A seasonality trigger for carbon injection at the Paleocene–Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum
(PETM) represents a ∼ 170 kyr episode of anomalous global
warmth ∼ 56 Ma ago. The PETM is associated with rapid
and massive injections of 13C-depleted carbon into the
ocean–atmosphere system reflected as a prominent negative
carbon isotope excursion (CIE) in sedimentary components.
Earth’s surface and deep ocean waters warmed by ∼ 5
◦C,
of which part may have occurred prior to the CIE. However,
few records document continental climatic trends and
changes in seasonality have not been documented. Here we
present the first high-resolution vegetation and paleoclimate
reconstructions for the PETM, based on nearest living
relative analysis of terrestrially derived spore and pollen
assemblages preserved in an expanded section from the
central North Sea. Our data indicate reductions in boreal
conifers and an increase in mesothermal to megathermal
taxa, reflecting a shift towards wetter and warmer climate.
We also record an increase in summer temperatures, greater
in magnitude than the rise in mean annual temperature
changes, and a shift to a summer-wet seasonality. Within the
CIE, vegetation varies significantly with initial increases in
epiphytic and climbing ferns, and development of extensive
wetlands, followed by abundance of Carya spp. indicative of
broadleaf forest colonization. Critically, the change in vegetation
we report occurs prior to the CIE, and is concomitant
with anomalous marine ecological change, as represented
by the occurrence of Apectodinium augustum. This suggests
that amplifications of seasonal extremes triggered carbon
injection.