A negative carbon isotope excursion (CIE) of -5 per mil in marine carbonates at the Paleocene-Eocene boundary (PETM, ~56 Ma) is interpreted as rapid release of isotopically light carbon. Which source is most consistent with this signal?
AEnhanced weathering of silicate rocks
BMassive release of methane from gas hydrates (delta-13C ~ -60 per mil) or thermogenic methane/CO2 from volcanic intrusion into organic-rich sediments (delta-13C ~ -30 to -50 per mil), both of which would shift ocean DIC toward more negative values
CIncreased productivity by marine phytoplankton
DRelease of 13C-enriched volcanic CO2
A rapid negative CIE requires addition of large quantities of 13C-depleted carbon to the ocean-atmosphere system. Methane hydrate dissociation (delta-13C ~ -60 per mil) and thermogenic carbon release are both very negative. Mantle-derived volcanic CO2 (delta-13C ~ -5 per mil) is similar to ocean DIC and cannot drive a significant negative excursion. The mass balance requires enough isotopically light carbon to shift the entire ocean-atmosphere reservoir.
Question 2 True / False
All photosynthetic organisms produce organic matter with the same carbon isotopic composition of approximately -25 per mil.
TTrue
FFalse
Answer: False
The delta-13C of organic matter depends on the photosynthetic pathway. C3 plants (most trees, rice, wheat) produce organic matter at -24 to -30 per mil. C4 plants (corn, sugarcane, tropical grasses) fractionate less, producing values of -10 to -14 per mil due to pre-concentration of CO2 by PEP carboxylase. Marine phytoplankton range from -18 to -28 per mil depending on CO2 availability and growth rate. Methanogens produce extremely 13C-depleted methane (-50 to -110 per mil). These differences enable source identification in sedimentary organic matter, food web studies, and paleodiet reconstruction.
Question 3 Short Answer
Explain why a positive delta-13C excursion in marine carbonates is interpreted as enhanced organic carbon burial.
Think about your answer, then reveal below.
Model answer: The ocean's dissolved inorganic carbon (DIC) pool has a steady-state delta-13C maintained by the balance between carbon inputs (volcanic/weathering CO2 at ~-5 per mil) and outputs (carbonate burial at ~0 per mil, organic carbon burial at ~-25 per mil). When organic carbon burial increases, more 12C is preferentially removed from the ocean, leaving the remaining DIC pool enriched in 13C. Marine carbonates precipitating from this enriched DIC record higher delta-13C values. The magnitude of the excursion reflects the fraction of total carbon burial that is organic -- a fundamental parameter in Earth's carbon cycle and atmospheric O2 regulation.
Positive delta-13C excursions record periods when the fraction of carbon buried as organic matter (f-org) increased, drawing down 12C and leaving the ocean isotopically heavier.