The variable nature of the coastal 14C marine reservoir effect: A temporal perspective for Rio de Janeiro

The radiocarbon (14C) marine reservoir effect (MRE) is very sensitive to physical and chemical processes affecting the local carbon (C) cycle. In coastal zones, these tend to be numerous and complex, hindering the derivation of local marine reservoir offsets (ΔR) for the establishment of archaeological chronologies. Although discussions of how the magnitude of the MRE responds to factors such as the presence of coastal upwelling are common in the literature, little has been done to assess how the effect is influenced by a changing relative sea level. Here, we address the MRE temporal variation on the coast of Rio de Janeiro in the light of updated 14C calibration curves and considering the Holocene relative sea-level curve for the region. To achieve this, we employ Bayesian modelling to analyze published and new 14C ages from Brazilian archaeological sites and pre-bomb samples. In this study, we show evidence for a Late Holocene increase in the MRE, varying from −577 ± 56 14C yr at ca. 5500 cal BP to −29 ± 25 14C yr at present, associated with a decreased relative sea-level.