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

Abstract

The stable carbon isotopic composition of nine individual amino acids were investigated in phytoplankton and zooplankton, estuarine plankton, terrigenous material, marine fecal material, and clay mineral isolates collected along the Washington coast. The clay fraction was isolated from suspended sediments of the Columbia River (the mineral source) and three shelf and slope stations (mineral deposition sites). In the sediments, terrigenous amino acids were replaced by those of marine origin, and microbial reworking of the amino acids further influenced their d13C compositions. Based on changes in isotopic composition, individual amino acids could be roughly divided into three groups. (1) Leucine and proline had isotopic shifts similar to bulk organic matter. On average, 80% of these river‐delivered amino acids were replaced by marine‐derived material. (2) Alanine, isoleucine, glutamic 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 marine substrates and/or preservation of ~50% of the terrigenous component in the marine environment. (3) Glycine and valine exhibited isotopic values outside the range of our sampled end members. Their unusual isotopic compositions are attributed to reworking of their isotopic signal during diagenesis. Microbial resynthesis of amino acids during growth on mixed substrates may account for nearly all the observed variation in amino acid isotopic composition. Similarity between the amino acid isotopic composition of the fecal material and that of the clay isolates suggests that alterations of the isotopic composition of the amino acids might occur while the amino acids are in distinct organic‐rich debris, prior to long‐term association with the sediment.