* S&P 500 ends below 1,900 for first time since Sept. 29
* Amazon among biggest drags on S&P 500, Nasdaq
* Indexes down: Dow 2.2 pct, S&P 2.5 pct, Nasdaq 3.4 pct (Updates to close)
By Caroline Valetkevitch
Jan 13 (Reuters) - U.S. stocks sank on Wednesday, pushing the S&P 500 to end below 1,900 for the first time since September and extending the year's sharp selloff, on nervousness over tumbling oil prices and U.S. earnings.
Stocks started the day higher but sentiment turned negative in afternoon trading as a brief rally in beaten-down oil prices stalled.
All 10 S&P 500 sectors ended in the red, with decliners outpacing advancing issues on the NYSE by a ratio of 7.64 to 1 and by 6.35 to 1 on the Nasdaq.
"We've been in capital preservation mode since the year began and as the market has shown an inability to rally with any conviction, that's only increased the level of nervousness and that seemed to have spilled over today in a very significant way," said Michael James, managing director of equity trading at Wedbush Securities in Los Angeles.
The declines ended a two-day rebound for the S&P 500 and resumed the steep selloff that began at the start of the year amid concerns about a slowdown in China and global growth.
The S&P 500 is now down 11.3 percent from its May 21 closing lifetime high, while the Russell 2000 small-cap index dropped 3.3 percent, putting it in bear market territory. The Russell index is down 22 percent from its June 2015 record close.
The Dow Jones industrial average was down 364.81 points, or 2.21 percent, to 16,151.41, the S&P 500 had lost 48.4 points, or 2.5 percent, to 1,890.28 and the Nasdaq Composite had dropped 159.85 points, or 3.41 percent, to 4,526.07.
The CBOE Volatility index, Wall Street's favorite gauge of uncertainty, gained 12.2 percent.
The market has put together 10 intraday rallies at the outset of 2016, and every single one has failed to sustain itself.
Analysts said nervousness about fourth-quarter earnings added to the bearish tone. CSX was down 5.7 percent at $22.35 after the railroad company's fourth-quarter profit fell on declining freight volumes.
"Today we gapped open again and the buyers retreated. There's no catalyst to really take it higher. If you start getting bank and other earnings that are really bad, nothing is going to hold," said Ken Polcari, director of the NYSE floor division at O'Neil Securities in New York.
Amazon fell 5.8 percent to $581.81 and was among the biggest drags on the S&P 500 and the Nasdaq.
About 9.8 billion shares changed hands on U.S. exchanges, compared with the 7.5 billion daily average for the past 20 trading days, according to Thomson Reuters data.
The S&P 500 posted 2 new 52-week highs and 108 new lows; the Nasdaq recorded 7 new highs and 510 new lows. (Editing by Chizu Nomiyama and Meredith Mazzilli)