Wary financial markets are watching the decline in copper, to see if the red metal is acting as a fire alarm for the global economy.
With a quiet economic calendar Wednesday, the Treasury's 10-year auction could also take on a higher profile than normal, especially ahead of next week's Fed meeting.
Stocks wobbled Tuesday, as concerns swirled that the economy is not growing fast enough, and traders were most worried that China could be stumbling. The Shanghai stock market was higher overnight but U.S. stocks fell with copper, a bellwether on the Chinese economy.
"Copper and iron ore are heavily used in China as collateral on loans," wrote Steve Scacalossi, director, head of sales global metals at TD Securities. "With the stress in the credit market and tightening controls unwinding of the carry trades would have an impact on the spot price—so it is difficult to gauge how much of the selloff is due to falling growth expectations and how much due to unwinding of carry trades."
Copper slumped to its lowest level in nearly four years, with May futures off 2.5 percent to $2.9520 a pound and now down 8 percent in three sessions. China is responsible for 40 percent of the world's copper imports.
Concerns about China are two-pronged. A recent report showed that February exports—a major engine for its economy—fell 18.1 percent from a year earlier. There are also worries about the country's financial system, and those concerns intensified when Shanghai Chaori Solar defaulted on its debt Friday, the first ever in China.
(Read more: China, copper and the U.S. stock market)
Traders circulated reports Tuesday that trading in another solar company, Baoding Tianwei Baobian Electric's stocks and bonds was suspended after it reported losses.
Since copper is used for collateral for bank loans, the concern is that more of the metal could flood the market if there are further defaults.
"There's a reasonable amount of concern out there about the general direction of the global economy," said Ian Lyngen, senior Treasury strategist at CRT Capital.
Events in Ukraine were also a concern, sending buyers into gold, as investors await the Sunday vote in Crimea on whether to join Russia and abandon Ukraine.
Bond yields were lower Tuesday, but the three-year edged higher. The Treasury's $30 billion three-year note auction was lackluster, and Lyngen expects 10-year yields to rise into the 1 p.m. ET auction of $21 billion in 10-year notes.
(Read more: What could heat up gold prices this week)
The 10-year was yielding 2.77 percent in late trading Tuesday. The Dow fell 67 points to 16,351, and the fell 9 to 1,867.
Markets have been watching every bit of economic data, in an attempt to glean whether the string of weaker data are the result of unusually severe winter weather or something else. Every bit of employment-related data are even more scrutinized because of the potential impact on the Fed's decision to cut back on its bond-buying program.
The Fed meets next week, with Fed Chair Janet Yellen presiding for the first time and conducting the post-meeting press briefing.
"Is anyone worried about the Fed? No, they're going to taper," said Lyngen.
(Read more: Stock market in 'euphoria mode': Citi strategist)
There are a few reports for markets to watch Wednesday. There are mortgage applications at 7 a.m. and oil inventories at 10:30 a.m. There is a release on the federal budget at 2 p.m.
"You do have auctions. That's what's driving the week, and the 10-year tends to be the most important of the week, and that will probably drive things tomorrow," said Ward McCarthy, chief financial economist at Jefferies. "The (bond) market has been trading well so far this week, which makes it look like it's going to be difficult to bid on this baby. We continue to see buying."
McCarthy said a lot of big bond market investors were caught short. "There are a lot of real money accounts that are gradually trying to get their positions a bit more balanced," he said.
—By CNBC's Patti Domm. Follow her on Twitter @pattidomm.