Amino acid biogeochemistry in the Laurentian Trough: vertical fluxes and individual reactivity during early diagenesis

The detailed composition of total hydrolysable (THAA), dissolved free (DFAA ) and combined (DCAA) amino acids was studied in settling particles and the solid phase and porewaters of underlying sediments in the Laurentian Troug h to evaluate their sources and individual reactivities during early diagen esis. Vertical fluxes of THAA measured at 150 m depth (234-980 mu mol/m(2)/ day) represented 3.8% of the average daily primary production and 8-16% of total organic carbon (TOC) and 24-42% of total nitrogen (TN) fluxes. THAA c oncentrations decreased from 89 +/- 39 to 39 +/- 4.4 mu mol/g from settling particles to the top 3 cm sediments, with no significant change of the %TH AA-C and %THAA-N. However, these parameters decreased with depth in the sed iments (10-13 to 7-8% and 30-45 to 22-28%, respectively) indicating a selec tive THAA removal. THAA composition of settling particles and sediments was relatively uniform and showed a marked enrichment in serine, threonine and glycine relative to fresh plankton which is ascribed to the selective pres ervation of diatom cell-walls. Serine was the more specific diatom tracer; it covaried with diatom lipid biomarkers, was relatively more abundant at a seaward site and increased downcore reflecting the selective preservation of diatom cell-walls. An increasing trend with sediment depth was also obse rved for aspartic acid whereas glutamic acid and histidine decreased. Porew ater DFAA and DCAA accounted for 3-25% of total DOC and showed low levels i n the surface zone of most intense solid phase THAA decay. Both fractions s howed clear compositional differences related to the prevailing source mate rial: DCAA, as solid phase THAA, were dominated by serine and threonine + g lycine, whereas DFAA were enriched in glutamic (Glu) and beta-aminoglutaric acids (beta Glu), probably originating from bacteria. These patterns chang ed with depth in the sediments: the proportion of serine and beta Glu incre ased in DCAA and DFAA, respectively, whereas that of glutamine, alanine and Glu decreased in the DFAA pool. The preferential downcore decay and conver sion of Glu into beta Glu was reflected by a consistent increase of beta Gl u/Glu ratios, particularly at a landward station where the higher rates of sedimentation and OM burial favor the continued metabolism of bacteria in d eeper sediment layers. (C) 1998 Elsevier Science Ltd. All rights reserved.