| The particulate organic carbon(POC) and dissolved organic carbon(DOC) in Pearl River estuary(PRE) were an important part of carbon cycle in this area, and studies on POC and DOC were also crucial to understand the carbon cycle for us. Many studies had shown that distribution of organic carbon(OC) in PRE was mainly controlled by river input, and the source of OC was predominantly terrigenous origin with only minor contribution from aquatic production in the 1980 s. In the late 1990 s, the extensive construction of reservoirs/dams in the Pearl River basin intercepted large amounts of sediment load and suspended particulate matter, resulting in decrease of OC in the PRE. Meanwhile, due to the fast development of industrial and agricultural activities and urbanization, the PRE received a very high load of anthropogenic nutrients in favor of phytoplankton growth.Thus the source of OC in the PRE might have shifted toward more contributions from aquatic productionin recent years. The PRE was located in a typical subtropical monsooon climate zoon, it’s still unclear whether the souces and properties of POC and DOC could be c hanged by the seasonal climate. Otherwise, the biomarker in suspended particulate organic matter and surface sediments was regarded as an effective tool to trace the source of organic matter, which could he lp us to evaluate accurate origin of organic matter. However, the studies focus on biomarkers of suspended particulate organic matter in PRE were insufficient, and what kind of sources of organic matter(OC) could be indicated by biomarkers and it’s environmental implication in this area deserved further investigation.In this paper, the POC, DOC, the OC of surface sediments, biomarkers of suspended surface particulate matter and surface sediments collected in PRE in different months were reported for their spatial and seasonal distribution. The isotope and biomarker tracer technique, together with hydrological data and water chemical parameters were applied to interpret sources of POC, DOC in different months and it’s biogeochemical process in PRE. We also discussed the indicating significance of biomarkers and it’s environmental implication in surface water particulate matter and surface sediments of PRE. The main conclusions were summed up as fo llowing:1. The sources of POC and DOC was different, the dominant source of POC was in situ primary production, and terrestrial organic matter was the main source of DOC. The source of POC and DOC exhibited seasonal changes in the PRE. In situ acquatic phytoplankton(>80%) was primary source for POC from freshwater to seawater in August, November and February; whereas soil-derived(~76%) OC was dominate for POC in freshwater and low salinity water(<15) in May, but faded away toward seawater. The contribution of terrestrial organic matter increased in August, which resulted in the raised of DOC concentration. DOC was more than POC in water of PRE in different month. The variation of POC and DOC concentration, and decrease of offset between δ13CPOC and δ13Cphyto, δ13CDOC and δ13Cphyto along salinity gradient was likely associated with biogeochemical change from net respiration in the upper estuary to net production in the lower and outer estuary. ―Fresh‖ alga utilized by microorganism was more efficiency than ―old‖ terrestrial organic matter, which might cause reduction of POC concentration being more than DOC(The concentration of POC and DOC declined by 67% and 39%, respectively, from freshwater to brackish water).2. The distribution of n-alkane, fatty acid and fatty alcohol in suspend particulate matter(SPM) suggested that acquatic phytoplankton was the primary source of these biomarkers(The contribution of phytoplankton to n-alkane was about 47%, to fatty acid was about 73%, to n-alkyl alcohol was about 75%). The input of terrigenous organic matter also had an effect on the distribution of long-chain n-alkane, n-saturate fatty acid and n-alkyl alcohol(The contribution of terrestrial OC to n-alkane was about 4%, to n-saturate fatty acid was about 3%, to n-alkyl alcohol was about 21%), and bacteria make a contribution to the fatty acid and fatty alcohol(The contribution of bacteria to fatty acid was about 24%, to n-alkyl alcohol was about 4%) in SPM of PRE. Spatially, fatty acid and fatty alcohol showed high values in freshwater, indicatived of an input of terrestrial organic matter controlled by precipitation and acquatic phytoplankton mediated by terrestrial nutrient. Sharp decreased of fatty acid and fatty alcohol occurred in salinity < 5(Total fatty acid declined by 28%, and Total fatty alcohol declined by 49% from freshwater to brackish water), was likely associated with degradation of organic matter, decreased of phyroplankon production, flocculattion and associated sedimentation in this zone. Fatty acid and fatty alcohol concentration increased in salinity of 15~30, which suggested net production of phytoplanton existed in this area. Seasonally, long chain n-alkane(nC25+nC27+nC29) concentration, carbon preference index(CPI24-34), terrestrial marine ratio(Ter/Mar) and O EP values of n-alkane increased, marine aqua tic ratio(Mar-aq) of n-alkane decresaed in May, indicated that input of terrestrial organic matter was increased in this month.3. The distribution of OC, n-alkane, fatty acid and fatty alcohol in surface sediments showed the signs from terrestrial organic matter in sediments was more obvious than in SPM, and signs from phytoplankton in sediments was diminished compare to SPM(Proportion of terrestrial OC in sediment OC increased by 37%, and phytoplankton declined by 37%; Proportion of terrestrial OC in sediment n-alkane increased by 22%, and phytoplankton declined by 4%; Proportion of terrestrial OC in sediment fatty acid increased by 5%, and phytoplankton declined by 20%; Proportion of terrestrial OC in sediment fatty alcohol increased by 48%, and phytoplankton declined by 46%). This might suggest that autogenous organic carbon(from acquatic phytoplankton) was more labile than terrigenous organic carbon in water and sediments, and it could be utilized preferentially by microorganism during sedimentation process of SPM and degradation process of sediments, while, more refractory terrigenous organic carbon could be sank and preserved in surface sediments.4. The iso GDGTs in surface water SPM and surface sediment s of PRE were mainly derived from Thaumarchaeota. Water temperature, NH4+ and dissolved oxygen(DO) concentration in water column have important impact on the distribution of iso GDGTs in the PRE. The unusually low TEX86-based temperatures of SPM and surface sediments in PRE might relate to the variation of archaeal community composition between the PRE and open SCS. Basing on TEX86 values of SPM collected from different seasons show a significantly positive linear correlation(r2 = 0.64, n = 43, p <0.001) with in situ surface water temperature in PRE, a new calibration relationship(formula) between TEX86 with surface water temperature was obtained in this area. The TEX86-based temperature of surface sediments in PRE calculated by this formula was close to the satellite-based annual mean surface water temperatures. The distribution of br GDGTs was affectd by mixture of terrestrial and in situ acquatic br GDGTs. Compared to SPM, the impact of acquatic br GDGTs to surface sediments was milder. This agreed with the result that the terrestrial signs of OC, n-alkane, fatty acid and fatty alcohol in surface sediments was more obvious than in SPM.Studying on the concentration and isotope of POC and DOC at different seasons in Pearl River-Lingdingyang-Marginal Sea convergence zone, which explored the soure and biogeochemistry process of OC. Researching on the composition and distribution of biomarkers of POM and surface sediments, which elaborated the indicating significance and it’s environmental implication for proxies of biomarkers. This work carried important scientific significance to further research on climate change, biogeochemistry processes and records in estuary and coast of Northern South China Sea. It provided basic data for carbon cycle, and offered references for the research of OC origin and paleoclimate reconstruction and paleoenvironment reconstruction of sedimentary records in this areas. |
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