Since marrow is supposed to be almost pure fat it would be a little less than 100 calories per tablespoon.
Some vegetable oils have 118 calories per tbsp, canola has 124. Mineral oil has 120, but none of its calories can be used by us. Lard has 115.2 calories from fat per tbsp (not sure if it was measured raw). Does raw bison fat have less? How many milliliters fit in a tbsp? Do some parts of the marrow (shank etc) have different counts of calories than others? Or can it have a different calorie count in each tbsp depending on if the animal was a different age or gender? How about grass or grain fed, different number of calories in the fat or marrow?
Catch a number by the tail. There must be calorie books, listing all the foods. There used to be for cooked SAD foods. Maybe there could be an ethnic one, one from somewhere where they still eat the whole animal. If there's not, one of us can bridge the gap and make one. I wonder if that would be good.. babbling..
