Investors are worried U.S. earnings growth may finally fall back to earth as turmoil in Europe and signs of a less robust Chinese economy hurt foreign support.

The euro zone's debt crisis and weakness in China have fueled investor concern that the global economy could tip back into recession, possibly dampening U.S. earnings growth at a time when the U.S. economy is still struggling to gain ground.

Overseas sales have helped U.S. companies beat earnings expectations in the last couple of years, with foreign sales totaling 30 percent on average for Standard & Poor's 500 companies.

If the euro region is crumbling, that's going to have a tremendous negative impact on companies like McDonald's , said Todd Schoenberger, managing director at LandColt Trading in Wilmington, Delaware.

I'm not expecting a big earnings quarter, he said. We've been getting the clues already.

The most recent company to trouble investors about the earnings outlook is Ingersoll Rand Plc , whose shares tumbled 12.1 percent to $28.09 on Friday after the industrial conglomerate cut its third-quarter and full-year earnings forecast to below market estimates.

Investor pessimism is already high.

The S&P 500 <.SPX> finished the quarter with its worst performance since 2008, and many strategists have slashed their forecasts for year's end.

The S&P 500 dropped 14.3 percent in the third quarter, losing about $1.7 trillion in market capitalization.

A disappointing third-quarter earnings period, which begins the second week of October, could only trigger more losses, analysts said. Stronger-than-expected earnings helped stocks claw back from 12-year lows in 2009.

This week, investors also will be bracing for data on the U.S. job market, among the weakest parts of the economy. The government's September employment report is due Friday, while U.S. manufacturing data from the Institute for Supply Management is due Monday. The ISM services-sector index is set for release on Wednesday.

CURRENCY CUSHION MAY BE THINNER

Companies reporting earnings have benefited for the last decade from weakness in the dollar, which helped overseas revenue figures.

With the euro down 7.4 percent this quarter, the biggest quarterly loss by percentage since mid-2010, companies could lose some of that currency cushion.

I think you'll see a lot of companies blaming problems on Europe, said Justin Walters, co-founder of Bespoke Investment Group in Harrison, New York.

Walters said excluding companies that report no international sales, the average percentage of overseas revenue for the S&P 500 is 41 percent.

The euro-zone debt crisis has investors worried about a repeat of the 2008 financial crisis.

In China, which has been a major engine of growth for the global economy, data has shown some weakness. On Friday, figures showed the country's manufacturing shrank for the third month in a row and had the longest contractional streak since 2009.

Analysts have slowly been reducing earnings forecasts for the quarter.

Third-quarter earnings are expected to have risen 13.3 percent from a year ago, according to Thomson Reuters data. The forecast was for 17 percent growth on July 1.

If there's a very drastic downturn in the European economic zone, that portion of U.S. earnings will be impacted, said Natalie Trunow, chief investment officer of equities at Calvert Investment Management in Bethesda, Maryland, which manages about $14.8 billion.

But she and other strategists are optimistic that the earnings period will not disappoint, and could even present a buying opportunity.

U.S. multinationals don't necessarily derive all of their additional earnings (from Europe), and in China, data seems to be showing a slowdown but not in hard-landing territory, Trunow said.

Other strategists said the dramatic cost-cutting that U.S. companies started in the 2008 financial crisis will help to keep bottom-line earnings numbers relatively healthy.

In our view, corporate America has learned to make money in this environment, said Hank Smith, chief investment officer at Haverford Trust Co in Philadelphia.

(Reporting by Caroline Valetkevitch; Editing by Jan Paschal)