The Middle Eocene Climatic Optimum (around 40 million years ago) was
a roughly 400,000-year-long global warming phase associated with an
increase in atmospheric CO2 concentrations and deep-ocean acidifcation
that interrupted the Eocene’s long-term cooling trend. The unusually
long duration, compared with early Eocene global warming phases, is
puzzling as temperature-dependent silicate weathering should have
provided a negative feedback, drawing down CO2 over this timescale. Here
we investigate silicate weathering during this climate warming event by
measuring lithium isotope ratios (reported as δ7
Li), which are a tracer for
silicate weathering processes, from a suite of open-ocean carbonate-rich
sediments. We fnd a positive δ7
Li excursion—the only one identifed for a
warming event so far —of ~3‰. Box model simulations support this signal
to refect a global shift from congruent weathering, with secondary mineral
dissolution, to incongruent weathering, with secondary mineral formation.
We surmise that, before the climatic optimum, there was considerable soil
shielding of the continents. An increase in continental volcanism initiated
the warming event, but it was sustained by an increase in clay formation,
which sequestered carbonate-forming cations, short-circuiting the
carbonate–silicate cycle. Clay mineral dynamics may play an important
role in the carbon cycle for climatic events occurring over intermediate
(i.e., 100,000 year) timeframes
