Sediment And Organic Carbon Burial In The Xin’anjiang Reservoir

River is an important conduit for transporting carbon from land to ocean.This conduit is not a neutral pipeline due to the emission of greenhouse gases to the atmosphere and carbon burial in sediments.In addition,the role of inland waters in the global carbon cycle has complicated by human activities including reservoir construction.Among inland waters,the artificial reservoir has the dual function of carbon burial and greenhouse gas emissions.This greatly affects the river carbon cycle.Artificial reservoirs are different from natural rivers and lakes because they usually have higher sedimentation rates and larger recharge coefficient.Therefore,reservoir sediments are important sites for carbon burial,and the quantitative study of reservoir carbon burial is of great significance for evaluating the role of reservoirs in regional and global carbon budget.In addition,the impact of reservoir on the global carbon cycle will gradually increase with the increase of the number of global reservoirs.However,the actual amount of reservoir carbon burial is highly uncertain due to the lack of comprehensive assessment of reservoir carbon burial.In this study,the combined method of seismic survey and sediment core sampling were used to study the carbon burial in a large reservoir(Xin’anjiang Reservoir)in eastern China.The Xin’anjiang Reservoir is an important type of hydropower development in the basin,and is very representative in terms of storage capacity,regulation type,water surface area and reservoir age.Combined with the methods of geochemistry and geophysics,this study investigated the underwater topography of the reservoir in detail,and estimated the total carbon burial rate in the reservoir.At the same time,the sources of nutrients and organic carbon in the sediments were analyzed,and the main factors controlling the carbon burial rates were discussed.The main results of this study are as follows:(1)The acoustic survey showed that the sediment distribution in the Xin’anjiang Reservoir was highly heterogeneous.Until the sampling year(2016),the sediment thickness of the reservoir varied from 0 to 1.9 m.The high sediment accumulation was apparent in the area with deep water depth.The sediments were mainly distributed in the middle of the reservoir,and the continuity was good while the distribution in the upper reaches of the reservoir was relatively small.This is because the variation of reservoir water level that leads to resuspension of sediment in the shallow area.Under the combined effect of downslope gravity and water currents,the sediment is easy to accumulate in the deeper area.The volume and mass of sediments in the Xin’anjiang Reservoir from 1960 to 2016 were estimated at1.44 × 108 m3 and 2.36 × 108 t.(2)The total organic carbon(TOC)content in the sediments of the Xin’anjiang Reservoir ranged from 0.2%to 5.97%with an average value of 1.87%.In the sediment profile of the upper reaches of the reservoir,the variation of TOC and inorganic carbon(IC)was similar to that of nutrients.The high TOC/TN ratio and δ 13C value(from-25.16 ‰ to-26.78 9‰)indicated that terrestrial input was the main source of organic matter in the upper reaches of the Xin’anjiang Reservoir.In the estuary region,the contents of BSi,IC,TP and TN showed the same variation pattern.The δ 13C values(-25.6 ‰ to-28.6 ‰)and low TOC/TN values indicated that primary productivity was the main contributor of organic matter in the sediments.Prior to 1997,TOC content was stable in the central area,and δ13C values further indicated that terrestrial input was the main organic Matter input in this period.Since 1997,the values of δ13C(-26.2 ‰ to-29.6 ‰)and TOC of organic matter indicated that organic matter was mainly derived from autochthonous organic carbon.(3)The sediment total phosphorus(TP)content in the upper reaches of the reservoir ranged from 0.13 g kg-1 to 1.03 g kg-1,and the TN value ranged from 0.03%to 0.2%.In the estuary region,the contents of TP and TN showed a similar variation.Prior to 1997,the nutrient content in this area was low and stable.Since 1997,the contents of TP and TN in the sediments have increased obviously from the bottom to the surface.The contents of TP and TN reached the maximum value of 1.6 g kg-1 and 0.4%,respectively.The TP and TN content were increased significantly to the surface of the sediment in the estuary and central regions of the reservoir,indicating that the increase of nutrient content in surface sediments in recent years was affected by new external sources.Due to the water environment protection oriented fishery policy,aquaculture may be the main reason for the increase of nutrient content in the reservoir area.(4)The maximum accumulation rate of TOC in the sediment of the Xin’anjiang Reservoir was 676.72 g C m-2 yr-1 with an average value of 34.01 g C m-2 yr-1(from the seismic survey).Since the reservoir construction,the estimated total organic carbon burial flux was 1.53 × 1010 g C yr-1.From the reservoir completion to 1967,the OC burial rate varied greatly,ranging from 15,78 g C m-2 yr-1 to 110.10 g C m-2 yr-1;from 1967 to 2016，the OC burial rate changed slightly,with an average of 40.4 g C m-2 yr-1.Since 2000,the burial rate of OC increased slightly with the increase of nutrient content and showed a similar variation trend with the content of autochthonous organic carbon.According to previous studies,cage culture has been developed in areas near the upstream of the main river,which may become the main reason of excessive nutrient input in these areas.In addition,higher terrestrial input and reservoir depth effect may also affect OC deposition.Higher water depth may also lead to the anaerobic condition at sediment-water interface,which shows higher OC deposition rate and lower mineralization rate.(5)There are many factors affecting the carbon burial rate of reservoir sediments,including TOC content,nutrient status,latitude,reservoir age,area,and sediment deposition rate.However,the results of this study showed that TOC content,nutrient status,and water surface areas are the key parameters affecting the OC burial in the sediment of the reservoir.Although large reservoirs are important sources of greenhouse gases to the atmosphere,they also have high carbon burial efficiency.Therefore,large reservoirs still play important roles in carbon sequestration.