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2018
Takeshita, Y, Johnson KS, Martz TR, Plant JN, Sarmiento JL.  2018.  Assessment of autonomous pH measurements for determining surface seawater partial pressure of CO2. Journal of Geophysical Research-Oceans. 123:4003-4013.   10.1029/2017jc013387   AbstractWebsite

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program currently operates >80 profiling floats equipped with pH sensors in the Southern Ocean. Theoretically, these floats have the potential to provide unique year-around estimates of pCO(2) derived from pH measurements. Here, we evaluate this approach in the field by comparing pCO(2) estimates from pH sensors to directly measured pCO(2). We first discuss data from a ship's underway system which covered a large range in temperature (2-30 degrees C) and salinity (33.6-36.5) over 43 days. This pH sensor utilizes the same sensing technology but with different packaging than those on SOCCOM floats. The mean residual varied between -4.64.1 and 8.64.0 (1 sigma) atm, depending on how the sensor was calibrated. However, the standard deviation of the residual, interpreted as the ability to track spatiotemporal variability, was consistently <5 atm and was independent of the calibration method. Second, we assessed the temporal stability of this approach by comparing pCO(2) estimated from four floats over 3 years to the Hawaii Ocean Time-series. Good agreement of -2.110.4 (1 sigma) mu atm was observed, with coherent seasonal cycles. These results demonstrate that pCO(2) estimates derived from profiling float pH measurements appear capable of reproducing spatiotemporal variations in surface pCO(2) measurements and should provide a powerful observational tool to complement current efforts to understand the seasonal to interannual variability of surface pCO(2) in underobserved regions of the open ocean.

2016
Johnson, KS, Jannasch HW, Coletti LJ, Elrod VA, Martz TR, Takeshita Y, Carlson RJ, Connery JG.  2016.  Deep-Sea DuraFET: A Pressure Tolerant pH Sensor Designed for Global Sensor Networks. Analytical Chemistry. 88:3249-3256.   10.1021/acs.analchem.5b04653   AbstractWebsite

Increasing atmospheric carbon dioxide is driving a long-term decrease in ocean pH which is superimposed on daily to seasonal variability. These changes impact ecosystem processes, and they serve as a record of ecosystem metabolism. However, the temporal variability in pH is observed at only a few locations in the ocean because a ship is required to support pH observations of sufficient precision and accuracy. This paper describes a pressure tolerant Ion Sensitive Field Effect Transistor pH sensor that is based on the Honeywell Durafet ISFET die. When combined with a AgCl pseudoreference sensor that is immersed directly in seawater, the system is capable of operating for years at a time on platforms that cycle from depths of several km to the surface. The paper also describes the calibration scheme developed to allow calibrated pH measurements to be derived from the activity of HCl reported by the sensor system over the range of ocean pressure and temperature. Deployments on vertical profiling platforms enable self-calibration in deep waters where pH values are stable. Measurements with the sensor indicate that it is capable of reporting pH with an accuracy of 0.01 or better on the total proton scale and a precision over multiyear periods of 0.005. This system enables a global ocean observing system for ocean pH.

2015
Takeshita, Y, Frieder CA, Martz TR, Ballard JR, Feely RA, Kram S, Nam S, Navarro MO, Price NN, Smith JE.  2015.  Including high-frequency variability in coastal ocean acidification projections. Biogeosciences. 12:5853-5870.   10.5194/bg-12-5853-2015   AbstractWebsite

Assessing the impacts of anthropogenic ocean acidification requires knowledge of present-day and future environmental conditions. Here, we present a simple model for upwelling margins that projects anthropogenic acidification trajectories by combining high-temporal-resolution sensor data, hydrographic surveys for source water characterization, empirical relationships of the CO2 system, and the atmospheric CO2 record. This model characterizes CO2 variability on timescales ranging from hours (e. g., tidal) to months (e. g., seasonal), bridging a critical knowledge gap in ocean acidification research. The amount of anthropogenic carbon in a given water mass is dependent on the age; therefore a density-age relationship was derived for the study region and then combined with the 2013 Intergovernmental Panel on Climate Change CO2 emission scenarios to add density-dependent anthropogenic carbon to the sensor time series. The model was applied to time series from autonomous pH sensors deployed in the surf zone, kelp forest, submarine canyon edge, and shelf break in the upper 100m of the Southern California Bight. All habitats were within 5 km of one another, and exhibited unique, habitat-specific CO2 variability signatures and acidification trajectories, demonstrating the importance of making projections in the context of habitat-specific CO2 signatures. In general, both the mean and range of pCO(2) increase in the future, with the greatest increase in both magnitude and range occurring in the deeper habitats due to reduced buffering capacity. On the other hand, the saturation state of aragonite (Omega(Ar)) decreased in both magnitude and range. This approach can be applied to the entire California Current System, and upwelling margins in general, where sensor and complementary hydrographic data are available.

2011
Gray, SEC, DeGrandpre MD, Moore TS, Martz TR, Friederich GE, Johnson KS.  2011.  Applications of in situ pH measurements for inorganic carbon calculations. Marine Chemistry. 125:82-90.   10.1016/j.marchem.2011.02.005   AbstractWebsite

This study examines the utility of combining pH measurements with other inorganic carbon parameters for autonomous mooring-based carbon cycle research. Determination of the full suite of inorganic carbon species in the oceans has previously been restricted to ship-based studies. Now with the availability of autonomous sensors for pH and the partial pressure of CO(2) (pCO(2)), it is possible to characterize the inorganic carbon system on moorings and other unmanned platforms. The indicator-based pH instrument, SAMI-pH, was deployed with an autonomous equilibrator-infrared pCO(2) system in Monterey Bay. California USA from June to August 2007. The two-month time-series show a high degree of short-term variability, with pH and pCO(2) changing by as much as 0.32 pH units and 240 mu atm, respectively, during upwelling periods. The pH and salinity-derived alkalinity (A(Tsalin)) were used to calculate the other inorganic carbon parameters, including pCO(2), total dissolved inorganic carbon (DIC) and CaCO(3) saturation states. The calculated pCO(2) was within 2 mu atm of the measured pCO(2) during the first day of the deployment and within 8 mu atm over the first month. The DIC calculated from pH-A-Ban and pCO(2)-A(Tsalin) were within 5 mu mol kg(-1) of each other during the first month. However, DIC calculated from pH-pCO(2) differed by similar to 50 mu mol kg(-1) from the other estimates over the same period, reflecting the sensitivity of the pH-pCO(2) calculation to measurement error. The data continued to diverge during the final month and this difference was likely driven by extensive biofouling. Because of the relative insensitivity of CO(3)(2-) concentration to these errors, aragonite saturation calculated from the pH-pCO(2) pair was within 0.15 of the pH-A(Tsalin) values over the entire deployment. These results show that in situ pH, when combined with other CO(2) parameters, can provide valuable insights into both data quality and inorganic carbon cycling. (C) 2011 Elsevier B.V. All rights reserved.

2010
Martz, TR, Connery JG, Johnson KS.  2010.  Testing the Honeywell Durafet (R) for seawater pH applications. Limnology and Oceanography-Methods. 8:172-184.   10.4319/lom.2010.8.172   AbstractWebsite

We report on the first seawater tests at 1 atm of the Honeywell Durafet (R) pH sensor, a commercially available ion sensitive field effect transistor (ISFET). Performance of this sensor was evaluated in a number of different situations including a temperature-controlled calibration vessel, the MBARI test tank, shipboard underway mapping, and a surface mooring. Many of these tests included a secondary reference electrode in addition to the internal reference supplied with the stock Durafet sensor. We present a theoretical overview of sensor response using both types of reference electrode. The Durafet sensor operates with a short term precision of +/- 0.0005 pH over periods of several hours and exhibits stability of better than 0.005 pH over periods of weeks to months. Our tests indicate that the Durafet pH sensor operates at a level of performance satisfactory for many types of biogeochemical studies at low pressure.

2009
Martz, TR, Jannasch HW, Johnson KS.  2009.  Determination of carbonate ion concentration and inner sphere carbonate ion pairs in seawater by ultraviolet spectrophotometric titration. Marine Chemistry. 115:145-154.   10.1016/j.marchem.2009.07.002   AbstractWebsite

We describe a novel method for determination of carbonate ion concentration in seawater by acidimetric titration with UV detection. Because CO(3)(2-) absorbs light at wavelengths of less than similar to 250 nm, it is feasible to titrate most carbonate-containing natural waters with acid and observe an increase in %Transmittance. The observed signal is proportional to the concentration of carbonate ion in the original sample. Present technology permits a theoretical precision in the determination of [CO(3)(2-)] in natural seawater background of similar to 0.5% (at 10 cm pathlength, 200 mu M CO(3)(2-), +/- 0.0001 AU). The procedure has been tested at 1 and 10 cm pathlengths using single and multipoint titration methods, respectively. Results using natural seawater test solutions indicate a resolution in [CO(3)(2-)] of 3.6% in a standard I cm cuvette using a very simple manual method, and 0.7% using a custom-built 10 cm closed titration cell. Estimates of the relative distribution of CO(3)(2-) between inner and outer sphere complexes with Mg(2+) and Na(+) were also determined and the equilibrium constants agree with published values. This method provides a new tool for studies of several fundamental aspects CO(2) chemistry, including the second dissociation constant of carbonic acid, CO(3)(2-) ion pairing, and can be used to directly measure the distribution of carbonate ion in seawater and many other types of natural waters. (C) 2009 Elsevier B.V. All rights reserved.