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Here is a concise summary of the article:

Isotope Symbols, Units, and Reference Standards

The article discusses the isotopic properties and notations used for carbon in the Community Land Model (CLM).

Key points:

• Carbon has two primary stable isotopes: 12C and 13C, with 12C being the most abundant.
• The isotope ratio (R) is the mass ratio of the rare isotope (13C) to the abundant isotope (12C).
• Carbon isotope ratios are often expressed using delta (δ) notation, which represents the difference between the isotope ratio of a compound and the Pee Dee Belemnite (PDB) standard.
• Isotopic fractionation can be expressed using alpha (α) or epsilon (ε) notation, which describe the ratio of isotope ratios between two reservoirs.
• The unstable isotope 14C is also included in CLM and can be described using delta (δ14C) or delta-delta (Δ14C) notation, with the latter correcting for mass-dependent fractionation.
• For 14C calculations in CLM, the fractionation is set to zero, such that δ14C = Δ14C.