The Usiminas shellmound on the Cabo Frio Island: Marine reservoir effect in an upwelling region on the coast of Brazil

The mixture of different water masses can strongly influence the marine reservoir effect (MRE) in localities under the effect of coastal marine upwelling. The Cabo Frio region, on the southeastern Brazilian coast, is well known for its seasonal marine upwelling and for its rich landscape of Holocene shellmounds. In this kind of archaeological settlement, common on the southern and southeastern Brazilian coasts, marine mollusk shells are frequently used for radiocarbon dating and can represent a valuable tool in the study of MRE. On the other hand, terrestrial mollusks have proven to be an important alternative to represent the atmospheric carbon isotopic concentration. In this work we analysed the most frequent terrestrial and marine mollusk shells from the Usiminas shellmound, on Cabo Frio Island. The radiocarbon signal of marine shells from Pinctada imbricata (Röding, 1798), Ostreidae (Rafinesque, 1815), Cymathium parthenopeum (Von Salis, 1793), Leucozonia nassa (Gmelin, 1791), Lithopoma olfersii (Philippi, 1846), and that of terrestrial shells from Thaumastus achilles (Pfeiffer, 1852) and Megalobulimus terrestris (Spix, 1827), collected from the archaeological layers in the sequence, were measured and a value of 67 ± 33 14C yr was obtained for the local offset from the average global marine reservoir age. The effect of upwelling in this region and in its surrounding area is discussed.