Abstract

The distribution of molybdenum, cadmium, zinc, vanadium and nickel, all redox-sensitive metals, was analyzed under a spatial (surface sediment samples) and temporal (core samples) perspective, in order to identify the potential of these metals for paleoxygenation interpretations in Mejillones bay, a hypoxic environment of northern Chile. Lithogenic supply (according to aluminum) was discarded. Biogenic flux is not the unique factor that controls the mechanism of metals preservation in sediments of this bay. Spatial variation of metals concentration was investigated in relation to the bottom water oxygenation in each sampling station. Results showed that only Ni and Cd present a strong correlation with dissolved oxygen, increasing their concentration toward the deepest area of the bay were an oxygen minimum zone prevails with values around 0.02 mL L-1. The temporal variation in redox-sensitive metals, recorded in three sediments core covering the last 2,000 years, showed that bottom oxygenation of the bay changed from a more oxygenated environment in the past to a less oxygenated environment at present. A biogenic flux across a very well developed oxycline, associated to an anoxic sediment-water interface, are proposed to be the principal factors to explain the metal concentrations preserved in Mejillones sediments during non-El Niño condition. When El Niño is present in the bay, the water column turn oxic, and the precipitation of redox-sensitive metals drops; so the concentration of metals decrease in the sediments. Thus, differences in concentration of metals in core top (sampled in 1996) and surface sediments (sampled in 2000) in the same area of the bay, were attributed to El Niño 1997-1998 that provoked a decrease in the flux of metals (principally nickel and cadmium) toward the sediments in a more oxygenated bottom environment. All these information show that among the five metals analyzed, only nickel and cadmium seem to record paleoxygenation conditions in Mejillones. However their use as proxy is limited to the cases when extreme changes from anoxic to oxic conditions occurred in bottom sediments of this bay.