Using cavity ringdown spectroscopy for continuous monitoring of δ 13C(CO 2) and CO 2 in the surface ocean

Abstract

The role of the global surface ocean as a source and sink for atmospheric carbon dioxide and the flux strengths between the ocean and the atmosphere can be quantified by measuring the fugacity of CO 2 (fCO 2) as well as the dissolved inorganic carbon (DIC) concentration and its isotopic composition in surface seawater. In this work, the potential of continuous wave cavity ringdown spectroscopy (cw-CRDS) for autonomous underway measurements of fCO 2 and the stable carbon isotope ratio of DIC [δ 13C(DIC)] is explored. For the first time, by using a conventional air-sea equilibrator setup, both quantities were continuously and simultaneously recorded during a field deployment on two research cruises following meridional transects across the Atlantic Ocean (Bremerhaven, Germany-Punta Arenas, Chile). Data are compared against reference measurements by an established underway CO 2 monitoring system and isotope ratio mass spectrometric analysis of individual water samples. Agreement within ΔfCO 2 = 0.35 μatm for atmospheric and ΔfCO 2 = 2.5 μatm and Δδ 13C(DIC) = 0.33‰ for seawater measurements have been achieved. Whereas "calibration-free" fCO 2 monitoring is feasible, the measurement of accurate isotope ratios relies on running reference standards on a daily basis. Overall, the installed CRDS/equilibrator system was shown to be capable of reliable online monitoring of fCO 2, equilibrium δ 13C(CO 2), δ 13C(DIC), and pO 2 aboard moving research vessels, thus making possible corresponding measurements with high spatial and temporal resolution. © 2012, by the American Society of Limnology and Oceanography, Inc.