Oxygen minimum zone variability during the last 700 years in a coastal upwelling area of the Humboldt system (Mejillones, 23? S, Chile). A new approach from geochemical signature

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 ?mol 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 ?mol 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 ?mol 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.