Catalysis of the reaction: polysaccharides containing beta-D-mannuronate residues = oligosaccharides with 4-deoxy-alpha-L-erythro-hex-4-enopyranuronosyl end. This reaction is the eliminative cleavage of polysaccharides containing beta-D-mannuronate residues to give oligosaccharides with 4-deoxy-alpha-L-erythro-hex-4-enopyranuronosyl groups at their ends.