[Yo, webmaster, I've tried to respond three times to High Hopes here, but every time I click Post Reply it just hangs. I will try it without hyperlinks on the research papers to see if it goes through.]
I searched for every published study on δ13C values in American pork, and the titles below were pretty much all I could find. I don't recall seeing a study by Ayotte that surveyed pork δ13C values (and I tried to check most of her work), but I'm happy to be corrected.
Application of Stable Isotope Ratio Analysis for Origin Authentication of Pork
Limitation: This study looked at pork that had been imported into S. Korea, which is very likely to be from mainstream sources and not pasture-raised.
Isotopic carbon turnover in pig hoof and rib
Limitation: This study only looked at how δ13C values change in pork as their diet is altered. It did not attempt to survey pork generally.
Carbon and nitrogen stable isotopes in fast food: Signatures of corn and confinement
Limitation: Did not look at pork. But the beef numbers and visuals are fantastic. Low δ13C values are rare, but they do occur. Must also keep in mind fast-food beef is the most homogenous mainstream beef available and does not represent a broad sample.
Excretion of 19-norandrosterone after consumption of boar meat
Limitation: It only studied wild boars in Germany.
FWIW, I am a generalist. Pretty good knowledge of the history of the FDA and regulatory capture. Worked in a research library for several years. Degree in Behavioral Sciences. Studied biology with Lorna Straus and Jerre Levy at UChicago, among others. Boring. I notice all of you jackals are anonymous.