Molecular and isotopic fingerprinting of aliphatic hydrocarbons in Conception Bay, Newfoundland

Bieger, Tilman (1992) Molecular and isotopic fingerprinting of aliphatic hydrocarbons in Conception Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The distributions and isotopic compositions of the aliphatic hydrocarbons of plankton, sediments and biota of a typical northern fjord (Conception Bay, Newfoundland) were studied by standard gas chromatography-mass spectrometry (GC-MS), and by the novel technique of compound-specific isotope ratio mass spectrometry (IRMS). Sediments contained compounds of terrestrial, marine biogenic, and bacterial origins, with petroleum appearing to contribute primarily an unresolved complex mixture (UCM), occasional pristane and phytane, and typical trace cyclic terpenoids. A suite of eight C₂₅ highly branched isoprenoid alkenes containing 3 to 5 degrees of unsaturation was the dominant feature in plankton, sediments, and some benthic biota in Conception Bay, as well as in samples from other coastal and offshore areas around Newfoundland. Although attempts to isolate the source of these compounds by culturing common local diatom species were unsuccessful, the molecular distribution and isotopic signature of these alkenes suggest that these hydrocarbons are synthesized by one particular organism during the spring bloom. Measurement of the δ¹³C (or carbon isotopic composition) of individual compounds proved to be a useful tool in recognizing the input of different sources of compounds to sedimentary organic matter pools. For example, variations in the isotopic compositions of even chain-length n-alkanes in sediments suggest the input of at least three main sources of n-alkanes (terrestrial leaf debris, marine algae, and a third source, proposed to be marine bacteria). The δ¹³C of biogenic hydrocarbons appeared to be relatively unchanged during early degradation in the water column and surface sediments. Although the existence of kinetic isotope fractionation during biosynthesis was indicated by co-variations in the distribution and isotopic signature of algal products in cultures, biochemical isotope effects generally appeared to be masked in the complex environmental samples analyzed. Compounds from distinct sources were generally found to all have distinct isotopic signatures. For example, known phytoplankton products featured δ¹³C values above -28%o, whereas terrestrial n-alkanes (nC₂₃₋₃₃) generally featured δ¹³C values below -30%o. The isotopic effects of petroleum input in Conception Bay appeared to be minor, but could not be quantitatively determined due to difficulties in separating compounds of interest from the large UCM typically found in petroleum sources. Petroleum input, however, could be effectively recognized using accepted molecular markers (hopanes, steranes, tricyclic terpanes). More detailed study of the isotopic signatures of individual compounds in sources such as terrestrial leaf wax alkanes and lubricating oils is needed in order to explain all of of the complex isotopic patterns seen in sediments.

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