An improved estimate of microbially mediated carbon fluxes from the Greenland ice sheet

Microbially mediated carbon fluxes on the surface of the Greenland ice sheet (GrIS) were
recently quantified by Hodson and others (2010) using measurements of the surface coverage of debris
(cryoconite) and rates of biological production associated with debris near the ice-sheet margin. We
present updated models that do not assume the same spatial uniformity in key parameters employed by
Hodson and others (2010) because they make use of biomass distribution and biological production data
from a 79 km transect of the GrIS. Further, the models presented here also include for the first time
biomass associated with both cryoconite holes and surficial algae. The predicted annual carbon flux for a
small (1600km2) section of ice surrounding the field transect is about four times that estimated using
spatially uniform biomass and production in this area. When surficial algae are included, the model
predicts about 11 times more carbon fixation via photosynthesis per year than the cryoconite-only
models. We therefore suggest that supraglacial carbon fluxes from the GrIS have previously been underestimated
by more than an order of magnitude and that the hitherto overlooked surficial algal ecosystem
can be the most crucial contributor. The GrIS is shown to be in a relatively stable state of net autotrophy
according to our model and so a strong link between bare-ice area and total carbon fluxes is evident. The
implication is a biomass feedback to surface albedo and enhanced ablation as a result. Climate
predictions for the year 2100 show that greater carbon fixation could also result from climate warming.
INTRODUCTION