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)

Nutrients– what are they? Macronutrients: NO 3 -, NO 2 -, ∑PO 4 3-,SiO 2, NH 4 + Micronutrients: Mostly transition metals (Fe, Mn, Mo, Cu…). We won’t talk about these (even though they can be important in coastal settings). Carbon cycle can be ~well understood with observations of O 2, nutrients, and carbonate system chemistry. O 2 covered in bio lab sections. With one notable exception, these are measured with wet chemical analyses, usually with spectrophotometric detection. We’ll have these done in a service lab.

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 CO 2(aq) (pCO 2 ), CO 2(aq), HCO 3 -, CO 3 =,(T CO2 ); B(OH) 4 -, H + (pH), OH -, NH 4 +, H 2 PO 4 -, HS -, H 3 Si(OH) 4 -, organic acids and bases… (T ALK )

What are the measurable parameters that define (constrain) the carbonate system in seawater? (Almost none of the individual species are directly measurable) Total CO 2 T CO2 ≡ [CO 2(aq) ] + [HCO 3 - ] + [CO 3 2- ] (aka DIC, ∑CO 2 ) Measured by acidification and potentiometric titration, coulometry, or IR- absorbance measurement of a strip-gas. Total Alkalinity (not C ALK !) T ALK ≡ [HCO 3 - ] + 2[CO 3 2- ] + [B(OH) 4 - ] + [OH - ] + [HS - ] + 2[S 2- ] + [H 2 PO 4 - ] + 2[HPO 4 2- ] + 3[PO 4 3- ] +∑organic bases - [H + ] - [NH 4 + ] - ∑organic acids … Measured by acid-titration

What are the measurable parameters that define (constrain) the carbonate system in seawater? (Almost none of the individual species are directly measurable) pCO 2 pCO 2 ≡ K h [CO 2(aq) ] NOT pCO 2 ! “p” denotes partial pressure. In µatm, nearly numerically equivalent to X CO2 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(a H+ ) = -log(γ -1 [H + ]). “p” is a mathematical operator. Many different scales to account for the difference between a H+ and [H + ] in complicated solutions like seawater. Measured by potentiometric electrode; color-change of pH-sensitive dyes.

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., 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 Well, if you’re careful… T ALK 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.

What are the advantages/disadvantages of each measurement? Total CO 2 Advantages: ‘Conservative’ parameter does not change as a result of differences between sampling/analysis and in situ conditions No definition or scale-convention uncertainties Desired precision AND accuracy (o 0.1%) can be attained New IR techniques allow rapid continuous analysis Of direct interest Disadvantages Requires very high precision and accuracy to be oceanographically useful (0.1%) Titration and manometry have poor accuracy AND precision (wrt required 0.1%) Coulometric techniques are difficult and user-sensitive, require expensive apparatus, and nasty chemical solutions Most techniques are useful only for discrete samples

Total Alkalinity Advantages: ‘Conservative’ parameter does not change as a result of sampling Simple, inexpensive apparatus Desired precision (o 0.1%) can be attained Of direct interest Disadvantages: Requires very high precision and accuracy to be oceanographically useful (0.1%) Slow analysis; can be run on discrete samples only Definition problems Desired accuracy is elusive

pCO 2 Advantages: No definition problems Lower relative accuracy and precision requirements than T ALK or T CO2 NDIR analyzers are stable and simple to operate Well-buffered gas not sensitive to sampling protocols Desired precision and accuracy (o 1 ppm) can be attained easily Sampling non-conservative-ness well defined. Well-suited for continuous analyses Of direct interest Disadvantages: Not ‘conservative’ wrt sampling No ‘cheap’ way to measure it

pH Advantages: Cheap, easy measurement to make (potentiometric) Very high precision can be obtained ( – pH units) by skilled analysts Disadvantages: Not ‘conservative’ wrt sampling Definition problems Buffer/calibration problems Not in itself an interesting measurement Accuracy 1-2 o worse than precision pH is a horrible measurement!

What are the best ways to measure the required two parameters? Cheapest: pH electrode and alkalinity titration Best: pCO 2 and T CO2 by GC/IR and Coulometry Fastest+Easiest+Best: pCO 2 by membrane-contactor equilibration and IR detection T CO2 by continuous complete strip followed by IR detection

How I prefer to do it: Following the ‘best’, but faster. pCO 2 : Continuous IR- absorption analysis of gas stream equilibrated with flowing sample stream. T CO2 : Continuous IR- absorption analysis of a gas stream stripping an acidified flowing sample stream.

pCO 2 : Relies on equilibration of the CO 2 in a recirculated gaseous headspace with the CO 2(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.

T CO2 : Relies on stripping CO 2 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 CO 3 2- and HCO 3 - into CO 2(aq). Mass balance controls the outlet gas CO 2 concentration. Requires precise flow control.