The common reference for delta 13C Marine Carbonate Standard was obtained from a Cretaceous marine fossil, Belemnitella americana, from the PeeDee Formation in South Carolina. This material has a higher 13C/12C ratio than nearly all other natural carbon-based substances. For convenience it is assigned a delta 13C value of zero, giving almost all other naturally-occurring samples a positive delta value.

The original sample was used up long ago, but a Vienna-based laboratory calibrated a new reference sample to the original fossil, giving rise to the widespread use of the term Vienna- PeeDee Belemnite standard, abbreviated to V-PDB.