Carbon dioxide seep sites expose shallow coral reefs in Papua New Guinea to volcanic CO2 resulting in gradients of seawater ranging from pH 8.0 (normal) to a more acidic pH of 7.5. Some areas of these reefs experience CO2 exposure equivalent to IPCC predictions for 2050 and 2100. In this project we will reconstruct seawater pH using radiocarbon as a novel tracer of CO2 input at a coral reef site that has been exposed to high CO2 due to volcanic seeps (seep CO2 has no 14C) for an unknown period of time (at least many decades, but possibly centuries). These results will help to understand the time it takes to change calcifying organisms into “winners” or “losers” as an analog for Ocean Acidification due to increased atmospheric CO2 input.