Publications

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2018
Bresnahan, PJ, Martz TR.  2018.  Gas diffusion cell geometry for a microfluidic dissolved inorganic carbon analyzer. Ieee Sensors Journal. 18:2211-2217.   10.1109/jsen.2018.2794882   AbstractWebsite

Variable gas diffusion cell geometries were tested for the extraction of dissolved inorganic carbon (DIC) from a microfluidic (tens to hundreds of microliters) seawater sample. With a focus on optimization of diffusion cell geometry, we compare five unique diffusion cell designs. Using 3-D printing technology to streamline the prototyping and testing process, we were able to conceive, design, fabricate, and thoroughly evaluate each design over the course of about one month. In total, 1043 DIC measurements were carried out in 109 experiments for the five working manifolds. We find that a small diameter, cylindrical diffusion cell design offers several advantages over its planar counterparts and a larger diameter cylindrical cell, most notably the ability to increase the ratio of the exchange membrane's contact surface area to solution volume (the "aspect ratio") without sacrificing channel integrity. Multiple designs approached short-term repeatability of <1%, but only the cylindrical diffusion cell design allowed for <0.2% repeatability using less than 200 mu L of sample.

2014
Martz, T, Send U, Ohman MD, Takeshita Y, Bresnahan P, Kim HJ, Nam S.  2014.  Dynamic variability of biogeochemical ratios in the Southern California Current System. Geophysical Research Letters. 41:2496-2501.   10.1002/2014gl059332   AbstractWebsite

We use autonomous nitrate (NO3-), oxygen (O-2), and dissolved inorganic carbon (DIC) observations to examine the relationship between ratios of C:N:O at an upwelling site in the Southern California Current System. Mean ratios and 95% confidence intervals observed by sensors over 8 months were NO3-:O-2=-0.110.002, NO3-:DIC=0.140.001, and DIC:O-2=-0.830.01, in good agreement with Redfield ratios. Variability in the ratios on the weekly time scale is attributable to shifts in biological demand and nutrient availability and shown to exhibit a spectrum of values ranging from near 100% New Production to 100% Regenerated Production.

2013
Takeshita, Y, Martz TR, Johnson KS, Plant JN, Gilbert D, Riser SC, Neill C, Tilbrook B.  2013.  A climatology-based quality control procedure for profiling float oxygen data. Journal of Geophysical Research-Oceans. 118:5640-5650.   10.1002/jgrc.20399   AbstractWebsite

Over 450 Argo profiling floats equipped with oxygen sensors have been deployed, but no quality control (QC) protocols have been adopted by the oceanographic community for use by Argo data centers. As a consequence, the growing float oxygen data set as a whole is not readily utilized for many types of biogeochemical studies. Here we present a simple procedure that can be used to correct first-order errors (offset and drift) in profiling float oxygen data by comparing float data to a monthly climatology (World Ocean Atlas 2009). Float specific correction terms for the entire array were calculated. This QC procedure was evaluated by (1) comparing the climatology-derived correction coefficients to those derived from discrete samples for 14 floats and (2) comparing correction coefficients for seven floats that had been calibrated twice prior to deployment (once in the factory and once in-house), with the second calibration ostensibly more accurate than the first. The corrections presented here constrain most float oxygen measurements to better than 3% at the surface.

2011
Strutton, PG, Martz TR, DeGrandpre MD, McGillis WR, Drennan WM, Boss E.  2011.  Bio-optical observations of the 2004 Labrador Sea phytoplankton bloom. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006872   AbstractWebsite

A unique time series of moored bio-optical measurements documented the 2004 spring-summer bloom in the southern Labrador Sea. In situ and satellite chlorophyll data show that chlorophyll levels in the 2004 bloom were at the upper end of those typically observed in this region. Satellite chlorophyll and profiling float temperature/salinity data show that the main bloom, which typically peaks in June/July, is often preceded by ephemeral mixed layer shoaling and a lesser, short-lived bloom in May; this was the case ;in 2004. The particulate backscatter to beam attenuation ratio (b(bp)[470 nm]/C(p)[660 nm]) showed peaks in the relative abundance of small particles at bloom initiation and during the decline of the bloom, while larger particles dominated during the bloom. Chlorophyll/C(p) and b(bp)/chlorophyll were correlated with carbon export and dominated by changes in the pigment per cell associated with lower light levels due to enhanced attenuation of solar radiation during the bloom. An NPZ (nutrients, phytoplankton, zooplankton) model captured the phytoplankton bloom and an early July peak in zooplankton. Moored acoustic Doppler current profiler (ADCP) data showed an additional mid-June peak in zooplankton biomass which was attributed to egg-laying copepods. The data reported here represent one of the few moored time series of C(p), b(bp) and chlorophyll extending over several months in an open ocean region. Interpretation of data sets such as this will become increasingly important as these deployments become more commonplace via ocean observing systems. Moreover, these data contribute to the understanding of biological-physical coupling in a biogeochemically important, yet poorly studied region.

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.