U.S. stocks closed sharply lower in another volatile session Wednesday. The Dow Jones industrial average fell 351.98, or 1.5 percent, to 23,323.66, its lowest level of 2018. The broader Standard & Poor's 500 index fell 39.20, or 1.5 percent, to 2,506.96. The technology-heavy Nasdaq composite index fell 147.08, or 2.2 percent, to 6,636.83.