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Se analizó la distribución de molibdeno, cadmio, zinc, vanadio y níquel, todos metales redox-sensitivos, bajo una perspectiva espacial (sedimento superficial) y temporal (testigos de sedimento), con el propósito de determinar su potencial como indicadores de paleoxigenación en la bahía Mejillones del Sur, un ambiente hipóxico del norte de Chile. El aporte litogénico (de acuerdo a aluminio) fue descartado. Además, el flujo biogénico no es el único factor que controla la preservación de metales en esta bahía. La variación espacial en la concentración de metales fue investigada en relación con la oxigenación de las aguas de fondo. Los resultados mostraron que solo el Ni y el Cd presentan una correlación significativa con el oxígeno disuelto, aumentando sus concentraciones hacia el sector más profundo de la bahía, en donde prevalecen condiciones de mínimo oxígeno, con valores cercanos a 0,02 mL L-1. La variación temporal en la concentración de metales redox-sensitivos, registrada en tres testigos de sedimento que cubren los pasados 2.000 años, mostró que, en general, la oxigenación de fondo de la bahía ha cambiado desde un ambiente más oxigenado en el pasado, hacia un ambiente menos oxigenado en el presente. Se propone que el flujo biogénico a través de una oxiclina bien desarrollada, asociado a una condición anóxica de la interfase agua-sedimento son los principales factores que permiten explicar la concentración de metales que se preserva en los sedimentos marinos de Mejillones durante condiciones no-El Niño/La Niña. Cuando El Niño está presente en la bahía, se profundiza la oxiclina, aumentando el nivel de oxigenación de las aguas subsuperficiales al interior de la bahía, lo que provoca que la precipitación de metales hacia los sedimentos disminuya, favoreciendo, a su vez, la liberación de metales desde la interfase agua-sedimento. Basado en esto, se plantea que las diferencias en la concentración de metales registrada entre las muestras de la parte superior de los testigos (correspondientes a antes de 1996) y las muestras superficiales (correspondientes al año 2000) y tomadas en la misma zona de los testigos, pueden ser atribuidas al efecto de El Niño 1997-1998, el cual provocó un brusco descenso en el flujo de metales (principalmente Ni y Cd) hacia los sedimentos, debido a una condición más oxigenada de la columna de agua. Este trabajo muestra que de los cinco metales analizados, solo en Ni y el Cd parecen tener un potencial como indicadores de paleoxigenación en la bahía Mejillones. Sin embargo, su uso está restringido para identificar cambios extremos desde una condición anóxica a una óxica en los sedimentos de fondo de esta bahía.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

Abstract: The distribution of, Co, Mo, Zn, Ni, Cd, U, Cu, Mn, V, Al, Fe, Ti, Total Organic Carbon (TOC) and Total Sulfur (TS) was studied in surface sediments accumulated at the upper limit of the oxygen minimum zone in the northern Humboldt Current (Mejillones Bay), using samples taken during two cruises in May 2002 and October 2004. The time series of dissolved oxygen revealed a seasonal fluctuation of the upper limit of the oxygen minimum zone into the bay. Similar bathymetric distribution curves of the metals measured in the two studied periods in the OMZ off northern Chile suggest that the factors responsible for this distribution are persistent over time. Authigenic metal content was calculated using lithogenic background by determining Metal/Al ratio from local aeolian dust, instead of average shale values, which are classically used for background correction. This more precise assessment allows examination of the source–sink relation of each metal in this coastal zone. Mo preserved in the bottom sediments originates from seawater and accumulates via particulate shuttle, whereas Cd is pre-concentrated in biodetritus. Ni, Mn, V and Fe show a combined behavior. Sulfate reduction is not the most important mechanism affecting metal accumulation in Mejillones Bay, probably due to the presence of Thioploca, which inhibits the pyrite formation and the metal-sulfide reaction. We propose the use of the authigenic Mo/U ratio and a preliminary statistical model to reconstruct the levels of paleoxygenation in the subsurface waters of Mejillones Bay, by analyzing the same metals measured throughout the sedimentary sequences (and using the same statistical procedure), at least to identify extreme variations of the OMZ in this bay of northern Chile. [Copyright &y& Elsevier]

Concentrations of Ni, Cd, Pb, Cu, and Zn were measured in the surface sediments of Mejillones Bay at 32 sites ranging in depth from 10 to 95 m. A similarity dendrogram analysis separated the study area into two large sedimentary facies: a coastal zone with low metal concentrations and a deep zone with high metal concentrations. The abundances of Zn, Ni, and Pb increased between two sampling periods (5 years apart) in association with greater anthropogenic activity along the coastline of the bay at the present. The results were analyzed with the index of geoaccumulation and by applying the sediment quality guideline. Although similar to preindustrial levels, current Cu and Ni concentrations pose a likely threat to communities of benthic organisms. Concentrations of Zn and Pb are enriched, probably in connection with the industrial development of the zone, but do not reach dangerous levels for the benthic communities. Cd concentrations are toxic to benthic life, even though those measured herein are similar to preindustrial levels. Thus, these Cd levels are a result of the bay's natural characteristics (e.g., high biological productivity, the microxic water-sediment interface). The results of the Chilean aquatic sediment quality standards and the international standard (sediment quality guideline) were similar but differed from those found using the index of geoaccumulation, which suggests that the proposed reference values for Chilean aquatic sediment should be reviewed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dissolved oxygen controls the preservation of Ni, V, U and Mo in marine sediments. • Paleoxygenation reconstructed with a novel statistical model based on trace elements. • Oxygen Minimum Zone intensity of northern Chile varied during the last millennia. Mejillones bay presents a permanent and shallow OMZ, whose upper boundary fluctuates between 30 m and 60 m water depth during a normal year. Under this condition, the bottom sediments preserve a geochemical signature of the past oceanic variability. Particularly, biological productivity and bottom dissolved oxygen can be reconstructed using 13C, 15N and trace elements metals measured along sediment cores. Trace elements like V, Ni, Mo and U preserved in surface sediment of Mejillones bay as a consequence of poorly oxygenated bottom waters, were used to develop a statistical model that allowed the reconstruction of the bottom water dissolved oxygen variability during the last 700 years. During these seven centuries, the marine sediments of Mejillones recorded three ocean-climatic periods. From 1350 to 1500 CE the bay was dominated by a strong and shallower OMZ, intense denitrification, but a low productivity and flux of biodetritus. Bottom waters recorded oxygen values around a mean of 3.52 µmol L−1, during this period. During the Little Ice Age (1500 to 1850) the OMZ was less intense, and bottom waters recorded mean dissolved oxygen of 5.35 µmol L−1. Finally, during the Current Warm Period (1900 to date) the OMZ has strengthened and is located at shallower depth, generating a bottom water environment with values of dissolved oxygen slightly lower than in the previous period, around 4.51 µmol L−1. Other studies show that during these same climatic periods the Sea Surface Temperatures changed from warm waters before the LIA, a succession of cold and warm periods during the LIA, and warm waters at the beginning of the CWP, but a prevalence of cold waters since the 1940s decade until the present. The sedimentological approach to reconstruct the paleoxygenation of bottom environments proposed for Mejillones bay must be considered as preliminary and of local-use potential, thus being necessary to explore similar methodologies in other environments in order to support and improve this approach. [ABSTRACT FROM AUTHOR]