Wall Street pulled back today, sending the Dow Jones industrials down more than 100 points after severe credit-related losses at Merrill Lynch & Co. and a steep drop in September’s existing home sales.

Merrill said it wrote down $7.9 billion in fixed-income instruments called collateralized debt obligations and from defaulting subprime mortgages — more than the $5 billion writedown the investment bank estimated earlier this month. The result was a net loss for the quarter of $2.3 billion, after total revenue plummeted 94%.

The worse-than-anticipated loss signaled to investors that the financial sector may be in a more dire situation than feared because of the credit squeeze that was triggered in part by spikes in mortgage defaults.

And the housing market appears far from recovery. The National Association of Realtors reported that existing home sales fell in September for the seventh straight month by a larger-than-expected 8% — the largest decline in records dating back to 1999.

Meanwhile, the technology sector appeared to be losing momentum from earlier in the week. Amazon.Inc. said late yesterday its quarterly profit more than quadrupled, but it only beat per-share estimates by a penny. Investors didn’t see enough reason to bring the Internet retailer’s shares, already at their highest level since 1999, even higher, so the Internet retailer plunged 13%.

The Dow fell 112.51, or 0.82%, to 13,563.72, giving back yesterdays 109-point gain.

Broader stock indicators also dropped. The Standard & Poor’s 500 index fell 16.52, or 1.09%, to 1,503.07, while the Nasdaq composite index lost 49.57, or 1.77%, to 2,749.69.