Due to thermohaline circulation, the Pacific water is more 'aged' so there is a higher amount of total dissolved inorganic carbon (e.g. CO2) in the deep Pacific when compared to the Atlantic. However, I don't know how this affects the relative 13C/12C enrichment of the Pacific. Factors I've thought about:
- Oxic respiration of buried organic matter is the source of CO2 in the deep ocean.
- Deep Pacific water has accumulated more respired CO2 since the Pacific is at the end of the deep circulation.
- Algae and microbes fractionate against 13C in favour of 12C.