Stocks tumbled on Thursday, with the Dow and S&P 500 posting their first monthly drop since January, as investors worried about Europe's economy, an Argentine default and a jump in U.S. labor costs prompted concerns about corporate margins.

"It's a combination of the ECI (Employment Cost Index) coming in hot, as well as the Argentinian situation, which we do not think is a contagion situation, but the straw that broke the camel's back," said Jim Russell, senior equity strategist for US Bank Wealth management.