Variability and composition of amino acids and amino sugars in sediment cores of the Changjiang Estuary

Abstract

Understanding the sources, compositions and degradation states of sedimentary organic matter (SOM) in estuarine regions can provide significant information about regional carbon and nitrogen cycles. Amino acids (AAs), amino sugars (ASs), total organic carbon (TOC), total nitrogen (TN) and stable carbon isotopic composition (δ13OC) were investigated in six sediment cores of the Changjiang Estuary. The concentrations of ASs ranged from 0.60 to 3.11 μmol g−1, and accounted for 1.7 ± 0.7% of TOC and 2.1 ± 0.9% of TN. The concentrations of AAs ranged from 5.18 to 11.66 μmol g−1 and accounted for 8.1 ± 1.7% of TOC and 17.4 ± 3.9% of TN. Relatively coarse and fresh SOM was mainly deposited in the nearshore (e.g., station B6), whereas fine and degraded SOM occurred in the offshore area (e.g., station A7-4). The relationships between median grain size and AA-C% and degradation index (DI) revealed that fine-grained particles adsorbed older SOM, but sandy sediments were correlated with relatively fresh SOM. Diatoms were the main contributor of marine SOM and high primary production increased the burial of fresh SOM. TOC and TN contents and their degradation states did not exhibit distinct trends with core depth, possibly due to frequent vertical disturbance. Additionally, bacterial OM contributed 0.4–23.7% to TOC and 0.6–30.6% to TN in the sediment cores. The hypoxia of the water might have further promoted the proportion of bacterial OM in the sediments.