High-resolution measurement of Southern Ocean CO2 and O2/Ar by membrane inlet mass spectrometry

This paper evaluates the simultaneous measurement of dissolved gases (CO2 and O-2/Ar ratios) by membrane inlet mass spectrometry (MIMS) along the 180 degrees meridian in the Southern Ocean. The calibration of pCO(2) measurements by MIMS is reported for the first time using two independent methods of temperature correction. Multiple calibrations and method comparison exercises conducted in the Southern Ocean between New Zealand and the Ross Sea showed that the MIMS method provides pCO(2) measurements that are consistent with those obtained by standard techniques (i.e. headspace equilibrator equipped with a Li-Cor NDIR analyser). The overall MIMS accuracy compared to Li-Cor measurements was 0.8 mu atm. The O-2/Ar ratio measurements were calibrated with air-equilibrated seawater standards stored at constant temperature (0 +/- 1 degrees C). The reproducibility of the O-2/Ar standards was better than 0.07% during the 9 days of transect between New. Zealand and the Ross Sea. The high frequency, real-time measurements of dissolved gases with MIMS revealed significant small-scale heterogeneity in the distribution of pCO(2) and biologically-induced O-2 supersaturation (Delta O-2/Ar). North of 65 degrees S several prominent thermal fronts influenced CO2 concentrations, with biological factors also contributing to local variability. In contrast, the spatial variation of pCO(2) in the Ross Sea gyre was almost entirely attributed to the biological utilization of CO2, With only small temperature effects. This high productivity region showed a strong inverse relationship between pCO(2) and biologically-induced O-2 disequilibria (r(2) = 0.93). The daily sea air CO2 flux ranged from -0.2 mmol/m(2) in the Northern Sub-Antarctic Front to - 6.4 mmol/m(2) on the Ross Sea shelves where the maximum CO2 influx reached values up to -13.9 mmol/m(2). This suggests that the Southern Ocean water (south of 58 degrees S) acts as a seasonal sink for atmospheric CO2 at the time of our field study. (C) 2007 Elsevier B.V. All rights reserved.