My understanding of the types of microbial action in food:
Curing: The "non-action" where meat is essentially rendered too salty for much of anything to grow. Sometimes fermentation is used as well, but my understanding is that carbohydrates are added if so (see below).
Fermentation: This is anaerobic degradation of carbohydrates. Yeast in bread and beer comes to mind, as does cheese. The acid preserves food. Odors are generally sour.
Putrefaction: This is anaerobic degradation of proteins. This sends food to the trash bin, preferably in a tightly sealed container.
Aerobic digestion: This is basically compost. The pile needs access to air to function and can generate considerable heat. It leaves very little organic material behind.
Meat has very little carbs, so how is fermentation functioning Hákarl? What stops putrefaction?