For slowly accumulating sediments, a major contrast exists in the radiocarbon-age differences among coexisting shells of planktic foraminifera between those experiencing little dissolution and those experiencing significant dissolution. In the former, the ages generally agree to within a couple of hundred years. In the latter, age differences as large as 1000 years are common. The most likely explanation appears to be the Barker Effect, which involves the preferential fragmentation of dissolution-prone G. sacculifer and G. ruber. The whole shells of these species picked for radiocarbon dating have shorter residence times in the bioturbation zone than those for dissolution-resistant species (including benthics). As low accumulation rate sediment cores often fail to yield reliable radiocarbon-based ocean ventilation ages, where possible, such studies should be conducted on high accumulation rate cores.