Reflecting financial uncertainties worldwide, share prices on the Tokyo Stock Exchange tumbled Monday with the benchmark 225-issue Nikkei average closing below 13,000 for the first time since Jan. 30, 1986.Trading closed at 12,948.12, down 275.57 points from Friday's close of 13,223.69 as sluggish afternoon trading failed to buoy prices after a sharp drop in the morning.Traders said one reason behind the Tokyo market's bearishness was Japan's failure to produce specific stimulus measures during the weekend Group of Seven meeting of financial ministers and central bankers of the world's leading industrialized countries.Deceleration in previously robust economies, including the U.S., led traders to shed stocks of major exporters, including blue-chip electronics manufacturers.The TOPIX index, which includes all first-section issues, also dropped to a 13-year low, falling 17.33, or 1.7 percent, to end at 996.69, its first sub-1,000 close since November 1985.Traders supported bank stocks in view of the G-7's call for Japan to take quick action and infuse them with public funds, but international blue chips were hit hard as foreign traders moved to expand selling to other issues, such as communications stocks NTT and KDD.