An Pretreatment Procedure For Measuring Stable Isotopic Composition Of Seawater Dissolved Inorganic Carbon:its Application In The Bohai And Yellow Seas

In aquatic ecosystems,carbon(C)isotopic composition of dissolved inorganic carbon(813Cdic)can provide information about its sources and sinks,and in turn,the role of anthropogenic and natural inputs in our ecosystems.The Yellow Sea and The Bohai Seas,as the continental shelf marginal sea system in China,the changes in its biogeochemical characteristics profoundly reflect the impact of external ocean and atmospheric changes on the marine environment,and also reflect human activities on the environmental factors of changes in distribution characteristics.In this thesis,we explored the seawater δ13CDIC pretreatment method,and re-developed the Picarro G2121-i CO2 isotope analyzer which is based on Cavity Ring Down Spectroscopy(CRDS).Finally,we established a pretreatment method for seawater δ13CDIC based on the "gas method".Meanwhile,we designed and optimized an acid-extraction pretreatment procedure for precisely detecting δ13CDIC.The CO2 was generated by adding 1 mL concentrated phosphoric acid to the 30 mL seawater samples,and collected in a gas sampling bag under the purge of nitrogen.Then,the gas sample was injected to a Picarro G2121-i instrument for δ13CDIC determination.Whether CO2 gas converted from DIC in seawater can be effectively collected is the key to the accuracy of δ13CDIC determination.Therefore,the recovery rate in this study will be an important index.We first determined the volume of seawater and phosphoric acid.Then,we investigated the influence of stiring,carrier gas flow and purge time on the determination of δ13CDIC during pretreatment at room temperature,meanwhile we also measured the recovery.The pretreatment conditions were optimized as follows:30 mL seawater sample,1 mL phosphoric acid(85%(V/V)),Remove residual CO2 in carrier gas is required,Stirring operation is required,150 mL/min carrier gas flow rate,4 minute 20 seconds purge time,10 minute isotope determination.Under these conditions,the sample recovery rate of dissolved inorganic carbon(DIC)was nearly 100%,and the standard deviation of δ13CDIC determination was estimated to be less than 0.1‰.Appling this procedure,we collected DIC and δ13CDIC samples in the Yellow and Bohai seas from July to August in 2019,and obtained the corresponding water environment parameters.By measuring δ13CDIC of different water layers,we established δ13CDIC data set in the open sea area of the Yellow and Bohai seas in summer for the first time.The results are summarized below:In the Yellow sea,the δ13CDIC of surface water ranged from-1.3‰-0.5‰,with an average of-0.3±0.4‰.The δ13CDIC of bottom water ranged from-2.0‰-0.4‰,with an average of-1.4±0.4‰.In the Bohai sea,the δ13CDIC of surface water ranged from-1.2‰-1.0‰,with an average of 0.2±0.7‰.The δ13CDIC of bottom water ranged from-1.3‰--0.5‰.with an average of-1.0±0.3‰.In a whole,Most surface δ13CDICvalues ranged between-0.5‰ and 0.5‰,δ13CDIC in bottom waters ranged from-2.0‰ to-0.5‰.By combining with DIC data,we explained the differences of δ13CDIC in the surface and bottom waters.The high value of surface δ13CDIC was attributed to the photosynthesis During the flourishing period of phytoplankton growth in summer,the surface phytoplankton tended to convert 12CO2 in seawater into organic carbon,leaving 13C-enriched CO2 in seawater,which showed the positive value of δ13CDIC in surface seawater.However,for the bottom water,the mineralization and decomposition of the bottom organic matter resulted in the low δ13CDIC of the bottom water.We roughly quantified the relationship between DIC andδ13CDIC,and obtained the following equation:δ13CDIC=-0.01×DICmeas+19.64.This expression combines δ13CDIC with the seasonal acidification that often occurs in the Yellow seaThis study provides an efficient and reliable technique for the precision determination ofδ13CDIC in seawater.