A high accuracy method for determining nitrogen, argon and oxygen in seawater

Abstract

An improved gas chromatographic system was constructed to analyze oceanic dissolved N 2, Ar and O 2 with a higher accuracy and shorter analytical time. To obtain a higher accuracy of N 2, Ar and O 2 measurements, the following was added to the system: (I) an air trapping system; (II) a N 2–CO 2 trapping system after the operation of the air trapping system; (III) an active carbon column system for separating N 2 and CO 2 completely and (IV) the introduction of automatic valves controlling most of the system. Compared to previous studies, the precision of the measurements of N 2, Ar and O 2 concentrations was higher at 0.04%, 0.05% and 0.02%, respectively, and our analytical time was shorter at 600 s. Using the improved analytical technique, concentrations of N 2 ( C N2, 561.69–611.81 μmol/kg) and Ar ( C Ar, 15.126–16.238 μmol/kg), saturation states of N 2 (ΔN 2, − 5.1–0.9%) and Ar (ΔAr, − 7.0 to − 1.1%) from 0 m to 3000 m depth in the western North Pacific were observed during March 2005. Based on these data, we propose a new concept for estimating the amount of bubble injection ( B). The total error in calculating B was estimated to be about 20%. We estimated B from 12 to 43 μmol/kg in this region using the observational values of N 2 and Ar. As each water mass had a significantly different value of B even with an error of 20%, it is possible to use it as an index of sea surface state for when each water mass is produced in the sea surface mixed layer. Moreover, based on our values of B, we estimated preformed dissolved oxygen (DO) ( C preDO, 309–332 μmol/kg) and the saturation state of C preDO (ΔpreDO, − 7.0 to − 1.2%) in this region. Thus, the difference between C preDO and DO content in the ocean interior may be a more useful index for biogenic organic decomposition in the ocean field compared to Apparent Oxygen Utilization (AOU). Until now, the estimation of oceanic uptake of anthropogenic CO 2 has used AOU as a major parameter. Therefore, it may be necessary to re-evaluate the oceanic uptake of anthropogenic CO 2 based on our new concept of B.