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Dear Alistair,

the newest version of the GLODAPv2 dataset contains a global, gridded field of Seawater pH, but also the original bottle data files from which this was created.

The data is available at

https://www.nodc.noaa.gov/ocads/oceans/GLODAPv2/

in a number of formats; I tend to use the netCDF version, which you can easily visualise, e.g. with Panoply. I have quickly created you a plot of the surface pH and the pH at 1200 m depth, just to illustrate. With Panoply this took me 20 seconds.

The processing of the data is described in

Article The Global Ocean Data Analysis Project version 2 (GLODAPv2) ...

Cheers, Christoph

Dear Christof,

Thank you. That is just what I need.

There is quite a contrast between the 1200m North Atlantic and the 1200m North Pacific. Is the reason for that known?

Dear Alistair,

yes, the reason is not too difficult, but it has two parts.

The first part is pure carbonate chemistry: the pH really is mainly a function of two quantities, dissolved inorganic carbon (DIC) and Alkalinity (Alk). With rising DIC, at constant Alk, pH decreases, and with rising Alk at constant DIC, pH increases. The deep Pacific has a higher DIC, compared to the deep North Atlantic, hence its pH is lower; the tendency is somewhat moderated by the fact that Alk is also higher in the North Pacific, compared to the Atlantic, but not as much as DIC.

The second part is: Why is DIC higher in the deep Pacific than in the deep Atlantic? This has to do with the pattern of the deep ocean circulation, the overturning: Deep water is mainly formed (i.e. it had its last surface contact with the atmosphere) in the North Atlantic (North Atlantic Deep Water, NADW) and around Antarctica (Antarctic Bottom Water, AABW). From there it spreads slowly through the worlds oceans in a somewhat complicated patter, which is best described in a paper by Lynne Talley in Oceanography Magazine in 2013. The upshot is that water in the deep North Pacific is the 'oldest' water in the sense that is has been away form the surface longest.

And on its long travel towards the North Pacific, there has been a constant rain of organic particles into that water, which has been remineralised by bacteria and animals, releasing CO2, i.e increasing DIC (and also nitrate, phosphate and silicate), and reducing O2. In the Atlantic, the water simply hasn't had enough time to accumulate the breakdown products of the constant rain of organic particles.

There is of course more that can be said on that, but to first order that is the explanation, I think.

Cheers, Christoph

Thanks for that Christoph.

I've now got GLODAPv2 and Panoply working under Linux, and now my maps match yours, but it has taken me a little more than 20 seconds .

Here is a plot of the oxygen at 1200m which matches your pH at 1200m and supports your argument that it is lack of oxygen which causes the low pH in the north Pacific.

Thanks again.

Dear Alastair,

great that you got it working. I found panoply quite powerful for visualisation (I especially like the many map projections available), but if you need to do calculations with the data (e.g. calculate spatial averages or calculate derived quantities) it is probably not the best tool. Sometimes then it makes sense to load the netCDF data into a tool like matlab, python or R. All of them have ways to read in netCDF data, so if you'd like to do something in that direction just ask; I have examples in all three languages (but with R I am really a newbie).

Cheers, Christoph

Hi Christoph,

My apologies for not getting back to you sooner.

I don't really want to get involved with Netcdy files if I can help it. I was just trying to check an idea I had that the depth of the CCD was controlled by pH. What I really need now is a relief map of calcite sediments.

But I have two other mini projects running at the moment so I have not been able to investigate that problem properly yet.

Cheers, Alastair.

I have just found:

Feely, R. A., Sabine, C. L., Lee, K., Berelson, W., Kleypas, J., Fabry, V. J. and Millero, F. J. (2004) ‘Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans’, Science, vol. 305, no. 5682, pp. 362–366 [Online]. DOI: 10.1126/science.1097329.

which has maps of the saturation horizon. So I will check those out against the Panoply pH maps.

Jorge,

Thank you for those links. They confirm that the pH is lower in deeper water and varies from place to place.

pH of Indian ocean : 8-1-8-2

pH of Atlantic ocean is 8-1

pH of Pacific ocean is 8-1

(In average basis)

Jorge Costa-de-Moura

I was looking for answers about the relationship between upwelling and OA and I found yours very explicative. So I am asking you: in Feeley 2008, the plots show increasing pCO2 values (and correspondingly increased DIC, decreasing pH) downward also within the upper 100 m of the water column, so how is it possible that CO2 was absorbed from the atmosphere into the surface ocean? pCO2 values of the upwelled waters then are 3 times higher than atmospheric pCO2 and should result in a flux from the atmosphere to the ocean (it that CO2 then re-adsorbed into the ocean offshore?). I don't get the whole thing.

And can you suggest me any study on the Peru-Chile upwelling system? is that also a site where OA affects the upwelling area? I always thought that upwelling areas are just zones of CO2 emission to the atmosphere. Thank you

It should not vary significantly. When they all are connected, the hydrogen ion concentration should be same.

If one pours a bucket of strong acid in a large reservoir at a place and measures pH after some time in the reservoir at different distances from the pouring site, the readings will be same.

By far the best coverage of global ocean pH whether deep or pelagic can be found here: http://www.abeqas.com/category/ocean-ph/

Everything else and most comments don't appear to be responsive to your request because they are not supported for the most part by direct electrochemical measurements of pH.