 1.1 | 0.0 | 34.2 | 0.0 | 9.2 | 12.1 | 1.7 | 0.0 | 7.7 | 1.2 | 8.5 | 20.4 | 91.1 | 12.1 | 13.7 | 0.0 | 0.0 | 234.9 | |
2010 | 4–6 | 0.0 | 1.1 | 0.0 | 6.2 | 0.0 | 37.4 | 0.0 | 0.0 | 56.4 | 7.4 | 17.2 | 18.2 | 0.0 | 10.9 | 15.5 | 0.0 | 18.5 | 36.6 | 169.8 | 14.5 | 17.1 | 0.0 | 0.0 | 426.8 | |
2010 | 6–8 | 0.0 | 0.8 | 0.0 | 2.1 | 0.0 | 17.4 | 1.2 | 12.8 | 29.2 | 3.1 | 10.7 | 10.3 | 0.0 | 3.6 | 8.4 | 1.2 | 8.5 | 17.6 | 102.6 | 7.5 | 26.0 | 0.0 | 1.2 | 264.1 | |
2009 | 10–12 | 0.0 | 1.3 | 0.0 | 4.4 | 0.0 | 15.2 | 2.7 | 21.9 | 6.1 | 5.5 | 14.2 | 10.2 | 0.0 | 4.6 | 10.2 | 2.0 | 9.7 | 14.5 | 147.8 | 21.6 | 28.2 | 1.3 | 2.2 | 323.6 | |
2006 | 16–18 | 0.0 | 0.7 | 0.0 | 3.1 | 1.1 | 23.1 | 6.3 | 15.3 | 41.0 | 3.7 | 8.2 | 9.0 | 0.0 | 3.3 | 6.8 | 1.5 | 7.5 | 21.1 | 91.0 | 9.1 | 19.6 | 1.0 | 1.0 | 273.5 | |
2011 | 0–2 | 0.0 | 0.3 | 0.0 | 1.0 | 0.0 | 6.1 | 0.0 | 0.0 | 5.2 | 1.3 | 5.4 | 4.9 | 0.0 | 1.9 | 0.0 | 0.1 | 4.2 | 13.0 | 84.8 | 10.4 | 29.6 | 2.6 | 0.0 | 170.7 | |
2010 | 0–2 | 0.0 | 0.8 | 0.0 | 1.0 | 0.7 | 12.6 | 0.8 | 8.8 | 11.2 | 2.4 | 8.1 | 11.6 | 2.0 | 3.8 | 10.4 | 0.0 | 9.7 | 20.9 | 125.0 | 33.2 | 22.0 | 1.1 | 1.0 | 287.0 | |
2010 | 2–4 | 0.0 | 0.2 | 0.0 | 0.0 | 0.5 | 5.4 | 0.4 | 5.1 | 8.3 | 1.6 | 6.9 | 11.4 | 2.3 | 2.6 | 11.4 | 1.4 | 8.4 | 3.1 | 110.6 | 7.1 | 17.1 | 1.0 | 1.1 | 206.0 | |
2010 | 4–6 | 0.0 | 0.2 | 0.0 | 1.1 | 1.0 | 22.7 | 0.9 | 9.0 | 11.0 | 1.9 | 7.9 | 12.0 | 1.8 | 2.5 | 9.8 | 0.0 | 10.7 | 12.7 | 154.5 | 7.1 | 15.2 | 2.0 | 0.8 | 285.0 | |
2010 | 6–8 | 0.0 | 0.0 | 0.0 | 0.8 | 0.6 | 6.6 | 0.3 | 4.9 | 11.0 | 1.3 | 6.1 | 9.5 | 1.8 | 1.1 | 7.1 | 0.0 | 8.0 | 52.4 | 153.1 | 7.8 | 14.0 | 0.9 | 1.6 | 288.9 | |
2009 | 8–10 | 0.0 | 0.3 | 0.0 | 0.2 | 0.0 | 6.8 | 0.7 | 7.2 | 13.4 | 2.6 | 7.6 | 12.1 | 2.4 | 3.7 | 10.4 | 1.6 | 11.1 | 0.1 | 69.9 | 10.1 | 18.7 | 1.0 | 0.9 | 180.8 | |
2002 | 14–16 | 0.0 | 0.1 | 0.0 | 0.1 | 0.1 | 6.2 | 0.3 | 4.2 | 4.4 | 1.4 | 4.3 | 8.1 | 1.2 | 2.0 | 6.8 | 0.0 | 6.0 | 1.6 | 63.8 | 3.6 | 7.8 | 2.0 | 0.1 | 124.1 | |
848.3 | 8.6 | 14.9 | 159.4 | 59.8 | 327.5 | 11.7 | 4.4 | 17.6 | 7.0 | 54.5 | 6.5 | 0.0 | 2.3 | 0.0 | 2.2 | 2.1 | 7001.3 | 2267.2 | 403.1 | 430.7 | 0.0 | 0.0 | 11629 |
Averages shown as arithmetic mean ± CI.
The intermittent increase in concentration observed only in the surface of all sediment cores except NT1200, indicates a recent sedimentary depositional event in the DeSoto Canyon. This is supported by the notion that PAHs are known to be relatively persistent in sediments due to their hydrophobicity and particle adsorption affinity [
Additionally, the relative composition of PAHs by their number of rings showed a larger variation in 2010 and 2011 than in previous years. DSH10 showed a distinct distribution of PAHs in 2011 (~15% increase in 4-rings, up to 25% increase in 3-rings, ~10% decrease in 6-rings;
Site | Year | Sed. Depth (mm) | PI | An/(Phe+An) | HMW/LMW | Parental/Alkyl | (ID+C)/HMW | (BAP+BGP)/HMW |
---|---|---|---|---|---|---|---|---|
2011 | 0–2 | 0.42 | 0.08 | 2.8 | 0.7 | 0.12 | 0.07 | |
2011 | 2–4 | 1.01 | 0.72 | 1.0 | 1.7 | 0.19 | 0.17 | |
2010 | 0–2 | 0.50 | 0.10 | 3.2 | 0.7 | 0.12 | 0.10 | |
2010 | 2–4 | 0.76 | 0.11 | 2.4 | 1.2 | 0.18 | 0.13 | |
2010 | 4–6 | 0.66 | 0.10 | 2.4 | 1.1 | 0.18 | 0.13 | |
2009 | 6–8 | 0.77 | 0.12 | 2.9 | 1.1 | 0.19 | 0.13 | |
2007 | 14–16 | 1.16 | 0.10 | 1.9 | 1.9 | 0.23 | 0.20 | |
2004 | 24–26 | 1.21 | 0.18 | 1.6 | 2.1 | 0.26 | 0.20 | |
2011 | 0–2 | 0.28 | 0.09 | 0.8 | 0.5 | 0.15 | 0.08 | |
2011 | 2–4 | 0.26 | 0.06 | 0.5 | 0.5 | 0.17 | 0.14 | |
2010 | 0–2 | 0.15 | 0.13 | 0.4 | 0.3 | 0.16 | 0.07 | |
2010 | 2–4 | 0.40 | 0.06 | 0.7 | 0.7 | 0.17 | 0.08 | |
2010 | 4–6 | 0.43 | 0.00 | 0.7 | 0.8 | 0.19 | 0.17 | |
2010 | 6–8 | 0.43 | 0.07 | 0.8 | 0.7 | 0.16 | 0.10 | |
2009 | 10–12 | 0.30 | 0.15 | 0.7 | 0.5 | 0.18 | 0.11 | |
2006 | 16–18 | 0.53 | 0.21 | 0.9 | 0.9 | 0.12 | 0.09 | |
2011 | 0–2 | 0.12 | 0.00 | 0.6 | 0.2 | 0.08 | 0.09 | |
2010 | 0–2 | 0.27 | 0.06 | 0.8 | 0.4 | 0.15 | 0.11 | |
2010 | 2–4 | 0.35 | 0.07 | 0.7 | 0.5 | 0.22 | 0.13 | |
2010 | 4–6 | 0.27 | 0.04 | 0.5 | 0.5 | 0.20 | 0.15 | |
2010 | 6–8 | 0.18 | 0.04 | 0.3 | 0.3 | 0.18 | 0.12 | |
2009 | 8–10 | 0.57 | 0.09 | 1.3 | 0.8 | 0.18 | 0.14 | |
2002 | 14–16 | 0.38 | 0.05 | 0.7 | 0.6 | 0.21 | 0.16 | |
References | DWH oil | 0.01 | 0.03 | 0.1 | 0.2 | 0.06 | 0.00 | |
GoM sed. |
3.92 | 0.28 | 4.0 | 4230 | 0.15 | 0.17 | ||
Petrogenic source | <0.30 | <0.10 | <1.0 | <1.0 | <0.15 | <0.17 | ||
Pyrogenic source | >0.30 | >0.10 | >1.0 | >1.0 | >0.15 | >0.17 |
Averages shown as arithmetic mean ± CI.
(1) GoM sediment data from sites with evidence of pyrogenic input collected in 2010 (OSAT 1, 2010).
Total concentration of biomarkers in the DeSoto Canyon varied from 41 to 776 ng g-1 (
Elevated concentrations of biomarkers are generally attributed to petroleum hydrocarbons in contaminated sediments due to their recalcitrant nature and source-specific compound distribution. The distribution profiles of biomarkers in our samples and in GoM oils are dominated by 17α(H), 21β(H)-hopane compound. In addition, C27–C29 diasteranes compounds are abundant with the 20R isomers more abundant than the 20S isomers. Typical biomarker ratios used in forensic analysis have shown to be useful in identifying the DWH event as the source-oil in samples collected from beaches (sand, rocks, tar balls) [
Site | Year | Sed. Depth (mm) | n | Ts/Tm | C29ααS/H | C29ββ(S+R)/C29αα(S+R) |
---|---|---|---|---|---|---|
2011 | 0–4 | 2 | 1.06 | 0.47 | 1.2 | |
2010 | 0–6 | 3 | 1.14 | 0.46 | 1.15 | |
-0.46 | -0.04 | -0.22 | ||||
Pre-2010 | Jun-90 | 19 | 0.77 | 0.27 | 1.22 | |
-0.21 | -0.05 | -0.19 | ||||
2011 | 0–4 | 2 | 0.87 | 0.29 | 1.63 | |
2010 | 0–8 | 5 | 0.78 | 0.34 | 1.34 | |
-0.18 | -0.06 | -0.21 | ||||
Pre-2010 | Oct-60 | 19 | 0.73 | 0.28 | 1.3 | |
-0.07 | -0.04 | -0.12 | ||||
2011 | 0–2 | 1 | 0.39 | 0.65 | 1.16 | |
2010 | 0–8 | 4 | 1.1 | 0.39 | 1.32 | |
-0.77 | -0.04 | -0.34 | ||||
Pre-2010 | Aug-65 | 18 | 0.55 | 0.31 | 1.16 | |
-0.09 | -0.03 | -0.15 | ||||
2011–1990 | 0–22 | 7 | 0.61 | 1.58 | 0.25 | |
-0.11 | -0.3 | -0.07 | ||||
References | NIST |
3 | 1.15 | 0.4 | 1.38 | |
-0.22 | -0.03 | -0.15 | ||||
MC252 |
3 | 1.29 | 0.58 | 1.37 | ||
-0.19 | -0.22 | -0.35 | ||||
DWH sed |
1 | 1.14 | 0.45 | 1.14 | ||
DWH sed |
1 | 0.81 | 0.37 | 1.07 |
Averages shown as arithmetic mean ± CI.
(1)Reference oil: NIST 2779
(2)Reference oil: British Petroleum (sample No. SOB-20100622-084)
(3)Contaminated sediment sample collected at DWH wellhead
(4)Contaminated sediment sample collected at DWH wellhead.
The results from the sites in the DeSoto Canyon indicate that higher amounts of hydrocarbons reached deepwater sediments in the study area in 2010–2011 for the shallower sites (DSH08 and PCB06) and in 2011 for the deeper site (DSH10) than in previous years. Specifically, aliphatic, PAH and biomarkers concentrations were higher in 2010–2011 (up to 337 μg g-1, 525 ng g-1, and 776 ng g-1, respectively) compared to previous years (up to 50 μg g-1, 320 ng g-1, and 523 ng g-1respectively) and the control site (up to 30.4 μg g-1, 45.0 ng g-1, and 85.9 ng g-1respectively) (Figs
Overall, the levels of hydrocarbons reported in our study for 2010–2011 can be classified as low to moderately polluted based on the proposed range of potential impact of 10–100 μg g-1 for aliphatics and 100–1100 ng g-1 for PAHs [
Organic matter in the continental shelf sediments of the GoM is derived from marine primary productivity, terrestrial and aged soil from watershed runoff from the Atchafalaya and Mississippi Rivers, and coastal plants debris and resuspended sediments [
Petroleum derived hydrocarbons in GoM sediments are consistently present and are suggested to originate from natural seeps, gas hydrate deposits, and oil exploration [
Petroleum derived hydrocarbons, distinct from DWH oil, were observed downcore (pre-2010) in all sites indicating the constant presence of oil at lower levels that enters the GoM annually through natural and anthropogenic activities [
Elevated concentrations of aliphatic, PAH and biomarker compounds were observed in most sediment depth intervals corresponding to the period 2010–2011. These compound groups indicate a mixture of hydrocarbons sources deposited during 2010–2011 that includes terrestrial, planktonic and weathered oil. Long-chain
Two mechanisms of hydrocarbon deposition to the seafloor have been proposed: a sedimentary depositional pulse driven by the formation and rapid settling of contaminated particles (“flocculent blizzard” hypothesis), and the direct contact of the deep plume with the continental slope surface sediments at depths between 1000–1200 m (“bathtub ring” hypothesis). In our study, we found several different lines of geochemical evidence that support each of these hypotheses and explain how hydrocarbons reached the sedimentary environment in the DeSoto Canyon during 2010–2011.
Aggregation of suspended particulate material with dispersed crude oil and dissolved hydrocarbons in the water column promotes transport of spilled oil in the environment and toxicity in sediments. This phenomenon of oil-mineral interaction and sedimentation has been studied and documented since the 1970’s in the Exxon Valdez and Arabian Gulf oil spills and in laboratory experiments [
The observed marine snow during the summer of 2010 in the GoM consisted of phytoplankton cells, bacteria, and oil-particle aggregates [
Additionally, oil-particle aggregates in 2010–2011 may have formed with hydrocarbons different from crude or dissolved oil in the water column. We observed elevated concentrations of HMW PAHs at DSH10, the deepest site in 2011 (
Poor correlation between TOC fluxes and hydrocarbon concentrations, and differential PAH abundance between HMW and LMW compounds at PCB06 site located at 1043 m depth (
The analyses of DeSoto Canyon sediments show the contribution of the DWH event to sediment hydrocarbon content and composition, and provides insight into the transport pathways for hydrocarbons to the seafloor and their fate during 2010–2011. Higher hydrocarbon concentrations, and associated mixed hydrocarbon compositions from various sources (including biogenic
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We dedicate this work to Benjamin Flower, who passed away in July 2012. Ben’s dedication, scientific vision and input to this work will always be remembered. The authors would like to thank the crew of the R/V Weatherbird II for their help during the field program, G. Toro for GIS support, S. Gilbert for useful comments, and the students who helped processing the samples in the laboratory: N. Zenzola, C. Michael, Q. Miller, Z. Means. We would like to thank the NSF Rapid Grant program for providing funds for the Deep Sea Instruments MC-800 Multi-corer. We are grateful to the British Petroleum/Florida Institute of Oceanography (BP/FIO)-Gulf Oil Spill Prevention, Response, and Recovery Grants Program for funding several of the initial research cruises and laboratory analysis during 2010 and 2011. This research was made possible by funding from BP/The Gulf of Mexico Research Initiative (GOMRI), specifically the Center for Integrated Modeling and Analysis of the Gulf Ecosystem (C-IMAGE) and the Deep Sea to Coast Connectivity in the Eastern Gulf of Mexico (Deep-C) consortia.