Gold futures fell more than $140 an ounce, to $1,360.60, on the second straight day of steep declines. (Kamran Jebreili/AP)

A wide range of financial markets tumbled Monday amid a stunning sell-off of gold and silver, new signs of global economic weakness and still deeper fears following the afternoon Boston Marathon attack.

It was an unsettling day in many corners of the financial world. Gold futures fell more than $140 an ounce, to $1,360.60, on the second straight day of steep declines. Gold, which had experienced a sharp run-up over the past decade, has fallen about 15 percent in just two trading sessions. It is the steepest two-day decline for the metal since 1983.

But while gold has usually fallen on days when the world economy seems more solid, Monday’s pattern was very much the reverse; China reported disappointing results for its first-quarter gross domestic product, and a New York Federal Reserve survey pointed to a weakening of the U.S. manufacturing sector.

There have been anecdotal reports of falling gold prices triggering a vicious cycle of panic selling as those who had bought the metal with borrowed money face margin calls. The silver market, which has been a hotbed of speculative activity in recent years, also declined sharply.

But the drop in prices for precious metals was only the most visible and dramatic decline in a day full of them. The Standard & Poor’s 500-stock index fell 2.3 percent on the day, and there were declines on most international stock markets.

The drop in the U.S. stock market accelerated as reports of the Boston Marathon bombing began rippling across news wires and TV screens shortly after 3:15 p.m, though most of the drop happened before that point. The S&P 500 was off about 27 points at the time of the attack, then fell an additional 10 points before the market close.

Prices for major commodities including oil and copper fell significantly, if less dramatically than the price of gold. Oil futures were down 2.8 percent, falling $2.58 per barrel to $88.71.

Conversely, Treasury bonds rallied as investors sought safety, pushing down yields. Ten-year Treasury bonds yielded 1.68 percent Tuesday, down from 1.72 percent at Friday’s close and 1.80 percent as recently as Wednesday. The dollar rose, with its index against six other major currencies up 0.13 percent for the day.

Put it all together, and the pattern on global financial markets is somewhat mystifying. Gold recently spiked when the international economic outlook was most unsettled, including hitting a high of $1,911 an ounce on Aug. 22, 2011, when the euro-zone crisis was in one of its most severe phases and the global stock market was tumbling. Spooked investors fled to gold as a safe haven.

The opposite of that pattern is underway now, with gold prices plummeting even as the economic outlook becomes murkier.

April has featured a wave of weaker-than-expected U.S. economic data, including on the job market, retail sales and manufacturing. Europe, meanwhile, seems to be in an in­trac­table recession, and the latest data suggest that the Chinese economy, while still growing rapidly, is not accelerating. Its first-quarter growth was at a 7.7 percent annual rate, down from 7.9 percent at the end of 2012.

Analysts have cited speculation that Cyprus, the island nation enduring a banking crisis, could sell off some of its gold holdings, putting downward pressure on prices and perhaps triggering sales by other governments that end up getting bailout agreements.

“The selloff in commodities appears to be an overreaction to recent events,” Paul Christopher, chief international strategist at Wells Fargo Advisers, said in a note to clients, citing the soft U.S. economic data and the latest report on Chinese economic output.

“China’s economy — a key user of industrial commodities — had an unexpectedly weak quarter, but investors had three months to react to the performance indicated in today’s report,” Christopher wrote.