4. Anderson, LS; Flowers, GE; Jarosch, AH; Adalgeirsdottir, GT; Geirsdottir, A; Miller, GH; Harning, DJ; Thorsteinsson, T; Magnusson, E; Palsson, F.Holocene glacier and climate variations in Vestfiroir, Iceland, from the modeling of Drangajokull ice cap.Quat. Sci. Rev., 2018, 190: 39-56 Holocene glacier and climate variations in Vestfiroir, Iceland, from the modeling of Drangajokull ice cap
Holocene; Glaciation; Glaciology; North Atlantic; Geomorphology; Glacial inception; Modelling; Little ice age; Holocene Thermal Maximum; Deglaciation
Drangajokull is a maritime ice cap located in northwest (Vestfiroir) Iceland. Drangajokull's evolution is therefore closely linked to atmospheric and ocean variability. In order to better constrain the Holocene climate and glacier history of Vestfiroir we model the past evolution of Drangajokull ice cap. Simulations from 10 ka to present are forced by general circulation model output, ice-core-based temperature reconstructions, and sea-surface temperature reconstructions. Based on these 10-thousand year simulations, Drangajokull did not persist through the Holocene. We estimate that air temperatures were 2.5-3.0 degrees C higher during the Holocene Thermal Maximum than the local 1960-1990 average. Simulations support Drangajokull's late Holocene inception between 2 and 1 ka, though intermittent ice likely occupied cirques as early as 2.6 ka. Drangajokull is primarily a Little Ice Age ice cap: it expanded between 1300 and 1750 CE, with the most rapid growth occurring between 1600 and 1750 CE. The maximum Holocene extent of Drangajokull occurred between 1700 and 1925 CE, despite the lowest late Holocene temperatures, occurring between 1650 and 1720 CE. Between 1700 and 1925 CE temperatures were likely 0.6-0.8 degrees C lower than the 1950-2015 reference temperature. The modern equilibrium line altitude (ELA) is bracketed by topographic thresholds: a 1 degrees C temperature increase from the modern ELA would eliminate the ice cap's accumulation area, while a reduction of 0.5 degrees C would lead to the rapid expansion of the ice cap across Vestfiroir. The proximity of Drangajokull to topographic thresholds may explain its late inception and rapid expansion during the Little Ice Age. (C) 2018 Elsevier Ltd. All rights reserved. DOI
3.Flowers, GE.Hydrology and the future of the Greenland Ice Sheet.Nat. Commun., 2018, 9 Hydrology and the future of the Greenland Ice Sheet
Detection, attribution and projection of mass loss from the Greenland Ice Sheet has been a central focus of the glaciological community, with surface meltwater thought to play a key role in feedbacks that could accelerate sea-level rise. While the prospect of runaway sliding has faded, much remains uncertain when it comes to the role of surface runoff and subglacial discharge in Greenland's future. DOI PubMed
2.Flowers, GE; Bjornsson, H; Geirsdottir, A; Miller, GH; Black, JL; Clarke, GKC.Holocene climate conditions and glacier variation in central Iceland from physical modelling and empirical evidence.Quat. Sci. Rev., 2008, 27: 797-813 Holocene climate conditions and glacier variation in central Iceland from physical modelling and empirical evidence
Lacustrine sediment cores from proglacial lake Hvitarvatn, central Iceland, reveal a detailed chronology of Holocene sedimentation, from which environmental conditions and the attendant fluctuations of Langjokull 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 delta O-18 record that are consistent with the empirical constraints show the most extensive advance of Langjokull 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 Hvitarvatn before similar to 1 ka BP 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 degrees 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 0.8-1.6 kg m(-3). These estimates increase to 1.4-2.0 kg m(-3) when sedimentation rates are highest. (C) 2008 Published by Elsevier Ltd. DOI
1.Flowers, GE; Bjornsson, H; Geirsdottir, A; Miller, GH; Clarke, GKC.Glacier fluctuation and inferred climatology of Langjokull ice cap through the Little Ice Age.Quat. Sci. Rev., 2007, 26: 2337-2353 Glacier fluctuation and inferred climatology of Langjokull ice cap through the Little Ice Age
Emerging paleoclimate records from proglacial lake Hvitarvatn, central Iceland, suggest that Langjokull ice cap attained its maximum Holocene extent within the last 400 years. With the aim of constructing glaciological models and appropriate model inputs for Holocene simulations of Langjokull, we begin by simulating the evolution of Langjokull through the Little Ice Age to present, a period for which we have some constraint on ice-cap geometry. Using modern measured mass balance distributions (1997-2003) and meteorological data from nearby Hveravellir, we derive a reference precipitation field for the period 1961-1990 over the ice cap. Our simulations suggest Langjokull attained its maximum Little Ice Age volume around 1840 with a second local maximum around 1890. The two outlet glaciers terminating in Hvitarvatn, Norourjokull and Suourjokull, advance slowly into the lake, occupying their maximum lake area in the late 19th century, and retreat comparatively rapidly in the mid- to late 20th century. Simulations of Norourjokull are much more faithful to the geomorphic evidence than are simulations of Suourjokull, potentially suggesting a difference in dynamics between these two glaciers. While only 35% of the Hvitarvatn catchment area is ice-covered, meltwater from Langjokull comprises similar to 70% of the water input to the lake. Two-thirds of this input from the ice cap is transported as groundwater. Simulated glacier-derived discharge to the lake through the Little Ice Age suggests that a sediment concentration of 1.5 kg m(-3) would have resulted in the transport of 1.5 x 10(11) kg of material to the lake over the last 300 years, comparable to the estimated mass of sediment in the most recently deposited sedimentary unit in the lake. (C) 2007 Elsevier Ltd. All rights reserved. DOI
