Input of Pelagic Tar into the Northwest Atlantic from the Gulf Loop Current: Chemical Characterization and its Relationship to Weathered IXTOC-I Oil

Abstract

Pelagic tar concentrations have been measured for samples collected monthly in the eastern Gulf of Mexico over a one year period. Analyses of the pelagic tar included gravimetric, isotopic (δ13C) and molecular (gas chromatographic) characterization of the total, aliphatic, and aromatic fractions. The concentrations of pelagic tar ranged from 0 to 26.5 mg∙m−2 of toluene extractable material, with an average of 1.60 mg∙m−2 in the offshore neuston tows and 0.05 mg∙m−2 in neuston tows taken on the west Florida continental shelf. Oblique tows to 100 m depth averaged 0.01 mg∙m−2. δ13CPDB values for the total tar in each tow averaged −27.2‰ with the aliphatic fractions averaging −27.5‰ and the aromatic fractions averaging −27.0‰. Approximately 40% of the tar samples showed a bimodal n-alkane distribution characteristic of crude oil sludge. Based upon an average annual Loop Current discharge of 28 × 106 m3∙s−1 through the Florida Straits, it is estimated that approximately 7000 t of pelagic tar are discharged annually from the Gulf of Mexico into the North Atlantic. Approximately half of this may be brought into the Gulf from the Caribbean via the Loop Current, while the remainder appears to originate in the Gulf of Mexico. Possible origins of the floating tar include input from the Caribbean Sea through the Yucatan Strait, production and transportation operations in the Gulf of Mexico, Mississippi River discharge, subsurface oil seeps, and major accidental discharges. Large amounts of oil from the IXTOC-I well blowout do not appear to have impinged on coastal waters of Florida.