While U.S. political deadlock is delaying tapering of unprecedented monetary stimulus, U.S. President Barack Obama announced what he called probably his most important economic decision, nominating Federal Reserve Vice-Chair Janet Yellen as the next chairman of the nation's central bank on Wednesday. Ms. Yellen not only replace Ben S. Bernanke, who has been leading the Fed for eight years, but also has become the first woman ever to head the 100-year-old U.S. central bank.