Abstract

Trace metals in anoxic sediments are enriched in different ways, and have recently been used to reconstruct ancient environments. Some of them are closely related with the primary productivity due to the fact that they are biological essential elements, reaching the sediments during the organic matter sedimentation. Others elements, such as U and Mo are conservative in the water column but affected by the redox environmental conditions, reaching the sediments by authigenic precipitation or adsorbed by organic or inorganic carriers. In anoxic sediments, the sulfide concentration could determine the immobilization of trace metal within the sediments by incorporation in the pyrite (e.g. Co); however, the kinetic reactions of other elements result in soluble metal sulfide formation prior to iron sulfide and subsequent pyrite formation (e.g. Cd). In this sense, the metal distribution in the sediments shows different geochemical pathways, and different environmental interpretations are plausible. In order to adequately interpret their distribution in the sediment through time it is thus necessary to understand the recent processes in which they are involved. The high concentrations of Cd, Co and Ba, reported in the upwelling zones off Peru and Chile, have been related with the primary production. Recently, the hypoxic areas along the margins (OMZ) of NW Africa, NE Pacific and Arabian Sea have been studied in regards to their role in the major ocean-atmospheric processes, using trace and minor metal distribution in the deep sediments as proxies for paleo-oxygenations and paleo-productivity. Efforts to understand the biological communities in the benthos and how physico-chemical properties are affecting them have been also reported. Central-south Chile (~35-38°S) has an important upwelling center due to the predominance of southwesterly winds (Arcos and Wilson, 1984), featuring high primary productivity rates (9.9-19.9 g C m-2 d-1) during the spring-summer period (Bernal et al., 1989; Fossing et al., 1995; Dellarosa, 1998; Daneri et al., 2000). Here, the OMZ extends over the shelf and it has been reported that during the 1997-98 warm ENSO event (i.e. El Niño) an unusual oxygenation during the summer period was observed. The appearance of the sediments was less reduced, in some cases until 10 cm depth, with positive values of redox potential at the surface (Muñoz et al., 2004a). The most evident response of the benthic fauna was a switch in the species composition, deeper penetration of the fauna into the sediments and an increased body size of the organisms. After this El Niño, the dissolved oxygen decreased gradually to reach usual low concentrations (<0.5 ml L-1); dropping well below 0.25 ml L-1 during 2003-2004, significantly lower than values reported in previous years (Gonzalez et al., 2005). Therefore, this area offers an ideal scenario to study the geochemistry of trace metals sensitive to redox conditions. Establishing the extension by which post-depositional processes affect the distribution of trace metals would allow us to evaluate their use as proxies of paleoceanographic processes. It is thus the aim of this project to understand the geochemistry of sensitive redox trace metals (Cd, Co, U, Mo) related to carbon fluxes to the sediments and/or authigenic enrichment in a seasonally hypoxic shelf area (36°S). Other elements related with primary production fluxes, not sensitive to redox conditions are also considered (Ba, Opal). Besides, the Fe, Mn and Al will be analyzed in order to establish the oxidative state of the sediments and detritus inputs to the shelf. To achieve this objective, fluxes that sustain metal distribution in the sediments must be estimated. The sedimentation and mass accumulation rates will be determined using 210Pb and 14C geochronology. Considering the anoxic-hypoxic properties of the shelf off Concepción, a net inclusion of trace metals into these sediments is expected, which could be reverted during the strongest anomalies in oxygenation as during the warm phases of ENSO. Short sediment cores will be studied in order to evaluate seasonal changes in recent sediments; and longer sediment cores will be used to identify decadal changes in ancient layers (> 100 years BP). It is important to mention that an important program, related with the Humboldt current system (HCS) along the Chilean Margin, is being developed since 2002 (the Center for Oceanographic Research in the eastern South Pacific, FONDAP-COPAS, University of Concepcion). This program is focused on the relevance of the HCS in the climate system and is composed of 6 subprograms; one of them related to paleoceanography and climate changes of the Late Quaternary and the Holocene, in which the PI of the present proposal is involved. In this subprogram, other key variables preserved in the sedimentary record (diatoms and foraminifers), that describe the variability associated with the ENSO cycle, the strength of the OMZ and the ventilation by AAIW (Antarctic Intermediate Water), are being studied. This proposal offers a new and complementary proxy that will contribute to understand the interdecadal variability associated with the ENSO cycle and/or the OMZ strength, establishing the role of the OMZ as a sink or source of these elements, and their implication in the biogeochemical cycles.