Abstract:

The goals of this project were to determine the concentration and chemical composition hydrocarbons throughout the water column focusing on the different layers comprising the deep-sea plumes/intrusions. Utilizing techniques in molecular organic and isotope geochemistry this research aims to develop a “chemical fingerprint” for the subsurface oil that future researchers can trace the flow and cycling of oil within the marine waters and sediments and within marine ecosystems and habitats. During two research cruises, August and December 2010 we were be able to evaluate how the concentration and chemical composition subsurface hydrocarbons are changing over time and space. This study of subsurface oils is also vitally needed for determining environmental baselines, assessing the persistence of the subsurface hydrocarbons in the plume layers with time and transport away from the blowout site, understanding ecosystem responses and evaluating the causes of biological and chemical changes associated with pre- and post-impact assessments and restoration.

Suggested Citation:

Hollander, David. 2014. Composition, Concentration, and Distribution of sub-surface oil, Northern Gulf of Mexico, May to December 2010. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N71J97PH

Purpose:

1. Determine the concentration and chemical composition of hydrocarbons (e.g., molecular abundance and distribution of organic compounds) in the different layers that comprised the deep-sea plumes 2. Develop a “chemical fingerprint” for the subsurface oil, using a combination of molecular distribution and CSIA so that future researchers can trace the cycling of oil within the marine waters and sediments and within marine ecosystems and habitats. 3. Evaluate how concentration and chemical composition subsurface hydrocarbons are changing temporally and spatially in the Northern Gulf of Mexico (GoM).

Data Parameters and Units:

concentration of total petroleum hydrocarbons (ug/l), concentration of individual n- alkanes, C15 to C38 (ng/l), molecular carbon isotopic composition of n-alkanes (‰)DSH09: 29.6365,-87.8685 DSH07: 29.2533,-87.7345 PCB06: 29.1283,-87.2666 PCB05 29.4416,-86.7833

Methods:

•Samples were collected via an in situ-filtration device (Maclane LV-142-8) fitted with a 142 mm nitex 53 um filter over a 142 mm GFF 0.7 um filter or via pressure filtration through inert FEP tubing and filter housings from water samples collected in stainless steel 18.9L canisters. •Filters were stored frozen and lyophilized prior to analysis. •Particulate-bound hydrocarbons were extracted from dried filters using a Dionex automated solvent extractor (95% dichloromethane, 5% methanol). • Separation of extract lipid fractions was conducted via selective elution through am silica-gel (neutral fractions, acid fractions) columns. The silica gel (high purity grade, 100-200 mesh, pore size 30A, Sigma Aldrich, USA) was combusted (450°C for 4h) and activated (2%) previous to assemble the columns. Ultra-clean silica bonded cyanopropyl (C3-CN, 50 μM, Interchim, USA) was used as well to improve separation of lipid fractions. Lipid fractions were collected by sequentially eluting the extracts with hexane (100%) to collect aliphatic hydrocarbons, and hexane/dichloromethylene mixture (3:1, v:v) to collect aromatic hydrocarbons. The aliphatic fraction was concentrated and spiked with d14-terphenyl as internal standard. All solvents used were at the highest purity available and used without further purification. •Two extraction blanks was included with each set of samples (15-20 samples) to ensure no contamination from chemicals, glassware and/or laboratory equipment. •A Varian 320-MS Triple-Quadrupole GC-MS was used for molecular/compound identification and quantification and a Thermo Delta+XL GC-C-irms was used for measurement of compound-specific δ13C.

Instruments:

All analyses were conducted on instrumentation (ASE, GC/MS and GC-C-IRMS) that is permanently housed at the University of South Florida College of Marine Science Paleooceanography, Paleoclimatology and Biogeochemistry (PPB) Lab in St. Petersburg, FL. . • Lipids were extracted from dried filters and MC-252 cruide oil using a Dionex automated solvent extractor (95% dichloromethane, 5% methanol) • Molecular/compound identification and quantification of total petroleum hydrocarbons (n-alkanes reported here) were measured using a Varian 320-MS Triple-Quad GC-MS. 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 n-alkanes (nC12-nC40) and isoprenoids pristine (Pr) and phytane (Phy) were conducted by comparing with reference standards. Branched alkanes and the unresolved complex mixture (UCM) were calculated using the mean response factors of n-alkanes. • A Thermo Delta+XL GC-C-irms was used to determine the compound-specific isotopic analyses (CSIA) of carbon (13C) from individual n-alkane molecules. CSIA of carbon was measured on a Thermo Delta+XL GC-C-irms coupled to an Agilent GC (using a DB5-MS column) and a Thermo GC combustion III interface using a temperature program. 13C values of individual compounds are determined relative to co-injected internal standards and to a reference gas (CO2). Molecular isotopic analyses are run in duplicate or triplicate.

Individual Files: