The Geochemical Character Of Carbon And Nitrogen In The Estuarine Sediments Of Changjiang And The Source Analysis Of The Organics

In order to study the organic source in the estuarine sediments of Changjiang, seven cores and surficial sediments were collected in the estuary and main geochemical indices were measured, including the total organic carbon (TOC), total nitrogen (TN), inorganic carbon and organic carbon isotope (δ13C). Spatial distribution of the grain size and the geochemical indices and the vertical variation of these indices were gotten. The C/N ratio and the δ13C were used to determine the organic sources. The analysis results show that:(1) Vertical profiles of the TOC, TIC, TN, C/N, and δ13C are smooth in the cores, showing that the estuarine sedimentary environment is relatively stable. Organic carbon content in the sediments is about0.3-0.8%, which is consistent with previous study. Compared with buried sediments, surficial sediments have much higher TOC, which may be associated with less decomposition of terrigenous organic matter in the surficial sediments. The spatial distribution of TOC shows the decreasing trend seaward and the large variation, going smaller downward, is present in the CJ14off the Jiuduan Sand. The vertical curves of TIC show less variation than the TOC but the value is within0.5-1.0%, higher than the TOC. The TN is low and less variable within0.04%-0.08%. The average C/N ratio of the cores is between7and16, showing that the organic matter in the Changjiang estuarine sediments is a mixture of terrigenous organic matter and marine algae. The C/N ratio goes down seaward in the surficial sediments and the vertical profiles are smooth except in the CJ01off Yuantuo Spur. The value of δ13C, within-26%o to-22%o, means the organic in Chanjiang estuary is a mixture of terrigenous organic matter and marine algae. The value of the δ13C in the CJ01, CJ05and CJ19is smaller suggesting more terrigenous input. The δ13C profile of the CJ19shows that the top36cm has higher value than the lower part. The increase of the δ13C in the top may be related to the introduction of Spartina alterniflora (a kind of C4plant) on the Nanhui tidal flat.(2) The sediments in the estuary consist of silt and clay, having little fine sand. At the CJ01, the finer grain size is related to the deposition of the offshore fine sediment transported by flood tide. The correlation analysis of these biogeochemical indices shows different correlation each other:The analysis shows that no correlation is present between the geochemical indices and the grain size except the finest core, the CJ08. In the CJ08, mean grain size has the positive correlation with the TOC and TN, with the coefficient of0.606and0.622. The TOC in the sediment has the negative correlation with the TIC, has mostly positive correlation with TN except the CJ14with the negative coefficient of-0.746, and has positive or negative correlation with the C/N. The TN has negative correlation with the C/N ratio. The513C ratio has good correlation with the TOC, TN and TIC in the CJ19, has medium correlation with the TOC in the CJ06, has no correlation with other indices in the other cores.(3) In the surficial sediments of the Changjiang estuary, the TOC is within the scale of0.17%-1.16%with the average of0.52%; the TN ranges from0.02%to0.08%with an average of0.057%and has higher value seaward; the C/N ratio is in the range of6.8to16with the average of9.6and decreases seaward; seabed materials consist of silt, clay and less sand. The TN and TOC show the good correlations with grain size. The different proportion of terrestrial organic sources might reflect the distance from the river mouth.(4) Both the C/N and the813C show that the organic matter is the mixture of terrigenous and marine sources in the Changjiang estuary. Although the two results is close, compared with the C/N ratio the δ13C is independent of grain size and biological disturbance. According to the813C, the proportion is57%in the CJ01,60%in the CJ05,55%in the CJ06,45%in the CJ08,66%in the CJ14,50%in the CJ16and60%in the CJ19. The proportion of the terrigenous organics in the estuary shows the decreasing trend seaward. This study suggests that the spatial distribution characteristics of carbon and nitrogen elements in the Changjiang esturay is not only controlled by the grain size, but also impacted by different contributions of organic matter input.