Abstract

This study aims to establish the variability of the main oceanographic and climatic features in recent sedimentary records of a transitional semi-arid ecosystem, susceptible to environmental forcing conditions. Coquimbo (30°S) is located south of the Atacama Desert, the driest desert in the world, and to the north of the wettest region in the hemisphere. The main features of this area are dry summers and short rainfall periods during winter. The region shows a wet-winter climate, generated by interactions between the southern Westerly winds and the South Pacific Anticyclone (SPA). In summer the SPA moves southwards while in winter it returns to the northern zone, creating storm fronts (Maldonado and Villagrán, 2002; 2006). This semi-arid zone is strongly affected by variations associated to El Niño-Southern Oscillation (ENSO), caused by seasonal latitudinal changes in the SPA that produce high variability and precipitations at Chilean mid-latitudes (Lamy and Kaiser, 2009). The warm phase of the ENSO cycle occurs over intervals of several years and has been associated with climatic and economic impacts on the Peruvian-Chilean coasts (Enfield, 1989; McPhaden, 1999). This event is characterized by the presence of oxygen-rich and nutrient-poor waters that prevent high primary production during the upwelling periods along the Chilean margin, as observed on the coast off Antofagasta, Iquique and Concepción (20-36°S) during the 1997–98 El Niño when nutrient-poor warm waters and substantially lower primary production were measured (Escribano et al., 2004). Furthemore, the impact of ENSO on the oxygenation at the bottom has been observed from the central Peruvian coast to the southern Chilean regions (12°S-36°S) (Escribano et al., 2004; Gutiérrez et al., 2008). In the Coquimbo region, made up of several bays (Fig. 1), high biological productivity is developing above a narrow shelf that helps the development of intense aquaculture and fisheries in the zone. A large counterclockwise circulation at the southern sites, between Tongoy and the Coquimbo Bays (Valle- Levinson and Moraga-Opazo, 2006; Moraga-Opazo et al., 2011) establishes retention zones that probably conduct the presence of this high pigment concentrations very close to the coast. Otherwise, low oxygen conditions are observed at shallow depths (30 m) in some periods by the intrusion of the oxygen minimum zone closest to the bays developing suboxic conditions suitable for the precipitation of redox sensitive trace metal (Calvert and Pedersen, 2007 and references therein) . Although, extensive anoxic areas have not developed here, the suboxic conditions and moderated organic particle fluxes to the bottom provide an adequate environment for forming sedimentary records that could show the main oceanographic and climatic variability in the SE Pacific region. From recent marine core sediments (2-3 m length) from the southern bays of Coquimbo (Tongoy, Guanaqueros and Coquimbo bays), we expect to identify changes in the environmental forcing from a decadal to a centennial scale using trace metals, and organic carbon. The time and mass accumulation rates will be determined using 210 Pb and 14 C geochronology. Complementary information will be obtained from sediment cores at the main wetlands of the zone in order to integrate atmospheric and oceanographic variables. Pollen and diatom records will be used as paleo-tracers establishing environmental changes at a centennial-millennial scale. Therefore this is a newly approach integrating terrestrial and marine paleo-tracers that allows to identify the main atmospheric and oceanographic variables that are defining the climatic variability of this region, and could help to better understand the frequency of regional phenomenon as El Niño events in the past.