Marine reservoir ages for coastal West Africa

Abstract

We measured the 14C age of pre-bomb suspension-feeding bivalves of known age from coastal West Africa across a latitudinal transect extending from 33∘ N to 15∘ S. The specimens are from collections belonging to the Muséum National d'Histoire Naturelle (Paris, France). They were carefully chosen to ensure that the specimens were collected alive or that they died not long before collection. From the 14C dating of the known-age bivalves, we calculated the marine reservoir age (as ΔR and R values) for each specimen. ΔR values were calculated relative to the Marine20 calibration curve, and the R values were calculated relative to Intcal20 or SHcal20 calibration curves. Except for five outliers, the ΔR and R values were generally homogenous with weighted mean values of −72 ± 42 14C years (1 SD, n=24) and 406 ± 56 14C years (1 SD, n=24) respectively. These values are typical of low-latitude marine reservoir age values. Five suspension-feeding species living in five different ecological habitats were studied. For localities where several species were available, the results yielded similar results whatever the species considered, suggesting that, in these locations, the habitat has only a limited impact on marine reservoir age reconstruction. We show that our measured marine reservoir ages follow the declining trend of the global marine reservoir age starting ca. 1900 CE, suggesting that the marine reservoir age of coastal West Africa is driven, at least to the first order, by the atmospheric CO2 14C ageing due to fossil fuel burning rather than by local effects. Each outlier was discussed. Local upwelling conditions or sub-fossil specimens may explain the older 14C age and thus the larger marine reservoir ages for these samples. Bucardium ringens might not be the best choice for marine reservoir age reconstructions.