Diagenetic transformation of nitrogenous organic matter in the Southwest Atlantic Shelf

Abstract

Amino Acids (AA) and Amino Sugars (AS) were assessed inseston from inner and shelf break waters of the Argentine Sea,and in sediment cores from the inner shelf. Glutamine, asparagine,alanine, serine, glycine and leucine accounted for ~60% of TotalParticulate AA (TPAA) in surface waters. The mol percentage of AAfrom algal frustules (e.g. glycine and serine) increased with depth,while cytoplasmic AA (e.g. alanine and asparagine) decreased,indicating selective bacterial AA uptake. Protein degradationproducts ornitine (orn), Beta-alanine (β-ala), and Gamma-AminoButyric Acid (γ-ABA) doubled their mol percentage from the surfaceto deep water. Percentage contribution of glucosamine to TotalParticulate AS (TPAS) decreased with depth from ~74% to ~60%.Galactosamine followed an inverse trend, reflecting an increase inbacterial nitrogen during seston sinking and degradation. TPAA/TPAS ratios increased from ~30% at surface indicating mixedphyto- and zooplanktonic sources, to ~63% in bottom layersas result of an increasing contribution of phytoplankton-deriveddetritus. Diagenetically fresh material predominated from surface to~50 m depth, changing to still relatively immature POM at 75 m120 m. At depths >750 m, POM reached higher maturity due tobacterial turnover, reflected in a diminished mol percentage ofproteinogenic AA and a higher of non-proteinogenic. Organic matterfrom surface sediments is diagenetically similar to seston at depths >1000 m, suggesting that POM attained a fairly stable condition shortlyafter deposition. Most parameters barely changed between surface (5 cm) and deep (>250 cm) sediment layers, likely due to muchlower rates of OM transformation in the anoxic environment.