Carbon’s radioactive isotope (radiocarbon) is a useful tool for oceanographers investigating carbon cycling in the modern ocean and ice age oceans (using foraminifera microfossils). Here we used sediment cores with excellent age constraints and abundant foraminifera microfossils to examine interspecies radiocarbon differences. All species demonstrate the same extreme radiocarbon depletion, and we argue that these observations represent important changes in seawater carbon chemistry.