VPDB (Vienna – Peedee Belemnite) and Its Importance in Carbon Isotope Studies

Peedee Belemnite (PDB) is a well-known standard used in carbon and oxygen isotope studies. Originally derived from the fossilized remains of Belemnitella americana, a species of belemnite found in the Late Cretaceous peedee Formation in South Carolina, USA, PDB has played a crucial role in geochemistry, paleoclimatology and biogechemistry.

Carbon Isotopes and PDB

Carbon exists in nature primarily as two stable isotopes, ¹²C and ¹³C. The ratio of these isotopes in different materials provides insights into various biological and geological processes.

To compare isotopic compositions, scientists use the δ¹³C notation, which express the difference in the ratio of ¹³C to ¹²C between a sample and a standard (PDB).

This value is in per mil (‰) also called per thousand.

• Positive δ¹³C values indicate enrichment in ¹³C relative to PDB or reference.
• Negative δ¹³C values indicate depletion in ¹³C compared to PDB or reference.

PBD to VPDB transition

Now the original PDB standard is not available, as the supply of Peedee Belemnite fossils has been exhausted. To maintain continuity in isotopic studies, the Vienna-Peedee Belemnite (VPDB) standard was introduced. VPDB is calibrated to match the original PDB scale, ensuring consistence in isotope ratio comparisons worldwide.

Applications

1. Paleoclimatology : δ¹³C values in marine carbonates help reconstruct past oceanic and atmospheric conditions.
   • K-Pg extinction (66Ma) A sharp -ve δ¹³C shift in marine carbonates corresponds with the impact event that led to the extinction of dinosaurs. This suggests a collapse in primary productivity, possibly due to reduced photosynthesis after the asteroid impact.
   • PETM (56Ma) The Paleocene-Eocene thermal maxima, A significant δ¹³C excursion indicates a massive release of carbon into the atmosphere, possible from methane hydrates or volcanic activity, leading to global warming and ocean acidification.
2. Fossil Fuel Studies : Carbon isotope signatures distinguish between different carbon sources, such as fossil fuels and biomass.
3. Biogeochemistry : Isotope ratio provide insights into carbon cycling in ecosystems.

Other examples

• Industrial Revolution and δ¹³C :- Since the mid-19th century, δ¹³C values in marine carbonated have shown a steady decline, reflecting increased CO2 emission from fossil fuel combustion. This phenomenon known as Suess Effect.
  • It is a result of burning of old carbon reservoirs (coal, oil, natural gas) that are depleted in ¹³C , thereby lowering δ¹³C values in the atmosphere and oceans.