Abstract
ABSTRACTQuantifying the local marine reservoir age (ΔR) and its change over time is critical for precise radiocarbon calibration of marine samples and for the study of the ocean carbon cycle. ΔR values are scarce for the African coast facing the Indian Ocean, and the few values available were obtained from pre-bomb shells collected during the 19th century. Here, the ΔR value for calibrated year 1110 ± 25 (1σ) CE was reconstructed from radiocarbon dating and Bayesian analysis of marine and terrestrial materials coexisting in a tsunami deposit discovered in Pangani Bay (Tanzania, western Indian Ocean coast). The reconstructed ΔR of –8 ± 40 (1σ, n = 3) is similar to pre-bomb regional estimates and provides new information to investigate regional ΔR change over time. The Bayesian analysis of the dated samples revises the age of the tsunami event found in Pangani Bay to 1064–1157 cal CE (95.4% confidence level) or 1110 ± 25 (1σ) cal CE, about one century younger compared to the previous estimate. Our results indicate that the new ΔR value and the proposed calibration approach can be used to refine existing chronologies in the region, with implications for paleo-environmental reconstructions and archaeological studies of Early Swahili societies.
Highlights
Deriving accurate calendar ages from the radiocarbon (14C) dating of marine organisms is challenging as marine 14C dates need a radiocarbon reservoir age correction, which is usually unknown
We used Bayesian modeling of a large set of terrestrial and marine 14C dates implemented in OxCal considering the geo-archaeological context of a Swahili settlement found in Pangani Bay (Tanzania) in order to reconstruct a new local marine reservoir age of –8 ± 40 (1σ) 14C yr with respect to the Marine20 calibration curve for the end/beginning of the 11th/12th century
The ΔR value is similar to that derived from a pre-bomb shell collected during the 19th century about 130 km from Pangani Bay and indicates that the marine reservoir age could have remained locally constant over the last millennium, more ΔR value reconstructions are needed to confirm this pattern
Summary
Deriving accurate calendar ages from the radiocarbon (14C) dating of marine organisms is challenging as marine 14C dates need a radiocarbon reservoir age correction, which is usually unknown. The radiocarbon reservoir age of a marine sample R t is the difference between its radiocarbon age (14Cm) and that of the atmospheric CO2 (14Catm) at the same calendar time (t) (Stuiver and Polach 1977; Ascough et al 2005; Jull et al 2013; Soulet et al 2016). The reservoir age in surficial ocean waters is primarily controlled by the time needed for atmospheric CO2 to exchange with surface waters plus the contribution deriving from the slow mixing with deep ocean waters containing vast stocks of old dissolved inorganic carbon (Bard 1988).
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