The Toarcian oceanic anoxic event (T-OAE; ∼183 million y ago) is
possibly the most extreme episode of widespread ocean oxygen
deficiency in the Phanerozoic, coinciding with rapid atmospheric
pCO2 increase and significant loss of biodiversity in marine faunas.
The event is a unique past tipping point in the Earth system, where
rapid and massive release of isotopically light carbon led to a major
perturbation in the global carbon cycle as recorded in organic and
inorganic C isotope records. Modern marine ecosystems are projected
to experience major loss in biodiversity in response to
enhanced ocean anoxia driven by anthropogenic release of greenhouse
gases. Potential consequences of this anthropogenic forcing
can be approximated by studying analog environmental perturbations
in the past such as the T-OAE. Here we present to our knowledge
the first organic carbon isotope record derived from the
organic matrix in the calcite rostra of early Toarcian belemnites.
We combine both organic and calcite carbon isotope analyses of
individual specimens of these marine predators to obtain a refined
reconstruction of the early Toarcian global exogenic carbon cycle
perturbation and belemnite paleoecology. The organic carbon isotope
data combined with measurements of oxygen isotope values
from the same specimens allow for a more robust interpretation of
the interplay between the global carbon cycle perturbation, environmental
change, and biotic response during the T-OAE. We infer
that belemnites adapted to environmental change by shifting their
habitat from cold bottom waters to warm surface waters in response
to expanded seafloor anoxia.