Good idea! How about C13/C12?
It seems to me there would be better readings for the carbon isotopes, since the current atmosphere is mostly CO2.
Why is it hard to believe that Mars could lose many atmospheres in 4 billion years. That is a hell of a long time. Losing 4 atmospheres would require a loss rate of about 1 x 10 ^ -6 millibar/year, or 1 millibar per million years. What is more difficult to imagine is how a planet can retain its atmosphere for 4 billion years. The loss rate needs to be practically nil.