Amino acid abundances and stereochemistry in hydrothermally altered sediments from the Juan de Fuca Ridge, northeastern Pacific Ocean

The Juan de Fuca Ridge is a hydrothermally active, sediment covered, spread ing ridge situated a few hundred kilometres off the west coast of North Ame rica in the northeastern Pacific Ocean. Sediments from seven sites drilled during the Ocean Drilling Program (ODP) Legs 139 and 168 were analyzed for total hydrolyzable amino acids (THAA), individual amino acid distributions, total organic C (TOC)and total N (TN) contents. The aim was to evaluate th e effects of hydrothermal stress on the decomposition and transformation of sedimentary amino acids. Hydrolyzable amino acids account for up to 3.3% o f the total organic C content and up to 12% of the total N content of the u pper sediments. The total amounts of amino acids decrease significantly wit h depth in all drilled holes. This trend is particularly pronounced in hole s with a thermal gradient of around 0.6 degrees C/m or higher. The most abu ndant amino acids in shallow sediments are glycine, alanine, lysine, glutam ic acid, valine and histidine. The changes in amino acid distributions in l ow temperature holes are characterized by increased relative abundances of non-protein beta-alanine and gamma-aminobutyric acid. In high temperature h oles the amino acid compositions are characterized by high abundances of gl ycine, alanine, serine, ornithine and histidine at depth. D/L ratios of sam ples with amino acid distributions similar to those found in acid hydrolysa tes of kerogen, indicate that racemization rates of amino acids bound by co ndensation reactions may be diminished. (C) 2000 Elsevier Science Ltd. All rights reserved.