Standard names: Saturation state for surface sea water with respect to aragonite/calcite

[abiardeau]

Hello,

I am Aurore BIARDEAU from Mercator Ocean international, working on Copernicus Marine data.

Date : 05-03-2024

We are currently using the following variable to refer to the saturation state for surface sea water wih respect to aragonite (and the same for calcite), and we would like to know if we could make it CF compliant :

• standard_name : aragonite_saturation_state_in_sea_water (and calcite_saturation_state_in_sea_water)
• no unit

This variable corresponds to a concentration product ratio. It is not a mole fraction (so the CF variable mole_concentration_of_aragonite_expressed_as_carbon_in_sea_water would be wrong here). We use here the strict definition of the solubility product and therefore OMEGA, the quantity commonly accepted by the marine chemistry community.

I can provide more information from my expert colleagues if needed,

Kind regards,
Aurore

[github-actions]

Thank you for your proposal. These terms will be added to the cfeditor (http://cfeditor.ceda.ac.uk/proposals/1) shortly. Your proposal will then be reviewed and commented on by the community and Standard Names moderator.

[JonathanGregory]

Dear Aurore @abiardeau

Thanks for your question. Is the quantity you are interested in the ratio of products of mole concentrations:

[Ca²⁺][CO₃²⁻]/[CaCO₃(aragonite)]

I believe this is the reaction quotient for the dissolution of aragonite

CaCO₃(aragonite) ⇌ Ca²⁺ + CO₃²⁻

Is that right? (I'm trying to remember about chemical thermodynamics from school a very long time ago!)

If that's right, I don't think we have any similar quanity so far in the standard name table. I tend to think we ought to call it reaction_quotient_for_dissolution_of_aragonite_in_sea_water, which would be precise. Would that make sense?

You also say it's at the "sea surface". Could you be more precise about the meaning of that?

I don't understand why this quantity is dimensionless, since concentrations are not dimensionless. Since the canonical unit for a mole concentration is mol m-3, why isn't the unit for this ratio mol m-3 mol m-3 mol-1 m3 = mol m-3?

Best wishes

Jonathan

[abiardeau]

Dear @JonathanGregory ,

Thanks for you prompt answer.
According to my colleague, the ratio is : ([Ca²⁺][CO₃²⁻])_seawater / ([Ca²⁺][CO₃²⁻])_SAT
That being a ratio (which we could call saturation state), there would be no unit.
I will ask for more information about that, my chemical lessons were a long time ago too !

[efisher008]

Hi Aurore,

I have added the name to the editor, you can view it here:
https://cfeditor.ceda.ac.uk/proposal/5275/edit

Would you be able to provide a description for the proposed name? I would suggest including the following phrase from the phrasebank for aragonite: "Aragonite is a mineral that is a polymorph of calcium carbonate. The chemical formula of aragonite is CaCO3. Standard names also exist for calcite, another polymorph of calcium carbonate."

Hoping the discussion continues on this.

Best regards,
Ellie

[efisher008]

Hi @abiardeau,

Are you happy for me to update the description of your proposed name aragonite_saturation_state_in_sea_water with the suggestion definition of aragonite? And what do you think of Jonathan's proposed alternative name, reaction_quotient_for_dissolution_of_aragonite_in_sea_water? If this does not in fact match the equation (ratio) you describe, perhaps it is not quite suitable in this context - but it would be great to hear your thoughts.

Could you also provide more information on what you mean by "surface sea water"? In the CF phrasebank: "The phrase "sea water surface" means the upper boundary of the liquid portion of an ocean or sea, including the boundary to floating ice if present.".
Would that be correct in your usage? If this quantity is defined only at the sea surface, that should be made clear in the name (I think the standard format would therefore be something like aragonite_saturation_state_in_sea_water_surface or aragonite_saturation_state_in_sea_surface).

Best,
Ellie

[efisher008]

Hi Aurore @abiardeau,

Do you have any comments on the latest discussion and suggestions?

Best wishes,
Ellie