Oil falling below $100 a barrel for the first time in 15 months is a signal that Russia can’t stay out of the bond market indefinitely after the Finance Ministry canceled its eighth auction in a row. Crude dropped to $98.96 in London today, what would be the weakest level on a closing basis since April 17 last year and a 12 percent retreat in the third quarter. Ruble-denominated debt handed investors the biggest loss in the period in dollar terms among 31 emerging markets monitored by Bloomberg . With no auctions since mid-July, a steeper decline in Russia’s main export earner could flip the nation’s $19 billion budget surplus into a deficit by year-end, according to Bank of America Corp. U.S. and European Union sanctions over Ukraine drove 10-year ruble borrowing costs 120 basis points higher since June. That compares with a 66 basis-point drop in Brazilian yields. […]