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Papadimitriou, S, Loucaides S, Rerolle VMC, Kennedy P, Achterberg EP, Dickson AG, Mowlem M, Kennedy H.  2018.  The stoichiometric dissociation constants of carbonic acid in seawater brines from 298 to 267 K. Geochimica Et Cosmochimica Acta. 220:55-70.   10.1016/j.gca.2017.09.037   AbstractWebsite

The stoichiometric dissociation constants of carbonic acid (K-1C(*) and K-2C(*)) were determined by measurement of all four measurable parameters of the carbonate system (total alkalinity, total dissolved inorganic carbon, pH on the total proton scale, and CO2 fugacity) in natural seawater and seawater-derived brines, with a major ion composition equivalent to that of Reference Seawater, to practical salinity (S-P) 100 and from 25 similar to degrees C to the freezing point of these solutions and -6 degrees C temperature minimum. These values, reported in the total proton scale, provide the first such determinations at below-zero temperatures and for SP >50. The temperature (T, in Kelvin) and SP dependence of the current pK(1C)(*) and pK(2C)(*) (as negative common logarithms) within the salinity and temperature ranges of this study (33 <= S-P <= 100, -6 degrees C <= t <= 25 degrees C) is described by the following best-fit equations: pK(1C)(*) = -176.48 + 6.14528 S-P(0.5) - 0.127714 S-P + 7.396 x 10(-5) S-P(2) + (9914.37 - 622.886 S-P(0.5) P + 29.714 S-P) T-1 + (26.05129 - 0.666812 S-P(0:57)) lnT (sigma = 0.011, n = 62), and pK(2C)(*) = -323.52692 + 27.557655 S-P(0:5) + 0.154922 S-P - 2.48396 x 10(-4) S-P(2) + (14763.287 - 1014.819 S-P(0:5) - 14.35223 S-P) T-1 + (50.385807 - 4.4630415 S-P(0:5)) lnT (sigma = 0.020, n = 62). These functions are suitable for application to investigations of the carbonate system of internal sea ice brines with a conservative major ion composition relative to that of Reference Seawater and within the temperature and salinity ranges of this study. (C) 2017 Elsevier Ltd. All rights reserved.

Williams, NL, Juranek LW, Feely RA, Johnson KS, Sarmiento JL, Talley LD, Dickson AG, Gray AR, Wanninkhof R, Russell JL, Riser SC, Takeshita Y.  2017.  Calculating surface ocean pCO(2) from biogeochemical Argo floats equipped with pH: An uncertainty analysis. Global Biogeochemical Cycles. 31:591-604.   10.1002/2016gb005541   AbstractWebsite

More than 74 biogeochemical profiling floats that measure water column pH, oxygen, nitrate, fluorescence, and backscattering at 10 day intervals have been deployed throughout the Southern Ocean. Calculating the surface ocean partial pressure of carbon dioxide (pCO(2sw)) from float pH has uncertainty contributions from the pH sensor, the alkalinity estimate, and carbonate system equilibrium constants, resulting in a relative standard uncertainty in pCO(2sw) of 2.7% (or 11 mu atm at pCO(2sw) of 400 mu atm). The calculated pCO(2sw) from several floats spanning a range of oceanographic regimes are compared to existing climatologies. In some locations, such as the subantarctic zone, the float data closely match the climatologies, but in the polar Antarctic zone significantly higher pCO(2sw) are calculated in the wintertime implying a greater air-sea CO2 efflux estimate. Our results based on four representative floats suggest that despite their uncertainty relative to direct measurements, the float data can be used to improve estimates for air-sea carbon flux, as well as to increase knowledge of spatial, seasonal, and interannual variability in this flux. Plain Language Summary The Southern Ocean is a key player in the global flow of carbon, yet it is hard to reach, and there are relatively few measurements there, especially in winter. Measuring the amount of carbon dioxide gas in seawater is key to advancing our understanding of the Southern Ocean. More than 74 robotic floats that use sensors to measure seawater properties have been deployed throughout the Southern Ocean, and each has a lifetime of around 5 years. It is currently not possible to directly measure carbon dioxide gas from these floats; however, it is possible to estimate carbon dioxide from things that the float can measure, like pH, a measure of ocean acidity. Here surface ocean carbon dioxide is estimated from several floats and compared to two ship-based estimates. In some locations, the floats closely match the existing estimates, but in other locations the floats see significantly higher surface ocean carbon dioxide in the wintertime, reinforcing the idea that the Southern Ocean's role in the global carbon cycle needs a closer look. Our results show that despite not measuring carbon dioxide directly, these floats will help scientists learn a lot about the Southern Ocean's part in the global flow of carbon.

Papadimitriou, S, Loucaides S, Rerolle V, Achterberg EP, Dickson AG, Mowlem M, Kennedy H.  2016.  The measurement of pH in saline and hypersaline media at sub-zero temperatures: Characterization of Tris buffers. Marine Chemistry. 184:11-20.   10.1016/j.marchem.2016.06.002   AbstractWebsite

The pH on the total proton scale of the Tris-HCl buffer system (pH(Tris)) was characterized rigorously with the electrochemical Flamed cell in salinity (S) 35 synthetic seawater and S = 45-100 synthetic seawater-derived brines at 25 and 0 degrees C, as well as at the freezing point of the synthetic solutions (-1.93 degrees C at S = 35 to -6 degrees C at S = 100). The electrochemical characterization of the common equimolal Tris buffer [R-Tris = m(Tris)/m(Tris-H+) = 1.0, with m(Tris) = m(Tris-H+) = 0.04 mol kg(H2O)(-1) = molality of the conjugate acid-base pair of 2-amino-2-hydroxymethyl-1,3-propanediol (Tris)] yielded pH(Tris) values which increased with increasing salinity and decreasing temperature. The electrochemical characterization of a non-equimolal Tris buffer variant (R-Tris = 0.5, with m(Tris) = 0.02 mol kg(H2O)(-1) and MTris-H+ = 0.04 mol kg(H2O)(-1)) yielded pH(Tris) values that were consistently less alkaline by 03 pH unit than those of the equimolal Tris buffer. This is in agreement with the values derived from the stoichiometric equilibrium of the Tris-H+ dissociation reaction, described by the Henderson - Hasselbalch equation, pH(Tris) = pK(Tris)* + logR(Tris), with pK(Tris)* = stoichiometric equilibrium dissociation constant of Tris-H+, equivalent to equimolal pH(Tris). This consistency allows reliable use of other R-Tris variants of the Tris-HCl buffer system within the experimental conditions reported here. The results of this study will facilitate the pH measurement in saline and hypersaline systems at below-zero temperatures, such as sea ice brines. (C) 2016 The Authors. Published by Elsevier B.V.

Alin, SR, Feely RA, Dickson AG, Hernandez-Ayon JM, Juranek LW, Ohman MD, Goericke R.  2012.  Robust empirical relationships for estimating the carbonate system in the southern California Current System and application to CalCOFI hydrographic cruise data (2005-2011). Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007511   AbstractWebsite

The California Current System (CCS) is expected to experience the ecological impacts of ocean acidification (OA) earlier than most other ocean regions because coastal upwelling brings old, CO2-rich water relatively close to the surface ocean. Historical inorganic carbon measurements are scarce, so the progression of OA in the CCS is unknown. We used a multiple linear regression approach to generate empirical models using oxygen (O-2), temperature (T), salinity (S), and sigma theta (sigma(theta)) as proxy variables to reconstruct pH, carbonate saturation states, carbonate ion concentration ([CO32-]), dissolved inorganic carbon (DIC) concentration, and total alkalinity (TA) in the southern CCS. The calibration data included high-quality measurements of carbon, oxygen, and other hydrographic variables, collected during a cruise from British Columbia to Baja California in May-June 2007. All resulting empirical relationships were robust, with r(2) values >0.92 and low root mean square errors. Estimated and measured carbon chemistry matched very well for independent data sets from the CalCOFI and IMECOCAL programs. Reconstructed CCS pH and saturation states for 2005-2011 reveal a pronounced seasonal cycle and inter-annual variability in the upper water column. Deeper in the water column, conditions are stable throughout the annual cycle, with perennially low pH and saturation states. Over sub-decadal time scales, these empirical models provide a valuable tool for reconstructing carbonate chemistry related to ocean acidification where direct observations are limited. However, progressive increases in anthropogenic CO2 content of southern CCS water masses must be carefully addressed to apply the models over longer time scales.

Dickson, AG.  2010.  Standards for ocean measurements. Oceanography. 23:34-47. AbstractWebsite

The Intergovernmental Oceanographic Commission (IOC) has been involved for 50 years in promoting the close coordination needed to ensure comparability of oceanographic measurements. In particular, IOC played a key role in encouraging the development of chemical standards and reference materials for oceanic measurements. This paper briefly reviews this history, and also the early work of the author's laboratory in producing reference materials for oceanic carbon dioxide measurements. The success of the latter program in improving the state of the art of such measurements encouraged others to develop, produce, and distribute additional reference materials for dissolved organic carbon, trace metals, and nutrients. The widespread use of these various reference materials is playing a significant role in ensuring the comparability of ocean data from a variety of laboratories, thus enabling the data to be put to use in global studies.

Seidel, MP, DeGrandpre MD, Dickson AG.  2008.  A sensor for in situ indicator-based measurements of seawater pH. Marine Chemistry. 109:18-28.   10.1016/j.marchem.2007.11.013   AbstractWebsite

Indicator-based spectrophotometric pH methods are now proven and commonly used for analysis of ocean samples; however, no autonomous system for long-term in situ applications has been developed based on this method. We describe herein an autonomous indicator-based pH sensor for seawater applications adapted from a design originally developed for freshwater pH measurements (SAMI-pH). The new SAMI-pH uses a different pH indicator, flow cell design, detection system, and mixing configuration to improve upon the freshwater performance. A new method was also tested that utilizes an indicator concentration gradient in the sample to correct for the pH perturbation caused by the indicator. With these design changes, laboratory tests found the precision improved from +/- 0.004 to +/- 0.0007 and the accuracy improved from -0.0030 to +0.0017 based on comparisons with benchtop UV/Vis measurements. In situ testing of two SAMI-pH instruments was completed off the pier at Scripps Institution of Oceanography. The average pH offset between the two instruments over the 22 d deployment period was 0.0042 +/- 0.0126 (n =883), with the precision primarily regulated by large spatial and temporal variability at the site. The results demonstrate that the SAMI-pH can provide drift-free and precise pH measurements in adverse measurement conditions (extensive fouling and large tidal variability). With the current battery power (18 alkaline D-cells), the system can be deployed for periods up to similar to 2 months with a 0.5 h measurement frequency. (c) 2008 Elsevier B.V. All rights reserved.

Wolf-Gladrow, DA, Zeebe RE, Klaas C, Kortzinger A, Dickson AG.  2007.  Total alkalinity: The explicit conservative expression and its application to biogeochemical processes. Marine Chemistry. 106:287-300.   10.1016/j.marchem.2007.01.006   AbstractWebsite

Total alkalinity (TA) is one of the few measurable quantities that can be used together with other quantities to calculate CO2 concentrations of species of the carbonate system (CO2, HCO3-, CO32-, H+, OH-). TA and dissolved inorganic carbon (DIC) are conservative quantities with respect to mixing and changes in temperature and pressure and are, therefore, used in oceanic carbon cycle models. Thus it is important to understand the changes of TA due to various biogeochemical processes such as formation and remineralization of organic matter by microalgae, precipitation and dissolution of calcium carbonate. Unfortunately deriving such changes from the common expression for TA in terms of concentrations of non-conservative chemical species (HCO3-, CO32-, B(OH)(4)(-), H+, OH-, etc.) is rarely obvious. Here an expression for TA (TA(ec)) in terms of the total concentrations of certain major ions (Na+, Cl-, Ca2+ etc.) and the total concentrations of various acid-base species (total phosphate etc.) is derived from Dickson's original definition of TA under the constraint of electroneutrality. Changes of TA by various biogeochemical processes are easy to derive from this so-called explicit conservative expression for TA because each term in this expression is independent of changes of temperature or pressure within the ranges normally encountered in the ocean and obeys a linear mixing relation. Further, the constrains of electroneutrality for nutrient uptake by microalgae and photoautotrophs are discussed. A so-called nutrient-H+-compensation principle is proposed. This principle in combination with TA(cc) allows one to make predictions for changes in TA due to uptake of nutrients that are consistent with observations. A new prediction based on this principle is the change in TA due to nitrogen fixation followed by remineralization of organic matter and subsequent nitrification of ammonia which implies a significant sink of TA in tropical and subtropical regions where most of the nitrogen fixation takes place. (C) 2007 Published by Elsevier B.V.

Lee, K, Tong LT, Millero FJ, Sabine CL, Dickson AG, Goyet C, Park GH, Wanninkhof R, Feely RA, Key RM.  2006.  Global relationships of total alkalinity with salinity and temperature in surface waters of the world's oceans. Geophysical Research Letters. 33   10.1029/2006gl027207   AbstractWebsite

A simple function of sea surface salinity (SSS) and temperature (SST) in the form A(T) = a + b (SSS - 35) + c (SSS - 35)(2) + d (SST - 20) + e (SST - 20)(2) fits surface total alkalinity (A(T)) data for each of five oceanographic regimes within an area-weighted uncertainty of +/- 8.1 mu mol kg(-1) (1 sigma). Globally coherent surface A(T) data (n = 5,692) used to derive regional correlations of A(T) with SSS and SST were collected during the global carbon survey in the 1990s. Such region-specific A(T) algorithms presented herein enable the estimation of the global distribution of surface A(T) when observations of SSS and SST are available.

Lamb, MF, Sabine CL, Feely RA, Wanninkhof R, Key RM, Johnson GC, Millero FJ, Lee K, Peng TH, Kozyr A, Bullister JL, Greeley D, Byrne RH, Chipman DW, Dickson AG, Goyet C, Guenther PR, Ishii M, Johnson KM, Keeling CD, Ono T, Shitashima K, Tilbrook B, Takahashi T, Wallace DWR, Watanabe YW, Winn C, Wong CS.  2002.  Consistency and synthesis of Pacific Ocean CO2 survey data. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 49:21-58. AbstractWebsite

Between 1991 and 1999, carbon measurements were made on twenty-five WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different laboratories and four countries analyzed at least two of the four measurable ocean carbon parameters (DIC, TAlk, fCO(2), and pH) on almost all cruises. The goal of this work is to assess the quality of the Pacific carbon survey data and to make recommendations for generating a unified data set that is consistent between cruises. Several different lines of evidence were used to examine the consistency, including comparison of calibration techniques, results from certified reference material analyses, precision of at-sea replicate analyses, agreement between shipboard analyses and replicate shore based analyses, comparison of deep water values at locations where two or more cruises overlapped or crossed, consistency with other hydrographic parameters, and internal consistency with multiple carbon parameter measurements. With the adjustments proposed here, the data can be combined to generate a Pacific Ocean data set., with over 36,000 unique sample locations analyzed for at least two carbon parameters in most cases. The best data coverage was for DIC, which has an estimated overall accuracy of similar to3 mu mol kg(-1). TAlk, the second most common carbon parameter analyzed, had an estimated overall accuracy of similar to 5 mu mol kg(-1). To obtain additional details on this study, including detailed crossover plots and information on the availability of the compiled, adjusted data set, visit the Global Data Analysis Project web site at: (C) 2001 Elsevier Science Ltd. All rights reserved.