Extensive loss of past permafrost carbon but a net accumulation into present-day soils

Atmospheric concentrations of carbon dioxide increased between
the Last Glacial Maximum (LGM, around 21,000 years ago) and
the preindustrial era1. It is thought that the evolution of this
atmospheric carbon dioxide (and that of atmospheric methane)
during the glacial-to-interglacial transition was influenced by
organic carbon that was stored in permafrost during the LGM and
then underwent decomposition and release following thaw2,3. It has
also been suggested that the rather erratic atmospheric δ13C and
Δ14C signals seen during deglaciation1,4 could partly be explained
by the presence of a large terrestrial inert LGM carbon stock,
despite the biosphere being less productive (and therefore storing
less carbon)5,6. Here we present an empirically derived estimate of
the carbon stored in permafrost during the LGM by reconstructing
the extent and carbon content of LGM biomes, peatland regions
and deep sedimentary deposits. We find that the total estimated soil
carbon stock for the LGM northern permafrost region is smaller
than the estimated present-day storage (in both permafrost and
non-permafrost soils) for the same region. A substantial decrease
in the permafrost area from the LGM to the present day has been
accompanied by a roughly 400-petagram increase in the total soil
carbon stock. This increase in soil carbon suggests that permafrost
carbon has made no net contribution to the atmospheric carbon
pool since the LGM. However, our results also indicate potential
postglacial reductions in the portion of the carbon stock that is
trapped in permafrost, of around 1,000 petagrams, supporting
earlier studies7. We further find that carbon has shifted from being
primarily stored in permafrost mineral soils and loess deposits
during the LGM, to being roughly equally divided between
peatlands, mineral soils and permafrost loess deposits today.