Significance Plankton in the sunlit surface ocean photosynthesize, fixing dissolved CO ₂ into particulate organic carbon (POC). This POC sinks and is respired, releasing CO ₂ into subsurface waters that are sequestered from the atmosphere. The depth scale over which this regeneration happens strongly affects atmospheric CO ₂ , but estimates to date have been sparse and challenging to interpret. We use a new geochemical method to determine POC regeneration depth scales at unprecedented resolution in the South Pacific Ocean, finding shallow regeneration in both oxygen-deficient zone and oligotrophic gyre settings. Our results imply decreased future ocean carbon storage due to gyre expansion and two opposing feedbacks to expanding oxygen-deficient zones, the net effects of which on ocean carbon storage require future research.