Sources And Distribution Of Sedimentary Organic Matter Along Andong Salt Marsh,Hangzhou Bay

Salt marsh is a coastal wetland located between land and ocean.Salt marshes usually have a high rate of carbon accumulation and serve as carbon sink,and this carbon has been known as a component of the 'blue carbon' These wetland ecosystems provide an important pool to sequester CO2 from the atmosphere and preserve it for millennia.However,a substantial amount of greenhouse gases have been discovered to emit from the wetland,especially methane,so salt marshes can also be regarded as the natural sources of greenhouse gases.The dual functions of these wetland ecosystems as carbon sink and sources can greatly effect the global carbon cycle and local ecosystems.Hence,studying the sources of organic carbon(OC)and lignin in salt marsh would be helpful to understand its roles in mitigating the global carbon cycle and climate change.In this study,lignin,C/N ratio and stable carbon isotope are applied to study the origin,distribution and digenesis of organic matter in the surface sediment along the Andong salt marsh.In order to better understand the characteristics of organic matter and lignin along the salt marsh,sediments from the Qiantang River and the Changjiang Estuary were compared.Furthermore,two core samples from the Andong salt marsh allow us to determine the effect of anthropogenic activities and extreme weather conditions on the Hangzhou bay coastal wetland in the recent three decades.The OC contents and lignin parameters showed differences along the Andong salt marsh,Qiantang River and Changjiang Estuary,although lignins were generally composed of mixture of woody and non-woody angiosperms.The Qiantang River showed higher input of terrestrial OC while the Changjiang Estuary has a stronger marine OC signal among the three systems.Compared with the Changjiang Estuary and Qiantang River,the Andong salt marsh has moderate values of elemental composition(TOC,C/N)and lignin parameters.This is because most of the sediment along the Andong salt marsh originated from the Qiantang and Changjiang Rivers and adjacent coast.In addition,the TOC,particle size and C/N ratio along the salt marsh were affected by hydrogeology and tidal current of Hangzhou Bay,which also caused the coarse particles to sediment farther offshore and the area nearer to the Qiantang River having more input of terrestrial OC.Results from the core samples indicated that the input of terrestrial OC were higher in Andong salt marsh before 2004.Non-woody tissue increased dramatically after 1997 due to the 1997 high tide occurring in Hangzhou Bay which was caused by extreme weather as a result of Typhoon Winnie.After 2004,vertical distributions of elemental composition and lignin phenols became more consistent and stable due to the presence of dams.The Three Gorges Dam and 2006 drought decrease the sediment discharge from Changjiang River,Less sediment discharge along the Andong salt marsh decreased the contents of organic matter and lignin phenols in the higher tidal flat(core C2)while the lower tidal flat samples(Cl)did not show such trend because it would continue to receive sediment input from the river erosion.The construction of Hangzhou Bay Cross Sea Bridge would also cause increased terrestrial OC signal at Cl(near the bridge)from 2003 to 2007.