High-precision δD and δ~(18)O measurement of water samples using gas equilibrium method and its application in tracing water mixing in nearshore areas of Beihai city

A procedure for measuring high-precision hydrogen and oxygen stable isotopes of water samples by gas equilibrium method was established based on authors' GV Isoprime Ⅱ isotope ratio mass spectrometry(IRMS) coupled with Dual Inlet System.The reproducibility is generally between 1.0‰ and 2.0‰ for δD and better than 0.10‰ for δ18O by repeated measurements with several international and national water stable isotopic standard references and a working standard of authors' laboratory.Using the procedure,δD and δ18O of natural water samples collected in nearshore areas of Beihai city in the northern Beibu Gulf during November 2007 were measured.The river water in the mouth of the Nanliujiang River and the alongshore surface seawater around Beihai city and Weizhou Island were sampled and used in the study,aiming to trace the interaction between natural water bodies in the nearshore areas.The results indicate that the δD and δ18O of the river water samples are very close to those of the winter rain water at a nearby Global Network of Isotopes in Precipitation(GNIP) station,suggesting the main source of the river water is precipitation.Similarly,the δD and δ18O of the water samples from the nearshore areas of Beihai city show a very good positive linear correlation(δD=6.43δ18O-4.79,r2=0.98).The slope of this regression line is significantly lower than those of the Global Meteoric Water Line and the Local Meteoric Water Line(around 8).The negative end of this regression line is overlapped with local precipitation,and the positive end seems to pass through the composition of seawater,suggesting the presence of mixing between precipitation and seawater.The mixing process is also supported by the positive linear correlation between the hydrogen and oxygen isotopes and salinity of the water samples.In addition to salinity,a moderately positive linear correlation(r2=0.56,N=8,p0.03) can also be found between δ18O and water temperature,indicating that evaporation may also affect water δ18O in the areas.Therefore,the δD and δ18O of water samples may help to trace the interaction between seawater and freshwater in nearshore areas.Moreover,the relationship between water H-O isotopic compositions and salinity may potentially be used in reconstructing paleo-salinity records in the areas.