Interpreting carbon-isotope excursions: Strangelove oceans

Large negative excursions in marine carbonate δ ¹³ C are commonly associated with period boundaries and mass extinctions. Explanations for these events must be consistent with limitations imposed by carbon-isotope mass balance. At steady state (i.e., for excursions lasting more than 10 ⁵ yr), the surface ocean δ ¹³ C is set by the organic fraction of the total carbon burial rate and the magnitude of the photosynthetic isotope effect. The carbon-isotope composition of the deep ocean is most reflective of internal oceanic processes. The cessation of organic export from the surface ocean, such as is presumed to have caused the Strangelove ocean condition of the Cretaceous/Tertiary (K/T) boundary, leads to an isotopically homogeneous ocean in decades to centuries. If this condition persists, the ocean's isotopic composition approaches that of the riverine weathering input (in 10 ⁵ yr). Failure to approach this value during the K/T event suggests continued production and burial of organic carbon, dominantly in either terrestrial or shallow-marine environments. -from Author