Carbon cycle observing in the coastal ocean

1. Carbon cycle observing in the coastal ocean Approaches: Remote-sensing In-water sensing In-water autonomous analysis Ship- or station-based autonomous analyses Sampling and lab-based analyses (Your approach)

2. Carbon cycle can be ~well understood with observations of O2, nutrients, and carbonate system chemistry. O2 covered in bio lab sections. Nutrients– what are they? Macronutrients: NO3-, NO2-, ∑PO43-,SiO2, NH4+ With one notable exception, these are measured with wet chemical analyses, usually with spectrophotometric detection. We’ll have these done in a service lab. Micronutrients: Mostly transition metals (Fe, Mn, Mo, Cu…). We won’t talk about these (even though they can be important in coastal settings).

3. The carbonate system in seawater. What is it? Dissolved, inorganic, carbon species, and all acid-base reactive chemicals which, through equilibrium acid-base processes, affect their distributions CO2(aq)(pCO2), CO2(aq), HCO3-, CO3=,(TCO2); B(OH)4-, H+ (pH), OH-, NH4+, H2PO4-, HS-, H3Si(OH)4-, organic acids and bases… (TALK)

4. What are the measurable parameters that define (constrain) the carbonate system in seawater? (Almost none of the individual species are directly measurable) Total CO2 TCO2≡ [CO2(aq)] + [HCO3-] + [CO32-] (aka DIC, ∑CO2) Measured by acidification and potentiometric titration, coulometry, or IR-absorbance measurement of a strip-gas. Total Alkalinity (not CALK!) TALK ≡ [HCO3-] + 2[CO32-] + [B(OH)4-] + [OH-] + [HS-] + 2[S2-] + [H2PO4-] + 2[HPO42-] + 3[PO43-] +∑organic bases - [H+] - [NH4+] - ∑organic acids … Measured by acid-titration

5. What are the measurable parameters that define (constrain) the carbonate system in seawater? (Almost none of the individual species are directly measurable) pCO2 pCO2 ≡ Kh[CO2(aq)] NOT pCO2! “p” denotes partial pressure. In µatm, nearly numerically equivalent to XCO2 in ppm. Measured by GC/IR-analysis of equilibrated gas headspace, or by color-change of pH-sensitive dye enclosed within gas-permeable membrane. pH pH ≡ -log(aH+) = -log(γ-1[H+]). “p” is a mathematical operator. Many different scales to account for the difference between aH+ and [H+] in complicated solutions like seawater. Measured by potentiometric electrode; color-change of pH-sensitive dyes.

6. Measurement of any two parameters in the same sample of known T, S, P will allow calculation of the rest through a combination of mass and charge balances, and equilibrium relationships. Canned software packages are available to assist with the calculations: Lewis, E., Wallace, D. W. R., 1998. Program Developed for CO2 System Calculations. ORNL/CDIAC-105. Carbon Dioxide Information Analysis Center, Oak Ridge NationalLaboratory, U.S. Department of Energy, Oak Ridge, Tennessee. See http://cdiac.ornl.gov/oceans/co2rprt.html Well, if you’re careful… TALK can be problematic because of its never-ending definition, and pH can be difficult because of different scale usage. These issues are particularly hard to deal with in estuarine settings.

7. What are the advantages/disadvantages of each measurement? Total CO2 Advantages:‘Conservative’ parameter does not change as a result of differences between sampling/analysis and in situ conditionsNo definition or scale-convention uncertaintiesDesired precision AND accuracy (o 0.1%) can be attainedNew IR techniques allow rapid continuous analysis Of direct interest DisadvantagesRequires very high precision and accuracy to be oceanographically useful (0.1%) Titration and manometry have poor accuracy AND precision (wrt required 0.1%) Coulometrictechniques are difficult and user-sensitive, require expensive apparatus, and nasty chemical solutions Most techniques are useful only for discrete samples

8. Total Alkalinity Advantages: ‘Conservative’ parameter does not change as a result of sampling Simple, inexpensive apparatusDesired precision (o 0.1%) can be attainedOf direct interestDisadvantages: Requires very high precision and accuracy to be oceanographically useful (0.1%)Slow analysis; can be run on discrete samples onlyDefinition problemsDesired accuracy is elusive

9. pCO2 Advantages:No definition problemsLower relative accuracy and precision requirements than TALK or TCO2NDIR analyzers are stable and simple to operateWell-buffered gas not sensitive to sampling protocolsDesired precision and accuracy (o 1 ppm) can be attained easilySampling non-conservative-ness well defined.Well-suited for continuous analysesOf direct interestDisadvantages:Not ‘conservative’wrt samplingNo ‘cheap’ way to measure it

10. pH Advantages:Cheap, easy measurement to make (potentiometric)Very high precision can be obtained (0.0001 – 0.001 pH units) by skilled analystsDisadvantages:Not ‘conservative’wrt samplingDefinition problemsBuffer/calibration problemsNot in itself an interesting measurementAccuracy 1-2 o worse than precisionpH is a horrible measurement!

11. What are the best ways to measure the required two parameters?Cheapest: pH electrode and alkalinity titrationBest:pCO2and TCO2 by GC/IR and CoulometryFastest+Easiest+Best:pCO2by membrane-contactor equilibration and IR detection TCO2 by continuous complete strip followed by IR detection

12. How I prefer to do it: Following the ‘best’, but faster. pCO2: Continuous IR-absorption analysis of gas stream equilibrated with flowing sample stream. TCO2: Continuous IR-absorption analysis of a gas stream stripping an acidified flowing sample stream.

13. pCO2: Relies on equilibration of the CO2 in a recirculated gaseous headspace with the CO2(aq) in a flowing stream of unperturbed seawater Equilibration is the idealized concept of determining the content of a gas stream by thermodynamic equilibrium with dissolved gases in a liquid stream, with no change in the liquid stream’s dissolved gas concentration. Can’t in reality be maintained in a continuous system, but we can get close with low gas:liquid flow ratios. No acidification allows carbonate buffering of the liquid stream’s chemistry.

14. TCO2: Relies on strippingCO2 from a flowing stream of acidified seawater; dependent on a mass balance. Stripping is the idealized concept of complete removal of a dissolved gas from the liquid phase. Can’t in reality be maintained in a continuous system, but we can get close with very high gas:liquid flow ratios. Acidification of the seawater is necessary to turn CO32- and HCO3- into CO2(aq). Mass balance controls the outlet gas CO2 concentration. Requires precise flow control.