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2018
Carter, BR, Feely RA, Williams NL, Dickson AG, Fong MB, Takeshita Y.  2018.  Updated methods for global locally interpolated estimation of alkalinity, pH, and nitrate. Limnology and Oceanography-Methods. 16:119-131.   10.1002/lom3.10232   AbstractWebsite

We have taken advantage of the release of version 2 of the Global Data Analysis Project data product (Olsen et al. ) to refine the locally interpolated alkalinity regression (LIAR) code for global estimation of total titration alkalinity of seawater (A(T)), and to extend the method to also produce estimates of nitrate (N) and in situ pH (total scale). The updated MATLAB software and methods are distributed as Supporting Information for this article and referred to as LIAR version 2 (LIARv2), locally interpolated nitrate regression (LINR), and locally interpolated pH regression (LIPHR). Collectively they are referred to as locally interpolated regressions (LIRs). Relative to LIARv1, LIARv2 has an 18% lower average A(T) estimate root mean squared error (RMSE), improved uncertainty estimates, and fewer regions in which the method has little or no available training data. LIARv2, LINR, and LIPHR produce estimates globally with skill that is comparable to or better than regional alternatives used in their respective regions. LIPHR pH estimates have an optional adjustment to account for ongoing ocean acidification. We have used the improved uncertainty estimates to develop LIR functionality that selects the lowest-uncertainty estimate from among possible estimates. Current and future versions of LIR software will be available on GitHub.

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.

2017
McLaughlin, K, Nezlin NP, Weisberg SB, Dickson AG, Booth JA, Cash CL, Feit A, Gully JR, Johnson S, Latker A, Mengel MJ, Robertson GL, Steele A, Terriquez L.  2017.  An evaluation of potentiometric pH sensors in coastal monitoring applications. Limnology and Oceanography-Methods. 15:679-689.   10.1002/lom3.10191   AbstractWebsite

A wealth of historical coastal water pH data has been collected using potentiometric glass electrodes, but the accuracy and stability of these sensors is poorly understood. Here we compared pH measurements from five potentiometric sensors incorporated into profiling Sea-Bird instrument packages and compared them to spectrophotometric measurements on discrete bottle samples collected at two to three depths associated with each cast. Differences ranged from -0.509 to +0.479 with a mean difference of -0.055 pH units. Ninety-two percent of the measurements were within +/- 0.2 pH units, but 1% of the measurements had differences greater than 0.322. Sensor performance was affected by depth, but most of the difference was associated with calibration shortcomings. Sensor drift within a day was negligible; moreover, differences between bottle samples and electrode measurements within a sampling day were smaller than differences across days. Bootstrap analysis indicated that conducting a daily in situ calibration would reduce the mean difference to 0.002 pH units and increase the number of samples within a 0.2 pH unit error to 98%.

Okazaki, RR, Sutton AJ, Feely RA, Dickson AG, Alin SR, Sabine CL, Bunje PME, Virmani JI.  2017.  Evaluation of marine pH sensors under controlled and natural conditions for the Wendy Schmidt Ocean Health XPRIZE. Limnology and Oceanography-Methods. 15:586-600.   10.1002/lom3.10189   AbstractWebsite

The annual anthropogenic ocean carbon uptake of 2.6 +/- 0.5 Gt C is changing ocean composition (e.g., pH) at unprecedented rates, but our ability to track this trend effectively across various ocean ecosystems is limited by the availability of low-cost, high-quality autonomous pH sensors. The Wendy Schmidt Ocean Health XPRIZE was a year-long competition to address this scientific need by awarding $2 million to developers who could improve the performance and reduce the cost of pH sensors. Contestants' sensors were deployed in a series of trials designed to test their accuracy, repeatability, and stability in laboratory, coastal, and open-ocean settings. This report details the validation efforts behind the competition, which included designing the sensor evaluation trials, providing the conventional true pH values against which sensors were judged, and quantifying measurement uncertainty. Expanded uncertainty (coverage factor k=2, corresponding to 95% confidence) of validation measurements throughout the competition was approximately 0.01 pH units. A custom tank was designed for the coastal trials to expose the sensors to natural conditions, including temporal variability and biofouling, in a spatially homogenous environment. The competition prioritized the performance metrics of accuracy, repeatability, and stability over specific applications such as high-frequency measurements. Although the XPRIZE competition focused on pH sensors, it highlights considerations for testing other marine sensors and measuring seawater carbonate chemistry.

Loucaides, S, Rerolle VMC, Papadimitriou S, Kennedy H, Mowlem MC, Dickson AG, Gledhill M, Achterberg EP.  2017.  Characterization of meta-Cresol Purple for spectrophotometric pH measurements in saline and hypersaline media at sub-zero temperatures. Scientific Reports. 7   10.1038/s41598-017-02624-0   AbstractWebsite

Accurate pH measurements in polar waters and sea ice brines require pH indicator dyes characterized at near-zero and below-zero temperatures and high salinities. We present experimentally determined physical and chemical characteristics of purified meta-Cresol Purple (mCP) pH indicator dye suitable for pH measurements in seawater and conservative seawater-derived brines at salinities (S) between 35 and 100 and temperatures (T) between their freezing point and 298.15 K (25 degrees C). Within this temperature and salinity range, using purified mCP and a novel thermostated spectrophotometric device, the pH on the total scale (pH(T)) can be calculated from direct measurements of the absorbance ratio R of the dye in natural samples as pH(T) = -log(k(2)(T)e(2)) + log(R - e(1)/1 - R-e3/(e2)) Based on the mCP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and - log( k(2)(T)e(2)) of purified mCP is described by the following functions: e(1) = -0.004363 + 3.598 x 10(-5)T, e(3)/e(2) = -0.016224 + 2.42851 x 10(-4)T + 5.05663 x 10(-5)(S-35), and -log( k(2)(T)e(2)) = - 319.8369 + 0.688159 S -0.00018374 S-2 + ( 10508.724 - 32.9599 S + 0.059082S(2)) T-1 + ( 55.54253 - 0.101639 S) ln T -0.08112151T. This work takes the characterisation of mCP beyond the currently available ranges of 278.15 K <= T <= 308.15 K and 20 <= S <= 40 in natural seawater, thereby allowing high quality pHT measurements in polar systems.

McLaughlin, K, Dickson A, Weisberg SB, Coale K, Elrod V, Hunter C, Johnson KS, Kram S, Kudela R, Martz T, Negrey K, Passow U, Shaughnessy F, Smith JE, Tadesse D, Washburn L, Weis KR.  2017.  An evaluation of ISFET sensors for coastal pH monitoring applications. Regional Studies in Marine Science. 12:11-18.   10.1016/j.rsma.2017.02.008   AbstractWebsite

The accuracy and precision of ion sensitive field effect transistor (ISFET) pH sensors have been well documented, but primarily by ocean chemistry specialists employing the technology at single locations. Here we examine their performance in a network context through comparison to discrete measurements of pH, using different configurations of the Honeywell DuraFET pH sensor deployed in six coastal settings by operators with a range of experience. Experience of the operator had the largest effect on performance. The average difference between discrete and ISFET pH was 0.005 pH units, but ranged from -0.030 to 0.083 among operators, with more experienced operators within +/- 0.02 pH units of the discrete measurement. In addition, experienced operators achieved a narrower range of variance in difference between discrete bottle measurements and ISFET sensor readings compared to novice operators and novice operators had a higher proportion of data failing quality control screening. There were no statistically significant differences in data uncertainty associated with sensor manufacturer or deployment environment (pier-mounted, flowthrough system, and buoy-mounted). The variation we observed among operators highlights the necessity of best practices and training when instruments are to be used in a network where comparison across data streams is desired. However, while opportunities remain for improving the performance of the ISFET sensors when deployed by less experienced operators, the uncertainty associated with their deployment and validation was several-fold less than the observed natural temporal variability in pH, demonstrating the utility of these sensors in tracking local changes in acidification. (C) 2017 Elsevier B.V. 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.

Carter, BR, Feely RA, Mecking S, Cross JN, Macdonald AM, Siedlecki SA, Talley LD, Sabine CL, Millero FJ, Swift JH, Dickson AG, Rodgers KB.  2017.  Two decades of Pacific anthropogenic carbon storage and ocean acidification along Global Ocean Ship-lebased Hydrographic Investigations Program sections P16 and P02. Global Biogeochemical Cycles. 31:306-327.   10.1002/2016gb005485   AbstractWebsite

A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (C-anth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30 degrees N, and P16 is a meridional section crossing the North and South Pacific at similar to 150 degrees W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to similar to 1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s-2000s) and recent (2000s-2010s) decades along P02, at rates of 0.53 +/- 0.11 and 0.46 +/- 0.11 mol Cm-2 a(-1), respectively. By contrast, Canth storage accelerated from 0.29 +/- 0.10 to 0.45 +/- 0.11 mol Cm-2 a(-1) along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60 degrees S and 60 degrees N of 6.1 +/- 1.5 PgC decade(-1) in the earlier decade and 8.8 +/- 2.2 PgC decade(-1) in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by -0.08 to -0.14 and aragonite saturation state by -0.57 to -0.82.

Takeshita, Y, Martz TR, Coletti LJ, Dickson AG, Jannasch HW, Johnson KS.  2017.  The effects of pressure on pH of Tris buffer in synthetic seawater. Marine Chemistry. 188:1-5.   10.1016/j.marchem.2016.11.002   AbstractWebsite

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (Delta V) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The Delta V of Tris in seawater media of salinity 35 (Delta V-Tris*) was determined between 10 and 30 degrees C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined Delta V-Tris* were fitted to the equation Delta V-Tris*= 4528 +0.04912t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9 cm(-3) mol(-1). Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200 bar, and can be expressed as: DpH(Tris) = -(4.528 + 0.04912t)p/ln(10)RT. where T is temperature in Kelvin, R is the universal gas constant (83.145 cm(3) bar K-1 mol(-1)), and Pis gauge pressure in bar. On average, pH of Tris buffer changes by approximately -0.02 at 200 bar. (C) 2016 Elsevier B.V. All rights reserved.

2016
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.

2015
Williams, NL, Feely RA, Sabine CL, Dickson AG, Swift JH, Talley LD, Russell JL.  2015.  Quantifying anthropogenic carbon inventory changes in the Pacific sector of the Southern Ocean. Marine Chemistry. 174:147-160.   10.1016/j.marchem.2015.06.015   AbstractWebsite

The Southern Ocean plays a major role in mediating the uptake, transport, and long-term storage of anthropogenic carbon dioxide (CO2) into the deep ocean. Examining the magnitude and spatial distribution of this oceanic carbon uptake is critical to understanding how the earth's carbon system will react to continued increases in this greenhouse gas. Here, we use the extended multiple linear regression technique to quantify the total and anthropogenic change in dissolved inorganic carbon (DIC) along the S04P and P16S CLIVAR/U.S. Global Ocean Carbon and Repeat Hydrography Program lines south of 67 degrees S in the Pacific sector of the Southern Ocean between 1992 and 2011 using discrete bottle measurements from repeat occupations. Along the S04P section, which is located in the seasonal sea ice zone south of the Antarctic Circumpolar Current in the Pacific, the anthropogenic component of the DIC increase from 1992 to 2011 is mostly found in the Antarctic Surface Water (AASW, upper 100 m), while the increase in DIC below the mixed layer in the Circumpolar Deep Water can be primarily attributed to either a slowdown in circulation or decreased ventilation of deeper, high CO2 waters. In the AASW we calculate an anthropogenic increase in DIC of 12-18 mu mol kg(-1) and an average storage rate of anthropogenic CO2 of 0.10 +/- 0.02 mol m(-2) yr(-1) for this region compared to a global average of 0.5 +/- 0.2 mol m(-2) yr(-1). In surface waters this anthropogenic CO2 uptake results in an average pH decrease of 0.0022 +/- 0.0004 pH units yr(-1), a 0.47 +/- 0.10% yr(-1) decrease in the saturation state of aragonite (Omega(Aragonite)) and a 2.0 +/- 0.7 m yr(-1) shoaling of the aragonite saturation horizons (calculated for the Omega(Aragonite) = 1.3 contour). (C) 2015 Published by Elsevier B.V.

McLaughlin, K, Weisberg SB, Dickson AG, Hofmann GE, Newton JA, Aseltine-Neilson D, Barton A, Cudd S, Feely RA, Jefferds IW, Jewett EB, King T, Langdon CJ, McAfee S, Pleschner-Steele D, Steele B.  2015.  Core principles of the California Current Acidification Network: Linking chemistry, physics, and ecological effects. Oceanography. 28:160-169.   10.5670/oceanog.2015.39   AbstractWebsite

Numerous monitoring efforts are underway to improve understanding of ocean acidification and its impacts on coastal environments, but there is a need to develop a coordinated approach that facilitates spatial and temporal comparisons of drivers and responses on a regional scale. Toward that goal, the California Current Acidification Network (C-CAN) held a series of workshops to develop a set of core principles for facilitating integration of ocean acidification monitoring efforts on the US West Coast. The recommended core principles include: (1) monitoring measurements should facilitate determination of aragonite saturation state (Omega(arag)) as the common currency of comparison, allowing a complete description of the inorganic carbon system; (2) maximum uncertainty of +/- 0.2 in the calculation of Omega(arag) is required to adequately link changes in ocean chemistry to changes in ecosystem function; (3) inclusion of a variety of monitoring platforms and levels of effort in the network will insure collection of high-frequency temporal data at fixed locations as well as spatial mapping across locations; (4) physical and chemical oceanographic monitoring should be linked with biological monitoring; and (5) the monitoring network should share data and make it accessible to a broad audience.

Bockmon, EE, Dickson AG.  2015.  An inter-laboratory comparison assessing the quality of seawater carbon dioxide measurements. Marine Chemistry. 171:36-43.   10.1016/j.marchem.2015.02.002   AbstractWebsite

Seawater CO2 measurements are being made with increasing frequency as interest grows in the ocean's response to changing atmospheric CO2 levels and to climate change. The ultimate usefulness of these measurements depends on the data quality and consistency. An inter-laboratory comparison was undertaken to help evaluate and understand the current reliability of seawater CO2 measurements. Two seawater test samples of different CO2 content were prepared according to the usual method for the creation of seawater reference materials in the Dickson Laboratory at Scripps Institution of Oceanography. These two test samples were distributed in duplicate to more than 60 laboratories around the world. The laboratories returned their measurement results for one or more of the following parameters: total alkalinity (A(T)), total dissolved inorganic carbon (C-T), and pH, together with information about the methods used and the expected uncertainty of the measurements. The majority of laboratories reported A(T) and C-T values for all their measurements that were within 10 mu mol kg(-1) of the assigned values (i.e. within +/- 0.5%), however few achieved results within 2 mu mol kg(-1) (i.e. within +/- 0.1%), especially for C-T. Results for the analysis of pH were quite scattered, with little suggestion of a consensus value. The high-CO2 test sample produced results for both C-T and pH that suggested in many cases that CO2 was lost during analysis of these parameters. This study thus documents the current quality of seawater CO2 measurements in the various participating laboratories, and helps provide a better understanding of the likely magnitude of uncertainties in these measurements within the marine science community at the present time. Further improvements will necessarily hinge on adoption of an improved level of training in both measurement technique and of suitable quality control procedures for these measurements. (C) 2015 The Authors. Published by Elsevier B.V.

2014
Takeshita, Y, Martz TR, Johnson KS, Dickson AG.  2014.  Characterization of an ion sensitive field effect transistor and chloride ion selective electrodes for pH measurements in seawater. Analytical Chemistry. 86:11189-11195.: American Chemical Society   10.1021/ac502631z   AbstractWebsite

Characterization of several potentiometric cells without a liquid junction has been carried out in universal buffer, aqueous HCl, and artificial seawater media. The electrodes studied include Ion Sensitive Field Effect Transistor (ISFET) pH electrodes, and Chloride-Ion Selective Electrodes (Cl-ISE) directly exposed to the solution. These electrodes were compared directly to the conventional hydrogen electrode and silver?silver chloride electrode in order to report the degree to which they obey ideal Nernstian laws. These data provide a foundation for operating the ISFET|Cl-ISE pair in seawater as a pH sensor. In order to obtain the highest quality pH measurements from this sensor, its response to changes in pH and salinity must be properly characterized. Our results indicate near-ideal Nernstian response for both electrodes over a wide range of pH (2?12) and Cl? molality (0.01?1). We conclude that the error due to sub-Nernstian response of the cell ISFET|seawater|Cl-ISE over the range of seawater pH and salinity is negligible (<0.0001 pH). The cross sensitivity of the Cl-ISE to Br? does not seem to be a significant source of error (<0.003 pH) in seawater media in the salinity range 20?35.

Thompson, RW, Dickson AG, Kahng SE, Winn CD.  2014.  Nearshore carbonate dissolution in the Hawaiian Archipelago? Aquatic Geochemistry. 20:467-481.   10.1007/s10498-014-9230-2   AbstractWebsite

Inorganic carbon measurements made in the late 1980s suggest that alkalinity in the waters surrounding the Hawaiian Archipelago is elevated relative to the oligotrophic waters of the North Pacific. These observations have been interpreted as evidence for a "halo" of elevated carbonate saturation state produced by the dissolution of highly soluble magnesium calcites and aragonite on the island platform or in the water column surrounding the islands. If present, this "halo" has implications for air-sea carbon dioxide exchange in Hawaiian waters and may impact the response of coral reef communities to the acidification of the surface waters of the global ocean. The purpose of this study was to assess the magnitude and extent of the elevated calcium carbonate saturation state observed on previous expeditions to this region. Transects were conducted near several atolls in the Northwestern Hawaiian Islands from shallow water adjacent to the forereef to the open ocean 15 km from the island. Hydrographic profiles were collected at each station, and discrete water samples were collected for the measurement of carbon system parameters necessary to compute calcium carbonate saturation state. Our data were compared with observations made at the Hawaii Ocean Time-series site at Station ALOHA and with hydrographic data collected on the WOCE lines in the North Pacific around the archipelago. We did not detect a carbonate dissolution halo around the islands. We conclude that the previously observed halo was probably an analytical artifact, or possibly a result of extreme variability in carbon chemistry surrounding the islands.

Carter, BR, Talley LD, Dickson AG.  2014.  Mixing and remineralization in waters detrained from the surface into Subantarctic Mode Water and Antarctic Intermediate Water in the southeastern Pacific. Journal of Geophysical Research-Oceans. 119:4001-4028.   10.1002/2013jc009355   AbstractWebsite

A hydrographic data set collected in the region and season of Subantarctic Mode Water and Antarctic Intermediate Water (SAMW and AAIW) formation in the southeastern Pacific allows us to estimate the preformed properties of surface water detrained into these water masses from deep mixed layers north of the Subantarctic Front and Antarctic Surface Water south of the front. Using 10 measured seawater properties, we estimate: the fractions of SAMW/AAIW that originate as surface source waters, as well as fractions that mix into these water masses from subtropical thermocline water above and Upper Circumpolar Deep Water below the subducted SAMW/AAIW; ages associated with the detrained surface water; and remineralization and dissolution rates and ratios. The mixing patterns imply that cabbeling can account for similar to 0.005-0.03 kg m(-3) of additional density in AAIW, and similar to 0-0.02 kg m(-3) in SAMW. We estimate a shallow depth (similar to 300-700 m, above the aragonite saturation horizon) calcium carbonate dissolution rate of 0.4 +/- 0.2 mmol CaCO3 kg(-1) yr(-1), a phosphate remineralization rate of 0.031 +/- 0.009 mu mol P kg(-1) yr(-1), and remineralization ratios of P:N:-O-2:C-org of 1:(15.5 +/- 0.6):(143 +/- 10):(104 +/- 22) for SAMW/AAIW. Our shallow depth calcium carbonate dissolution rate is comparable to previous estimates for our region. Our -O-2:P ratio is smaller than many global averages. Our model suggests neglecting diapycnal mixing of preformed phosphate has likely biased previous estimates of -O-2:P and C-org:P high, but that the C-org:P ratio bias may have been counteracted by a second bias in previous studies from neglecting anthropogenic carbon gradients.

Bockmon, EE, Dickson AG.  2014.  A seawater filtration method suitable for total dissolved inorganic carbon and pH analyses. Limnology and Oceanography-Methods. 12:191-195.   10.4319/lom.2014.12.191   AbstractWebsite

High biomass and heavy particle loads may interfere with carbonate chemistry analyses of samples from experimental aquaria and cultures used to investigate the impact of ocean acidification on organisms, as well as from biologically productive coastal regions. For such samples, a filtration method is needed that does not change the dissolved CO2 content, and consequently does not alter the total dissolved inorganic carbon and pH of the sample. Here, a filtration method is presented in which the sample seawater is pumped by a peristaltic pump through a replaceable 0.45 mu m filter in a 50 mm polycarbonate filter holder and then into the sample bottle. Seawater samples of known carbonate composition were filtered to confirm that the filtration method did not alter the CO2 content, and compromise the subsequent sample analysis and data usefulness. Seawater samples with added phytoplankton concentrations in the range of 1-5 x 10(5) cells mL(-1) were also filtered successfully. Finally, seawater with added biogenic CaCO3 was tested to prove that the method could successfully filter out such particles and produce dependable results. This approach will help to ensure more consistent and reliable carbonate chemistry measurements in coastal environments and from ocean acidification aquaria and cultures, by providing a well-tested method for sample filtration.

2013
Bockmon, EE, Frieder CA, Navarro MO, White-Kershek LA, Dickson AG.  2013.  Technical note: Controlled experimental aquarium system for multi-stressor investigation of carbonate chemistry, oxygen saturation, and temperature. Biogeosciences. 10:5967-5975.   10.5194/bg-10-5967-2013   AbstractWebsite

As the field of ocean acidification has grown, researchers have increasingly turned to laboratory experiments to understand the impacts of increased CO2 on marine organisms. However, other changes such as ocean warming and deoxygenation are occurring concurrently with the increasing CO2 concentrations, complicating the understanding of the impacts of anthropogenic changes on organisms. This experimental aquarium design allows for independent regulation of CO2 concentration, O-2 levels, and temperature in a controlled environment to study the impacts of multiple stressors. The system has the flexibility for a wide range of treatment chemistry, seawater volumes, and study organisms. Control of the seawater chemistry is achieved by equilibration of a chosen gas mixture with seawater using a Liqui-Cel (R) membrane contactor. Included as examples, two experiments performed using the system have shown control of CO2 at values between approximately 500 and 1400 mu atm and O-2 at values from 80 to 240 mu mol kg(-1). Temperature has been maintained to 0.5 degrees C or better in the range of 10-17 degrees C. On a weeklong timescale, the system has achieved variability in pH of less than 0.007 pH units and in oxygen concentration of less than 3.5 mu mol kg(-1). Longer experiments, over a month in duration, have been completed with control to better than 0.08 pH units and 13 mu mol kg(-1) O-2. The ability to study the impacts of multiple stressors in the laboratory simultaneously, as well as independently, will be an important part of understanding the response of marine organisms to a high-CO2 world.

Carter, BR, Radich JA, Doyle HL, Dickson AG.  2013.  An automated system for spectrophotometric seawater pH measurements. Limnology and Oceanography-Methods. 11:16-27.   10.4319/lom.2013.11.16   AbstractWebsite

Spectrophotometric pH measurements stand to benefit greatly from the consistency and speed made possible through automation. Here we describe a simple, fast, and precise automated spectrophotometric pH measurement system for seawater samples. The system requires 4 min per analysis, consumes 60 mL seawater from a sample bottle, and requires little operator interaction to obtain repeatability comparable with the best results published with other techniques (+/- 0.0004). The system and the suggested sample handling methods are assessed using over 5000 at-sea measurements obtained during a hydrographic cruise in the Indian Ocean. We estimate the overall measurement uncertainty of the existing, pre-2011, body of at-sea spectrophotometric pH measurements-made using these methods or otherwise-to currently be in the range of 0.01 to 0.02 pH units. However, a new approach for using purified dyes at a range of temperatures and salinities (Liu et al. 2011) stands to greatly reduce this uncertainty for future spectrophotometric pH measurements: our assessment suggests that the overall uncertainty should improve to similar to 0.005 pH units if dye impurities and the indicator's temperature and salinity sensitivity are adequately addressed. Any such improvement in measurement accuracy may provide a basis from which to determine adjustments appropriate for the existing body of spectrophotometric pH measurements made using commercially available (and impure) dyes.

2012
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.

Feely, RA, Sabine CL, Byrne RH, Millero FJ, Dickson AG, Wanninkhof R, Murata A, Miller LA, Greeley D.  2012.  Decadal changes in the aragonite and calcite saturation state of the Pacific Ocean. Global Biogeochemical Cycles. 26   10.1029/2011gb004157   AbstractWebsite

Based on measurements from the WOCE/JGOFS global CO2 survey, the CLIVAR/CO2 Repeat Hydrography Program and the Canadian Line P survey, we have observed an average decrease of 0.34% yr(-1) in the saturation state of surface seawater in the Pacific Ocean with respect to aragonite and calcite. The upward migrations of the aragonite and calcite saturation horizons, averaging about 1 to 2 m yr(-1), are the direct result of the uptake of anthropogenic CO2 by the oceans and regional changes in circulation and biogeochemical processes. The shoaling of the saturation horizon is regionally variable, with more rapid shoaling in the South Pacific where there is a larger uptake of anthropogenic CO2. In some locations, particularly in the North Pacific Subtropical Gyre and in the California Current, the decadal changes in circulation can be the dominant factor in controlling the migration of the saturation horizon. If CO2 emissions continue as projected over the rest of this century, the resulting changes in the marine carbonate system would mean that many coral reef systems in the Pacific would no longer be able to sustain a sufficiently high rate of calcification to maintain the viability of these ecosystems as a whole, and these changes perhaps could seriously impact the thousands of marine species that depend on them for survival.

Bates, NR, Best MHP, Neely K, Garley R, Dickson AG, Johnson RJ.  2012.  Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean. Biogeosciences. 9:2509-2522.   10.5194/bg-9-2509-2012   AbstractWebsite

Fossil fuel use, cement manufacture and land-use changes are the primary sources of anthropogenic carbon dioxide (CO2) to the atmosphere, with the ocean absorbing approximately 30% (Sabine et al., 2004). Ocean uptake and chemical equilibration of anthropogenic CO2 with seawater results in a gradual reduction in seawater pH and saturation states (Omega) for calcium carbonate (CaCO3) minerals in a process termed ocean acidification. Assessing the present and future impact of ocean acidification on marine ecosystems requires detection of the multi-decadal rate of change across ocean basins and at ocean time-series sites. Here, we show the longest continuous record of ocean CO2 changes and ocean acidification in the North Atlantic subtropical gyre near Bermuda from 1983-2011. Dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO(2)) increased in surface seawater by similar to 40 mu mol kg(-1) and similar to 50 mu atm (similar to 20%), respectively. Increasing Revelle factor (beta) values imply that the capacity of North Atlantic surface waters to absorb CO2 has also diminished. As indicators of ocean acidification, seawater pH decreased by similar to 0.05 (0.0017 yr(-1)) and omega values by similar to 7-8%. Such data provide critically needed multi-decadal information for assessing the North Atlantic Ocean CO2 sink and the pH changes that determine marine ecosystem responses to ocean acidification.

2010
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.

Turk, D, Zappa CJ, Meinen CS, Christian JR, Ho DT, Dickson AG, McGillis WR.  2010.  Rain impacts on CO2 exchange in the western equatorial Pacific Ocean. Geophysical Research Letters. 37   10.1029/2010gl045520   AbstractWebsite

The ocean plays a major role in the global carbon cycle through the atmosphere-ocean partitioning of atmospheric carbon dioxide. Rain alters the physics and carbon chemistry at the ocean surface to increase the amount of CO(2) taken up by the ocean. This paper presents the results of a preliminary study wherein rain measurements in the western equatorial Pacific are used to determine the enhanced transfer, chemical dilution and deposition effects of rain on air-sea CO(2) exchange. Including these processes, the western equatorial Pacific CO(2) flux is modified from an ocean source of +0.019 mol CO(2) m(-2) yr(-1) to an ocean sink of -0.078 mol CO(2) m(-2) yr(-1). This new understanding of rain effects changes the ocean's role in the global carbon budget, particularly in regions with low winds and high precipitation. Citation: Turk, D., C. J. Zappa, C. S. Meinen, J. R. Christian, D. T. Ho, A. G. Dickson, and W. R. McGillis (2010), Rain impacts on CO(2) exchange in the western equatorial Pacific Ocean, Geophys. Res. Lett., 37, L23610, doi:10.1029/2010GL045520.

Dickson, AG.  2010.  The carbon dioxide system in seawater: equilibrium chemistry and measurements. Guide to best practices for ocean acidification research and data reporting. ( Riebesell U, Fabry VJ, Hansson L, Gattuso JP, Eds.).:17-40., Luxembourg: Publications Office of the European Union Abstract
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