MEXICO CITY — Mexico's U.S.-backed naval special forces have captured a man believed to be one of the two top leaders of the Gulf cartel, a drug-trafficking organization that once dominated the northeast border region but has recently engaged in devastating battles with the vicious Zeta paramilitary force, authorities said Tuesday.Mario Cardenas Guillen, alias El Gordo ("Fatso"), was paraded before reporters in Mexico City on Tuesday after his capture Monday in the northern border state of Tamaulipas.