Comparativrve Research On Biogeochemistry Of Sedimentary Organic Carbon In The Changjiang And Mississippi Estuaries

River-dominated ocean margins (RiOMars) are land-ocean systems that are influenced significantly by major rivers with high sediment and/or water discharge rates which play an important role in transporting both dissolved and particulate matter from terrestrial environments to the coastal ocean. It has been estimated that over 80% of the total organic carbon preserved in marine sediments occurs in terrigenous-deltaic regions, primarily in RiOMar environments. They are natural"recorders"of global environmental change which impacted significantly by anthropogenic activities. Sedimentary pigments are useful biomarkers in environmental changes of estuarine systems. They can be utilized to reconstruct past phytoplankton communication and production, to reflect eutrophication historical changes resulting from nutrients input, and to indicate source and fate of organic matter.In this study, TOC, TN, molar C/N,δ(13)^C,δ(15)^N and sedimentary chloropigments in sediment cores from the Changjiang and Mississippi RiOMars were analyzed to show natural factors and anthropogenic activities influencing on transportation and burial of organic carbon, and eutrophication historical changes in the two RiOMars. The main results were shown below:(1) Using a suitable solvent for extracting pigments from sediment for high performance liquid chromatography (HPLC) analysis is critical for obtaining qualitative and quantitative estimates of phytoplanktonic and benthic algal biomass, as well as community composition. Five methodological factors (sample dehydration, extraction solvent, extraction duration, number of extractions, and ratio of solvent volume: sample weight) were studied using an L₉ (3⁴) orthogonal design in a sedimentary pigment extraction experiment on samples collected from the Changjiang large-river delta-front estuary (LDE), using HPLC analysis. The results showed that the optimal extraction method for sedimentary pigments should include freeze-drying samples prior to extraction. The effects of different factors on sedimentary pigment extraction were separated by the L₉ (3⁴) orthogonal design experiments and showed that the extraction solvent was most important, with extraction duration second most important, and numbers of extraction and ratio of solvent volume: sample weight were least important. The mixed solvent treatment comprised of acetone, methanol and water (80: 15: 5, by volume) was best for polar pigment extraction, with 100% acetone better for apolar pigments. For most pigments employed in this study (i.e., peridinin, fucoxanthin, alloxanthin, diatoxanthin, zeaxanthin, pheophytin-a andβ-carotene), 3 h was found to be enough time for extraction from these deltaic sediments. However, for chlorophyll-a, the most important pigment used for estimating algal biomass, 12 h was needed. A small amount of solvent (3 mL) with duplicate extractions obtained the greatest amount and diversity of pigments. Unfortunately, no one extraction method was found to be suitable for all pigments in sediments. The choice of extraction procedure should be made in accordance with the objective of each study, taking into consideration the properties of sediments and pigments in question.(2) Organic carbon in sediment cores of both the Changjiang and Mississippi RiOMars came from terrestrial (allochthonous) and marine (autochthonous) source. The contribution of terrestrial source in marine sedimentary organic carbon was about 1/3 in the Changjiang RiOMar estimated byδ(13)^C, while in the Mississippi RiOMar, it's unable to estimate this contribution accurately due to theδ(13)^C values of both marine source and terrestrial source were ambiguous. The correlations of each individual pheopigment (group) showed that, in the Changjiang RiOMar, both senescence of phytoplankton and grazing by zooplankton were key progresses for the degradation of chlorophyll-a, while in the Mississippi RiOMar, grazing was more important. This point was supported by the difference of physical dynamic conditions, such as current speed, in the sediment-water surface between the Changjiang and Mississippi RiOMars. All sites studied in this research were located in the primary depositional pathway of sediment and organic carbon. Sedimentation, transformation and burial of huge amounts of organic matters were occurred in estuarine sites. Even though organic matters deposited in the erosive area, they were not burial there but transported to other location. The inner shelf of the East China Sea and the Mississippi Canyon which are far away from the estuaries were the major burial area of organic matter. This was closely connected with the inherited nature of RiOMar environment.(3) Eutrophication is one of the foremost threats in RiOMars which were impacted by anthropogenic activities significantly. We used sedimentary chloropigments and other geochemical parameters records to study historical trends of eutrophication over the last decades in the Changjiang and Mississippi RiOMars. The results revealed that the sedimentary chloropigments in accumulation layer are suitable biomarkers of environmental status, i.e., eutrophication, changes in the studied Changjiang and Mississippi RiOMar ecosystems. Long-term plentiful terrigenous inputs of nutrients (most components were resulted from anthropogenic activities, i.e., fertilizer and manure) through large rivers were major origin of eutrophication. The historical trends of eutrophication effects were recorded well by stable pheopigment biomarkers. In the Changjiang RiOMar, eutrophication effects recorded were mainly the enhanced phytoplankton abundance associated with increasing anthropogenic nutrients input, while in the Mississippi RiOMar, they were not only the elevated algal biomass but also the higher preservation rate of organic matter varied by hypoxia the phenomenon associated with eutrophication. Since the nutrients inputs to the Changjiang RiOMar were equal to or higher than those to the Mississippi RiOMar, and they could be even much higher in forthcoming decades, it's important to slow down the eutrophication process by reducing the increasing rate of anthropogenic nutrient inputs to the RiOMar ecosystems and improve the utilization rate of fertilizer and manure.