A Study On The Distribution,Origin And The Early Diagenesis Of Organic Matter In Pearl River Estuary And Adjacent Shelf

In this study, the isotopic composition (δ¹³C andδ¹⁵N), organic carbon (OC) and total nitrogen content (TN) of sediment cores as well as plankton and suspended particulates in the Pearl River estuary and adjacent shelf were determined, and the organic matter accumulation rate was estimated. The temporal and spatial distribution of sedimented terrestrial and aquatic organic matter was studied, and its implications to the variation of ecological environment were discussed. Moreover, the magnitude and mechanism of chemical and isotopic fractionation of organic matter were investigated based on the analysis of the relative content andδ¹³C of the component compound classes extracted from plankton and selected sediment cores. Main conclusions were drawn as follows:(1) The contents of TOC and TN decrease with the distance from Pearl River outlets. Within the Pearl River estuary (Lingding bay), the TOC content is higher and relatively stable in the lower part of the sediment cores, and increases upward with larger variations, indicating increased organic matter input due to human activities in the last two decades.(2) Theδ¹³C_org values of the sediment cores in the adjacent shelf is similar with small variations, indicating thatδ¹³C_org can be a reliable proxy to the sources of organic matter. OC/N ratios can help discriminate the source of sedimented organic matter, but significant alteration would occur due to diagenesis.δ¹⁵N cannot be used to indicate the origin of sedimented organic matter in the studied area.(3) Generally, terrestrial organic matter accounts for more than half of that in the sediment cores sampled in the Lingding bay, with its proportion decreases away from the Pearl River outlets, while the organic matter in the sediment cores of the adjacent shelf was predominantly algal-derived.(4) Calculations using two end-member (terrestrial and aquatic) mixing model of organic matter indicated that there was no significant difference between the sediments of the Pearl River estuary and the adjacent shelf in the content of aquatic organic matter of the sediment cores.(5) The accumulation rate of aquatic organic carbon was 1.26-1.63 mg/cm².a in the adjacent shelf, which was much lower than that in Pearl River estuary (6.1-13.5mg/cm².a), indicating that the primary productivity in Pearl River estuary is much higher than that in the adjacent shelf due to the riverine input of nutritions.(6) The fraction of THAA TCHO and lipid decreases from plankton to POM to surface sediments and to the buried sediments, with a simultaneous increase of that of acid-insoluble organic matter, which caused the alteration of organic matter during early diagenesis.(7) The different degradation rates of organic compounds and the formation and degradation of bacteria organic matter are the major mechanisms causing the chemical and isotopic alteration of organic matter during early diagenesis.(8) Because of the lower carbohydrate content in the plankton and the rapid degradation of lipid, the carbon isotopic fractionation caused by selective degradation of compounds could be relatively small and was partially canceled by the addition of bacterial synthesized organic matter, resulting in the observed conservative nature ofδ¹³C_org during early diagenesis. On the other hand, the addition of bacteria organic matter may account for the depletion and variation ofδ¹⁵N and OC/N ratio caused by early diagenesis.