Stable isotope ratios of carbon and nitrogen were shown as the ratio of heavy to light carbon and nitrogen relative to Vienna Peedee Belemnite standard limestone (13C/12C) and atmospheric dinitrogen standards (15N/14N) in parts per thousand:X ‰ = (Rsample/Rstandard − 1) × 1000‰, where X = 13C or 15N and
R = 13C/12C or 15N/14N, respectively [15]