Brazil's depressed currency rebounded Thursday after the head of its central bank vowed to use all instruments in its arsenal to curtail the Real's collapse. Earlier in the day the Real tumbled to an all-time low of 4.248 to the U.S. dollar, but bounced back to 4.023 after central bank President Alexandre Tombini, in an unscheduled press briefing, did not rule out selling part of the country's $371 billion foreign reserves to calm the exchange rate market.