Holocene climate conditions and glacier variation in central Iceland from physical modelling and empirical evidence

Lacustrine sediment cores from proglacial lake Hvítárvatn, central Iceland, reveal a detailed chronology of Holocene sedimentation, from which environmental conditions and the attendant fluctuations of Langjökull ice cap have been interpreted. We apply a numerical ice-sheet model to determine the climatic conditions under which the empirical reconstruction is glaciologically plausible. Modelling constraints are derived from core records of diatom concentration, benthic diatom fraction, and ice-rafted debris occurrence, as well as lake bottom morphology and the present-day ice-cap geometry. Holocene simulations driven by the NGRIP δ18O record that are consistent with the empirical constraints show the most extensive advance of Langjökull ice cap to be its most recent, beginning somewhere between 5 and 3 ka BP. Ice advance in response to the 8.2 ka BP cold event is followed by several thousand years of nearly ice-free conditions in the mid-Holocene. All simulations suggest that the maximum Holocene stand of the ice cap occurred during the Little Ice Age (LIA); those consistent with the constraints show little to no ice advance into Hvítárvatn before ∼1kaBP and indicate the lake area occupied by ice was much greater during the LIA than at any previous time. The most plausible simulation results were obtained for a maximum Holocene warming of 3-4∘C relative to the 1961-1990 reference, twice the Arctic average, and for Holocene Thermal Maximum (HTM) precipitation amounts comparable to or slightly greater than the modern. Reconciling the simulated subglacial discharge record to the empirically derived sediment volumes and emplacement times requires mean Holocene sediment concentrations of 0.8-1.6kgm-3. These estimates increase to 1.4-2.0kgm-3 when sedimentation rates are highest.