John A. Calder

Early diagenesis of fatty acids and hydrocarbons in a salt marsh environment

Abstract In a salt marsh environment, plant-produced fatty acids and aliphatic hydrocarbons undergo significant modification upon being deposited in sediment. The major changes include alteration of the distribution pattern of straight chain components, increase in the concentration of branched components and decrease in the concentration of unsaturated components. Changes are similar in both oxidizing and reducing environments. Carbon isotope measurements indicate that there is little horizontal movement of sediment organics. Spartina alterniflora has a δC 13 = − 12.5% (vs NBS 20) and Juncus romerianus has a δC 13 = −23.2%. Sediment organic matter taken from cores up to 60 cm in depth retains a δC 13 value similar to the plant species growing at the core site. Microbial metabolism appears to be responsible for the observed in situ changes in sediment lipids. The in situ changes did not result in significant alteration of δC 13 values.

Organic carbon 13C12C variations in estuarine sediments

The δC13 value for sedimentary organic carbon in four estuaries of the Gulf of Mexico increases with radial distance from the river mouth. Mass balance calculations indicate that terrestrial organic carbon is limited to sediments within a relatively short distance from the river mouth. This distance is a function of the discharge rate of the river. For the Mississippi River, terrestrial organic carbon is limited to sediments within 69 km of the mouth of Pass a Loutre and 61 km of South Pass. These data indicate that the low δC13 (< −22%.) values reported for Pleistocene sediments in the Gulf of Mexico may be the result of factors in addition to the postulated large influx of terrestrial organic carbon.

The solubility of medium molecular weight aromatic hydrocarbons and the effects of hydrocarbon co-solutes and salinity

Abstract The solubilities of several medium molecular weight aromatic hydrocarbons were determined at 25°C in aqueous binary, ternary, and quaternary systems. The binary systems consisted of distilled water plus one each of the following: naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, biphenyl, 2-ethylnaphthalene, 1,5-dimethylnaphthalene, 2,3-dimethylnaphthalene, 2,6-dimethylnaphthalene, acenaphthene and phenanthrene. The ternary systems consisted of distilled water plus one of the following hydrocarbon pairs: naphthalene/biphenyl, naphthalene/acenaphthene, naphthalene/phenanthrene, biphenyl/phenanthrene, biphenyl/2-methylnaphthalene, acenaphthene/phenanthrene and 2-methylnaph-thalene/phenanthrene. Quaternary systems consisted of distilled water and one of the following hydrocarbon mixtures: naphthalene/biphenyl/phenanthrene, 2-methylnaphthalene/biphenyl/phenanthrene and naphthalene/acenaphthene/phenanthrene. In binary systems at S %. = 0, solubilities ranged from 31.3 ppm for naphthalene to 1.07 ppm for phenanthrene. The alkyl naphthalenes exhibited solubilities which were a function of molar volume and substituent size and position. In ternary and quaternary systems, solubilities often deviated from values obtained in binary systems presumably due to solute-solute interactions and/or formation of solid solutions. Aromatic hydrocarbons are ‘salted out’ by increasing concentrations of inorganic salts. At S %. = 35, the solubility of naphthalene is 22.0 ppm and of phenanthrene, 0.71 ppm. The empirical salting parameters are identical in both binary and quaternary systems.

Geochemical implications of induced changes in C13 fractionation by blue-green algae

Abstract The photosynthetic fractionation of carbon isotopes by blue-green algae in laboratory culture is dependent in a non-linear fashion on the CO 2 concentration in the feed gas. For the three species tested, the minimum fractionation occurred at a CO 2 concentration of 0.2% in air and was approximately zero for the two marine species tested. Enrichment of C 12 in the reduced carbon is not an inevitable result of photosynthetic carbon fixation. Temperature and pH had no detectable effect on fractionation. The maximum fractionation observed in the laboratory cultures or in recent blue-green algal mats was 18‰. Differences in the isotope ratio of coexisting oxidized and reduced carbon in Precambrian stromatolites are as great as 31‰. Present carbon isotopic evidence is not consistent with the idea that blue-green algae were major contributors to the organic matter in Precambrian sediments.

Dissolved hydrocarbons in the eastern Gulf of Mexico loop current and the Caribbean Sea

Concentrations of dissolved non-polar hydrocarbons extracted from waters taken at several stations and depths in the Gulf of Mexico and Caribbean Sea ranged from traces to 75 μg1−1, with the highest occurring in the Florida Strait. In all cases except in the Florida Strait, this fraction was characterized by relatively large amounts of n-alkanes having between 15 and 20 carbon atoms and relatively small amounts of n-alkanes with more than 20 carbon atoms. In the Florida Strait there were much larger concentrations of n-alkanes above C20. There was an unresolved envelope in the gas chromatograms of all the samples that extended approximately from the C15 to the C30 position, with the maximum between the C20 and C23 positions.

SEASONAL VARIATIONS OF HYDROCARBONS IN THE WATER COLUMN OF THE MAFLA LEASE AREA

Abstract A series of 15 stations in the northeast Gulf of Mexico were occupied during summer, fall and winter 1975-76. Samples were collected and analyzed by gas chromatography for dissolved hydrocarbons and those associated with suspended particulate material. Average concentration of total resolved hydrocarbons was 0.4 yg/1 dissolved and 0.3 yg/1 particulate. Concentrations were higher near shore. Unresolved components were present in both dissolved and particulate phases, especially near the Mississippi River and Sound which may be the source of this material. Biogenic hydrocarbons, nC15, nC17, pristane and squalene in the particulate phase may be reflective of in situ biomass. A series of n-alkanes (nC21 to nC32) in both dissolved and particulate phases persisted during all seasons. Squalene was the dominant molecule in the dissolved unsaturated/aromatic fraction at most stations, but was very low in concentration at the offshore stations in the fall. Total dissolved hydrocarbons did not correlate with particulate organic carbon. Total particulate hydrocarbons did not correlate with particulate organic carbon or chlorophyll a (Chi. a ).