GULF OF MEXICO — It’s thousands of square miles wide, virtually devoid of oxygen and it has been blamed for an increase in shark attacks: the Gulf of Mexico “Dead Zone” is getting bigger and forcing marine life — including sharks — into shore. The zone has been caused by a flood of nutrients, such as agricultural fertilizers, which boost algae production in the sea. These growths consume huge amounts of oxygen, creating a “marine desert.” Similar “Dead Zones” are being discovered across the world, and a major United Nations report in 2003 found that the number had doubled each decade since the 1960s.