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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.

2015
Nam, S, Takeshita Y, Frieder CA, Martz T, Ballard J.  2015.  Seasonal advection of Pacific Equatorial Water alters oxygen and pH in the Southern California Bight. Journal of Geophysical Research-Oceans. 120:5387-5399.   10.1002/2015jc010859   AbstractWebsite

Chemical properties of the California Undercurrent (CU) have been changing over the past several decades, yet the mechanisms responsible for the trend are still not fully understood. We present a survey of temperature, salinity, O-2, pH, and currents at intermediate depths (defined here as 50-500 m) in the summer (30 June to 10 July) and winter (8-15 December) of 2012 in the southern region of the Southern California Bight. Observations of temperature, salinity, and currents reveal that local bathymetry and small gyres play an important role in the flow path of the California Undercurrent (CU). Using spiciness (p) as a tracer, we observe a 10% increase of Pacific Equatorial Water (PEW) in the core of the CU during the summer versus the winter. This is associated with an increase in p of 0.2, and a decrease in O-2 and pH of 30 mu mol kg(-1) and 0.022, respectively; the change in pH is driven by increased CO2, while total alkalinity remains unchanged. The high-p, low-O-2, and low-pH waters during the summer are not distributed uniformly in the study region. Moreover, mooring observations at the edge of the continental shelf reveal intermittent intrusions of PEW onto the shelf with concomitant decreases in O-2 and pH. We estimate that increased advection of PEW in the CU could account for approximately 50% of the observed decrease in O-2, and between 49 and 73% of the decrease in pH, over the past three decades.

2012
Martz, T, Takeshita Y, Rolph R, Bresnahan P.  2012.  Tracer Monitored Titrations: Measurement of Dissolved Oxygen. Analytical Chemistry. 84:290-296.   10.1021/ac202537f   AbstractWebsite

The tracer monitored titration (TMT) technique is evaluated for measurement of dissolved oxygen. The TMT developed in this work uses a simple apparatus consisting of a low-precision pump for titrant delivery and an optical detector based on a white LED and two photodiodes with interference filters. It is shown that the classic Winkler method can be made free of routine volumetric and gravimetric measurements by application of TMT theory, which allows tracking the amounts of titrant and sample using a chemical tracer. The measurement precision of the prototype setup was 0.3% RSD.

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.