Porewater chemistry gives some indication as to specific diagenetic reactions occurring in sediments. Reduction of manganese and iron oxides can occur due to abiotic reactions with sulfide or microbially mediated as electron acceptors in the degradation of organic matter. Sedimentary porewaters obtained from the Bay of Concepcion (36°38'S; 73°02'W) and continental shelf (36°31'S; 73°08'W) were analised to determine the distribution of remobilised Fe2+ and Mn2+. Sampling of the area coincided with the presence of the strong "1997-98 El Niño event". Porewaters were extracted under a nitrogen atmosphere and analised by atomic absorption spectrometry. Subsurface maxima are observed for Fe2+ during "El Niño event" in the bay and adjacent continental shelf (86.8 ± 9.3 µM y 59.3 ± 35.5 µM, respectively). A year later when the oxygen minimum is present, the production of Fe2+ on bay surface sediments increases 3-fold. Maxima concentrations for Mn2+ appear on surface sediments during and after "El Niño event" within the bay. It is postulated that vertical displacement of maxima concentrations observed for Fe2+ and Mn2+ in these sediments are modulated by changes of the oxygen minimum depth in the water column and the amount and quality of organic matter deposited in surface sediments