What is the mean amount (in grams) of dissolved carbon in a ton of sea water?
This graph at Wikipedia/Wikimedia is part of the answer :



source: 


en.wikipedia.org/wiki/Oceanic_carbon_cycle#/media/File:WOA05_GLODAP_pd_DIC_AYool.png
en.wikipedia.org/wiki/File:WOA05_GLODAP_pd_DIC_AYool.png


2.15 mol/m3 at the surface in 1990 seems like a approximative value, which makes approximately 27 g. 

Now, the two questions left are:


How did this value evolve since 1990?
Does this concentration change a lot with depth?