New York (CNN Business) Glee about a big tech merger sent stocks soaring Monday, but then dread about the threat of new tariffs on China sent them plunging. The Dow swung more than 900 points from its highs of the day to its lows and finished with a more than 245-point loss.

Stocks turned lower in the afternoon after Bloomberg reported that the White House is considering more tariffs on Chinese goods by December if the next round of negotiations between President Donald Trump and Chinese leader Xi Jinping do not go well.

"We'll see what happens," said White House Press Secretary Sarah Sanders on Monday about the meeting. "We're not going to get ahead of those conversations."

The S&P 500 fell 0.7% and the Nasdaq dropped 1.6%. The Dow, which had been up more than 350 points earlier Monday morning, is now more than 9% below its all time highs. Both the Dow and S&P 500 gave up their gains for the year as well.

The Dow was dragged lower in particular by Boeing ( BA ), which has significant exposure to China and also has the highest stock price in the Dow. Shares fell 6.6%. (The Dow is weighted by stock price, not market value like most other indexes.)