U.S. stocks declined sharply on Wednesday, with the fourth quarter starting off on a dour note after the S&P 500's seventh quarterly gain, as investors fretted global concerns, mixed U.S. economic data and earnings ahead.

Benchmark indexes posted their worst beginning to October since 2011 as investors sought safety in U.S. Treasury bonds and gold,with the CBOE Volatility Index, a measure of investor uncertainty, rising. The Russell 2000 fell into correction territory, down 10 percent from its July record.