The limb of the HCS that veers off the coast of Peru creates a decrease in ventilation within the system. This lack of ventilation is the primary driver of an intense oxygen minimum zone (OMZ) which is formed in the sub-surface to intermediate depths. In the north, the EUC ventilates the OMZ, and in the south the PCU advects low oxygen waters southward towards northern Chile. This OMZ is the fourth largest permeant hypoxic zone in the world's oceans. It occupies an area about 2.18 ± 0.66 × 106 km3. The core of this zone is centered off Peru, creating a shallow upper boundary that reaches from about down to . Another factor contributing to the OMZ is sinking and decay of primary productive resources.