Previous work based on paired charcoal/shell samples has suggested that seasonal and inter-annual upwelling along the southern coast of Brazil have caused variations in the reservoir effect through time. However these samples were collected from a midden on a paleo-lagoon with extensive freshwater input. The possibility of an “old wood” effect from unidentified charcoal also contributed to the uncertainty in the results. This work aims to examine the possible upwelling influence on the paleo-reservoir age of Brazilian surficial coastal waters.
Archaeological sites chosen are located in a large headland in Arraial do Cabo, southeastern coast of Brazil with open ocean conditions and a well-known strong and large upwelling of the Malvinas/Falkland current. Paired marine/terrestrial samples were selected from well-defined archaeological layers at Boqueirão, Usiminas and Ilha de Cabo Frio which are being studied by the Maria Cristina Tenório group from the Museu Nacional at Rio de Janeiro. To avoid a possible “old wood” effect, carbonized seeds (mostly date palm) were selected for the terrestrial samples. For the marine samples only the gastropod Astraea tecta olfersii was found in association with the seeds with the exception of one pelcypod Pinctada imbricata shell fragment at Ilha de Cabo Frio. However in the absence of carbonate rocks or significant freshwater input, no species difference would be expected.
The 14C age differences between carbonized seed and shell samples varied from 281 ± 44 to 1083 ± 51 14C yr, but there were also significant age differences between carbonized seed samples (977 14C yr) and shell samples (228 14C yr) from the same archaeological layer at the Ilha de Cabo Frio site that cannot be explained only by a reservoir effect. Three pairs of carbonized palm seed and gastropod shells collected at different points of the archaeological bed at the Usiminas site yielded marine/terrestrial age differences between 565 ± 44 and 358 ± 44 14C yr (DR = 19 ± 37 to 217 ± 44) in a relatively short period of time (cal AD 1310 -1520 at 95% probability) For the Boqueirão site, samples collected from a campfire remain related to a burial yielded the smallest difference at 281 ± 44 14C yr (DR = -75 ± 34). It is inconclusive whether the DR variability is due to (a) upwelling induced changes in the reservoir effect, (b) archaeological uncertainties or (c) another unexpected factor.
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