Calculation And Analyses Of Distribution Of PCO₂ In The Surface Water And Estimation Of Submarine Groundwater Discharge Using Radium Isotopes In Laizhou Bay,China

Submarine groundwater discharge?SGD?refers to the flow of all water from the sea floor to the nearshore waters on the edge of the continental shelf,regardless of the composition and driving force of the water.SGD is an important part of the global water cycle,and it is also an important path for the transport of water and various chemicals from the land to the ocean.It has an important impact on the ecological environment of coastal water.At the same time,continental shelf is close to land,and can be easily impacted by anthropogenic activities.The oceans absorb about 30%to50%of the CO₂ emitted by human beings every year,keeping the earth at a temperature suitable for human survival.Temperature,biological activities and runoff make it complicated to estimate its CO2 sink/source mechanism.In this paper,surface water PCO₂ in Laizhou Bay in spring was calculated based on the measured DIC,it is found that PCO₂ increase from the coastal waters to central sea area.The values of PCO₂ range from 173 to 3875?atm,with an average of 1428?atm,difference of partial pressure of CO₂ between sea water and atmosphere ranges from-216 to 3485?atm.The whole Laizhou Bay seawater acts as a source of CO₂ in spring.Temperature has a great influence on the spatial distribution of PCO₂.The temperature is higher in nearshore zone so that the solubility of CO₂ is reduced.The temperature is lower in the central sea area so that the solubility of CO₂ there is increased.Owing to the influence of human activities,the degree of organic pollution is relatively high in the Bay.The spatial distribution of PCO₂ is greatly affected by organic degradation.There is strong organic degradation in the area with high PCO₂and low DO.At the same time,due to the affluent nutrient brought by runoff,in the southern coastal area where Chla and DO values are high there are strong phytoplankton activities.Because of the input of the Yellow River,there formed a high PCO₂ zone in the estuary which attenuates offshore seawards.The spatial distribution of PCO₂ in spring is mainly affected by organic degradation and phytoplankton activities.The gas exchange coefficients calculated from the three methods proposed by Liss&Merlivat,Wanninkhof,and McGillis et al.and average wind speed 10 metres above sea level are used to estimate the CO₂ flux of the Laizhou Bay.The averaged CO₂ flux of the three methods is 4.68mmol m^-2 d^-1.This means that overall,in spring,the whole bay seawater acts as a source of CO₂ and can release 0.34×10³tC to the atmosphere every day.The radium isotopes ²²³Ra and ²²⁸Ra are used as geochemical tracers to establish radium mass balance models of Laizhou Bay water in this study.The flushing time of Laizhou Bay water in spring is estimated to be 50.8 days,and the flux of submarine groundwater discharge is estimated to be 2.2×10⁸m³/d².4×10⁸m³/d.By establishing the mass balance of water and salt in Laizhou Bay,it is estimated that the submarine fresh groundwater discharge flux in Laizhou Bay is 0¹.54×10⁷ m³/d.