A zone of rapid transition between areas of aerobic and anaerobic decomposition in oceanic sediments. Its depth within the sediment depends on the quantity of organic matter available for decomposition and the rate at which oxygen can diffuse down from the overlying water. For example, in organic muds, relatively impermeable to oxygen-carrying water, the upper aerobic layer may only be a couple of millimetres deep, while in permeable sands with a low rate of organic input aerobic conditions can extend for tens of centimeters.