Micro-organisms and bugs are a amjor factor in the surival of archaeological artifacts. Just how well artifacts survive depend a lot on how alive the soil in which they are buried is. You could do an experiment that demonstrates the preservation and decay of certain artifacts in different soil conditions. The acids in the soil affect the survival of artifacts. Gardeners test the acidity of soil all the time to determine what grows best in their flower beds.
Archaeologists test the acidity of the soil to test what survives best.
Law of Superposition. You will also have top choose an archaeological site to serve as your example. Pompeii would make a good story. You will then reenact your archaeological story inside your small aquarium. Build your story up layer by layer using different soils.