Environmental archives are needed to study the variability of natural systems and the impact of man on them. Microbial mats, modern homologues of stromatolites, can be found in extreme environments such as the Ebro River Delta and were studied as potential environmental archives of atmospheric deposition. 210Pb, a radiotracer widely used in geochronology studies, was used both to determine the growth rates of a microbial mat from this environment and to estimate the 210Pb atmospheric flux in the northwestern Mediterranean Sea. The 210Pb profile showed the presence of three distinct peaks related to low growth-rate periods. This variability indicated the sensitivity of the system to external forcing. The annual atmospheric flux of 210Pb 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 rates, 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.