I recently graduated from an environmental studies MS program. People had backgrounds in biology, engineering, environmental science, history, sociology, psychology, etc. You have to decide what realm your research will fall within (biology, political science, history), and you have to find an angle to observe a certain type of environmental problem. My background was geography/environmental science and my MS thesis was grounded in political science, so you don't have to stick with your undergrad focus. If you want to stick with marketing, I'm assuming you'll be going the route of marketing sustainable goods and how to achieve specific environmental goals through marketing. But I don't know much about that world
As long as you complete some prerequisite coursework and hammer out a good statement of purpose, I think you will easily get into a master's program.