Get an aerator that will cycle the water of your size pond. It needs to be down with the muck. You also may need several heads or slowly move it around over time. Make sure that you don't run it constantly at first as too much of that stuff will cause a fish kill.

You will increase the D.O. , reduce or iliminate the muck, and get rid of the smell.

An Aerator is the best investment you can make for your pond and its wildlife. I puit one in this summer on a less than 1 year old pond and was amazed at the smell that came up from the bottom initially. Now there is no smell at all. We now run it 24 hours a day during the warm months. Not sure yet what i'll do as it gets cold. Need to consult some experts and see what they say.