Sources, sinks, concentrations and sub-lethal effects of light aliphatic and aromatic hydrocarbons in the Gulf of Mexico.

The spatial distribution and sources of light hydrocarbons in the Gulf of Mexico are evaluated from sniffing surveys of over 5,000 miles of cruise tracks aboard the R/V Gyre, R/V Alaminos, and R/V Miss Freeport and from over 300 discrete water samples. The surveys were performed using a modified Beckman Process Gas Chromatograph with a FID detector. The methodology of hydrocarbon sniffing is after Brooks et al., (1973) and the discrete analysis is after Swinnerton and Linnenbom (1967) and McAullife (1971). The six significant inputs of light hydrocarbons to the Gulf are: (1) natural seepage of oil and gas, (2) offshore drilling and production, (3) transportation losses, (4) coastal contributions, (5) air-sea exchange, and (6) biologically-derived hydrocarbons. It was found that hydrocarbon sniffing was a sensitive means of detecting man-derived hydrocarbon pollution. The significance of each of the major inputs is evaluated. Coastal waters of the Gulf of Mexico, in general, are not in equilibrium with the atmosphere with respect to low-molecular-weight hydrocarbons. This is chiefly the result of large man-derived inputs. Offshore production and transportation activities are the chief inputs of light hydrocarbons to the coastal regions, contributing the whole spectrum of light gaseous, liquid and aromatic hydrocarbons. Coastal contributions from refineries and runoff are also significant man-derived inputs. In the deep-water Gulf of Mexico natural sources of methane and olefins in the upper few hundred meters can rival man's input of these hydrocarbons. Natural seepage of gas results in a detectable increase in gaseous hydrocarbon concentrations in the immediate vicinity of the seep. Many of the natural seeps located on topographic highs in the northwestern Gulf appear to have a biogenic origin. The hydrocarbon composition of two collected seep gases contained 99.98% methane and del-C 13 values (C13PDB exp 60) representative of biogenic methane. The use of the seeps in exploration necessitates differentiation between biogenic and petrogenic seepage. The effect of light liquid and simple aromatic hydrocarbons on the carbon fixation rates of mixed ocean cultures and pure laboratory cultures was examined by HC14O3 uptake. The aromatic hydrocarbons require a higher hydrocarbon concentration to effect a given reduction in carbon fixation than do the light liquid hydrocarbons. The aromatic hydrocarbons, however, exhibit a sublethal enhancement in carbon fixation. This can occur at concentrations 1 to 2 orders of magnitude below the levels causing a reduction in carbon fixation. This same effect is not observed with the light liquid hydrocarbon.