text: Stratified waters, in combination with slow vertical mixing, are essential to maintaining euxinic conditions. Stratification occurs when two or more water masses with different densities occupy the same basin. While the less dense surface water can exchange gas with the oxygen-rich atmosphere, the denser bottom waters maintain low oxygen content. In the modern oceans, thermohaline circulation and upwelling prevent the oceans from maintaining anoxic bottom waters. In a silled basin, the stable stratified layers only allow surface water to flow out of the basin while the deep water remains anoxic and relatively unmixed. During an intrusion of dense saltwater however, the nutrient-rich bottom water upwells, causing increased productivity in the surface, further enhancing the nutrient trap due to biological pumping. Rising sea level can exacerbate this process by increasing the amount of deep water entering a silled basin and enhancing estuarine circulation.