Proof-of­‐Concept: Automated high­‐frequency measurements of PCO₂ and TCO₂ and real­‐time monitoring of the saturation state of calcium carbonate Public Deposited


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  • The rapid increase in atmospheric carbon dioxide (CO₂) over the last 250 years has led to the absorption of approximately 550 billion tons of anthropogenic CO₂ by the global ocean. This oceanic uptake of CO₂ has resulted in decreasing pH and alterations to carbonate chemistry, threatening many ecologically and economically important marine species. The majority of biological production takes place on highly dynamic coastal margins, which require instrumentation capable of high-frequency measurements. In practice, measurements of sufficient resolution often do not include all required analytical parameters necessary to constrain the carbonate chemistry in order to investigate biogeochemical processes relevant to ocean acidification. This report provides a proof-of-concept for the development of an instrument designed to make autonomous measurements of the partial pressure of CO₂ (PCO₂) and total CO₂ (TCO₂) in a continuous sample stream at high frequency, based on combination of two existing measurement techniques. The objective is to provide measurements sufficient to constrain the carbonate chemistry in ocean waters while capturing the variability seen over short timescales in estuaries and on coastal margins. By constraining the carbonate chemistry and performing real time calculations of the saturation state of calcium carbonate and other carbonate parameters, this instrument can be utilized as a monitoring tool for fisheries in need of high resolution time series carbonate data. In our combined system, PCO₂ is determined by measuring the infrared absorbance due to CO₂ in the re-circulated gaseous headspace of a shower-type equilibrator. For TCO₂ analysis, a low-flowing seawater sample stream is acidified and passed through a microporous membrane contactor. The evolved CO₂ diffuses into a high-flowing CO₂-free strip-gas stream and is measured by infrared absorbance in the same manner as the PCO₂ method. The results of laboratory testing indicated the instrument is able to resolve TCO₂ changes with 0.5% precision. The system responds to changes in TCO₂ with a time constant of 12 seconds. TCO₂ analysis of gravimetrically prepared liquid carbonate standards and discrete field samples that were cross-analyzed at the Hales lab at Oregon State University indicated the internal accuracy of the system is better than 1%. PCO₂ measurements made with the combined PCO₂/TCO₂ system were within 3.5% of measurements made on synchronously-collected discrete samples preserved with HgCl₂ and subsequently analyzed in the Hales lab. However, absolute accuracy has yet to be validated for both PCO2 and TCO₂ measurements. Field observations carried out at Whiskey Creek Shellfish Hatchery at Netarts Bay on the Oregon coast illustrate the instrument’s ability to capture the high variability seen in the bay. The maximum rates of change seen in carbonate conditions were 123μM hr⁻¹ for TCO₂ and 103μatm hr⁻¹ for PCO₂, corresponding to environmental changes of 2.4°C hr⁻¹ in temperature, and 2.6 psu hr⁻¹ in salinity. Our interpolation method developed to model alkalinity between synchronized TCO₂ and PCO₂ measurements predicts the saturation of calcium carbonate minerals with an internal precision of 1.6%. The error of the resultant high-resolution time series of calcium carbonate saturation is estimated to be less than 3.6%. I conclude that this instrument is capable of producing quality time series of carbonate data at sufficient resolution to be a powerful tool for coastal biogeochemical research and deepening our understanding of the impacts of ocean acidification.
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