Oceanographic studies using dissolved gases as chemical tracers

This article reviews the author's hitherto progress on marine geochemical studies using the concentration, radioactivity, and stable isotope ratio of dissolved gases as chemical tracers in the ocean. Shipboard gas chromatography was developed for the simultaneous measurement of major dissolved gases (O2, N2, Ar and ΣCO2). A Carpenter-modified titration method was introduced for the improvement of the accuracy of O2 data, which played an important role in discovering and monitoring temporal variations in deep convection systems in the Japan Sea, an ideal miniature of global oceans. Radiocarbon (14C) measurements together with precise ΣCO2 and δ13C data enabled the author to clarify the timescale of the abyssal circulation of the Japan Sea, the characteristics of bottom waters in the Philippine Sea, and the invasion rate of bomb 14C from the atmosphere to the western Pacific seawater. Minor dissolved gases, 222Rn and CH4 in seawater, were also analyzed using the originally designed shipboard measurement systems. The activity of 222Rn (in excess over the 226Ra-supported portion) in bottom seawaters was applied for the estimation of CO2 flux from a submarine volcano in Kagoshima Bay, and for the mapping of vertical eddy diffusivity just above the bottom over a wide area of the northwestern Pacific Ocean, where the distribution and characteristics of benthic boundary layers were also researched. Data on CH4 in deep seawaters were accumulated at and around hydrothermally active sites or cold seepage zones associated with plate boundaries, to discover new active vents and elucidate the chemical characteristics of emanating fluids from the seafloor. (author)