The distribution of δ13C in total dissolved inorganic carbon in the central North Pacific Ocean along 175°E and implications for anthropogenic CO2 penetration

We measured the quantity of δ13C in total dissolved inorganic carbon in seawater collected between 48°N and 15°S along 175°E in the central North Pacific during the Northwest Pacific Carbon Cycle Study program (NOPACCS) in 1993 and 1994, focusing on the distribution of δ13C on isopycnal surfaces of 25.6, 26.0, 26.4, 26.8 and 27.2 σθ in the main pycnocline of the North Pacific. The distribution of δ13C was controlled largely by biological cycling. Using apparent oxygen utilization as a measure of biological effects on δ13C variation, we calculated preformed values of δ13C at the time when the water left the surface. The preformed δ13C showed a trend of increasing southward in the subtropical region on all the surfaces. Variations in preformed δ13C on the surfaces between 25.6 and 26.4 σθ in the subtropical region most likely reflect the penetration of 13C-depleted anthropogenic CO2, because there was no other effect acting on the preformed δ13C. We applied another approach, using dissolved phosphate, to correct for biological effects on δ13C distribution. The results of both methods allowed us to determine the changes in δ13C due to the penetration of anthropogenic CO2.