Diagenetic Transformation of Nitrogenous Organic Matter in The Southwest Atlantic Shelf
Amino Acids (AA) and Amino Sugars (AS) were assessed in seston 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 Total Particulate AA (TPAA) in surface waters. The mol percentage of AA from 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 degradation products ornitine (orn), Beta-alanine (β-ala), and Gamma-Amino Butyric Acid (γ-ABA) doubled their mol percentage from the surface to deep water. Percentage contribution of glucosamine to Total Particulate AS (TPAS) decreased with depth from ~74% to ~60%. Galactosamine followed an inverse trend, reflecting an increase in bacterial nitrogen during seston sinking and degradation. TPAA/ TPAS ratios increased from ~30% at surface indicating mixed phyto- and zooplanktonic sources, to ~63% in bottom layers as result of an increasing contribution of phytoplankton-derived detritus. 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 to bacterial turnover, reflected in a diminished mol percentage of proteinogenic AA and a higher of non-proteinogenic. Organic matter from surface sediments is diagenetically similar to seston at depths > 1000 m, suggesting that POM attained a fairly stable condition shortly after deposition. Most parameters barely changed between surface (5 cm) and deep (>250 cm) sediment layers, likely due to much lower rates of OM transformation in the anoxic environment.