Pb-210 atmospheric flux and growth rates of a microbial mat from the northwestern Mediterranean Sea (Ebro River Delta)

Environmental archives are needed to study the variability of natural syste ms and the impact of man on them. Microbial mats, modern homologues of stro matolites, can be found in extreme environments such as the Ebro River Delt a and were studied as potential environmental archives of atmospheric depos ition. Pb-210, a radiotracer widely used in geochronology studies, was used both to determine the growth rates of a microbial mat from this environmen t and to estimate the Pb-210 atmospheric flux in the northwestern Mediterra nean Sea. The Pb-210 profile showed the presence of three distinct peaks re lated to low growth-rate periods. This variability indicated the sensitivit y of the system to external forcing. The annual atmospheric flux of Pb-210 was 81.2 +/- 1.4 Bq m(-2) yr(-1), which is similar to other Values found in the literature. The age profile showed two layers of differing growth rate s, being 0.99 +/- 0.10 mm yr(-1) from the surface down to 10 mm depth. The accumulated mass profile showed a change at about 9 mm depth, corresponding to year 1983 +/- 1. It is noteworthy that this is coincident with a strong El Nino Southern Oscillation event during 1982-1983, which has been shown to affect other ecosystems, including some in the Mediterranean area.