Reworking of amino acid in marine sediments: Stable carbon isotopic composition of amino acids in sediments along the Washington coast

The stable carbon isotopic composition of nine individual amino acids were investigated in phytoplankton and zooplankton, estuarine plankton, terrigen ous material, marine fecal material, and clay mineral isolates collected al ong the Washington coast. The clay fraction was isolated from suspended sed iments of the Columbia River (the mineral source) and three shelf and slope stations (mineral deposition sites). In the sediments, terrigenous amino a cids were replaced by those of marine origin, and microbial reworking of th e amino acids further influenced their delta C-13 compositions. Based on ch anges in isotopic composition, individual amino acids could be roughly divi ded into three groups. (1) Leucine and proline had isotopic shifts similar to bulk organic matter. On average, 80% of these river-delivered amino acid s were replaced by marine-derived material. (2) Alanine, isoleucine, glutam ic acid, aspartic acid, and phenylalanine had intermediate isotopic shifts. These isotopic compositions are consistent with both the expected isotopic fractionation associated with microbial resynthesis of amino acids using m arine substrates and/or preservation of similar to 50% of the terrigenous c omponent in the marine environment. (3) Glycine and valine exhibited isotop ic values outside the range of our sampled end members. Their unusual isoto pic compositions are attributed to reworking of their isotopic signal durin g diagenesis. Microbial resynthesis of amino acids during growth on mixed s ubstrates may account for nearly all the observed variation in amino acid i sotopic composition. Similarity between the amino acid isotopic composition of the fecal material and that of the clay isolates suggests that alterati ons of the isotopic composition of the amino acids might occur while the am ino acids are in distinct organic-rich debris, prior to long-term associati on with the sediment.