Revista Chilena de Historia Natural 77 (1): 121-138, 2004
RESEARCH ARTICLE
Redox-sensitive metals evaluation as proxis of paleoxygenation in a hypoxic marine
environment of northern Chile
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<sup>-1</sup>. 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.
Key words:
redox-sensitive metals,
paleoxygenation, marine sediments, Oxygen Minimum Zone, Mejillones bay
