Canada's benchmark stock index lost almost 2.8 per cent on another bleak Monday for commodities like oil, gold and copper.

The S&P/TSX Composite Index lost 373 points to close at 13,004. That's the lowest level for Canada's benchmark stock index since October 2013.

The TSX is now down by almost six per cent since the start of September. If that holds until Wednesday, the last day of the month, it will be the worst month for the index since 2012, and the June-to-September period would be the worst three-month period for Canada's benchmark stock index since 2011.

Almost all of the sub-sectors were lower. Commodities were especially hard hit as the December gold contract fell $13.40 to $1,132.20 US an ounce and the November crude oil contract was down $1.23 at $44.47 US a barrel.

The gloom in commodities was largely tied to more news out of China about that country's slowing economy. Profits at Chinese industrial firms dropped by the largest amount on record since Beijing started releasing the data in 2011.

"Whenever the market is down, the first place to look these days is China," John Manley, chief equity strategist at Wells Fargo Fund Management, told The Associated Press.

"Right now, we need evidence that China is not slowing that much and that profits are still going to be OK."

Alcoa splitting in two

In corporate news, one of the world's largest mining companies, Alcoa, was a bright spot for mining stocks with the stock rising about six per cent on the NYSE. But that optimism was only because the company announced it was splitting itself into two, to insulate its growing and profitable aerospace and automotive business from its sagging base metals business, which primarily consists of aluminum assets.

Prior to Monday's bounce, Alcoa shares had lost more than 40 per cent this year as the price of metals cratered.

The Canadian dollar lost almost half a cent amid the gloom, to close at 74.66 cents US.

The financial sector was also lower, as were health-care stocks. The main culprit was Canada's largest drug company, Valeant Pharmaceuticals, which was off by almost 10 per cent. Valeant shares have slid for five straight trading days, ever since drug prices became a campaign issue in the U.S. election last week. The company's shares have lost 22 per cent since the sell-off began. Valeant has fallen from being Canada's biggest company to its third-largest by market capitalization in the process.

Traders agreed it was an ugly day.

Barry Schwartz, chief investment officer, Baskin Wealth Management said the key for investors is to look long-term. He said he sees buying opportunities because many stocks are at bargain valuations.

"I'm hoping that calmer people will come into the market and realize that some of these prices are too good to be true," he told CBC News.