Climate forced atmospheric CO2 variability in the early Holocene: A stomatal frequency reconstruction

The dynamic climate in the Northern Hemisphere during the early Holocene could be expected to have impacted on the global
carbon cycle. Ice core studies however, show little variability in atmospheric CO2. Resolving any possible centennial to decadal
CO2 changes is limited by gas diffusion through the firn layer during bubble enclosure. Here we apply the inverse relationship
between stomatal index (measured on sub-fossil leaves) and atmospheric CO2 to complement ice core records between 11,230 and
10,330 cal. yr BP. High-resolution sampling and radiocarbon dating of lake sediments from the Faroe Islands reconstruct a distinct
CO2 decrease centred on ca. 11,050 cal. yr BP, a consistent and steady decline between ca. 10,900 and 10,600 cal. yr BP and an
increased instability after ca. 10,550 cal. yr BP. The earliest decline lasting ca. 150 yr is probably associated with the Preboreal
Oscillation, an abrupt climatic cooling affecting much of the Northern Hemisphere a few hundred years after the end of the
Younger Dryas. In the absence of known global climatic instability, the decline to ca. 10,600 cal. yr BP is possibly due to
expanding vegetation in the Northern Hemisphere. The increasing instability in CO2 after 10,600 cal. yr BP occurs during a period
of increasing cooling of surface waters in the North Atlantic and some increased variability in proxy climate indicators in the
region.
The reconstructed CO2 changes also show a distinct similarity to indicators of changing solar activity. This may suggest that at
least the Northern Hemisphere was particularly sensitive to changes in solar activity during this time and that atmospheric CO2
concentrations fluctuated via rapid responses in climate.