SOURCES AND DIAGENETIC STATUS OF ORGANIC MATTER IN THE HAURAKI GULF, NEW ZEALAND USING DISTRIBUTION AND CARBON ISOTOPIC COMPOSITION OF AMINO ACIDS

Amino acids are one of the more labile classes of organic matter in marine sediments. These compounds are the structural components of proteins and constitute the largest reservoir of organic nitrogen in most organisms. Their distribution and abundance have been used to assess diagenetic status of organic matter and their isotopic compositions have been linked to organic matter source identification. Organic matter in marine environments is derived from both allochthonous and autochthonous sources. Detailed source apportionment is difficult using bulk chemical characterization owing to the contribution of organic matter from multiple sources. Chemical and isotopic characterization of individual organic compounds, however, can yield detailed information on organic matter sources in complex systems.

Sediment samples were recovered in 1999 from the Hauraki Gulf of New Zealand aboard the HMS Tangaroa. Near-shore and shelf environments were sampled along major currents to investigate the source and distribution of organic matter in the gulf. Amino acids were isolated from sediments by acid hydrolysis and subsequently analyzed by gas chromatography/mass spectrometry following derivatization to their respective N-TFA-isopropyl esters.

Initial results indicate a predominance of L- over D- amino acids in all sediment core top samples, which may reflect algal and terrestrial sources and little input from bacterial sources. The distribution of amino acids is similar to those found in other marine sediments, which show an abundance of glycine, alanine, and aspartic acid. Results also reveal a greater concentration of amino acids in near-shore sites compared to offshore sediments, which may reflect diagenesis during transport through the water column. Sediments from the Firth of Thames estuary exhibit the highest amino acid concentrations, which may be indicative of phytoplankton blooms. Further down-core analyses will help determine the extent of organic matter diagenesis in these sediments and the isotopic compositions of amino acids will help identify their sources.