Molecular and isotopic analyses of Tagish Lake alkyl dicarboxylic acids

The Tagish Lake meteorite soluble organic suite has a general composition that differs from those of both CI and CM chondrites. These differences suggest that distinct processes may have been involved in the formation of different groups of organics in meteorites. Tagish Lake alkyl dicarboxylic acids have a varied, abundant distribution and are, with carboxylated pyridines, the only compounds to have an occurrence comparable to that of the Murchison meteorite. This study has undertaken their molecular and isotopic characterization, with the aim to understand their origin and to gain insights into the evolutionary history of the meteorite parent body. Tagish Lake alkyl dicarboxylic acids are present as a homologous series of saturated and unsaturated species with three- through ten-carbon atom chain length. Linear saturated acids are predominant and show decreasing amounts with increasing chain length. A total of 44 of these compounds were detected with the most abundant, succinic acid, present at similar to40 mnol/g meteorite. Overall the molecular distribution of Tagish Lake dicarboxylic acids shows a remarkable compound-to-compound correspondence with those observed in the Murchison and Murray meteorites. In both Tagish Lake and Murchison, the imides of the more abundant dicarboxylic acids were also observed. The hydrogen and carbon isotopic compositions of individual Tagish Lake dicarboxylic acids were determined and compared to those of the corresponding acids in the Murchison meteorite. All deltaD and delta(13)C values for Tagish Lake acids are positive and show a substantial isotopic enrichment. deltaD values vary from, approximately, +1120parts per thousand for succinic acid to +1530parts per thousand for methyl glutaric acid. delta(13)C values ranged from +12.6parts per thousand for methyl glutaric acid to +22.9parts per thousand for glutaric acid, with adipic acid having a significantly lower value (+5.5parts per thousand). Murchison dicarboxylic acid showed similar isotopic values: their delta(13)C values were generally higher by an average 17% and deltaD values were lower for succinic and glutaric acids, possibly due to contamination. The molecular and isotopic data collected for these compounds restrict their possible origin to processes, either interstellar or of very cold nebular regions, that produced significant isotopic enrichments. Saturated or partially unsaturated nitriles and dinitriles appear to be good precursor candidates as their hydrolysis, upon water exposure, would produce dicarboxylic acids and other carboxylated species found in Tagish Lake. This evolutionary course could possibly include pre-accretionary processes.