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: NO3-, NO2

-, ∑PO43-,SiO2, NH4

+

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 O2, nutrients, and carbonate system chemistry. O2 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

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)

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-] + [CO3

2-] (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[CO3

2-] + [B(OH)4-] + [OH-] + [HS-] + 2[S2-]

+ [H2PO4-] + 2[HPO4

2-] + 3[PO43-] +∑organic bases - [H+]

- [NH4+] - ∑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)

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.

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.

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 analysisOf 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%)

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 samplingSimple, inexpensive apparatus

Desired precision (o 0.1%) can be attainedOf direct interest

Disadvantages:

Requires very high precision and accuracy to be oceanographically useful (0.1%)

Slow analysis; can be run on discrete samples onlyDefinition problems

Desired accuracy is elusive

pCO2

Advantages:No definition problems

Lower relative accuracy and precision requirements than TALK or TCO2

NDIR analyzers are stable and simple to operateWell-buffered gas not sensitive to sampling protocols

Desired precision and accuracy (o 1 ppm) can be attained easilySampling non-conservative-ness well defined.

Well-suited for continuous analysesOf direct interest

Disadvantages:Not ‘conservative’ wrt samplingNo ‘cheap’ way to measure it

pH

Advantages:Cheap, easy measurement to make (potentiometric)

Very high precision can be obtained (0.0001 – 0.001 pH units) by skilled analysts

Disadvantages:Not ‘conservative’ wrt sampling

Definition problemsBuffer/calibration problems

Not in itself an interesting measurementAccuracy 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:pCO2 and TCO2 by GC/IR and Coulometry

Fastest+Easiest+Best:pCO2 by membrane-contactor equilibration and IR

detectionTCO2 by continuous complete strip followed by IR

detection

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.

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.

TCO2: Relies on stripping CO2 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 CO3

2- and HCO3

- into CO2(aq).

Mass balance controls the outlet gas CO2 concentration. Requires precise flow control.