Environmental biogeochemistry of the Northwest Arm and the Trinity Bay, Newfoundland : novel molecular and carbon isotopic approaches

Pulchan, K. Jerry (2001) Environmental biogeochemistry of the Northwest Arm and the Trinity Bay, Newfoundland : novel molecular and carbon isotopic approaches. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this study, marine sedimentary organic matter of Trinity Bay and the Northwest Arm in spatial and temporal terms) was analyzed by pyrolysis and tetramethylammonium hydroxide chemolysis - techniques that degrade complex organic materials into moieties that are amenable to analysis. In addition, chemolysates were analyzed for their molecular isotopic signatures. The goal was to interpret the variation in the pyrolysate and chemolysate compositions in the context of sources and fates of organic matter and the influence of the surrounding communities on the distribution of terrestrially-derived material in the bay. -- Pyrolysis products of organic matter present in near-shore sediments (Trinity Bay, Newfoundland) were characterized using GC/MS. The major products observed by "on-line" pyrolysis/GC/MS were nonadecene, phenol, methyl phenol, indole, and methylindole giving limited structural information about the sources of sedimentary organic matter. Spatial variation of the pyrolysates of the sediment were interpreted in the context of sources of organic matter, that were related to the historical economic activities of the study area. -- A "batch-wise" TMAH thermochemolysis (in-situ methylation) technique was developed for the analysis of marine sediments. Standard phenolic aldehyde and acids and fatty acids were quantitatively methylated while triacylglycerol were converted to fatty acid methyl esters. Thermochemolysis of carbohydrates produced 1,2,4-trimethoxybenzene as an interesting chemolysate marker. Polyunsaturated fatty acids could be analyzed using the thermochemolysis conditions employed. -- TMAH thermochemolysis products of sediments and model compounds were characterized using GC/MS and GC/C/IRMS. Thermochemolysates of sediments included aromatic and aliphatic compounds determined as methyl esters and ethers. The dominant aliphatics are fatty acids that ranged from C₁₄ to C₂₈, with a predominance of even carbon numbered species, and are either saturated, mono-unsaturated or branched. The aromatic compounds are phenols of which the most abundant is methylated vanillic acid, an important chemical marker in evaluating the terrestrial contribution to the organic carbon pool. -- The spatial variation of the thermochemolysates indicates very high abundances of fatty acids and phenols in the near-shore sediments of the Northwest Arm rather than in the offshore sediment cores where fatty acids to phenol ratios were lowest for near-shore sediments. ²¹⁰Pb dates of sediments of two near-shore cores enabled calculation of annual preserved fluxes of organic carbon, total fatty acids and phenols. The fluxes of phenols were very high for one core, H1, and were related to saw-milling and logging activities of the community of Hickman's Harbour. -- Compound-specific isotope analysis (CSIA) (δ¹³C) of thermochemolysates of standard fatty acids and phenols indicate that was was only minor isotopic fractionation (0.4‰ or better) during high temperature methylation conditions. Also, there appears to be minor isotopic fractionation during base hydrolysis of ester bonds of a triacylglycerol. Theoretical mass balance calculations on phenols produced by TMAH thermochemolysis of a lignin dimer indicates that one of the phenols may have retained its isotopic signature after synthesis of the dimer. The δ¹³C compositions of chemical markers were calculated after correction for the contribution of the methylating agent (TMAH) to the isotopic signature of individual the chemolysates. The isotopic composition of the phenolic markers, vanillin and vanillic acid show a strong similarity (depleted δ¹³C) to those same markers generated from adjacent terrestrial soil cover. Such supporting isotopic evidence, along with structural evidence that these TMAH products resemble guiacyl units of lignin, show that the TMAH thermochemolysates, methylated vanillin and vanillic acid are excellent biomarkers for terrestrially-derived carbon in a marine environment. In contrast, methylated coumaric acid shows a wide range in δ¹³C values among locations, and with a more δ¹³C-enriched isotopic composition with a wide range (-28 to -l9‰) originates from terrestrial and marine sources. Finally, the results of δ¹³C values of fatty acids indicate that they are derived from multiple sources; δ¹³C evidence also indicates that there are more contributors to the fatty acid pool in near-shore than in offshore sediments.

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