Jiro Ishii

Early diagenesis of amino acids in Okhotsk Sea sediments

Abstract Amino acids, organic carbon, and total nitrogen were determined in five sediment cores from the Okhotsk Sea. The factors controlling the early diagenesis of organic nitrogen compounds are discussed. The amino acid contents were in the range of 2 to 8 mg g −1 of dry sediment at the surface. A rapid linear decrease from the surface to 1.5 m was observed in all five cores. The amounts of amino acids in the subsurface layer were 37.6±8.1% at 1 m and 27.8±14.5% at 1.5 m of those at the surface. Below 1.5 m the amino acid contents remained nearly constant at about 25%. The ratio of amino acid nitrogen to total nitrogen was 24±9% in the surface, 12.5±3.5% at 1 m, 11.0±6% at 1.5 m, and 11.2±2.5% at 1.5 to 3.0 m. The profiles of amino acid groups showed that the fraction of acidic amino acids decreased with depth and basic amino acids were constant or increased slightly with depth. Laboratory decomposition experiments under conditions of high pressure and temperature suggested that the acidic and aromatic amino acids were predominantly decomposed, whereas neutral and basic amino acids were maintained at a constant value. It is presumed that the relative changes of amino acid composition depend primarily on preferential hydrolyses at acidic or aromatic amino acid sites and deamination during the diagenesis.