CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT   The calculation of a conventional radiocarbon age is made relating the 14C activity of the analyzed sample to that of the CO2 of the atmospheric reservoir which it was in equilibrium. The difference in activities is assumed due to 14C radioactive decay. But the sample could have not been in equilibrium with the atmospheric reservoir, or did it with other one, like sea water, rivers, lakes, estuaries, etc., where the 14C concentration is commonly different. These differences origin the so-called reservoir effect (RE). Reservoirs other than the atmospheric are usually depleted in 14C, leading to an apparent aging of samples. As a consequence of the RE, inorganic species of carbon dissolved in ocean water present at global scale a 14C depletion equivalent to about 400y. This phenomenon presents important local variations known as local reservoir effect (LRE), that in case of to vary with time, could be used to asses environmental changes. The minimal required conditions for the evaluation of the LRE in a temporal section are the availability of marine and terrestrial organism of well known age, belonging to the same locality and that the set of sample pairs had been proved to be coetaneous. The difference between their ages is proportional to the RE. The methodology here presented, is based in the calculation of the present LRE. It is worthy to note that the correction of a sample age is only possible when the specific reservoir 14C activity in the moment that the sample was formed is known. This work showed that 14C activity is very sensitive to changes in coastal circulation patterns, due to the variations of continental water input, coastal morphology, upwelling, coastal drift, etc. These aspects make 14C an excellent tool for characterizing the conditions of a specific area. 14C -LRE has been very useful to compare parameters from the past and present times, or to asses differences between various moments in the past. We have noticed a trend of increasing LRE with latitude, but it is often masked by local coastal fluctuations. We will continue calibrating this effect in a more detailed way and also recommend start to calibrate this effect in archaeological sites, where synchronous marine and continental materials can be compared.

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