Atom Trap Trace Analysis Of ³⁹Ar And Its Application In Physical Oceanography

The noble gas radioisotope 39Ar has a half-life of 269 years.Its uniform distribution in the atmosphere and chemical inertness make it an ideal tracer in groundwater,mountain glacier and ocean circulation researches.The radiometric 39Ar dating range is from 50 to 1000 years,covering the dating gap between 3H(or CFCs,SF6,etc.)and 14C.Due to the extremely low isotopic abundance(8×10-16),the analysis of 39Ar in environmental samples poses great challenges to the modern analytical technology.Recently,the Atom Trap Trace Analysis method(ATTA)has made breakthroughs in 39Ar analysis.This laser based atom counting method has both ultrahigh sensitivity and ultra-high selectivity.Analysis on kg-size water or ice samples has been realized using the ATTA method.The advancement of ATTA has made the 39Ardating available to the earth and environmental science community.This dissertation focus on the 39Ar dating method based on ATTA and its application in physical oceanography.The first part of the dissertation is about the technical developments of 39Ar dating.We present the calibration of a newly-built Ar-ATTA system and the data processing method for the 39Ar analysis.The atmospheric 39Ar input history has been reconstructed based on a cosmic-ray record and the anthropogenic contributions of 39Ar in the atmosphere have been estimated based on measurements of Ar samples collected in the 1950s.Based on these studies an input correction curve for 39Ar-dating in the past 2500 years is given.The second part of the dissertation is about the application of the 39Ar-dating method in physical oceanography studies.A multi-tracer study at the Yap-Mariana Junction in the western Pacific using 85Kr,39Ar and 14C has been carried out.The vertical profiles of 85Kr and 39Ar are measured using the state-of-the-art Atom Trap Trace Analysis(ATTA)apparatus.Combined with the calculated 14C data we are able to study the abyssal mixing at the Yap-Mariana Junction with the Transit Time Distribution(TTD)method.An enhancement of diapycnal mixing near the ocean floor is revealed.This dissertation is organized as follows:Chapter 1 gives the background information about 39Ar-dating and its application.In Chapter 2,the calibration of the Ar-ATTA system and the data processing method for the 39Ar analysis are presented.In Chapter 3,the atmospheric input history of 39Ar is studied and an age correction curve for 39Ar-dating is given.In Chapter 4,the multi-tracer study at the Yap-Mariana trench is presented.