Abstract

This work analyzes the seasonal and spatial variability of the Oxygen Minimum Zone (OMZ) and its influence on the composition of the organic matter deposited on the bottom sediments of Mejillones Bay. The obtained results indicate that the upper limit of the OMZ fluctuated during the year between 20 and 45 m depth. In summer, this limit ranged from 20 to 30 m depth, whereas in winter it ranged from to 40 to 55 m depth. According to a Rock-Eval analysis, the organic matter present in the deepest sediments (100 m) is better preserved than the shalow material (35 m). This analysis revealed that the sediment deposited in the bay is marine phytoplanktonic in origin (Type II) and immature. The ratio of total sulfur to total organic carbon ranges from 0.8 to 3.9% at station A (35 m) and from 5.7 to 10.3% at the deeper stations (70 and 100 m). The C:N ratio in the sediments varies between cold and warm periods, with respective averages of 9.13 and 7.74 and a wintertime peak of 11%. The ANDEVA results indicate that the dissolved oxygen (P < 0.001), total carbon (C) (P < 0.001), total nitrogen (N) (P < 0.008), total organic carbon (TOC) (P < 0.001), and hydrogen index (HI) (mg HC g COT-1) (P < 0.019) differ significantly by depth. The dissolved oxygen concentrationwas found to be related to the conservation of organic matter in the sediments: the organic matter in the deeper sediments was preserved better due to the effect of the OMZ.