Burial Fluxes And Source Apportionment Of Carbon In Culture Areas Of Sanggou Bay Over The Past 200 Years

Many studies on carbon burial flux in sediments have been carried out in the marginal seas, however, long-term carbon burial records and source apportionment are rarely reported in culture areas. Sources of various forms of carbon and their proportion characteristics also have no detailed analysis. With the concept of carbon sink fisheries being put forward, promoting fishery biological carbon sequestration and developing low-carbon economy is becoming a hot spot of concern. As the main carrier of carbon sink fisheries, shellfish and algae have an important influence on the marine carbon sequestration capacity. To better understand the characteristics of carbon burial and source in sediments of culture areas, four sediment cores from culture area of Sanggou Bay were set as the research objects. Total carbon(TC), total organic carbon(TOC), total inorganic carbon(TIC), Shell inorganic carbon(Shell-IC), total nitrogen(TN), grain-size, isotope values(δ13C,δ15 N), marine-derived organic carbon(Ca), carbon and nitrogen ratio(C/N) and other parameters were analyzed. Combined with sediment chronology, we analyzed the source of different carbon forms, assessed the carbon burial fluxes(BF) and preliminarily explored the factors that may affect the amount of carbon burial, and provided a scientific basis for researches on ocean carbon cycle. The main results were as follows:1. Interdecadal variation characteristics of sediment carbonTC contents of Sanggou Bay varied between 0.32% and 7.42% with an average of 68%; the average content of TIC and TOC were 1.13% and 0.5%, respectively. The vertical distributions of different forms of carbon in S1 and S2 core sediments could be divided into three stages during the last two hundred years. Different forms of carbon content were relatively steady before 1900s; however, along with the rise of the industrial revolution, natural small shellfish thrived due to the increase of nutrients in the inner bay, leading to a rapid increase of carbon content; and then, from 1960s, with the development of large-scale artificial farming, carbon content gradually reached a steady state again because of the growing effort of harvesting. S3 and S4 stations are located at the center of Sanggou Bay where is rarely influenced by the circulation and terrigenous input, leading to a stable sedimentary environment.There was no large amount of shell debris buried in sediments, vertical variation of various forms of carbon in each layer neither changed a lot. The vertical variation of carbon content in S4 station was slightly more significant than that in S3 station due to the stronger effect of current to S4 station which is closer to the outer bay.2. Source apportionment of sediment carbonOrganic matter in Sanggou Bay sediment was a mixture of land-derived and marine autogenic organic matter. We used C/N ratio to trace the variations of organic matter source, the average ratio C/N which indicated the mixed sources of organic materials was 11. The C/N ratio of S1 and S2 station were slightly higher than that of S3 and S4 station. S1 and S2 station in the north and south sides of the Bay were more influenced by terrestrial organic matter input. Because the sedimentary environment was stable, S3 andS4 station in the center of the bay were weakly influenced by terrigenous input. We also used the method of stable isotope(δ13C,δ15N) to analyze the source of organic carbon of S3 and S4 station. The results of the two methods lead to the same conclusion on Ca content, which improved the applicability of C/N ratios for indicating the source of organic matter. Shells in S1 and S2 core sediments, picked out with microscope, and indicated that Shell-IC was an important source of TIC burial in this region.3. Estimation of carbon burial fluxesThe average sedimentation rates of Sanggou Bay was 0.66 cm/a, significantly higher than some near coastal area, such as the Yellow Sea and the East China Sea(about 0.2 cm/a). TC burial flux(BFTc) of Sanggou Bay ranged from 31 to 895 g/(m2.a) with an average of 147.79 g/(m2.a). The averages of TOC burial flux(BFTOC) and TIC burial flux(BFTIC) were 44.64 g/(m2.a) and 103.15 g/(m2.a), respectively. BFTIC accounted for a large proportion of TC burial fluxes in both cores and the average BFTIC/BFTc ratios were about 70%, suggesting that TIC is a major part of buried carbon in Sanggou Bay; in S1 and S2 station the ratio can be even up to 90% during flourishing period(1880 a-1948 a) of natural small shellfish. The ratio of BFTIC /BFTc in Sanggou Bay was higher than that(about 15%) in the central Yellow Sea. The high ratio of TIC/TC in Sanggou Bay may result in great errors when using TOC or Ca to estimate the burial fluxes of carbon. BFTIC/BFTC ratios of S3 and S4 station(the center of the bay) were higher than that in S1 and S2 station(close to the land).Before the 1960s, the ratios of BFCa/BFTOC in S1 and S2 station were under 20%; after that time, with the expansion of farming activities, the ratios kept increasing; BFTIC/BFIC ratios of S3 and S4 core sediments were upward trend. In recent years, the ratios were about 50%, which may indicated that BFca was closely associated with aquaculture activities.4. The factors influencing carbon burialThe propagation and wreckage of shellfish and algae in culture areas dominated the organic carbon burial. The negative correlation between granularity and BFTOC indicated that the burial of organic carbon was accompanied by fine particulate matter. In warm and rainy environment, prosperous shellfish and algae and strong river input make the sediments rich in organic matter. BSi negatively correlated with BFTIC. As the main food of natural shellfish, BSi may affect shellfish production and thus BFTIC. When compared with Pacific Decadal Oscillation(PDO), BFTIC of S1 station, influenced greatly by the Yellow Sea Coastal Current, had a certain response to PDO in some specific periods. In addition, environmental factors such as temperature and precipitation had significant influence on BFTIC.High sedimentation rate and biological wreckage settlement distinguished the source and burial fluxes of carbon in Sanggou Bay from other coastal areas.