Abstract
The ocean is a major sink for anthropogenic carbon dioxide (CO2), with the CO2 uptake causing changes to ocean chemistry. To monitor these changes and provide a chemical background for biological and biogeochemical studies, high quality partial pressure of CO2 (pCO2) sensors are required, with suitable accuracy and precision for ocean measurements. Optodes have the potential to measure in situ pCO2 without the need for wet chemicals or bulky gas equilibration chambers that are typically used in pCO2 systems. However, optodes are still in an early developmental stage compared to more established equilibrator-based pCO2 systems. In this study, we performed a laboratory-based characterization of a time-domain dual lifetime referencing pCO2 optode system. The pCO2 optode spot was illuminated with low intensity light (0.2 mA, 0.72 mW) to minimize spot photobleaching. The spot was calibrated using an experimental gas calibration rig prior to deployment, with a determined response time (τ63) of 50 s at 25°C. The pCO2 optode was deployed as an autonomous shipboard underway system across the high latitude North Atlantic Ocean with a resolution of ca.10 measurements per hour. The optode data was validated with a secondary shipboard equilibrator-based infrared pCO2 instrument, and pCO2 calculated from discrete samples of dissolved inorganic carbon and total alkalinity. Further verification of the pCO2 optode data was achieved using complimentary variables such as nutrients and dissolved oxygen. The shipboard precision of the pCO2 sensor was 9.5 μatm determined both from repeat measurements of certified reference materials and from the standard deviation of seawater measurements while on station. Finally, the optode deployment data was used to evaluate the physical and biogeochemical controls on pCO2.
Highlights
Atmospheric emissions of carbon dioxide (CO2) as a result of fossil fuel combustion and cement production averaged ca. 8.9 Gt C yr−1 (91% of total emissions) over the period between 2004 and 2013, while land use changes contributed an average of 0.9 Gt C yr−1 (9% of total emissions)
Dissolved inorganic carbon and total alkalinity Discrete seawater samples were collected from CTD casts and three times daily from the underway supply
An experimental pressure of CO2 (pCO2) optode was characterized in a laboratory before being deployed as a high resolution autonomous underway sensor
Summary
Atmospheric emissions of carbon dioxide (CO2) as a result of fossil fuel combustion and cement production averaged ca. 8.9 Gt C yr−1 (91% of total emissions) over the period between 2004 and 2013, while land use changes contributed an average of 0.9 Gt C yr−1 (9% of total emissions). The ocean is a significant sink for anthropogenic CO2, with an average global oceanic CO2 uptake of ca. The high latitude North Atlantic, for example, is a globally significant sink for CO2, both storing anthropogenic CO2 and acting as a conduit to the rest of the worlds’ oceans (Álvarez et al, 2003; Schuster et al, 2009). This is supported by data from the Bermuda Atlantic Time-series Study (BATS), which suggests that the North Atlantic mode waters formed at high latitudes have higher anthropogenic CO2 levels than those at lower latitudes (Gruber et al, 2002)
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